23j(-yJ;:

UCRL-JC-125938

PREPRINT

Mapping Steam and Water F1OW

M.

Petroleum Reservoirs

WilL C. Schenkel, T. Daley, J. Peterson, E. Majer,A.S. Murer, R.M. Johnston and L. Klonsky

in

This paper was pmpamd for submittal to the1997 Society of Petdeum Engineers In@nationaf

Thermal Opemtions & Heavy Oil SymposiumBakq/i& CA

February 10-12,1997

November 1996

DISCLAMER

This document wae prapred ae an account of WOWeponaod by an agency of the United Statea Govarn.rmmt.Neither the United Statee Government nor the UNveraity of CaMornia nor any of their employees, makea anywarranty, express or implied, or aeeumeaany legal liabilitym rqomfMity for the accuracy, completexwm orusefulness of any infmmaticm, apparatue, product, or process dkloae4 or repreeanta that its uae would notint%ngaprivately owned nghta. Raferencahereinto any apedfic mmmerdd produ~ proceaa, or eerviceby tradename, trademark, manufacturer, or otherwise, do= not necay mns~~~ or ~PIY i~ md~menbrecommendation, or favoring by the United StateaGovernment or the Univemity of California. The viewe andopinioneof authors expeeed herein do not nace@udy state or reflect tlrme of the United StateeGovernment ortheUNvareity of CaMbrn4 and ahd not be used for dVdi3@ m prOdUCt andoraementpuqxwaa.

..

Mapping Steam and Water Flow in Petroleum Reservoirs

Michael Wilt and Clifford Schenkel, Lawrence Livermore National Laboratory; Tom Daley, John Peterson, and ErnestMajer, Lawrence Berkeley National Laboratory; A.S. Murer, Mobil Exploration and Producing U.S.; R.M. Johnston, SPE,,CalResouroes LLC.; and Louis Klonsky, Chevron USA Production Co.

AbstractOver the past 5 years, we have applied high-resolutiongeophysicalmethods(crosswellseismicandelectromagnetic(EM),andpassiveseismic)to mapandcharacterizepetrokxynreservoirsin the SanJoaquinValleyand to monitorchangesduring secondaryrecoveryoperations.The two techniquesprovidecomplementaryinformation.Seismicdatarevealthe“reservoirstructure, whereas EM measurementsare moresensitiveto theporefluid”distribution.

Seismicsurveysat the SouthBelridgefieldwereusedtomapfracturegenerationandmonitorformationchangesduetothe onset of steam flooding. Early results show possiblesensitivityto changesin gas saturationcausedby the steamflooding.CrosswellEM surveyswereappliedat a shallow

~lijd~

pilot at Lost Hills for reservoircharacterizationand steam-flood monitoring.Imagesmade frombaselinedata clearlyshowthe distributionof thetargetoil sands;repeatedsurveysduringthesteamfloodallowedusto identifitheboundariesofthe steam chest and to accurately predict breakthrough.Applications of the EMtechniquesin steel-casedwellsareat .

IntroductionAlthoughlargequantitiesof petroleumareproducedthroughwater and steam‘flooding,the process is typi@y poorlyunderstood.This leadsto inefficientrecoveryand associatedproduction problems, such as premature water/steambreakthrough fracturing of reservoir Wk and well failures. Ina new effort to undemtsnd these fluid displacement processesand associated reservoir changes, we are applying crosswellgeophysical methods to monitor secondary recoveryprocesses.

The goal of this project is to jointly use high-resolutiongeophysicalmethwk to map and characterize petroleumreservoirsduringsecondaryrecoveryoperations.Weviewtheintroductionof steamandwaterfloodsinpetrolew rese~oirsas naturaltrscemto mapfluid flow and to define the reservoirstructure. Efficient use of such tools can help us determine theflow mechanisms, map the creation and destruction of llactumporosity. and track the injected flow through natural channelsthat connect (and isolate) petroleum, deposits. It is an idealmechanism for detailed reservoir characterization; thereservoir is defined (and redefined) as it is produced.

In 1994, a partnership was formed between LawrenceBerkeley and Lawrence Livermore national laboratories(LBNL ‘md LLNL) and a group of San Joaquin Valley oilfiroducers including~ alResources, Chevron, Mobil, andindependent Bakersfield Energy. The partners agreed to applygeophysical tec~ques to characterize several pilot areas andto monitor existing and incipient steam- and water-floodprojects within the valley. The multiyear project consists of aseries of geophysical surveys in several pilot areas and jointefforts to interpret the data in terms of reservoir properties and

m- ‘early stage, but preliminaryresultsat Lost Hills show~*@C@SS d~g oil Production.sensitivity to formation resistivity in a water-flood pilot. In this paper, we will briefly describe the technologies

Finally, passive seismic surveys during hydrofracture applied and examine some early field results from several of

operations measured events correlatable in f=quency content the pilot field projects.

and magnitude with the size and orientation of inducedfractures.

Geophysical MethodsThe two main geophysicalseismics and crosswell EM.

tools employed are crosswellThese provide complementary

high-resolution data on fimdamental reservoir properties.Seismic measurements oflen reveal the reservoir structure andthe location of fracture zones and bedding planes. EM datadefine the resistivity of the rockj which is in turn sensitive tothe distribution of pore fluids. We apply these techniques tosmall- to moderate-scale field experiments, so we can improveour understanding of fluid flow during recovery. Data arecollected in observation and production wells at depthsencompassing the injection and production zones. We use coreand log data to help interpret results. ,.

In’addition to these active source techniques, we also applypassive seismic monitoring to image fracture creation andgrowth during hydrofiacturing and steam injection.

CrossWell EM System. The LLNL/LBNL crosshole EMsystem consists of separate transmitter and receiver stationsthat deploy tools in wells separated by up to several hundredsof meters (Fig. 1). The transmitter uses a vertical-axismagneticcore wrappedwith 100to 300 turns of wire andtuned to broadcast a low-frequencysinusoidal signal atfrequenciesfrom100Hzto20HIz.l

Thetransmittersignalinduceselectricalcurrentsto flowinthe formationbetween the wells. These cuments in turngenerate a secondarymagnetic field in proportion to theelectrical resistivity of the rock where they flow. Bymeasuring the fields from the receiver borehole, we can itierthe resistivi~ distribution between wells.

At the receiver borehole, a custom-designed coil detectsthe vertical magnetic field and sends it up the logging cable toa lock-in amplifier located at the surface. The lock-inamplifier operates like a radio by measuring only those signalsthat are coherent with the transmitted signal while rejectingincoherent background noise. An in-field computer is used tokeep track of the transmitted signal, the detected magneticfield and the depths of both source and receiver coils.

By positioning both the transmitter and receiver tools atvarious levels above, below, and within the zone of interestwe can create an image of the resistivity distribution betweenthe wells. The EM data are interpreted by computer modeligin which the rock between the wells is divided into two-dimensional, square blocks 1 to 5 m on a side. Each block isassigned an electrical resistivity value, estimated from theborehole resistivity log (if available). The computer thenmodifies the resistivity of these blocks until the calculated andmeasured EM data agree to within the measurement error,usually 1 to 2’%0. This process usually requires 10 to 12 hoursper datasct on a 50-MHz workstation to produce a detailedimage of the underground strata.

Crosswell Seismic System. The crosshole seismic systemconsists of a high-frequency piezoelectric source and boreholesensors. The receivers may be deployed in combination with

the source fortomography or

active experiments such as crossholeseparately for passive listening during

hydrofractmsor reservoir injection.The seismic source is a high-voltage piezoelectric

transducer, which can be operated in a pulse mode or in acontrolled signal sweep mode. It operates at t%equenciesup to15000 Hz and is effective at well separations of 200 m ormore, depending on the rock type. The high-frequencycharacteristics of this source make it ideal for resolving small-scale features such as fractures and injection tints.

We deploy both hydrophores and three-componentclamped geophones for signal detection. The hydrophorestring is often used with the piezoelectric source in water-filled boreholes; this tool deploys rapidly. .The clampedgeophones are lypically used for passive monitoring where itis necessary to detect lower-level signals and obtainmuhicomponent data.

Field ExperimentsSeismicand EM field projects were initiated as early as 1994and continue in four separate field areas. Most projects are inearly to intermediate stages of completiorh but several havejust begun secondary operations. In the following sections, weprovide selected highlights tim some of these.

Seismic Measurements at the CalResources PiloL SouthBelridge Field. CalResources is presently applying a high-energy steam drive for oil recovery in diatomite reservoirs atthe South Belridge field. This pilot is the third in a series tounderstand the steam-flow mechanism in this very tight rock.Previous results were generally encouraging for e@anced oilproduction, but there was evidence of steam b@as andbreakthrough into intervals outside the targeted injectionzone.2

Well drilling for the third pilot began in April 1995. Thenew wells included two fiberglass and two steel-casedobservation wells in addition to the injection and productionwells in the 5/8-acre pilot. Because of the unexpectedencounter of a hot zone within the overlying Tulare sands,both fiberglass wells were lost almost immediately afterdrilling. We were therefore unable to collect EM data for thissite.

Three crosshole seismic surveys were completed: oneimmediately after the onset of steam in June 1995, the secondin August 1995, and the third in February 1996. The initialsurvey included passive seismic monitoring duringhydrofiacturing. The results of this monitoring showed noobservable activity. The lack of microseismicity wassurprising in light of the high scismicity observcii earlier in adifferent part of the field during steam injection.

Initial crosshole measurements showed Strong attenuationof the P-wave phase despite good energy in later arriving S

waves. The weak P-wave energy was quite surprising in viewof strong P-wave energy during crosshole surveys at Mobilleases withii the same field.

In the follow-up survey two months iater, we had similarresults-that is, weak P-wave energy but strong S waves (Fig.2a). The third survey, made six months after the steaminjection had beguIL had dramatically different resuits. In tiiscase, the P waves are noticeably stronger over much but notail, of the recorded interval. This change clearly indicates thatsignificant changes are occurring in the reservoir because ofthe steamflooding.

Although there are several possible explanations for thisbehavior, such as near-borehole changes in rock properties, wesuspect that the cause may be reservoir pressurization. Asreservoir pressure increases because of the steam flood, weexpect a comesponding decrease in gas saturatio~ which inturn dramatically increases the seismic signai levels. Theseismic data may therefore be a rough indicator of locaireservoir pressurization. Notice aiao in Fig. 2 that the changeis not uniform over the entire section. P-wave data at somelevels are still quite small or missing, thereby indicating thatthe pressurization is not unWorm.

Crosshoie EM Measurements at Lost Hiiis. A series ofcrosshole EM surveys was conducted at Mobil Oil leases atLost Hills in central California beginning in 1993. Themeasurements were made to demonstrate the technology forcharacterizing oil reservoirs and monitoring steam floods.Two fiberglass-cased boreholes were driiled about 55 m apartnear a steam injector in shaiiow, heavy oil sands. Steam wasinjected at depths of 65, 90, and 120 w corresponding toupper, middle, and lower layers of the target Tulareformation.’

The resulting crosshoie EM induction images (Figs. 3a and3b), collected before steaming and six months after, clearlyshow the distribution of the high-resistivity oil sands (darkgray) and the intervening shale layers (light gmy) The arrowsindicate points of steam injection. The image in Fig. 3aindicates that the upper oil sand is a thick unit dipping gentlyeastward. The middle and lower sands are thinner and morediscontinuous between the weUs. The image shown in Fig. 3bis visibly different only at depths below 70 u where theresistivity has decreased significantly as a resuit of the steaminjection. In all other parts of the image, the before and afterresistivity values are unchanged. The resistivity decline iscaused by the temperature increase and the replacement of oilby water and steam.

Figure 3C is a “difference” image made by subtracting thebaseline image (Fig. 3a) from the monitoring image (Fig. 3b).This figure, which highlights the parts of the section that havechanged during the steam flooding, shows that the resistivityhas decreased dramatically in the middle and lower oil sands,indicating the presence of substantial steam there. The image

also indicates that aimost no steam has gone into the upper oilsand. In addition, Fig. 3C indicates that the steampreferentially flows to the west in the middle sand but to theeast in the lower unit.

The EM surveys showed the steam flood to be much lessuniform than the operator anticipated, providing valuableinformation on the progress of the flood and the parts of thereservoir affected by the steaming. In fact, our predictionshave been confiied by recent temperature and inductionresistivity logs in fiberglass-cased observation wells. Wecontinue to monitor the progress of this steam flood and areconcentrating efforts on the upper oil sand.

Water Flood Monitoring with Crosshole EM in Steel-Cased Weiis. Since most avaiiable boreholes in operating oilfields use steel caa~ it is difficult to do crosswell EMsurveys, because the casing causes a high attenuation andphase delay of the transmitted signal. Recent ~h suggeststhat, although dit%cult, it is possible to make effectivecrosswell measurements, even in steel weiis. Extending thecrosshole EM technique to steel-cased wells is an importantobjective of our reaemch because it dramatically broadens theuse of this technique.

In 1994, we began a collaborative project with Mobil totest the crosshole EM techrdque for mapping the resistivitystructure between welis and to track resistivity changes due tothe water injection. Fibe@ass monitoring well 003 is located”10 m from a water injector and 90 m from a steel-casedproduction well. Induction resistivity logs in 003 show that theaverage resistivity of the diatomite is 2.5 to 3 ohm-~ varyingbetween 1.5 and 4 ohm-m in the production interval from 550to 770 m. In additio~ repeat induction logs show that thewater injection has decreased the resistivity from 20 to 40°/0.

For the initiai test, we positioned both source and receivertools at the same vertical level and adjusted the frequency ofthe transmitter beginning at’ the lowest frequency detectablewith our receiver (20 Hz). Then we increased the frequencyuntil the signal was attenuated by the casing to below thesection threshold (about 500 Hz). These fkquency soundingswere made at 10 different levels, corresponding to differentcasing segments and small dfierences in formation resistivity.The sounding data were found to repeat over time to about3%.

In Fig. 4, we show a crosshole ffequency sounding from adepth of 550 m. The data are plotted with numericalcalculations for steel-casing modeis with and without a 3 ohm-m formation. The casing parameters were obtained by usingthe Schlumberger ME’IT casing evaluation log for thethickness and by trial-and-error fitting of the Iower-fkequencysection (<50 Hz) of the crosshole soundings to obtain theconductivity and magnetic permeability of the casing. At theseiower frequencies, the calculated modeis with and withoutconducting formation efkcts agree to within a few percent.

At frequencies above 50 HZ the effect of the formationcauses the total response curve to deviate from the curve withthe casing alone. The observed data seem to correspond muchmore closely to the model with the formation present. TMspromising result, coupled with other research, stronglysuggests that the formation resistivity may be obtained withcrosshole EM even in steel-cased wells.

In future surveys, our goal is obtain a completetomographic resistivity section through one steel well casing.In additio~ we expect that within a year or two, the ongoingEM monitoring will yield information on the subsurfacerr%stivity changes due to water flooding.

Passive Monitoring Microseismic Activity at Lost Hills. Inan experiment separately supported by Chevron, a two-stagefracture stimulation of the lower Dlatomite formation at LostHills was monitored for microseismic activity. We used twomonitoring wells, each located approximately 10 m from theinjector. In one well, we deployed a clamped three-componentgeophone and in the other, a five-level string of hydrophores.The seismic data were recorded on a high-frequency system(bandwidth of 12 kHZ).

The objective of this test was to collect data for use indesigning a fkacture and fracture-growth imaging experiment.In the fiture, test fractures will be imaged using low-costhydrophores to monitor microseismic events.

Figure 5 is a histogram of events detected during the fmtstage of fracturing. A striking increase in the number of eventsis observed afler the shut-in following the fwst stage of thefracturing. It is notable that the observed seismic events have ahigh-fi-equency content and that events of a similar size tend tocluster in time. We also observed that the average frequencyof events was approximately 500 Hz with seismic energyobserved up to 6000 Hz. We can use this information to studythe size and orientation of induced fractures. With only twoobservation wells, the location of these events is imprecise.Future deployment would use four or more observation,wells.

*This work was performed underby Lawrence Lfverntore National

Discussion and ConclusionsIn thk paper, we have shown geophysical data that reIate toreservoir changes during steam and water flooding and duringhydrofracturing operations. Although the projects are clearlyat an early stage, the results indicate that the geophysicalmeasurements may provide a diagnostic tool for monitoringreservoir behavior during secondary and tertiary operations.The next stage is to attempt to match these behaviors withknown or suspected reservoir behavior during production.

AcknowledgmentsThis work was supported by Chevron USA and the U.S.Department of Energy, Assistant Secretary for Fossil Energy,Offke of Oil and Gas Technology through the Natural Gasand Oil Technology Partnership Program at LawrenceLivermore National Laboratory and Lawrence BerkeleyNational Laboratory under Contract DE-AC03-76SFOO098.

References1.

2.

3.

Wilt M., Lee, K. H., Becker, A., Spies, B., and Wang, B.:“Crosshole EM in Steel-Cased Boreholcsj” Expanded Abstractsfor the Society of Exploration Geophysicists Annual Meeting(1996), Denver, CO.Kovscek, A. R., Johnson, R.M., and Patzek, T.W.: “Evaluationof Rock/Fracture Interactionsduring Steam Injection throughVerticalHydrofiactures~paperSPE 29622 (1995).Wilt M., Lee, K., Alumbaugh, D., Morrison, H.F., Becker, A.,Tseng, H. W., and Torres-Verdin, C.: ‘CrossholeElectromagnetic Tomography—A New Technology for OilField Characterization,” The Leading Edge of Exploration,Societyof Exploration Geophysicists (1995), pp. 173-177.

S1Metric Conversion Factorsacre x 4.047 E+03 = m2

.m x3.281 E+oo = h

,.

the auspices of the U.S. Department of Energy ILaboratory under contract No. W-7405 -Eng-48.

Motor generator

Signalsource

Transmitter

(a)

homed lines Laptop computer

J\Depth encoder p<w

/\ Current monftor =

.-l 0 4’YR”)Lock-in detector

Standard logging cable

T,

I

M wlndlngs

runingcapacftor

—

1100 1200 1300 1400 1500

(b)

40

50

60

Boreholereceiver tool

1100 1200 1300 1400 1500

Fig.2<roSshole-selsmlcracordlngsfollowingtheInltlatlonofnsteamfloodntthesouthBelrldgeflddInJuly 1995. (a) Data from theAugust 1995 survey show highly attenuated P-wave phase and strong S-wave energy. (b) Data from February 1996 show large Increasm inP-wave energy. This change is believed to be related to a decrease in gas saturation caused by reservoir pressurization.

(a) Baseline 9/93

20.

40.

2.20.

140.

(b) 4/84

20.0

40.0

60.0

80.0

100.0

120.0

140.0

(c) Percent difference

2ew 6035 35E20.0

40.0

;

~60.0

5%0 80.0

100.0

120.0

0:0 10’.0 20’.0 30’.0 40’.0 50’.0Location (m)

-30.0 -16.0 10.0 30’.0

Percent difference resisitivity

Location (m)

5.0 10.0 15.0

Resistivity (ohm-m)

~;lFig. Mrosshote resistivity tomographic sections from Lost Hills.kteam flood. (a) 7he baseline image maps the@and d[stribution~between wells. (b) 7he image after 6 months of steam flooding Is similar to baseline results except at depths below 70 m. (c) 7he differenceImage, made by subtracting (b) from (a), shows that the resistivity has decreased in the lower two target sands but has remained unchangedIn the upper sand.

,,

LH1 rec = 550 m

~ —— Free-space amp— 3 ohm-in amp

105 —..”..........................”...............“....,...””......”......... ..... ..................... ...................

106 .................................................................... ................... ... ...............................

t

1\\

10-7 1 I a 1 1 1 a e , 9 1 1 I n I10‘ 102 103

Frequency (Hz)

Casing prop thick = 1 cm (0.4 in.)Cond =3.6x106Mag perm = 80

Fig. 4-Croeshole EM sounding measurements through a ateef-caeedborehofe at the Lost Hilis fieid. The piot indicates that the collected dataare consistent with a model that accounts for both the steel casing and the formation reefstfvitym

30

25

5

n;000 1100 1200 1300 1400 1500 1600 1700 1800

Time of day (24-hour clock)

Fig.&Hiatogram of microaeismic events recorded during a two-staga induced fracturing of dietomite at Lost Hiiis. Monitoring began duringfracturing at 1000 hours, and no events were observed until severai hours after fracturing waa compieted. At 1700 hours, the second stagefracturing wea compieted; this activity yteided no events.

.

Technical Inform

ation Departm

ent • Lawrence Liverm

ore National Laboratory

University of C

alifornia • Livermore, C

alifornia 94551