2. Water Saturation Modelling From petrophysics to volumes and simulation FORCE seminar 30-31 March 2004 Jan C. Rivenæs J.I. Kristiansen, Tor Barkve, Morten Fejerskov, Stephen Rodgers, Julio Marre, Geir Håskjold FORCE seminar 30-31 March 2004 Jan C. Rivenæs J.I. Kristiansen, Tor Barkve, Morten Fejerskov, Stephen Rodgers, Julio Marre, Geir Håskjold

3. T&K Page: 2 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

4. T&K Page: 3 Philosofical background Philosephor Thales from Miletus ( ~ 500 B.C ): Everything is water. Petrophysiscist Julio Marre ( ~ 2000 A.C ) Much is water. Hydro Boss Torgeir Kydland ( ~ 2000 A.C ) Too much is water!

5. T&K Page: 4 In the beginning, there was water… Migration of oil: A drainage process

6. T&K Page: 5 An integrated work process GEOLOGY PETROPHYSICS CORE ANALYSIS LAB RESERVOIR CAN WE FIND A common language? A common understanding? A common model?

7. T&K Page: 6 The Sw modelling 2003 project GEOLOGY PETROPHYSICS CORE ANALYSIS RESERVOIR GEIR HÅSKJOLD TOR BARKVE JAN CHR RIVENÆS JULIO MARRE MORTEN FEJERSKOV STEPHEN RODGERS JAN INGOLF KRISTIANSEN

8. T&K Page: 7 Historic view on Sw modelling 1935: Realisation of existence of capillary water Garrison – 1935 Schilthuis – 1938 1940: Methods for predicting water saturation Leverett – 1941 Archie – 1942 70’ties: The first reservoir simulator Beta II – 1975 Eclipse - 1982 80’ties: Geological modelling using computers Mapping of zone averages Integrate Sw function over zones thickness - 1985 90’ties: 3D modelling and visualisation 2003: Sw modelling is still a "black box"… ??

9. T&K Page: 8 Main deliverables from Sw modelling STOOIP/GIP Function of bulk rock volume, NTG, PHI, Sw and PVT HCPV distribution In which segments are the oil/gas accumulated? Input to reservoir simulation Initial hydrocarbon distribution prior to production Integrated part of SCAL model

10. T&K Page: 9 General workflow Petrophysical modellingGeological modellingReservoir simulation Establish a Sw function honoring both log and core data and define fluid levels Defined segment contacts and populate the Geo-model with water utilising an established Sw function and mapped rock properties Initialize the Sim-model honoring upscaled saturations from the Geo-model and SCAL

11. T&K Page: 10 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

12. T&K Page: 11 Water saturation - Sw Matrix (sand, silt) Dry clay Clay bound water Free water S w = 1 S h = 0 Fraction of water in the pore volume Free Water Hydrocarbon S w < 1 S h > 0 S w S h = 1 - S w

13. T&K Page: 12 Vertical equilibrium The initial fluid distribution is defined by vertical equilibrium between capillary pressure and gravity forces. Pc Depth Sw

14. T&K Page: 13 k = 50 md 10 md k = 100 md 1 D 500 md 200 md Height above FWL Water saturation FLUID DISTRIBUTION AND CONTACTS: ROCK QUALITY DEPENDENCE OF WATER SATURATION- HEIGHT PROFILE Saturation depends on rock quality (average pore throat radius) as well as height above the FWL.

15. T&K Page: 14 The basic levels… Depth 1 FWL OWC Free water level FWL: Depth at which oil/water capillary pressure is zero. Free oil level FOL: Depth at which gas/oil capillary pressure is zero. Oil/water contact OWC: Minimum depth where the water saturation is equal to it’s maximum value.

16. T&K Page: 15 Water saturation definitions Drainage Imbibition Sw S WIR S WCR Formation Pressure (FMP) Depth FOL FWL S WIR

17. T&K Page: 16 Is OWC precise? FWL OWC No, refer to fluid contacts based on pressures instead Good sand: OWC  FWL Poor sand: OWC  FWL

18. T&K Page: 17 FOL and appearant FWL for gas FWL FWLG FOL GAS OIL WATER

19. T&K Page: 18 Wettability and capillary pressure OIL ROCK WATER The contact angle can be used to distinguish between a water wet, mixed wet, and oil wet system. The wettability is a function of rock and fluid properties, but also on history. Drainage: Non-wetting phase saturation is increasing. Imbibition: Wetting phase saturation is increasing.

20. T&K Page: 19 Sources for Sw data Sw Routine core analysis Special core analysis Pressure measurements Sw logs (CPI) Production and production logging

21. T&K Page: 20 Sw and the other properties… Porosity Apparent, non-apparent, absolute, effective, total, residual, intergranular, intermatrix, primary, secondary,... Permeability etc Appearant, effective, relative, Klinkenberg, air, horisontal, vertical, syntetic, well-test, mobility, wettability Net-to-gross Cut-off, reservoir, non-reservoir net sand, net pay, Vshale,… Scale Core plugs, log scale, geomodel, simulation model, upscaling, modelling approach,… ?

22. T&K Page: 21 The petrophysical porosities Matrix (sand, silt) Dry clay Clay bound water Free water Free Water Hydrocarbon ee tt cc Log analysis:  e = PHIE and  t = PHIT  c (low temperature humidity drying) ~  e  c (high temperature oven drying) ~  t Use  e (PHIE) and proper  c 

23. T&K Page: 22 Should we use NTG and cut-off? No contribution? If you model effective props (PHIE and Keff), cut-off in a 3D model is not needed NTG should not be modelled as a property… However, for up-scaling Sw, post-model cutoff may be needed in some cases

24. T&K Page: 23 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

25. T&K Page: 24 COOPERATION EXTERNAL CONDITIONS WORKFLOW ORGANISATION Sw … An integrated work process GEOLOGY PETROPHYSIC S CORE ANALYSIS LAB RESERVOIR

26. T&K Page: 25 A question of scale… Core scale Log scale Geo model scale Flow model scale

27. T&K Page: 26 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

28. T&K Page: 27 Workflow for petrophysical Sw-model GEOLOGY PETROPHYSICS CORE ANALYSIS LAB RESERVOIR ZONATION FACIES CORE DATA: k, w, Pc RAW LOGS FMP FLOW ZONES CPI’s Sw MODELS FLUID LEVELS

29. T&K Page: 28 Traditional Leverett model building J (S w )-model Using log data: SWE+PHIE+KHLOG+H  J-model  test Using core data: Pc(Sw)+  c+K  J-model  test

30. T&K Page: 29 Sw-modelling preferences

31. T&K Page: 30 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

32. T&K Page: 31 Workflow for geological Sw-model GEOLOGY PETROPHYSICS CORE ANALYSIS LAB RESERVOIR CPI’s Sw MODELS FLUID LEVELS CORE DATA: k, w ZONATION & REGIONS GEO MODEL Sw VOLUMES

33. T&K Page: 32 3 steps for the geomodeller Receive function(s) Implement in modelDeliver products Drainage part

34. T&K Page: 33 Handling input from petrophysicists Sw formulas on mystical forms, in mystical units SWE, SWT, PHIE, PHIT, PHIcore, Kair, Ksynth (Use whatever you want) ?? Until now Sw formulas on standard forms, in NH standard units SWE, PHIE, K ! 2004 …

35. T&K Page: 34 Quality control is important Large (thick) cells may give problems Ln(SwD) H GEO MODEL

36. T&K Page: 35 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

37. T&K Page: 36 Work flow for the simulation Sw-model GEOLOGY PETROPHYSICS CORE ANALYSIS LAB RESERVOIR GEO MODEL SW VOLUMES SCAL DATA: PC AND REL.PERM J FUNCTIONS FLUID LEVELS $$$$$

38. T&K Page: 37 Upscaling - prone for confusion… I can perhaps use your upscaled Sw, but only partially! Partially...? Don’t you like me? Talk together: The way R.T. choose to incorporate Sw in Eclipse may influence the upscaling The upscaling is a multidisciplinary task, but in practice R.T. is in charge! RT GEO

39. T&K Page: 38 The world as seen by a reservoir dog The geo model alone is not sufficient to define all necessary Sw data for the simulation model. The upscaled Geo Sw cannot be used directly in the simulation model. Sw data related to mobility must be taken from SCAL data. SCAL data are not always sufficiently available. SCAL and Geo data may seem inconsistent.

40. T&K Page: 39 Requirements for the simulation Sw model The initial volumes should be (approximately) stable. Capillary pressure/gravity equilibrium. “Correct” dynamic response. Imbibition or drainage data? Initial volumes should match geo model. Drainage Pc data. LESS SW! STABILITY NOW Use Pc! Give us SWCR

41. T&K Page: 40 Vertical equilibrium in the simulator FWL Pc To achieve true vertical equilibrium: Depth values must be taken at grid cell centers. Cell refinement can be used in Eclipse initialization but leads to an initially instable model. ALWAYS CHECK INITIAL MODEL STABILITY.

42. T&K Page: 41 Accurate volumes versus stability Inaccurate representation of fluid contact Grid cells with saturation change larger than 0.05 after 100 days simulation without wells. FWL Pc = 0

43. T&K Page: 42 … Sw for R.T. is less straightforward … If using upscaled Sw from geomodel… Need to use rel.perm. end-point scaling No capillary pressure No water production in transition zone Problems around FWL (anyway) Or If using Sw from Capillary pressures… Core plug J-curve does not apply to sim. scale Achieve volume match with Geo may be difficult Simulator will produce water in transition zone –Which is commonly not observed in practice! Approach dependent on use of model!

44. T&K Page: 43 Summary The reservoir challenge is the combination of the SCAL data and the geo model data. The petrophysicist has already handled this problem, but on a different scale. A Sw implementation in the simulation model should preferably be based on a J function approach. Sw modelling for reservoir is time consuming. Plan for this!

45. T&K Page: 44 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

46. T&K Page: 45 Upscaling Best practice: Upscale porosity arithmetic, weighted on bulk cell volume Compute PoreVolume for each cell Upscale Sw weighted on pore-volume, arithmetic Consider use of "cut-off" in Geomodel before upscaling Challenge What input does the res.eng want? –Sw –Swir? (a rock parameter, ~independent of contacts) –Swcr? (a rock parameter, ~independent of contacts) Be precise on delivery to R.T.

47. T&K Page: 46 Some upscaling of Sw issues The contact problem Cell below or above contact? The resolution problem Cut-off (filtering) or not? Sampling near faults Core-plug to ressim cell problem

48. T&K Page: 47 The contact problem Large (thick) cells may give problems Cell properties refers to center point; i.e. the whole cell gets one value Oil cell?100% water? FWL

49. T&K Page: 48 Geomodel: Integrating J over geocell H=dZ Breath … relax … The IPLIB routines do this for you!

50. T&K Page: 49 Sw model implementation, QC Usually the SCAL data for the Sw model development, and the RMS PHIE and Kh data are different : -1- The synthetic Kh can differ substantially from the SCAL data -2- The synthetic Kh is blocked before being used for Sw computation (up scaling process) Grid structure influence the fluid contacts :

51. T&K Page: 50 The resolution problem (tiger problem) Having geocell vertical grid 0.5 to 1m is close to Sw log resolution!

52. T&K Page: 51 The “tiger” problem Same Sw(H) curve? What about relperm start point? Should we try to ”back-compute” PHI Net Sw Net, K Net and NTG (for Sw calculations only?) 1m PHI=0.25, K=3000 PHI=0.13, K=10 Heterolithic coarse sand/silt PHI=0.19, K=1500 Homogenous sandstone PHI=0.19, K=1500 Blocked/Upscaled ?? Pc

53. T&K Page: 52 ”Cut-off” (filter) in Sw upscaling Sw in geo model is often correlated with the permeability k. In a stochastical model, some grid cells may have low k, but high porosity. This may lead to artificially high mobile water saturations in the simulation model. Cure: Set all cells with K < x and w <y inactive. I.e. a cut off before upscaling (On Oseberg Øst, x=1mD, y=0.05 p.u.) This is espacilly important if some kind of end-point scaling is used, but should also be considered elsewhere k=0.01 mD w = 0.2 Sw = 1 k=100 mD w = 0.2 Sw = 0.1 k=67 mD w = 0.2 Sw = 0.33

54. T&K Page: 53 Water sat upscaling: Sampling crucial! Up Down GeoModel SimModel

55. T&K Page: 54 Quality control on upscaling effects Ln(SwD) H GEO MODELSIM MODEL

56. T&K Page: 55 QC of Sw upscaling Plot the upscaled Sw from the Geo model in the same functional format as the petrophysical model. Below FWL, Ln(SwD) should be close to 0

57. T&K Page: 56 The quest for the correct Pc… How should I define the Pc curve in the simulator to reproduce the upscaled Geo Sw distribution SWGEO? How should lithological variation in Pc data be described on the simulation model scale? K, w, Sw Pc? K, w, Sw J J curves are scale dependent. Eclipse takes simple form of J curves only. Core plug ?

58. T&K Page: 57 The end point scaling solution If Pc h 0 in the Eclipse input, Sw = SWIR above the FWL. FWL Sw Depth SWIR 1 For each cell in Eclipse, define: Pc h 0 SWIR = SWGEO

59. T&K Page: 58 GEOLOGY PETROPHYSICS CORE ANALYSIS LAB RESERVOIR CAPILLARY PRESSURES J FUNCTIONS Pc selection for the simulation model SCYLLA CHARYBDIS PETROPHYSICAL MODEL: J-CURVES GEO-MODEL SCALE MAY HAVE TO BE SIMPLIFIED PRIMARY DRAINAGE DATA NO LITHOLOGICAL GROUPING ON SIM.SCALE SCAL DATA: CAPILLARY PRESSURE DATA CORE SCALE DRAINAGE OR IMBIBITION? NO LITHOLOGICAL GROUPING ON SIM.SCALE

60. T&K Page: 59 Topics Introduction Definitions Overall workflow Sw in Petrophysics Sw in Geology modelling Sw in Reservoir Simulation Upscaling issues Summary

61. T&K Page: 60 Summary (remember this) Use effective porosities (PHIE) Avoid net-to-gross as a property Use FWL instead of OWC Use appearant FWL for gas above oil Use J-functions (and start simple) J-functions should be derived at the proper scale … Since permeability is so scale-dependent Treat 3D blocks near FWL correctly Consider use of appearant properties RT have the most difficult task Sw modelling is truly interdisciplinary… NH work: Best practice document Yet, there are many unresolved issues…

62. T&K Page: 61 Philosofical epilougue Res.eng. Tor Sw means “Sucking worries” Geologist Jan Sw means “Simple wrap” Petrophysicist Jan Ingolf Sw means ”Some work”

63. T&K Page: 62 Thank you for participating Feedback from you are welcome

64. T&K Page: 63 THE END