Observing reservoir dynamics and changing

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New ideas in Improved Recovery and EOR Finding Petroleum, 18 April 2012

Observing reservoir dynamics and changing reservoir management Kes Heffer, Reservoir Dynamics Ltd. Acknowledgements: David Bamford, New Eyes Exploration Stuart Crampin, The Univ of Edinburgh, BGS Ian Main, The Univ of Edinburgh John Greenhough, The Univ of Edinburgh Schlumberger Geomechanics Centre BP, Statoil, Nexen

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Talk outline 1. 2. 3. 4.

Stress-aligned microcracks at near-critical densities from shear-wave splitting Flow Directionality & recovery in reservoirs Other evidence for near-criticality of stress Interwell correlations in rate fluctuations – – –

5. 6.

General characteristics Reservoir physics Rate diffusivities

Approximately chronological

Results & microseismicity in Valhall Aggregated results from 6 North Sea fields – consistency with 1. and 2.

7. 8.

Effect on recoveries – field data Conclusions

Finding Petroleum; 18 April 2012

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Shear wave splitting indicates stress-aligned microcrack / fractures near critical density Stuart Crampin et al. http://www.geos.ed.ac.uk/homes/scrampin/opinion/ Shmax Anisotropic PoroElastic Shmin model (APE)

Stress-related

Most frequent crack densities in all rocks S-wave vel. anisotropy Crack density Crack radius

Highly sensitive Long-range

Finding Petroleum; 18 April 2012

Increasing differential horizontal stress 3

Field Directionality: The Prize – theory Areal sweep efficiency and anisotropic permeability (classic: Caudle 1959) For a 5 spot pattern of wells: Permeability axes

Physical model with Kmax/Kmin = 16

favourable P

P unfavourable I

P

P

To breakthough:

Areal sweep efficiency, %

0.1

100 90 80 70 60 50 40 30 20 10 0 1.0 Mobility ratio

Isotropic Favourable Unfavourable To 90% watercut:

Isotropic Favourable Unfavourable

10.0

Orientation of well pattern relative to permeability axes can change recoveries by 10’s of % points

Finding Petroleum; 18 April 2012

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Flood directionality and Stress Field data (“unfractured” reservoirs)

Shmax

Shmax

47 field cases (“unfractured”) Finding Petroleum; 18 April 2012

Numerical FE Modelling 5

Flood Directionality & Stress State Field evidence ‘Unfractured’ Reservoirs (47 cases)

Shmax

‘Fractured’ Reservoirs (33 cases)

0 350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190

1020 3040 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Finding Petroleum; 18 April 2012

All Reservoirs (80 cases)

0

0 350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190

180

1020 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170

350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190

180

1020 3040 50 60 70 80 90 100 110 120 130 140 150 160 170

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Flow rate fluctuations

Producers

Injectors

Finding Petroleum; 18 April 2012

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Interwell correlations between rate fluctuations general characteristics Stress-related Shmax Long-range

Fault-related First principal component of matrix of rate correlations between all wells in field B – independent mode ‘explaining’ largest proportion of fluctuation variance Finding Petroleum; 18 April 2012

Injector-Producer pairs only >0.5x106 pairs; 8 field areas broadband fluctuations high frequency fluctuations __ zero correlation 8

Reservoir physics • Communications are not just Darcy fluid flow, but… • …coupled fluid flow and geomechanics • incorporating pre-existing microcracks, fractures and/or faults • influenced by modern-day stress state • involving changing permeabilities

• … near a critical point • long-range interactions Finding Petroleum; 18 April 2012

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Extraction of rate diffusivities from timebehaviour of rate correlations

For triangles between wells: Extract tensors related to diffusive rate behaviour: Takes some account of spatial relationship of wells & of timebehaviour of correlations.

Finding Petroleum; 18 April 2012

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Case studies (6) in North Sea Diffusivity of flow rates: orientational distributions relative to maximum horizontal stress Highly anisotropic horizontal stresses Strike-slip/thrust stress state

~Isotropic horizontal stresses Normal stress state

Finding Petroleum; 18 April 2012

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Aggregated orientational frequency distributions of major diffusivity axes - for 6 N Sea fields Orientational component of correlation in xx strain

XX-S TRES S VARIATION AROUND A CRACK (AT LARGE DIS TANCE RELATIVE TO CRACK LENGTH)

Heffer & King, 2006

SHmax

tens ile S IN(T)S IN(3T) compres s ive

X

ZERO 0 340

350

10

x

20

330

30

320

After Pollard

40

zero at 33.5 from x

310

50

300

60

290

70

max at 33.2 deg from y

280

80

270

90

260

100

250

Healey et al, 2006

110

240

120

230

z

130 220

140 210

150 200

190

170 180

160

t 33 2 d

f

x Reches & Lockner, 1994 Finding Petroleum; 18 April 2012

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Flood progress through interacting fractures in matrix of medium permeability 10o permeability 30o

______ most favoured breakthrough directions for injected fluid in 47 ‘unfractured’ fields worldwide

Healey et al, 2006 Fracture dilation Min. path distribution around a well ~ isobars of pressure field. Equivalent to flood progression. Finding Petroleum; 18 April 2012

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Flowstreams through interacting fractures in matrix of low permeability 0o permeability 30o

______ most favoured breakthrough directions for injected fluid in 33 ‘fractured’ fields worldwide

Healey et al, 2006 Fracture dilation Min. path distribution around a well ~ isobars of pressure field. Equivalent to flood progression. Finding Petroleum; 18 April 2012

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Possible mechanism (2): reactivation of polymodal faults (Reches, 1978, 1983; Krantz, 1988, 1989) PLANE STRAIN

3-D STRAIN

Dihedral angle related to ratio of intermediate and minimum extensional strains – related to local stress tensor Finding Petroleum; 18 April 2012

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Valhall: Microseismic events and interpreted diffusivity axes from rate correlations

Fast shear waves = fracture strikes Teanby et al. (GJI 2004)

Finding Petroleum; 18 April 2012

Zoback & Zincke (PAGeophys, 2002) Focal mechanism: normal slip

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The Prize – field data Cumulative production per well

Shmax

Oil

|_____| Increased oil recovery 1 MMBO Increased oil rate Water-oil |_____| ratio Fewer wells WOR 1.0 Lower watercuts

Oil production is greatest perpendicular to stress WOR greatest sub-parallel to stress Finding Petroleum; 18 April 2012

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Watercut

The Prize – theory Lower watercuts Increased oil recovery 1.0 Increased oil rate 0.8 Fewer wells

mobility ratio = 1

favourable

0.6

unfavourable

Save ~$0.6/bbl* of produced water

0.4

isotropic

0.2 0.0 0

20

40

60

80

100

% total area swept

*SPE 73853 Khatib & Vermeek, 2002 e.g. $22million p.a. at 100,000 bwpd Finding Petroleum; 18 April 2012

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Rate correlations can identify faults /fracture paths involved in the processes to aid reservoir modelling

Input to reservoir model history-matching

North Sea ‘unfractured’ field: 1st principal component from rate correlations superimposed on fault trace map Finding Petroleum; 18 April 2012

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Time-lapse monitoring: Magnus daily rate data: changes with time in first principal component Years of development

/

/

Strike-slip / thrust

/

/

/ Stress/ regime

normal

/

/

/

/

Strike-slip / thrust Average reservoir pressure

Effective stresses

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Conclusions • Coupled geomechanics-flow near a critical point is an integral part of reservoir physics affecting commercially important behaviour; in particular it influences flood directionalities • Inter-well correlations in rate fluctuations are max. in shearing directions; (micro-) fracture interactions explain this as well as being consistent with flood directionalities. Some shear slip also likely. • Analysis of inter-well correlations in rate fluctuations offers a low cost means of interpreting specific flowpaths between wells, esp. for input to reservoir model history-matching; also allowing time-lapse. • Modes of deformation can change during field’s life. • Integration with s-wave data will give most info’. Finding Petroleum; 18 April 2012

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Acknowledgements of grants NERC CONNECT grant GR3/C0022 with matching funding from BP COFFERS project, under the Industry Technology Facilitator (sponsors: Amerada Hess, BG Group, BP, Conoco-Phillips, DTI, Kerr-McGee, StatoilHydro, Shell and Total) RESURGE project, sponsored by the Technology Strategy Board

Finding Petroleum; 18 April 2012

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