4D MULTICOMPONENT SEISMIC CHARACTERIZATION OF

GLORIETA PADDOCK CARBONATE RESERVOIR

AT VACUUM FIELD, NEW MEXICO

by

ii

Catalina Acua

A thesis submitted to the Faculty and the Board of Trustees of the Colorado

School of Mines in partial fulfillment of the requirements for the degree of Master

of Science (Geophysics).

Golden, ColoradoDate ___________________

Signed: ___________________________Catalina Acua

Approved: __________________________Dr. Thomas L. DavisProfessor of GeophysicsThesis Advisor

Golden, ColoradoDate____________________

___________________________

iii

Dr. Terence K. YoungProfessor and HeadDepartment of Geophysics

ABSTRACT

A 4D-3C seismic volume has been interpreted to characterize the Glorieta

Paddock reservoir at Vacuum Field, New Mexico. This reservoir has been under

water flood since 1992. Seismic velocity, amplitude and anisotropy attributes

have been analyzed in a time-lapse manner.

Compressional and shear waves have been able to detect changes within the

reservoir due to water flooding and the results are supported by production data.

The results achieved in this study suggest that it is possible to monitor water

flood in a carbonate reservoir using multicomponent data.

iv

TABLE OF CONTENT

ABSTRACT iii

LIST OF FIGURES vi

LIST OF TABLES. x

AKNOWLEDGEMENTS.. xi

1. INTRODUCTION 1

1.1 Vacuum Field 11.2 Data Set. 21.3 Seismic Data Processing 61.4 Characterization Methodology.... 10

1.4.1 Shear wave method.. 111.4.2 4D or seismic monitoring method... 13

2. GEOLOGIC BACKGROUND.. 16

2.1 Tectonic history of Permian Basin and Delaware Basin 162.2 Origin of Delaware Basin: 192.3 Local geology and rock characteristics of the main producers.. 20

3. DATA CALIBRATION.. 27

3.1 Introduction 273.2 Calibration P-waves. 27

v3.3 Calibration S-waves. 32

4. MALJAMAR SEISMIC INTERPRETATION.. 37

4.1 Introduction.. 37 4.2 Structural interpretation. Coherence cube.. 41

5. PRODUCTION DATA AND RESERVOIR ENGINEER.. 53

5.1 Production History.. 535.2 Reservoir performance. 545.3 Reservoir fluid data 585.4 Data available. 585.5 Production maps. 60

6. 4D MULTICOMPONENT SEISMIC ANALYSIS.. 68

6.1 Coherence cube P-waves..... 686.2 P-waves.. 73

6.2.1 Structure maps in time. 736.2.2 Time Delay analysis. 73

6.3 Multicomponent S1 and S2... 786.3.1 Time Delay analysis. 796.3.2 Anisotropy analysis 826.3.3 Amplitude analysis. 91

6.4 Gassmann modeling.. 976.5 Reliability of the data. 103

7. CONCLUSIONS AND RECOMMENDATIONS 110

7.1 Conclusions. 1107.2 Recommendations. 110

vi

8. REFERENCES. 115

TABLE OF FIGURES

1.1 Regional map showing geological location of Vacuum Field 3

1.2 Location of Vacuum Glorieta Unit.. 4

1.3 Index map showing relative location of seismic survey areas... 5

1.4 Shear wave splitting.. 12

2.5 Subsidence curves from the Permian Basin. 19

2.6 Stratigraphy column of the Permian Basin 21

2.7 Type log for the well Bridges State #95. 22

2.8 Core photos of the Paddock formation.. 24

2.9 Depositional setting for the Paddock interval.. 25

2.10 Core photo of the Glorieta interval 26

3.11 Location of the wells with synthetic seismograms 28

3.12 Synthetic seismogram for the well WS-13 (P-waves). 303.13 Synthetic seismogram for the well VGWU-127 (P-waves).. 313.14 Comparison between Trace 49 and corridor stack for S1-wave . 36

vii

3.15 Time depth table for P and S waves.. 36

4.16 Tie between VSP-127 and seismic data from Maljamar. 384.17 Inline from Maljamar survey .. 394.18 The Glorieta map in time 40

4.19 Volumetric visualization of the Glorieta map in time 41

4.20 Time slice 934 ms from coherence cube.. 44

4.21 Faults interpreted in TS 934 using faults from seismic sections .. 45

4.22 Block diagram showing linked faults and relay ramps 46

4.23 Block diagram of the possible spatial distribution of relay ramps .. 47

4.24 Time slice from the coherence cube at 918 ms 48

4.25 Interpretation of faults using just coherence analysis . 494.26 Time slice from the coherence cube at 1046 ms.. 50

4.27 Time slice showing how discontinuities disappear in depth 51

4.28 Diagram. Transference zone between two overlapping faults 52

5.29 Historical production of the field before unitization ..................................55

5,30 Historical production of the field after unitization ......................................57

5.31 Water injection wells with water injection profiles ....................................615.32 Total oil production maps (1997 and 1998) ...............................................625.33 Total gas production maps (1997 and 1998) ............................................635.34 Total water production maps (1997 and 1998) .........................................635.35 Water cut maps (1997-1998) .......................................................................64

viii

5.36 Total water injection maps (1997 and 1998) .............................................655.37 Average water injection pressure maps (1997 and 1998) ......................65

5.38 FVF vs reservoir pressure (oil and gas) .....................................................666.39 Time slices from the Vacuum survey coherence cube ............................70

6.40 Structural model for the Paddock dolomite ...............................................71

6.41 The Maljamar and Vacuum coherence cubes comparison ....................726.42 Line extracted from P-wave and S-wave volumes ...................................74

6.43 The Glorieta and the Paddock structural maps in time ...........................74

6.44 P-wave relative velocity differences ...........................................................75

6.45 P-wave relative velocity differences & structural model ..........................77

6.46 Relative Sl and S2 velocity increase ............................................................80

6.47 Relative S1 and S2 velocity decrease ..........................................................80

6.48 Inline 66 extracted from S1 and S2, pre and post, volumes .....................84

6.49 Anisotropy maps (pre and post) .................................................................856.50 Anisotropy difference map ..........................................................................87

6.51 Inline 55. Intervals used in anisotropy computations ...............................89

6.52 Anisotropy maps computed for two different windows ............................90

6.53 Interpretation of a "middle" event in all S-volumes ..................................92

6.54 Time delay in S2. Relationship with "middle" event .................................... 93

ix

6.55 RMS amplitude difference maps (S1 and S2 wave) .................................... 946.56 RMS amplitude ratio (Sl/S2) pre and post .................................................... 95

6.57 RMS amplitude ratio difference ..................................................................... 96

6.58 Anisotropy maps computed in the overburden (800-1500ms) ...............1066.59 Anisotropy maps computed in the overburden (10000-1500ms) ...........1076.60 Anisotropy maps computed in the overburden (800-1000ms) ...............1086.61 Crossline 80. base and repeat S-wave traces side by side ....................109

xLIST OF TABLES

1.1. Phase VII compressional wave data processing flow .................................7

1.2. Phase VII shear wave final processing sequences ....................................9

1.3. Phase VII shear wave final statics values ..................................................10

5.4. Oil properties and initial reservoir characteristics ......................................59

6.5 Bulk modulus and density for oil and water..................................................99

6.6 Bulk and shear modulus of the mineral for dolomite and limestone .......... 99

6.7 Parameters used in the Vp and Vs calculations .........................................100

6.8 Compressional and shear velocities calculated with Gassmann .............102

xi

ACKNOWLEDGMENTS

I wish to express my appreciation to the following faculty members at Colorado

School of mines: Dr. Thomas Davis for his advice, support, guidance and his

enormous encouragement. Without his foresight and expertise I would not have

appreciated the significance of this dataset and the results of this study. Thanks

Tom.

I want to thank the members of my committee: Dr. Michael Batzle and Dr.

Robert Benson. Especially for Mike I have the deepest gratitude feeling. I thank

him as a scientific, for all the hours he dedicated to explain me several aspects of

the fascinating geophysics as a science in his, no less fascinating, lab. I also

thank him as a person, for his unconditional support and understanding of many

other students and me, more as persons than as students.

Much appreciation to TEXACOs people: Mike Raines, Robert Martin and

Kevin Hickey for their support and expertise.

My very special thanks to the members of RCP Phase VII and Phase VIII.

Especially people like Gwenola, Luca, Raul, Reynaldo and David were crucial to

the success of my research, and were totally necessary to make my life happier

during these two years.

xii

I want to express my sincere thanks to Lucy and Ed Jenner for their

unconditional and valuable help reading and correcting my thesis. I could not ask

for better people to do that annoying job: professionally perfectly capable, andpersonally just great. Thanks for being an example of a wonderful couple. The daily support of my friend Ronny made my life at Colorado School of

mines something special and beautiful. Thanks Ronny for helping me,

understand me and for always being there when I needed you.

I thank my parents for their education, their encouragement and their love.

Without them none of my goals could have been reached.

Finally, and above all, I thank Maite, my daughter, and Werner, my love;

without their love, understanding and unbelievable patience, the successful

culmination of this enormous task would have not been possible.

11. INTRODUCTION

1.1 VACUUM FIELD

Vacuum Field is a mature operating field located 20 miles west of the town of

Hobbs in Lea County, New Mexico. This field is located on the northwestern shelf

of the Delaware Basin, on a large east-northeast anticlinal structure formed by

drape of sediments over a basement controlled fault block. Regionally, the study

area forms part of the Northwestern Shelf of the Permian Basin region. The

Permian Basin is a segmented foreland basin composed of two structural

depressions known as the Delaware and Midland basins, separated by the north-

south trending uplift, the Central Basin Platform (Figure 1). The data comespecifically from Central Vacuum Unit, which includes approximate 7 sections (7square miles) and is operated by Texaco. The Glorieta Paddock interval is a carbonate reservoir with an extended

production history (Chapters 2 and 5). RCP Phase VII was originally conceived in an attempt to develop seismic

techniques to monitor associated with CO2 flooding of the San Andres Formation

the primary producing unit in the Permian Basin. The data available imaged

2deeper intervals so that it could be used to determine the reservoir performance

of Glorieta Paddock interval as well.

The Glorieta Paddock interval is being produced by Texaco and Phillips, in

two production units called Vacuum Glorieta West and Vacuum Glorieta East

respectively (Figure 2). This study is focused on the southeastern portion ofVacuum Glorieta West Unit, since the seismic data available was recorded in the

overlapping of both the Central Vacuum and the Glorieta Units (Figure 2).

1.2 DATA SETS:

RCP Phase VII started July 1, 1997 and concluded December 30, 1999. The

project involved a multidisciplinary study of a CO2 injection program for SanAndres reservoir. This project was extended to the study of deeper intervals ofGlorieta - Paddock and Drinkard - Abo in August 1999.

Several data sets have been made available to RCP Phase VII study in

Vacuum Field, specifically for the Glorieta Paddock interval. Beside the seismic

and well data provided for Central Vacuum Unit, a complete set of production

data including well logs have been provided for Vacuum Glorieta West Unit in the

interval of interest.

The seismic data available includes a large seismic survey and a subset of

two surveys recorded by RCP in Phase VII to monitor the reservoir (Figure 3).

3Figure 1. Regional map showing geological location of Vacuum Field andstructural provinces of the Permian Basin. Black spots are oil and gas fieldsproducing from the different units in the Permian Basin (after Mattocks, 1998).

4Figure 2. Location of Vacuum Glorieta Unit and the respective sub-division of Vacuum Glorieta West Unitoperated by Texaco since the unitization in 1992. The 4D area corresponds to the overlapping of CentralVacuum Unit and Vacuum Glorieta West Unit.

5Figure 3. Index map showing relative location of seismic survey areas and well location for Phase VII SanAndres and the Glorieta Paddock reservoir characterization.

6This subset consists of two 3D-3C (3-dimensional multicomponent) seismicsurveys. The baseline survey was acquired in December 1997, prior to the CO2

injection program in San Andres. The monitoring survey took place on December1998.

In conjunction with each seismic survey, a downhole 3D-3C seismic survey, aVSP and a walk-away VSP were acquired in the well VGWU-127 (Figure 3). Apassive seismic recording system was also used to detect induced seismicity in

the reservoir interval prior to the second 3D seismic survey.

1.3 SEISMIC DATA PROCESSING

Since the main objective of this study is to pursue a reliable 4D interpretationof the Glorieta - Paddock, the processing flows applied to the baseline and

repeat survey are key in the development of the dynamic interpretation.

The main objective was to enhance the shear wave quality. Compressionalwaves were processed with the same parameters shown in Table 1. The primary

difference for Phase VII was a new approach in the calculation of residual statics.

Shallow evaporite dissolution beds formed playa lakes at the surface

influencing the position of the seismic events at depth (Mndez, 1999). Newstatics were calculated for both baseline and repeat volumes in P-wave and S-

wave. The new static values applied to P-wave volume are listed in Table 3. To

preserve the repeatability of the data during the 4D interpretation the seismic

7P-wave

Geometry descriptionSource/Receiver trace editRefraction static estimationMinimum phase conversionQ-compensationSurface consistent deconvolution (source/ receiver domain)Cross-correlation of common-trace pairs (P-wave source / verticalcomponent)Apply source and receiver phase corrections to match surveysResidual statics / Velocity estimationsIndividual trace editSurface consistent amplitude correctionsCMP stackFX Filter (spatial deconvolution: Inline Crossline)Spectral shapingBandpass filter3D migration (phase shift algorithm with single velocity model from stackingvelocities and VSP survey)

Table 1. Phase VII Compressional wave data processing schematic flow.

8 processing flows for both P-wave and S-wave volumes (Table 2) were the samefor the baseline and the repeated surveys. It is important to state that the

deconvolution operator used was the same for both the baseline and the repeat

survey. With this it is anticipated that the changes in frequency and phase that

could be observed in the wavelet would be due to changes intrinsic to the

reservoir through time.

The final shear wave processing sequences were determined through

multiple processing tests and even the complete reprocessing of the pre-injectionvolumes. It was determined that the playa-lakes were also affecting the shear

wave image locally. As a consequence, new processing tests and the

reprocessing of the multicomponent pre-injection data set was planned toreevaluate the effects of playa-lakes on seismic signal. A new strategy, involving

increased trace editing as well as new deconvolution operators and velocity

analysis was employed to improve the shear wave image. From the 650,000

traces, approximately 10,000 were killed in the vicinity of the playa-lakes during

the initial pre-injection data processing. After reprocessing the pre-injection survey, the new data showed morecontinuous reflections and more reliable time delays for anisotropy and time-

lapse analysis (Mndez, 1999; Cabrera, 1999). The same polarization angle of118 degrees used in Phase VI was determined for Phase VII using VSP

polarization information at well VGWU-17 to separate the S1 and S2 components.

9WHERE PROCESSING PARAMETERWAS DETERMINED

Pre PostPROCESS APPLIEDS1 S2 CO2 CO2 Phase VI

- Reformat SEGD field tapes to internal format * * * *- Build identical geometries S1H1, S1H2, S2H1, S2H2 * * * *- Reverse traces * * * * *- Rotate 118 degrees S1H1, S1H2, S2H1, S2H2 datavolumes

* * * *

- True amplitude correction, time raised power .8 * * * *- Surface consistent shot and receiver amplitude * * * *- Trace kills * * * *- Shift to processing datum, based on flattening staticsfrom Phase VI

*

- Surface consistent shot and receiver flattening statics * *- Residual statics, pass 1 and pass 2 * * *- NMO. 2nd pass Phase VII * * * *- Shot domain TX dip filtering * * * *- Receiver domain TX dip filtering * * * *- Inverse NMO, 2nd pass Phase VII * * * *- Bandpass Butterworth filter 6-18 Db, 60-72 Db * * * *- Minimum phase conversion * *- Notch filter 30 Hz noise for area receiver * * * *- Convolve receiver deconvolution filters * *- Convolve shot deconvolution filters * *- Residual statics, 3rd pass * * *- NMO final Phase VII * * *- Top mute * * *- CDP stack * * * *- FXY filter 8-50 Hz, 50% addback * * * *- TXY filter reject mode, pass real data * * * *- CDP fold taper * * * *- Finite difference migration, VSP velocity field * * * *

Table 2. Phase VII Shear wave final data processing sequences.

10

A more extensive discussion about the processing algorithms applied to each

survey, S1 and S2, pre-CO2 and post CO2, can be found in Mndez, 1999.

Table 2 shows the final processing sequences for both shear volumes and

where each processing parameter was determined. The new statics values

applied are listed in Table 3 together with P-wave statics values for Phase VII.

VOLUME STATICS PROCESS STATICS RANGEPhase VII total refraction statics -75 to 105 ms

P-wavePhase VII total statics -75 to 109 ms

Phase VI flattening statics +50 to 85 msPhase VII additional flattening statics +20 to 27 ms

Phase VII residual statics +14 to 14 msS-wave

Phase VII total statics +60 to 120 ms

Table 3. Phase VII P-wave and S-wave statics values, after Mndez (1999).

1.4 CHARACTERIZATION METHODOLOGY

11

In order to characterize the Glorieta Paddock interval with the data available

several methods were applied. Well log data were used to create synthetic

seismograms to tie depth with time for the compressional and multicomponent

volumes. Production data were analyzed to explain the seismic results obtained

from the 4D point of view and shear wave anisotropy, shear and compressional

velocity and shear amplitude analyses were made in a time-lapse sense.

1.4.1 Shear wave method

In an anisotropic medium, shear waves are split into fast (S1) and slow (S2)models. Particle motion within the S1 component is parallel to the orientation of

the features creating anisotropy (e.g. fractures), with the S2 particle motionoriented perpendicularly (Figure 4). The time difference between correspondingevents on the two principal time series directions and a slow component,

perpendicular to the fracture direction, provides a measure of average anisotropy

over extended depth intervals (from Martin and Davis, 1987). Shear wave anisotropy estimates are computed by normalizing processed

fast (S1) and slow (S2) shear wave isochron differences as follows:

121

tStStS

AnisotropyD

D-D= (1)

12

Where DtS1 is the isochron of the interval of interest in the S1 volume and DtS2 is

the correspondent isochron in the S2 volume. The subtraction is normalized by

DtS1 and multiplied by 100 so that the result is given in percentage.

13

Figure 4. Shear wave splitting. When a shear wave enters an azimuthallyanisotropic medium (the anisotropy is here represented by a set of alignedfractures), it is effectively split into a fast (S1) component parallel to fractureorientation, and a slow (S2) component, polarized perpendicular to the fracturetrend. The time delay between S1and S2 component is illustrated (Martin andDavis, 1989).1.4.2 4D seismic monitoring method

Time-lapse seismic is the link between seismic observables and reservoir

variables such as fluids, pressure or temperatures through time.

Most of the information regarding the 4D method was taken from Jack (1997),and it is extensively discussed in his book Time Lapse in Reservoir

Management.

A reservoir under a secondary recovery program (water flood) may change itproperties through time. The properties that potentially altered include:

1. Pore pressure: It can fall as fluids are extracted from the reservoir, or it can

increase as water is injected.2. Reservoir pore fluids: may change as fluids are extracted. Pore fluid

properties could be very sensitive to pressure changes. Density and

saturation could vary.

3. Temperature: is usually fairly constant, but injection of cold water into areservoir causes a reduced temperature behind the flood front. Cooling of the

14

reservoir rock may change rock properties. The effects of the cold water bank

may show up on the time-lapse seismic in some cases.

4. Reservoir medium properties: there will be a change of density, porosity, and

compressibility with pressure due to deformation of the rock frame and the

pore fluids.

5. Microearthquakes and induced fractures: can occur during production due to

temperature reduction or effective stress changes caused by water injectionor compaction of the reservoir rock. The direction of the fracturing will be

controlled by local stress.

6. Reservoir rock properties: could change as a result of chemical interaction

between an injection fluid and the rock.The seismic observables that need to be analyzed and related with the

changes within the reservoir include:

1. Times on surface seismic data: Changes in event time due to a change in

seismic velocity due to extraction (production or injection) might be visible.

Seismic features are analyzed in terms of time differences (Dt or isochron) for

one specific interval (i.e. the reservoir interval) between the baseline and therepeat surveys.

2. Amplitudes: If we assume consistent data processing, amplitudes may vary

due to changes in the reservoir. Amplitude analysis in a time-lapse sense was

15

not meaningful for compressional waves, nonetheless amplitude analysis for

shear waves revealed some important information. This analysis was pursued

taking the ratios between S1 and S2 RMS amplitudes for each survey.

3. Anisotropy measurements: anisotropy measured in a time-lapse sense is

nothing but the difference in anisotropy measured between baseline and

repeat surveys. Just for convention, an anisotropy difference is given by:

)()( postAnisotropypreAnisotropyAnisotropy -=D (6)

Therefore, a negative difference of anisotropy means an increase in

anisotropy through time. On the other hand, a positive difference of anisotropy

means a decrease in anisotropy through time.

4. Frequency and phase: changes in frequency and phase could arise due to

changes in rock absorption (inelastic attenuation). They might also occur dueto effects resulting from changes in velocity within the rock and variable

conditions during the acquisition.

16

2. GEOLOGIC BACKGROUND

2.1 TECTONIC HISTORY OF PERMIAN BASIN AND DELAWARE BASIN

The tectonic evolution of the Permian Basin has been examined by many

authors including Horak (1985); Galley (1958); and Hills (1988) and it can beoutlined as follows:

Passive margin (late Precambrian to Mississippian, 850-310 ma.): theancestral Permian Basin occupied a passive margin characterized by weak

crustal extension and low rate subsidence (Horak, 1985). A broad, shallow,gently dipping depression known as the Tobosa Basin developed (Galley, 1958).Relatively uniform and widespread shelf carbonates and thin shales occupied the

basin (Hills, 1988). Collision phase (Late Mississippian through Pennsylvanian, 310-265 Ma): Thesignificant structural features that characterize the gross geometry of the

Permian Basin resulted during this period, as a result of the collision of North

America and Gondwana Land. This collision gave rise to the Oachita Marathon

fold belt. The ancestral Tobosa Basin was intensively deformed along high

angle basement faults and pre-existing zones of weaknesses (Horak, 1985).High heat flow, rapid basin subsidence and sedimentary filling took place during

17

this phase. Given its equatorial location at this period, broad carbonate shelves

developed along the western, northern, and eastern margins of the Permian

Basin. By Late Paleozoic time the original Tobosa Basin was divided into the

Delaware Basin, Central Basin Platform and Midland Basin.

Permian Basin phase (265-230 Ma.): during this phase, rapid filling of thebasin with fine to coarse-grained clastic and the development of extensive reef-

fringed carbonate/evaporite platforms and shelves proceeded until only the

Delaware Basin remained as a small depocenter (Horak, 1985). During thisperiod the Glorieta Paddock interval (with clastic and carbonate character) wasdeposited.

Stable Platform phase (Mesozoic, 230-80 Ma.): Mobility rates were low andstructural deformation was limited (Horak, 1985). Laramide deformation (Late Cretaceous through Early Eocene, 80-50 Ma.):The western side of the Permian Basin was elevated approximately 4000 ft.

Volcanic phase (Early Eocene through Middle Oligocene, 50-30 Ma.):Extension and crustal thinning followed the Laramide, which resulted in

widespread volcanic activity.

Basin and Range tectonism (recent, 24-0 Ma.): Rifting, crustal thinning, andhigh heat flow characterized the region from the western Delaware Basin across

the southwestern U.S. to California. Figure 5 illustrates subsidence curves from

18

different areas of Permian Basin and highlights the major tectonic phases andperiods of most active basin development.

2.2 ORIGIN OF DELAWARE BASIN:

The Delaware Basin of western Texas and south eastern New Mexico is a

major petroleum producing province about 200 miles long and 100 miles wide. Late Mississippian: In excess of 7000 ft of pre-Pennsylvanian sediments

were deposited in the central portion of the basin. Upthrusting of the Central

Basin Platform began at the end of the Mississippian.

Pennsylvanian: Rapid subsidence of the basin occurred. Clastics including

turbidites deposition dominated the early Pennsylvanian.

Permian: Rapid subsidence permitted the accumulation of up to 8000 mts of

sediment.

During the Early Permian considerable areas in the northern, north western,

and northeastern Delaware basin were shallow enough and free enough from

clastics for limestone shelves to develop. During the Middle Permian, subsidence

continued although not as rapidly as during the preceding Early Permian. The

amount of clastic influx decreased and carbonates formed. Evaporite deposits

were also common in restricted lagoons. The increase of clastic source in Late

Leonardian time occurred due to the renewed uplifts on the northwest. Late

Permian is characterized by tectonic calm.

19

Figure 5. Basement mobility profiles for each of the major provinces illustrate thetime and duration of successive tectonic phases which have affected thePermian Basin. Major tectonic phases and periods of most active basindevelopment are displayed (From Horak, 1985)

20

Deposition at this time consists predominately of limestone, although clastics

and evaporites are also common. During this period fault reactivation may have

occurred. Reef and other sedimentary features often formed on these structures

and were important for oil and gas traps. At the end of Permian time, stress

relaxation continued and local subsidence formed many salt pans on the floor of

the receding sea. In the northeastern part of Delaware basin, evaporation of

lagoonal brines proceeded to completely fill depressions, leaving large deposits

of potassium salts (Hill, 1970). Post Permian: lower clastic deposition rate. Compressional stage

accompanied the uplift of Delaware Mountains. Minor volcanism and intrusions,

as well as considerable faulting were the effects of this uplift.

2.3 LOCAL GEOLOGY AND ROCK CHARACTERISTICS OF THE MAIN PRODUCERS

The Vacuum Glorieta Field is located on the Vacuum structural High. Although

designated the Vacuum Glorieta Pool by the New Mexico Oil Conservation

Division, production is primarily from the Leonardian Paddock zone (Figure 6).The upper seal of the reservoir is the overlaying Glorieta interval, and the lower is

the Blinebry interval. These tops are shown in a type-log for the well Bridges

State #95 (Figure 7). The Paddock formation is subdivided into the Upper and Lower Paddock. The

Upper Paddock is subdivided into Upper Paddock Limestone and Upper

21

Era Period Epoch Formation GeneralLithologyApproximate

Thickness(ft)

Dewey Lake Redbeds/Anhydrite 200-400

Rustler Halite 100OchoanSalado Halite/Anhydrite 1000

Tansill Anhydrite /Dolomite 200

Yates Ss Sh /Anhydrite 200

Seven Rivers Dolomite /Anhydrite 500

Queen Sandy Dolomite/Anhydrite/Shale 200-500Arte

sia

Gro

up

Grayburg Dolomite / Anhy. /Shale / Sandstone 300

Guadalupian

San Andres Dolomite /Anhydrite 1500

Glorieta Sandy Dolomite 100

Upper Limestone /DolomitePaddock

Lower Dolomite

300

Blinebry Dolomite /Sandy Dolomite 1000

Tubb Dolomite /Sandy interval 400

Yeso

Drinkard Dolomite 300

Leonardian

Abo Dolomite / Anhy. /Shale 1000

Pale

ozo

ic

Permian

Wolfcampian Wolfcamp Limestone /Dolomite 0-1500

Figure 6. Stratigraphy column of the Permian Basin in the vicinity of the studyarea on the Northwest Shelf for the formations deposited during Permian period.The age, general lithology, and approximate thickness of each formation arelisted. The main producer in Vacuum Glorieta West Unit is shaded. Modified fromPranter (1999); Broadhead (1993), and Talley (1997).

22

Figure 7. Type log for the well Bridges State #95. The top of the reservoir (TheGlorieta), Upper and Lower Paddock, as well as Blinebry (the bottom of thereservoir, are picked).

23

Paddock Dolomite. The Upper Paddock Limestone has about 0-150 ft of

thickness and is the main producer. The Upper Paddock Dolomite is very tight

and is the permeable barrier between the limestone and Lower Paddock, which

is another producer interval. The Lower Paddock is a dolomite and it is extremely

fractured (Martin, 2000, Pers. Comm.). The production interval of the MainPaddock is primarily within the upper 100 ft of the formation.

The Main Paddock as seen in core studies is predominantly dolomite,

nonetheless an irregular limestone interval is also observed (Upper PaddockLimestone). The dolomite is tan to light brown, fine to medium crystalline, withvuggy and intercrystalline porosity. Core photos (core intervals and thinsections)of the Paddock interval are displayed in Figure 8. From observed cores, core

descriptions, mapping and petrophysical log analysis, the productive interval of

the Main Paddock is interpreted as having been deposited as an oolitic shoal

environment along an east-west trending shelf edge (Figure 9). The Glorieta section overlying the Paddock consists of dolomitic sandstones.

It is Late Leonardian in age and is regarded as lowstand deposit, restricted to a

shallow shelf environment. The Glorieta Formation is composed of shelf

dolomites and minor interbedded sands in Vacuum study area. It also contains

isolated, lenticular porous zones. The Paddock and Glorieta is naturally fractured

(Broadhead, 1993)

24

Figure 8. Core photos and thin sections of the Paddock Formation. Figure (A)shows a core from the Lower Paddock Dolomite, which is extremely fractured.Figure (B) is a thin section of Upper Paddock porous limestone, vuggy and/ormoldic porosity can be seen. Figure (C) is a thin section of Upper Paddock non-porous dolomite, which is the permeable barrier between the Upper Paddockporous limestone and Lower Paddock fractured dolomite.

6

4

6

4

25

Figure 9. Schematic figure showing the depositional setting for the Paddockinterval (Burman, 1991).

The Glorieta Formation is composed of cyclically deposited siliciclastics,

carbonate, and carbonate-evaporite units. Environments of deposition range from

supratidal, through shallow sub-tidal to open-marine conditions. The siliciclastics

are dominantly eolian-derived sediments that were deposited on the shelf.

Although the Glorieta interval is not considered as a producer, three wells

within the Vacuum Glorieta West Unit are producing hydrocarbons from this

interval (Hickey, 2000, Pers. Comm.). Porosity of the productive Glorietareservoirs in south east New Mexico generally ranges from 5% to 12%, with

26

permeabilities from 2 to 9 md. (Broadhead, 1993). A core photo from theGlorieta interval is displayed in Figure 10.

Figure 10. Thin section of the dolomitized sandstone of the Glorieta interval. Lowporosity can be appreciated.

6

4

27

3. DATA CALIBRATION

3.1 INTRODUCTION

As a first step, P-wave and S-wave seismic surface data were tied to the well

data was. Five wells with sonic logs (one of these with accompanying densitylog), and one multicomponent vertical seismic profile (VSP) data set was used(well VGWU-127), although two of them were available (wells CVU-200 andVGWU-127).

3.2 CALIBRATION P-WAVES

Calibration data for P-wave was extensively studied by Baylock (1999) for theSan Andres interval. His results were assumed to be reliable and were taken as

a base or datum for the calibration within Glorieta - Paddock interval.

Synthetic seismograms were constructed from the sonic logs for the 5 wells

within the seismic area. In addition others synthetic seismograms were used from

2 wells located on the edge of the study area. Only one well had a density log, for

the remainder, a constant density of 2.76 g/cm3 was assumed. Figure 11 shows

the map of the wells used for P-wave calibration.

28

Figure 11. Location of the wells with synthetic seismograms. The small square isthe area of the 4D study.

A synthetic created for the Warren State-13 well in the south central part of

the area is displayed in Figure 12. This well shows the whole section, which

0 2000 ft

29

includes the San Andres interval and Glorieta Paddock interval. It is possible to

observe a good tie at the San Andres level, and a relatively good tie at the

GLORIETA - PADDOCK level, with a correlation coefficient of 61. After Baylocks

work it is important to note that although there are successive changes in

processing flows (see Chapter 6.5), the polarity of the data is still the same:normal polarity SEG standard (peak = negative impedance contrast), or reversepolarity Landmark - Syntool (peak = positive impedance contrast) convention.The correlation also suggests that the data is approximately zero phase.

All the synthetic seismograms were constructed with the following common

parameters: the wavelet convoluted with the reflectivity model was a trapezoid of

8-14-60-80 or 8-14-40-60 frequency bandwidth, zero phase, reverse polarity

Syntool convention or normal polarity SEG convention, and 100 ms operator

length. The datum for the seismic is 4000 ft., the velocity for the overburden (therock layers between the well elevation and the datum was taken from the VSP

VGWU-127, and is of 11,500 ft/sec.

Figure 13, displays the synthetic constructed for the VSP VGWU-127

correlated with the surface seismic and the corridor stack from the VSP.

30

Figure 12. Synthetic seismogram calculated for the well WS-13, which display agood tie for both San Andres and Glorieta - Paddock intervals. Panel (a) displaysthe sonic log used in the synthetic seismogram construction; panel (b) is theimpedance curve calculated overlain the reflection coefficient series; panel (c) isthe synthetic seismogram, and panel (d) is the synthetic overlain the trace # 74from the surface seismic P-wave volume.

(A) (D)(C)(B)

31

Figure 13. Panel (a) displays is the synthetic seismogram constructed for the wellVGWU-127; panel (b) is the correlation coefficient between the seismic trace #49and the synthetic; panel (c) is the synthetic overlain the trace # 79 from thesurface seismic P-wave volume; panel (d) is the correlation coefficient betweenthe corridor stack and the synthetic; and panel (f) is the same synthetic overlainthe VSP corridor stack for P-waves. Note the good correlation synthetic-seismicin contrast with the poor correlation synthetic-corridor stack.

(A) (D)(C)(B) (F)

32

Correlation between synthetic and surface seismic is good, with a correlation

coefficient of 60.

The VSP corridor stack was reprocessed in order to get the same resolution

as in the surface seismic data as well as time-shifted. Several shifts were applied

and 76 ms appeared to be the best choice. The wavelet shape or seismic

signature of the corridor is, in general, the same as in the synthetic for the

intervals between San Andres top and the Paddock, nonetheless differences can

be observed. The correlation coefficient is 45.

From all the synthetics made it can be concluded that Glorietas top is a

trough and can be found between 840 to 860 msecs. The Paddock top is

immediately below the Glorieta, and it is picked between 850 to 870 msecs.

A similar investigation by Li (2000) over the New Mexico State #18 well, whichmainly focussed on deeper intervals, has shown the same time picks for the

Glorieta and Paddock in compressional waves.

3.3 CALIBRATION DATA FOR SHEAR WAVES:

Since there was no shear wave sonic within the Glorieta Paddock interval

some assumptions and extrapolations were taken into account. The deepest

sonic dipole belongs to the New Mexico State #18 well, and it reaches 5200 ft in

depth. The shear synthetic seismogram for this well was constructed with the

following parameters: the wavelet convoluted with the reflectivity model was a

33

trapezoid of 8-12-20-30 frequency bandwidth, zero phase, normal polarity

Syntool convention or reverse polarity SEG convention, and 100 ms operator

length.

The S-wave corridor stack for the VSP in well VGWU-127 was available but

the Glorieta - Paddock interval was seen as part of the reflected wave field.

Direct waves end at about 5600 ft. Two different procedures were made to

address the problem of no shear wave data within Glorieta - Paddock interval.

First, a visual correlation between the S-wave corridor stack and the closest

seismic section to the VGWU-127 well was established. Figure 14 shows in one

panel the trace # 49 and in the other the corridor stack for S-waves from the VSP

well. This correlation was possible through the extrapolation of the time-depth

table available for this well and calculated by Michaud (1999) from the firstarrivals of the direct shear wave in the borehole (Figure 15). The extrapolationwas made assuming that below the Queen (3600 ft) the behavior of the curve islinear. Using this new Time-Depth curve, the corridor stack for S-waves and the

surface seismic can be relatively well correlated. For this synthetic it can be seen

that the Glorieta top can be picked around 1950 ms, and the top of the Paddock

would be the trough immediately below the Glorieta. This agrees with Baylock

(1999) where he established that shear wave volume has reverse polarity withrespect to the compressional volume. The reason for that is the way that shear

waves were acquired in the field for both, baseline and repeat survey.

34

The second approach was to use the deepest dipole sonic available (NewMexico State #18 well) and construct a synthetic seismogram for it. Thissynthetic was later compared with the VSP corridor stack for S-waves (wellVGWU-127) and with the surface seismic in the vicinity of this well. (Figure 14).The objective of this comparison was to confirm that the Time-Depth table for theS1-wave was valid at least till 5200 ft in depth. Another way to prove the validity

of the time-depth table was to calculate the Vp/Vs average for the interval

between San Andres top and the end of the curve, which is of 1.8, and it is agree

with published Vp/Vs values for carbonates.

35

Figure 14. Visual comparison between the trace # 49 extracted from the S1-wavevolume, and corridor stack for S1-wave. Panel (a) displays the sonic log used inthe synthetic seismogram construction; panel (b) is the synthetic seismogramcalculated for the well New Mexico State #18; panel (c) is the synthetic overlainthe trace # 79 from the surface seismic S1-wave volume; panel (d) is thesynthetic overlain the VSP corridor stack for S1-wave.

(A) (D)(C)(B) (F)

36

Figure 15. Time-Depth table calculated by Michaud (1999) using the first arrivalsof the VSP for compressional and shear wavelet. Notice the linear relationshipthat exists between S1 and S2 below Queen.

37

4. MALJAMAR SEISMIC INTERPRETATION

4.1 INTRODUCTION

The Maljamar P-wave seismic survey is a 30 mi2 extraction taken from alarger data set (about 120 mi2) and was donated for RCP use by Philips,Chevron, Marathon, Mobil, Shell, and Texaco (Figure 3). The Maljamar surveyrepresents a regional overview of the Permian shelf margin at Vacuum. The

Maljamar survey was interpreted in order to determine the regional structure andmajor stratigraphic patterns for the Glorieta Paddock reservoir. These datawere mainly used in the determination of the regional structural framework for the

Glorieta Paddock interval.

The VSP-127 synthetic seismogram (Figure 16) plus all the others calculatedand tied in the previous studies were used to initiate the current interpretation,

focussing the interpretative work on the definition of the structure of the Glorieta-

Paddock interval (Figures 17, 18 and 19). Structural interpretation in the Glorieta Paddock interval is key to the

prediction fractures within the interval. This work was pursued using the seismic

coherence cube.

38

Figure 16. Tie between the VSP-127 and the seismic data for Maljamar survey.The Glorieta is a trough and Paddock is a zero crossing.

39

Figure 17. Crossline #360 taken from the Maljamar volume, showing theinterpretation of Glorieta and Paddock seismic events. In the lower left corner therelative location in the Maljamar survey is displayed.

40

Figure 18. The Glorieta map in time. The black lines represent fault systems: onein the shelf break with east-west trend and the other in the slope with a north-south trend. The blue square is the 4D area.

41

Figure 19. Visualization of the Glorieta time structure map.

4.2 STRUCTURAL INTERPRETATION COHERENCE CUBE

As a first step, faults were interpreted in vertical sections, following obvious

discontinuities in the horizons. From this first step, a system of two major normalfaults, accompanied by one minor normal fault in the shelf break is clearly

observable (Figures 18 and 19).

900

1200

ms

N

42

The next step was the utilization of the coherence cube tool. With this attribute

seismic discontinuities were assumed to be related to faults if the general

tectonic history matched with them, otherwise they could be related with

stratigraphic features. Previous work in the structural interpretation of the

Maljamar survey using coherence, has been done (Galarraga, 1999), although itwas focussed in the San Andres interval.

The coherency cube used in this study was the same as used by Galarraga

(1999). The parameters and general considerations in the construction of thecube were the following:

- Landmark PostStack Continuity from Correlation algorithm was used: this

option measures data continuity via crosscorrelation. For each trace, data

within a sliding window is crosscorrelated with data from 2, 4 or 8 adjacenttraces. The correlation coefficients are analyzed to determine a continuity

attribute which is assigned to the central sample.

- The minimum correlation was used within the correlation option in order to

emphasize the largest local discontinuities.

- A vertical window size of 16 ms was used since the lateral resolution of the

faults appears to increase as the vertical window size is decreased. At the

same time the signal to noise ratio can be increased as the size of the vertical

window is also increased.

43

- In the tests made by Galarraga (1999) coherency algorithm was shown to bemore robust as more traces were used in the crosscorrelation and lineaments

of low coherency are better defined, therefore the maximum number possible

was used (8 traces).- FK filtering was applied: seismic discontinuities due to faulting can be

separated from other discontinuities related to noise and dipping seismic

reflections through the definition of the dip for each one of them. The dips of

the faults in the area are nearly vertical, while the seismic discontinuities from

stratigraphic reflection range from flat to 25o. A filter was created in the t-x

domain to eliminate events with dips of zero to five milliseconds per trace

(zero to 25o ). This filter was then transformed into the f-k domain and appliedto the f-k before being transformed back to the (t,x) domain with the undesiredevents removed.

In order to start the interpretative process of this attribute, the faults

interpreted in sections were overlain onto the time slices extracted from the

coherence cube, and a first interpretation was made for the Glorieta - Paddock

time interval (900-1200 ms). The fault interpretation in time slices was made intime slices every 16 ms, and it followed the faults previously interpreted in

sections (Figures 20 and 21). At this time the faults were interpreted as segments, not as large continuous

curvilinear planes. Justification for this interpretation comes from advances in

44

descriptive structural geology at outcrops and the better resolution in seismic

data. This new approach proposes that faults are presented as a system of

interlinked faults.

Figure 20. Time slice from the coherence cube (934 ms). The marks show wherethe faults interpreted in section cut the time slice: each color (green, blue, pink) isa different fault, each one interpreted as a continuous fault.

4D Area

45

Figure 21. Faults interpreted as a system of linked segments using the faultsinterpreted in section as a guide. Observe the overlapping between faults.

Much recent work involving detailed mapping of fault traces using 3D seismic

data, has demonstrated that faults are usually made up of many overstepping

segments, linked by areas of complex deformation, termed transfer zones or

relay ramps. These areas are difficult to interpret from limited subsurface data,

yet are often sites of hydrocarbon traps and their evolution is important to the

understanding of the formation of many oil and gas fields. (Peacock and

4D Area

46

Sanderson, 1994). See Figures 22 and 23 for graphic visualization of theconcept.

Figure 22. Block diagram showing the main features of linked faults and relayramps. Bedding is reoriented in the relay ramp to accommodate displacementtransfer between the overstepping segments. (Peacock, et. al., 1994)

47

Figure 23. Block diagram of the possible spatial distribution of relay ramps.Different levels have different displacement and different stages of relayevolution. (Peacock, et. al., 1994). The evolution of relay ramps is directly relatedwith the evolution of fractures in carbonate settings.

Using this approach, coherence data were used to interpret fault segments

that afterward are used to locate the relative position of the relay ramps. The

second step is to use the coherence data alone, not following the faults

interpreted in sections but following the discontinuities of the data itself (Figures24 and 25).

48

Figure 24. Time slice from the coherence cube at 918 ms., near the top of theGlorieta. The discontinuities were associated with faults and based on thecoincidence of the faults interpreted in section with those discontinuities.

4D Area

49

Figure 25. Interpretation of faults using just coherence analysis. The faults ingreen were interpreted as a separate group of faults because they neverintersected the Glorieta interval (they disappeared in depth). In yellow the topGlorieta Paddock seismic interpretation at 918 ms.

The interpretation through the Glorieta Paddock interval shows that the fault

zones are composed of arrays of overstepping and linked segments (Figures 26and 27).

4D Area

50

Figure 26. Time slice of the coherence cube at 1046 ms. Notice how the stronganomalies or discontinuities do not persist due to the decrease in seismicresolution with depth in some cases and to the end of the faults in some other.

51

Figure 27. At this time (1046 ms) is not possible to distinguish the character ofmany fault segments. The south faults (green color in figure 28) are no longervisible. However the character of the main faults in the shelf break is preserved, itis just weaker. The set of north-south trending faults (blue) is now observable.

The possible presence of relay ramps in these arrays has a particularly

important role in the definition of the fracture pattern for the carbonates of the

Glorieta Paddock interval.

4D Area

52

Studies made in other areas have shown that depending on the evolution of

relay ramp, transfer faults can be developed generating fractures in between the

main faults (Figure 28). However, the Vacuum survey P-wave data has a higherresolution, so that a more accurate interpretation of the linked fault system

pattern is expected, with a better description of the relay ramps role in the

fracture distribution and hydrocarbon trapping mechanism.

Figure 28. Some of the features common in the transference zone between twooverlapped faults. Fractures are generated in between the two faults and theycan have parallel as well as perpendicular direction respect to the main faulttrend (Trudgill and Cartwright, 1994).

53

5. PRODUCTION DATA AND RESERVOIR ENGINEERING

5.1 PRODUCTION HISTORY

The initial discovery of the field was in 1929 by the Socony Vacuum Oil

Company with the completion of Bridges State Well #1 in the San Andres. The

Vacuum Glorieta Pool was discovered on January 11, 1963 by Texacos #12

New Mexico O State NCT-1. The completed interval is in the dolomitized

Paddock member of the Upper Yeso Formation.

The Vacuum Glorieta Pool was established in 1963 after a meeting that

resolved the vertical limits of each pay zone within the reservoir. The type log

was designated as the Mobil State Bridges #95 (Figure 7, Chapter 3). The upperpool was designated the Vacuum Glorieta Pool, starting at the top of Glorieta (logdepth 5838 ft) and ending at the top of the Blinebry. The Geologic Subcommitteedesignated the top of the Paddock at 5950 ft. The major production in theVacuum Glorieta Pool is from the Paddock and a minor amount from the

Glorieta. After the initial discovery of the Vacuum Glorieta Pool, rapid

development extended the field to the north and east. These wells were drilled on

state-wide 40-acre spacing with a total of 185 wells having produced 61,816

MMBO plus 75,778 MMCFG and 35,837 MBW (as of January 1, 1990).

54

The resultant OOIP (Original Oil In Place) of 171 MMSTBO is calculated forthe Main Paddock using a 6% porosity cutoff and a 50% gamma ray, along with

capillary pressure provided by Shell and a geological oil/water contact map.

5.2 RESERVOIR PERFORMANCE

The predominant producing mechanism for the Vacuum Glorieta Field started

been solution gas drive with some pressure support from the surrounding aquifer.

Although the field has produced a significant amount of water, especially near the

edges, aquifer activity can best be described as encroachment rather than active

influx providing any significant pressure support.

The historical production plot for the field before unitization and water flooding

(Figure 29) shows that the initial production began in February 1963 with full fielddevelopment essentially completed in early 1967. Most wells in the field

produced at the maximum allowable rate of 107 BOPD until early 1974, when the

field began to decline as a whole. Water influx and pressure support from the

surrounding aquifer were limited until early 1974, as evidenced by the constant

water cut of 25% (once full field development had taken place in early 1967) andconstantly increasing gas-oil ratio from initial field discovery in early 1963. After

early 1974, the field had undergone significant fluid and pressure depletion and

55

Figure 29. Historical production of the field before unitization. Production from both Vacuum Glorieta Westand Vacuum Glorieta East are included.

56

some support. Generally water cut and reservoir pressure are higher and GOR's

lower on all flanks of the reservoir.

After the analysis of historical GOR, water cut and bottom hole pressure

(BHP) maps, it was determined that the Vacuum Glorieta West Unit has largeamounts of solution gas drive coming higher GOR's, lower reservoir pressure

and lower water production. This indicated that the west unit holds a great

percentage of secondary oil that needed to be waterflooded for an extended

period of time before any tertiary recovery (CO2 injection) could be applied. Logs suggested that the Glorieta is not connected with the Paddock and may

not be a candidate for unit wide water flooding or CO2 injection. Primary reservesfrom the Glorieta as determined by decline curve analysis were included in the

total projected recovery. The previous analysis led to the formation of two units of production: Vacuum

Glorieta West Unit (VGWU) operated by Texaco and Vacuum Glorieta East Unit(VGEU) operated by Philips. This process was accomplished in 1992 and waterflooding was initiated in VGWU later that same year after unitization.

A production history plot shows the behavior of the reservoir after the initiation

of the water flood program (Figure 30). From this plot it can be observed that gasproduction decreased while oil production increased uniformly after June 1994.

57

Figure 30. Historical production of the field after unitization and water flooding program started. (Hickey,2000)

58

In June 1996 oil production experienced an increase which was maintained

during the whole of 1997. Flushing fractured dolomite in the Lower Paddock with

water appears to be the reason for the increase (see reasons in the nextsection). Shortly after, 1997, a new increase in oil production occurred due to theimplementation of a horizontal-drilling program.

5.3 RESERVOIR FLUID DATA

Bottomhole fluid samples were obtained from different wells soon after the

reservoir discovery in 1964. PVT analysis were run on the samples by three

different labs. Table 4 summarizes the main characteristics of the oil samples

taken at that time. From the composition, it was demonstrated that the reservoir

fluid is black oil.

5.4 DATA AVAILABLE

Texaco provided production information from 62 wells from the Vacuum

Glorieta West Unit, as well as water injection information (amount of waterinjected and pressure of injection) for 54 injector wells. Production informationincludes water, oil and gas production per month from 1995 to 1999.

The data include production information for 9 horizontal wells. These wells are

part of a horizontal-drilling program started in 1997 in the Upper Limestone. The

objective of this program is to recover the oil bypassed within the porous

59

Initial reservoir pressure 2260 psi

Bubble point pressure 1331 psi

Oil FVF @ BPP 1.306 RB/STB

Oil FVF @ 14.7 1.019 RB/STB

Solution GOR @ BPP 552 SCF/STB

Oil viscosity @ BBP 0.662 cp

Oil viscosity @ 14.7 2.08 cp

Oil density @ BBP 0.7278 gm/cc (62.9o API)

Oil density @ 14.7 psia 0.811 gm/cc (43.0o API)Avg Reservoir Temperature 118.7 oF

Table 4. Oil properties and initial reservoir characteristics. Information displayedcorresponds to samples taken at the beginning of the field production history.

limestone. There is evidence that proves that the water originally being injectedto the Upper Paddock was no longer being absorbed by the formation. Over

saturation of this interval induced the water to flow into the Lower Paddock

fractured dolomite (Martin and Hickey, 2000, Per. Com.).

60

Water injection profiles (Figure 31) logged in water injector wells showedunequivocally that the water injected within the Upper Paddock Limestone since1994 was being repulsed and going to the next open interval, the Lower Paddock

Dolomite during 1997. Reservoir engineers suspected that the Upper Paddock

Limestone was definitely not drained appropriately and the water injected therecreated an over saturation of the formation. Thus the water started to go to the

Lower Paddock Dolomite and was rapidly canalized due to the high fracture

porosity in that formation.

5.5 PRODUCTION MAPS

Several production maps were constructed for each year in order to determine

production trends that could help to determine the behavior of the fluids in the

reservoir during the time-lapse period (1997-1998). The maps constructedinvolved total oil, gas and water production (Figures 32, 33 and 34), as well aswater cut, water injection and surface tubing injection pressure maps (Figures 35,36 and 37). All maps were generated based on a monthly report of fluid extractedand fluid injected, and their associated injection pressures. Analysis of the production data maps for the seismic time-lapse interval,

provides useful information about the reservoir behavior over time.

61

Figure 31. Water injection wells with water injection profiles shown on the right ofeach log. Note the change in water injection profiles from 1994 to 1997 (most ofthe water was initially being injected into the Upper Paddock limestone and nowis being injected into the Lower Paddock fractured dolomite.

62

Figure 32. Total oil production bubble map for 1997 (left) and 1998 (right) inbarrels. The size of the bubbles is relative to the amount of oil produced for eachwell. Injectors are green and producers are red. Main wells from the CO2 injectionprogram in San Andres are labeled as a reference. The big square is the areathat corresponds with the seismic survey Phase VI. The red square is the 4Dseismic area for Phase VII. The whole southern line of wells have been shut offbecause of high water production and mechanical problems (Hickey, 2000).

VGWU 103VGWU 103

VGWU 116 VGWU 116VGWU 118 VGWU 118

63

Figure 33. Total gas production bubble map for 1997 (left) and 1998 (right) inSTB.

Figure 34. Total water production bubble map for 1997 (left) and 1998 (right) inbarrels.

VGWU 103 VGWU 103

VGWU 103 VGWU 103

VGWU 116 VGWU 116

VGWU 116

VGWU 118 VGWU 118

VGWU 118 VGWU 118VGWU 114 VGWU 114

VGWU 126VGWU 126

64

Figure 35. Water cut map for 1997 (left) and 1998 (right). Notice how the oilproduction moved from the southeastern corner to the northwestern corner fromone year to the other.

Production and water injection volumes are given in stock tank barrels (STB).In the case of gas production they were given in MMSCF and converted to

reservoir barrels (RB) using PVT data provided by TEXACO (Figure 39). Thebottom hole pressure in the reservoir varies greatly but generally ranges between

1500 and 2500 psi. An average reservoir pressure is 2000 psi (Hickey, 2000). At2000 psi., a GOR of 350, temperature of 118 oF and 40 API oil, the FVI (Bo) isabout 1.2 RB/MSCF. This was the gas volume factor used in the calculation of

gas in reservoir barrels.

65

Figure 36. Total water injection bubble map for 1997 (left) and 1998 (right) inbarrels.

Figure 37. Average water injection pressure bubble map for 1997 (left) and 1998(right) in psi.

66

Figure 38. Variation of the Formation Volume Factor (FVF) versus reservoirpressure for oil and gas (reservoir fluids).

The analysis of the total oil production maps shows that oil moved toward the

northwestern portion of the study area from 1997 to 1998. The VGWU-103 well

shows a considerable increase in oil production while well VGWU-119 in the

eastern portion of the area, and well VGWU-116 south of 103, exhibited a

noticeable decrease. It is important to note that the whole bottom line of producer

wells in the area, including the VSP well VGWU-127, have been shut off since

1995, because of low production (mostly due to flooding) and mechanicalproblems in the two most eastern wells in the line.

67

Gas production maps show that increasing gas production toward the

northwestern portion of the area has occurred. Wells VGWU 118 and 116

experienced a gas production drop while VGWU-103 had an increase in gas

production.

Water production maps for the two years indicates that the aquifer is active at

the edges of the field . As previously stated, the bottom line of producer wells has

been shut off because they produced mostly water. One piece of evidence for

this is that well VGWU-126 still had some production, however, it was all water

(see Figure 35). In general water production behavior remained constant from1997 to 1998, nevertheless, well VGWU-116 had an obvious increase in water

production whereas well VGWU-114 to the west, relatively close to well #116,

experienced a dramatic decrease.

Water cut maps reveal important information about the spatial movement of

the oil between 1997 and 1998. From the 1997 water cut map it is possible to

observe that the majority of oil per barrel of total fluid produced was concentratedin the southeastern portion of the area. During 1998 oil appeared to move toward

the northwestern corner. The largest amount of oil per water produced are now

concentrated in that portion of the study area. A brief look over the water injectionmaps show that a great amount of water has been injected in the west side of thearea, and almost no water has been injected in the east portion.

68

6. 4D MULTICOMPONENT SEISMIC ANALYSIS

6.1 COHERENCE CUBE P-WAVES

A coherence cube was constructed for the compressional waves in the

baseline survey of the 4D seismic area. The main objective was to determinepossible structural patterns within the Glorieta Paddock interval using the

higher resolution Vacuum seismic survey. Correlation of the structural pattern

inside the 4D area within the Maljamar structural seismic interpretation was alsopursued.

The parameters used in the computation of this new coherence cube were

slightly different from those used by Galarraga (1999) in the Maljamar survey,and they can roughly be listed as follows:

- Continuity/Coherency algorithm used (Poststack/PAL Landmark):Correlation pattern of 4 traces,12 msec window and 0 dip search: 4 traces

pattern filtrate the information for more clarity, 12 msec window and 0 dip

because the area is relatively flat and the continuity changes expected are not

large.

- From Promax: trace/scalar, multiply by a scalar value of 200 (increases theamplitude values and changes the polarity, for easier display)

69

- F-K Dip Filter from Poststack/PAL: F-K fan filter

- Promax: Trace DC removal

The last two process are related with the enhance of the data display.

The interpretation of the coherency cube was made in the conventional way

with time-slices every 8 ms, starting with the stratigraphic top of the reservoir and

finishing with the bottom of the Paddock.

Figure 39 shows a sequence of time-slices within the reservoir. For the time-

slices near the top and within the Lower Paddock fractured dolomite (time-slices904, 912 and 920ms) patterns of low coherence have northwest-southeast andnortheast-southwest trends, which were interpreted as faults (Figure 40). Comparisons between coherence time-slices from both the Maljamar andVacuum surveys were made. The main fault trends observed in the Vacuum

coherency cube coincide with the regional structural trends interpreted in

Maljamar survey (Figure 41). Considering the coherence time-slice from Maljamar, it is possible to identify aset of faults near the Vacuum survey, one to the west with a NW-SE trend,

another to the south with a E-W trend and some others further south with NE-SW

trend. Coincidentally, these are the same trends interpreted for the coherence

cube in the Vacuum survey at the dolomite level. Furthermore, time-lapse and

production data seem to support the existence of the two faults interpreted in the

Vacuum survey (see sections 6.2 and 6.3.2).

70

Figure 39. Time-slices every 8 ms showing the coherent (red) or non-coherent (black) character of Glorieta Paddock interval. Notice two trends of non-coherent patterns.

71

Figure 40. Structural interpretation of the lower part of the Upper Paddock dolomite and the Lower Paddockfractured dolomite at Vacuum survey. Two sets of faults, one NW-SE and the other NE-SW, with juxtaposedfractures is proposed.

72

Figure 41. Comparison of Maljamar (left) and Vacuum (right) surveys coherencecube interpretation. The absence of structural features within the Vacuum areafor Maljamar coherence cube is due to the poor resolution of this survey. Label 1.indicates the presence of faults with NW-SE trend in Maljamar survey. Label 2.indicates the faults with NE-SE trend. Both trends are observed in the Vacuumcoherence cube as non-coherent patterns.

73

6.2 P-WAVES

6.2.1 Structure maps in time

The seismic events for the tops of the Glorieta and Paddock were clear and

continuous for both baseline and repeat compressional wave volumes (Figure42). Time structural maps show flexural trends that have similar directions onthose mentioned previously (Figures 43). A horizon located at about 920 ms was interpreted as the approximate bottom

of the Paddock. The upper limit of the interval studied was a horizon located

above the Glorieta (a trough). Top Paddock was not considered for thecalculations at any time (see section 6.3.1 for reasons).

6.2.2 Time delay analysis (relative velocity change) Compressional seismic velocity maps in a time-lapse sense were computed

over an interval between one cycle above the Glorieta event and the bottom of

the Paddock, approximately a 55 ms window. Figure 44 displays the velocity

difference maps for decreasing (negative values in percentage) and increasing(positive values in percentage) P-wave velocity. There is a general tendency for the P-wave velocities to decrease (about -4%). This decrease is larger in the areas surrounding the injector well VGWU122. Nonetheless there are two other localized tendencies: About the south

central portion of the area velocities are increasing (5%), and following this area

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Figure 42. Comparison between the same line (Line 66) extracted fromcompressional volume baseline (left) and repeat (right). The main horizonspicked are displayed.

Figure 43. Time structural maps of Glorieta top (left) and Paddock top (right).

75

Figure 44. Compressional velocity differences Pre-Post (baseline repeatsurvey). Left map displays the relative velocity decrease, the right map displaysthe relative velocity increase. The scales for both are percentages (See text forexplanations).

to the north west there is a narrower area where the velocities seem to be

unchanged. Almost no variation in velocity is displayed in the dark shaded area (-1 to 1%). An increase in pore pressure (Pp) within the reservoir, hence a decrease ineffective pressure, is proposed as an explanation for the general decrease in

compressional velocity. The bulk modulus is strongly affected by pressure

76

changes (Mavko, et al, 1998). When pore pressure increases K and m decrease,

therefore Vp decreases:

r

m34-

=k

Vp (7)

If the structural interpretation made using coherency analysis is overlain by

both velocity decreasing and velocity increasing maps an interesting correlation

is observed. Figure 45 displays the velocity decreasing map with the structural

trends proposed. The fault with NW-SE trend seems to fit the narrower area

where the relative velocities did not change with time. On the other hand, at the

junction of the two faults, there is an increase in velocity through time. Since the water injection within the Lower Paddock dolomite can be affectingthe reservoir, the results in velocity changes for P-wave can be explained as

follows:

In the NW-SE fault: there could be an increase in pore pressure (totaldecrease in effective pressure) then, as a consequence, the compressionalvelocity decreases. The same effect is observed in general for the whole area. At

the same time, it is possible to have a fluid substitution effect. Water is displacing

the small amounts of oil remaining in the fractured area. The water bulk modulus

is larger than the oil bulk modulus. On the other hand, water is denser than oil.

77

Figure 45. Compressional velocity difference decrease with structuralinterpretation proposed overlaid. Notice how the NW-SE fault matches with thenon-variability in Vp through time.

78

However, the bulk modulus increases more significantly than the density due to

water substitution. Hence, an increase in Vp is more likely to be expected. Both

effects, fluid substitution (Vp increase) and pore pressure increase (Vp decrease)could cancel each other and the combined effect would be no change in Vp. This

is what is being observed.

In the junction of the two faults: Production information (Chapter 5) indicatesthat in the southern portion of the area the bottom line of wells are shut in

because by 1996 they were producing mostly water. This fact suggests the

existence of better permeability patterns in the areas close to the southern fault.

This is evidence that in some wells the water started to flow to the Lower

Paddock dolomite in 1996. A total flush of the fractures associated to the

southern fault can be expected. Hence, assuming better permeability for that

area, especially for the junction of these two faults, a simple pore pressure effectis unlikely to occur. Fluid substitution of hydrocarbon to brine can be expected

and therefore Vp would increase. The results are in agreement with this

interpretation: Vp increases in the junction of the two faults.

6.3 MULTICOMPONENT S1AND S2 WAVES

Multicomponent analysis was done using time differences for the fast shear

wave (S1) and the slow shear wave (S2), as well as anisotropy measurementsand amplitude extraction.

79

6.3.1 Time delay analysis (relative velocity change) Relative velocity difference maps (post - pre) were calculated for S1 and S2.Figure 46 and Figure 47 display velocity increase and velocity decrease maps,

respectively. The scale for both is given as a percentage, positive for increase in

velocity and negative for decrease. The velocity difference maps were calculated

with the same methodology used for P-wave. The computations were made over

an isochron between a horizon above the Glorieta top and approximately the

bottom of the Paddock. The Glorieta top was not taken as the top of the

computation window since the first interpretation attempt showed that the

Glorieta seismic event was changing in time for S1 and S2 volumes (see Figure49). The Glorieta top was a horizon chosen as a non-variant, since there is nowater flooding going in the Glorieta. Nevertheless, it was evidently changing. For

that reason, a horizon one cycle above Glorieta (~1900 ms) was chosen as astatic horizon, since it was discernibly more stable. This horizon was flattened to

1900 ms and the calculations were recomputed between this horizon and the

bottom of Paddock (~2010 ms). The total window was then ~110 ms. The top of Paddock was not used either since there was a remarkable

variability or non-continuity of the event from one volume to another. Moreover

the seismic window between the top of Paddock and the bottom where it could

be defined, is extremely small resulting in unstable computations.

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Figure 46. Relative velocity increase (pre post) for S1 (left) and S2 (right)

Figure 47. Relative velocity decrease (pre post) for S1 (left) and S2 (right)

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The resultant velocity maps show that there is an increase in S1 velocities

(about 25%) in an area with NW-SE trend. Coincidentally, in the same area it ispossible to see that S2 velocities decrease by about 23 to 26%. With the

maximum values located toward the NW corner of the survey. Isolated spots of

small decreases in S1 and increases in S2 are located to the sides of the main

trend. The high percentage values of velocity variation for both shear waves are

due to the small window taken in the calculations. In general, the reservoir is

seismically thin, it means that the reservoir is defined in a small time window and

therefore has limited seismic response, but the interpretation was also limited by

the fact that no event could be picked below the bottom of Paddock in a reliable

way.

The fact that an event one cycle above Glorieta is being used as the top of the

interval is some what dangerous since the San Andres immediately overlies the

Paddock reservoir, and the San Andres is changing through time. There is a risk

of confusing responses if events within the San Andres are taken as a top of the

Paddock interval. However, one cycle above Glorieta top is still considered more

consistent.

The trend of increasing S1 and decrease S2 coincides with the NW-SE fault

proposed in the structural model, as well as with the junction of that fault and theNE-SW fault.

82

An increase in pore pressure, resulting in a decrease in effective pressure, is

interpreted to cause of the decrease in S2 velocity in the fractured area. The fluid

being injected opens up the fractures with NW-SE direction associated with thefault of the same trend. The aperture of the fractures in a perpendicular direction

with respect to S2 could cause the shear modulus (m) to decrease and

consequently VS2 will decrease. Furthermore, if there is an effect of fluid

substitution in the fracture area, which is known to be small due to the lack of

porosity within the fractures, that change will also affect VS2. As discussed in the

previous chapter, the density of the water is larger that the density of the oil, so

from equation (8) it can be seen that VS2 will decrease.

rm

=Vs (8)

Where m is the shear modulus and r the total density.

Talley (1997) in his work on the San Andres showed that the anisotropycomputations over the San Andres interval were due to an increase in S1 velocity.

He proposed that the shear wave traveling parallel to the open fracture direction

(S1) responds to changes in viscosity within the fractures, with less attenuationand higher velocity. This is just a hypothesis and barely applicable in this study.

6.3.2 Anisotropy analysis

83

Anisotropy measurements were the basic measurements made for the

Glorieta-Paddock reservoir. It is well known that these measurements are very

sensitive to horizon time interpretation errors. Although careful attention was

made into the picking process, the fact that this interval is deeper than the San

Andres interval means the resolution is lower. In addition, the interval is varying

in time due to the water injection and production, which made the interpretationunusually complicated. Picked horizons and how they vary for each volume are

shown in Figure 48.

One of the first complications observed was the fact that it was not possible to

pick the top Paddock on either the S1 post or both S2 pre and post volumes (asexplained in the previous section). A first attempt to calculate the anisotropybetween the Glorieta horizon and the bottom of the Paddock horizon, resulted in

anisotropy values in the order of 50 to 50%. Although it is known that small

intervals of highly fractured rock can have high anisotropy change, a test was

conducted to verify the validity of the interpretation.

An event one cycle above the Glorieta was chosen as the top of the interval,

and the results at this time were more reliable and stable. The result of pre and

post anisotropy obtained was in the order of 25 to 25% (Figure 49). It is a highanisotropy value compared with the San Andres interval (Cabrera, 2000) but thesize of the window is a factor to take into account.

84

Figure 48. Line 66 for each shear wave volume, S1 and S2 (pre and post) the Glorieta and Paddock bottompicks are displayed.

85

Figure 49. Anisotropy maps baseline (left) and repeat (right).

Baseline Repeat

86

f we analyze both the baseline and repeat anisotropy maps, a dramatic

change is observable for the NW corner of the seismic area. Calculating an

anisotropy difference map between pre and post (Figure 50) resulted in theanomaly becoming more evident. This anomaly has negative values and an

elongated shape with a NW-SE trend.

The baseline anisotropy map for the Glorieta Paddock interval shows the

NW-SE trend to have negative anisotropy. It means VS2 is faster than VS1 (timedelay for S2 is smaller than the time delay for S1). By definition or nomenclatureVS2 has to be slower than VS1, if the opposite happens an incorrect rotation of

the volumes could be the problem. At this point it is necessary to clarify that the

rotation of the volumes used in this study was constant, using an angle of 118o

for the fast shear waves. The anisotropy values changed radically in the post

anisotropy map. Here VS1 is bigger than VS2, as was expected. A possible

explanation could be that there was a set of open fractures with a NE-SW

orientation created by the main or regional stresses in the field.

By the time of the water flooding in the dolomite the NW-SE fractures

associated with the fault with the same trend could have been opened. Different

fracture sets have different aspect ratios, the response of each one of them to

pressure and fluid effects due to water injection varies greatly.

87

Figure 50. Baseline repeat anisotropy map difference. Faults are overlaid. Seetext for explanations.

88

The southeastern portion of the NE-SE fault does not have significant

anisotropy. It could be interpreted as either an absence of fractures or the

existence of a set of open conjugate fractures. The assumption of betterpermeability paths for this fault based on production data makes us to think that

the second interpretation is the best alternative. In the other extreme the fault has

negative anisotropy values in both the baseline and repeat maps, indicating that

VS2 is greater than VS1. Since the fractures interpreted here have a NE-SW

direction the chosen S1 direction would be the natural S2 direction. Moreover, for

the post map we observe that the southeastern portion of the NE-SW fault has

negative anisotropy values, indicating that a possible increase in water flooding

in that area possible caused the NE-SW fractures to open more than the NW-SE

fractures (increasing compliance in that direction). Since the anisotropy values are quite high, a test was conducted to prove the

validity of these results: a horizon at about 2750 ms was picked. This horizon

presented good continuity conditions and its time map for both S1 and S2 (pre andpost) showed small changes. A window between the horizon picked and flattened(above Glorieta) and the 2750 ms horizon was taken. Anisotropy calculationswere made over that interval, a total of 850 ms, and the results were visually

similar to the ones obtained for the 110 ms interval (trough above Glorieta andbottom of Paddock) (Figure 51). Figure 52 show a comparison between the

89

anisotropy difference maps for both intervals. The same anisotropy features but

with smaller anisotropy values (from 5 to 5%), can be observed.

Figure 51. Seismic line (L66) showing the top and the bottom of the two intervalsused in the anisotropy calculations

90

Figure 52. Anisotropy maps computed over an interval of 110 ms (left) and over an interval of 850 ms (right).

N

0 1000ft

91

The anomalies are there and the high values obtained for the Glorieta-

Paddock interval are possibly a consequence of the size of the interval used in

the calculations. A better multicomponent data quality (by reprocessing, forinstance) could assure better definition of the events below the bottom of thePaddock so that an interpretation of deeper events and new anisotropy

calculations can be conducted.

6.3.3 Amplitude analysis

During the interpretation of the S1 and S2 volumes (pre and post) a change inthe character of the reflectivity of the seismic response within the reservoir

interval was observed between the two surveys. A seismic event (a pick) barelyobservable in S1 baseline had greater amplitude in S1 repeat and even more

evident in S2 baseline. In the case of S2 repeat, the event appears as a

continuous horizon. Here it had a strong clear signature specifically for an area

where it was not observable in any of the other volumes. A map of this event was

derived for each volume. Observe that in S2 post, the reflector appears in an area

where it does not appear in any of the other volumes (Figure 53). Coincidentally,this area coincides in some lines with the increase in travel time for S2 (decreasein velocity) (Figure 54). RMS amplitude difference maps were computed for both S1 and S2 (RMSamplitude pre minus RMS amplitude post), and they reveal a significant

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Figure 53. Time structure maps of the middle event between the top and thebottom of the Paddock for both S1 (upper maps) and S2 (lower maps) shear wavevolumes, for baseline (left maps) and repeat (right maps) surveys. Notice howthe horizon covers the entire map in the repeat maps, especially in theinterpretation made on the S2 volume.

N

Baseline Repeat

S1

S2

93

Figure 54. Line 66 for both S2 baseline (left) and repeat (right) surveys displayingthe Glorieta top and the Paddock bottom. Note the time delay (decrease in S2velocity) at the bottom of the Paddock in the same area where there is anamplitude anomaly or an event that exists in the repeat survey but not in thebaseline. The interpretation of this middle event is also displayed (two firstmaps). The third map is the RMS amplitude ratio difference map (explainedafterward). Note how the anomaly coincides with an amplitude increase.

Baseline Repeat

94

difference in amplitude range for S2 with respect to S1 (Figure 55). To avoidinterpretation problems due to different amplitude ranges the amplitude ratio

between S1 and S2 was computed.

Figure 55. RMS amplitude difference maps (base repeat) computed for S1 (left)and S2 (right). Notice the large scale difference between each one of the shearwaves. The range of S2 amplitude values is roughly 3 times larger.

NBaseline Repeat

95

Figure 56. RMS amplitude ratio (RMS amplitude S1 / RMS amplitude S2) baseline(left) and repe