Reservoir Surveillance and Successful InfillReservoir Surveillance and Successful Infill Well Delivery in a Mature Asset

Adrian Zett, Mary Ward, Chris Pearse, Dawn Houliston - BP

Parijat Mukerji - Schlumberger

Aberdeen May 2011

Outline

Field Introduction

Surveillance History

U t i t & Ch llUncertainty & Challenges

Baseline Logging Objectives

Saturation Logs Evaluation

Future Work/RecommendationsFuture Work/Recommendations

Reservoir Surveillance and Successful Infill Well Delivery in a Mature Asset

Production Petrophysics plays a key role in reservoir surveillance and field management. This is particularly true for mature assets which present several challenges related to fluid contact movement, connectivity of reservoir layers and well productivity. Identification of infill targets therefore requires an integration of all subsurface data. ThisIdentification of infill targets therefore requires an integration of all subsurface data. This paper presents a case study from a mature North Sea field where cased hole surveillance helped minimize risks in a high cost infill project.

The Machar field, located in the UK Central North Sea is a fractured Cretaceous chalk e ac a e d, oca ed e U Ce a o Sea s a ac u ed C e aceous c aand Palaeocene sandstone oil reservoir.

Machar is a subsea field development and therefore petrophysical surveillance has been restricted due to limited well access and logistical challenges. During the infill drilling, it g g g g,was therefore decided to use the opportunity and capture cased hole saturation and production logs in the existing wells. This data enabled the asset teams to understand fluid displacement mechanisms and upon integration with LWD and other logs provided the basis for the side track strategy. In particular, location of the imbibition flood front, fracture conduits and differentiation between formation and injection water were critical in the delivery of a j ysuccessful producer.

Two wells have been drilled on the eastern flank, one in 2008 and another in 2010. Baseline petrophysical surveillance was part of the data acquisition program in both wells. The initial objective was to use such data in Time Lapse mode with later surveillance. However, in-depth work identified immediate use when integrating with LWD data.

Field BackgroundLocation: Central North Sea 150 miles• Location: Central North Sea, 150 miles east of Aberdeen

• Block number 23/26a (100% BP)

Water Depth: 95m• Water Depth: 95m

• Discovered 1976

• Phased Development:

• Phase I – Natural Depletion, 1994

• Phase II – Water Injection Pilot, 1995

• Phase III – Field Development, 1998100 Km

benWitch

p

• Gas Lift commenced 2003

• Subsea production manifold; 16”

19 20 21 22 7

Forties

Buchanan Graben Viki

ng G

rab

Moray Firth Graben

Jaeren

Witch Ground Graben

• Subsea production manifold; 16 production flowline, 12” injection line 35km to Central Processing Facility

2725 26 2928

23

30

AberdeenAberdeenAberdeenAberdeenAberdeenAberdeenAberdeenAberdeenAberdeen

Scotland

Aberdeen

es High

Wes Central

JaerenHigh

East Central G2725 26 2928 30

UK

1,000 Km

al GrabenGraben

(“Ben” Machar) Reservoir Summary

Ben Nevis height -1344m (tvdss)

• Salt diapir structure with Palaeocene Sand and Cretaceous Chalk reservoirs

• 7 current producers and 3 water injectors

• Large variations in reservoir thickness

• High level of reservoir uncertainty

• Main production from fractured chalk

• Initial pressure close to bubble point

• Pressure support crucial.

• Understanding fracture pathways critical

• 845m of core taken in reservoir section

Well LocationMachar SECPP SG1: Appraise-Select Technical DSP

A04Y(W125ST)

18Z(W181)

..12/z.16

OWC @ top FortiesOWC @ top Chalk

A04Z(W125)

A0

18Z(W181)

A05

A08(

W12

7)

A02(W123ME2

A01(W120)

A09Z(W128)

.. 13

13Z

A07(W126)

A05Z(W182)

6Y(W122)

123)

1RE

5

W129

...

20Y(W121)

A06(W124)

A09

10

10Z

29 (Machar East)

. .2.. .

TOP CHALK DEPTH 19

Block 23/26A

A03 (W1

Mapping from2007 PSDM

1

Metres TVDss19Y

19Z

.19

~1km

80).

Surveillance History

AA04Y(W125ST)

18Z(W181)

)

Producer completionInjector completionShut-in wells ..12/z.16

OWC @ top FortiesOWC @ top Chalk

A04Z(W125)

RST/PLT-1999; RST-2009

PLT-2000

2004

LT- 2

010

.13 A07(W12

A05Z(W1

A08(

W12

7)

A02(W123)ME2. .

RPM

-2RM

T/PL

TRST/PLT-1999RST/PLT-2001

RST/PLT-2006 RST Baseline-2010

A01(W120)

20Y(WA09Z(W128).13Z

(W126)(W182)

6Y(W122)

)

1RE

5.

RST/PLT-2008

RMT/PLT-2010

R

TDT/PLT-1994

RST–1998, 2000; RST/PLT-2001

T 200220Y(W121)

A06(W124)10

10Z

W129 (M

achar Eas

. .2.. TDT–1995, 96PLT-1998RST/PLT-2000

RST Baseline-2008

RPM/PLT-2002

RST–2001

RPM/PLT-2002

TOP CHALK DEPTHMetres TVDss .19

Block 23/26A

A03 (W180

ast)

Mapping fromPSDM finalised 2007RST/PLT–2000

RPM 2004Metres TVDss19Y

19Z ~1km

0). RPM-2004

Uncertainty & Challenges

• Well access • Subsea completion with long intervals for logging

• Interpretation UncertaintyInterpretation Uncertainty• Water salinity variation• Changes in Porosity• Presence of Fracture Conduits• Cement Quality• Cement Quality• Lithology Variations

Objectives of InterventionObjectives of Intervention

Acquire baseline saturation log through casing (Reservoir Saturation q g g g (Tool in Pulsed Neutron Capture & Spectroscopy mode)

Log up in sigma mode at 1800 ft/hr from 3045m to 2300m MDBRT

Logging SummaryLog up in sigma mode at 1800 ft/hr from 3045m to 2300m MDBRT

CO passes – 3 passes over each of the following intervals at 3ft/min:3045m to 2985m MDBRT2800m to 2700m MDBRT2650m to 2560m MDBRT

Well InformationBit Size 8 1/2”Bit Size 8 1/2

Casing Size 5.5”

Casing Internal Diameter 4.892

Casing Weight 17 lb/ft

M i T Mi d li d i d f l li h lMatrix Type Mixture sandstone-limestone derived from spectral lithologyevaluation

GR Porosity Sigma Resistivity Sw ResistivitySw Sigma

(v/v)

Variable Salinity Challenges

540

210 k

35 ppk(gapi)

(v/v) (cu) (ohmm) (v/v)

35 ppk

Depth(ft)

Sw uncertainty 560

580

210 ppk

600 210 ppk

620

640

660

680

oil water

680

700.50 .25 .75 1.20 .1 .3 .4 10 150 100 10210 1031.1 .50 .25 .75 1

Resistivity-Sigma Numerical Method

ppmpp

Rw(t) Σw(p,t)

Sw_Sig (Σm, Σhc, Φ, Σ, Σw , Shale)

Sw_Res(a, m, n, Φ, Rt, Rw, Shale)

At each depth, search ppm such that ||Sw Res – Sw Sig ||2 is minimum ||Sw_Res – Sw_Sig || is minimum

The numerical method uses a minimization algorithm to search for ppmth t i S S i t h d th that gives Sw_res ~ Sw_sig at each depth

Resistivity-Sigma Application (Well: ME2)

Pulsed Neutron Evaluation (Well: ME2)

Imbibition1 Imbibition1

Limitation 2 of

standalone SigmaSigma

Spectral Lithology Evaluation

GammaGamma--Ray SpectraRay Spectra

Elemental YieldsElemental Yields

Dry Weight % ElementsDry Weight % ElementsSi, Ca, Fe, S, Ti, Si, Ca, Fe, S, Ti, GdGd

Clay, Carbonate, Anhydrite, QFM (Quartz, Feldspar, Mica)Clay, Carbonate, Anhydrite, QFM (Quartz, Feldspar, Mica)

Spectral Lithology Evaluation (WELL: ME2)

ImbibitionMatrix Variations

Imbibition Height (Well: ME)

Outcome

Rebuilding of the reservoir model as imbibed zones not expected initiallyImpact on perforation strategy for future wellsImpact on perforation strategy for future wells

Role of fractures in injection water movementRole of fractures in injection water movementNeed to understand matrix variations while evaluating pulsed neutron logsSpectrolith processing provided key inputs for formation evaluation

Baseline Cased Hole Saturation complements the Formation Evaluation data acquired in Open Hole providing valuable information with regards to:- imbibed zones

nature of fluid/salinity profile- nature of fluid/salinity profile

Future work and Recommendations

• Surveillance logging (Pulsed Neutron Capture & Spectroscopy) in MacharEast or any new well in the field to help characterize and capture changes

• A dual baseline should be considered to:• Ensure continuity between OH data and subsequent time lapse

data.• This should reflect changes in porosity due to acid frac stimulation g p y

as well as real changes in saturation due to fluid displacement processes.

• First baseline immediately after setting the casing and another after acid frac.

References

Cao Minh C, et al 2010 SPE ATCE- Combining Resistivity and Capture Sigma Logs for Formation Evaluation in Unknown Water Salinity: A Case Study in A Mature Carbonate Field, paper SPE 135160

C bi D J ll R J H P ll d R 2007 Th M h Oil Fi ld W t fl di F t dCasabianca D, Jolly R. J.H, Pollard R, 2007, The Machar Oil Field: Waterflooding a Fractured Chalk Reservoir, Geological Society, London, Special Publications; v. 270; p.171-191.

Brown A, Davies M, Nicholson H, Gane B, 1999, The Machar Field, Unlocking the Potential of a North Sea Chalk Field, Offshore Europe Conference, Aberdeen, Scotland, SPE 56974.

Pearse C.H.J, Blackburn N, Ibram M, Jehanno Y, McRonald R, O’Hanlon M, Spicer P, Ward M, Zett A, 2009, Machar Field East Flank - A Case History Using Seismic Reprocessing and Reservoir Modelling to Unlock Prospectivity, 71st EAGE Conference & Exhibition, Amsterdam, W005.

Zett A, Mukerji P, 2008, Subsea Logging and Intervention – Challenges and Solutions on Machar Field, 14th SPE ICoTA European Well Intervention Round Table, Aberdeen, Scotland.

Zett A, Webster M, Jehanno Y, 2010, Production Petrophysics - Preserving program flexibility p y g p g yto ensure successful infill delivery in a Mature Field Environment, SPWLA 51st Annual Logging Symposium held Perth, Australia, June 19-23, 2010,paper 46513

Reservoir Surveillance and Successful InfillReservoir Surveillance and Successful Infill Well Delivery in a Mature Asset

Adrian Zett, Mary Ward, Chris Pearse, Dawn Houliston - BP

Parijat Mukerji - Schlumberger

Aberdeen May 2011