The Alba Field: Improved Reservoir Characterisation using 4D Seismic Data. Elaine Campbell Oliver Hermann Steve Dobbs Andrew Warnock John Hampson

Transcription

1 The Alba Field: Improved Reservoir Characterisation using 4D Seismic Data Elaine Campbell Oliver Hermann Steve Dobbs Andrew Warnock John Hampson Chevron 2005

2 Alba Field Location Equity: Chevron 23% (operator) ConocoPhillips 23% StatoilHydro 17% BP 13% Total 13% Cieco 8% Endeavour 2% Block 16/26, UK Central North Sea 130 miles NE of Aberdeen Discovered st Oil January 1994 Chevron

3 Alba Geology Eocene, high density turbidite Amalgamated channelised sands deposited in a pre existing scour Wings Wings Massive, homogeneous unconsolidated sandstone & intrareservoir shales Main Reservoir Sand re mobilisation, causing injectites and wing structures High porosity (35%) & perms (2.5 4 D) High vertical permeability Reservoir thickness ~ 250 ft Surlyk et al Chevron

4 Alba 22 Appraisal wells (incl. sidetracks) 74 Development wells 34 Active producers 7 active water injectors, full voidage replacement Heavy oil gravity 19 API, no gas cap Estimated >950 MMBOE OIP Cumulative production: >350 MMBOE Horizontal attic producers drilled as close as possible to top reservoir, aim to optimise sand length & stand off to OWC Chevron

5 Chronology of Seismic Surveys 1989 Acquired Alba streamer survey 1991 Acquired Britannia streamer survey 1994 Production started 1998 Acquired Alba OBC survey; not designed for 4D 2002 Veritas Q15 Spec streamer survey acquired; not designed for 4D, but similar orientation as Britannia 1991 July 2008 Acquired dedicated Alba 4D designed to repeat Vertitas 2002 streamer survey Oct 2008 Delivery of fast track co processing of 2002 and 2008 surveys Jan 2009 Delivery of full co processed 1991, 2002 and 2008 surveys Chevron

6 P Wave versus S Wave Seismic Surveys P Wave (PP) Seismic Data shows strong fluid effect but no clear image of the reservoir S Wave (PS) Seismic Data shows lithological changes, injected sands & large intra shale geobodies 12 Area Alba Extreme South PP91 Data SW Line 2285 NE Si99 Data Shear Impedance Inversion of S Wave (for Lithology) SW Line 2285 NE Chevron

7 Key Geological Uncertainties Rugose/injected character of the top reservoir Nature of field edges, Alba wings Scale & location of intra reservoir shale geobodies Orientation & nature of fault networks Understanding fluid transmissibility across faults has become increasingly important over time 15 AREA NORTH AREA CENTRAL AREA 12 AREA EXTREAME SOUTH Chevron

8 Previous Reservoir Modelling Effort Result : modelling effort Earth Model based on PS interpretation (lithology) Reasonable history match but problem areas contained geologically unrealistic iterations Accurate for well planning but often required several sidetracks to place wells optimally for good sand length high in the structure No hierarchy to fault transmissibility character (baffling) Si99 Shear Impedance Data Line 1382 Si04 Shear Impedance Data Line Chevron

9 2008 Reservoir Modelling Effort Re interpret the 1991, 2002 & newly acquired 2008 PP Seismic & build a new Earth Model which encompassed the optimal learnings from the PS & PP interpretations Early & continuous collaborative work between the earth scientist & reservoir engineer to validate static & dynamic options Dynamic simulations to validate the preferred static grid design Static guidance on fault transmissibility multipliers & pore volume multipliers Achieve a history match that maintains geological integrity of the Earth model Customised workflow based modelling, iterative aspect of the modelling project required the ability to rapidly update the static Earth model Chevron

10 2008 4D Acquisition Key Geological Requirements Interpret & understand the nature of the PP 2002 & 2008 OWC s across semi vertical faults previously interpreted from PS Seismic Interpret the polygonal fault network & observe relationship with injected sands Detailed re interpretation of top & base reservoir, injected sands & Alba wings using combination of PP & PS Seismic Interpret the nature & scale of intra reservoir shale geobodies Chevron

11 Interpretation Polygonal Fault Network Results: Polygonal Faults Mainly visible on PP datasets Cut the top of the reservoir interval & delineate Commonly seen at the field edges Generation mechanism dewatering of shale packages N Time display, vertical exaggeration 2:1 PP08 Chevron

12 Interpretation Vertical Fault Network Results: Vertical Faults More visible on PS data, dominantly at reservoir level Pronounced topographical feature in the central main field Fault trend offsetting reservoir, downthrown on the eastern flank Generation mechanism unknown most likely associated with compaction & sand remobilisation Impact on sand & fluid distribution recognised PS99 PP08 Chevron

13 Interpretation 2002 & 2008 OWC s Results Significant offsets of OWC s observed across semi vertical faults Steps in contact surface caused by vertical faults Baffling character (direct input into reservoir model) Pronounced production cones Chevron

14 Interpretation 2002 & 2008 OWC s Faults act as baffles & in places aid coning beneath producers Results: Seis Sim Comparison OWC from 4D seismic OWC from simulation OWC from 4D seismic Cross section of PP08 seismic showing contrast between simulated & 2008 OWC 2008 OWC Simulated 2008 OWC Line 2285 Chevron

15 Interpretation 2002 & 2008 OWC s Results: Seis Sim Comparison Away from fault zones coning under wells was steeper & sharper than simulated saturation surfaces 40% Sw surfaces 2008 extracted from PS model 2008 OWC shows sharp cone development under A23Z producer Line 2468 Chevron

16 Interpretation 2002 & 2008 OWC s Results Larger intra reservoir shale geobodies masked the OWC s? PP08 Data Si99 Data Shear Impedance Inversion of S Wave (for Lithology) OWC response broken, no faults interpreted, shear impedance indicates intra reservoir shale geobody PS99 Base Reservoir Cross section PS99 Earth Model Chevron

17 Interpretation of Top & Base Reservoir Results Top & base reservoir interpreted using PP Seismic dataset Injected sands & field edges interpreted using a combination of PP & PS Seismic dataset Alba North & 15 Area Injectite Horizons Base Reservoir Horizon Top Reservoir Horizon Chevron

18 Interpretation of Top & Base Reservoir Results Injectites difficult to interpret on PP data PS Shear Impedance is the best guide Alba North & 15 Area PS Top Injectite PP08 Data Line 1382 Si99 Data Shear Impedance Inversion S Wave (for Lithology) Line 1382 si99 Chevron

19 Ways to evaluate the 4D signal Chevron

20 Ways to evaluate the 4D signal or just look at the differences Only areas of change (where water has replaced the oil) are showing, the rest is noise. The quality depends on how closely the monitor survey replicates the baseline survey (repeatability). Bright red areas represent the flushed zone Survey 1 Survey Phase shift = Quadrature difference Chevron

21 Ways to evaluate the data With the difference data the flushed zones (red on previous slide) can be isolated using volume rendering and geobody detection. Chevron

22 Current Earth Model Results: Reservoir model is considered a realistic representation of the subsurface Increased team confidence to optimise placement of future wells & better predict well performance Significant time saving benefit due to continuous collaboration between the earth scientist and the reservoir engineer & use of workflow based modelling tools Si99 Shear Impedance Data Line 1382 PP08 Data Line 1382 PS99 PS PP08 PP08 Chevron

23 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

24 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

25 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

26 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

27 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

28 Simulation Model (PS) 2006 PS99 Simulation Model 2009 PS99 Simulation Model Chevron

29 Chevron 2005 Questions