Petrophysical Characterization and Distribution of Remaining Mobile Oil: South Cowden Grayburg Reservoir, Ector County

RI0260. Petrophysical Characterization and Distribution of Remaining Mobile Oil: South Cowden Grayburg Reservoir, Ector County, Texas, by F. J. Lucia. 54 p., 53 figs., 6 tables, 2000. ISSN: 0082335X: Print.

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About This Publication
This report uses the rock fabric method to construct a reservoir model of a Permian Basin Grayburg dolomite reservoir. Rock fabrics are related to porosity, permeability, and capillary pressure and are distributed within a sequence stratigraphic framework. Key findings are that 1) remaining oil saturation is found in intervals with between 0.1 and 1 md, below perforated intervals, and between injection wells when no producer is present; 2) the best production comes from areas where anhydrite has been dissolved, creating a touching-vug reservoir; and 3) the presence of a 100-ft ROZ indicates that this reservoir is in imbibition mode, not drainage.

ABSTRACT
The South Cowden reservoir, a large Permian Basin oil field in Ector County, West Texas, produces from a dolomitized carbonate-ramp succession in the Grayburg (Permian) Formation. The field can be divided into a western area, characterized by interparticle porosity and by diagenetic products that conform moderately well to depositional textures, and an eastern area, characterized by interparticle and touching vug porosity and by poor conformance of diagenetic products to depositional textures.

Thin-section analysis demonstrates that the reservoir interval can be characterized by grain-dominated dolopackstones, medium crystalline mud-dominated dolostones, and a touching-vug pore system composed of dissolved-anhydrite nodules, fusumolds, and microfractures. Two rock-fabric-specific porosity-permeability transforms are defined, one for the grain-dominated and medium crystalline, mud-dominated dolostones and one for the touching-vug pore system.

In the western area the porous and permeable intervals conform to high-frequency cycles (HFC), and flow barriers occur at some cycle boundaries. In the eastern area, late dissolution of anhydrite has occurred, and no evidence has been found to relate the removal of anhydrite to HFC's. As a result, there is little conformance of petrophysical properties to HFC's, horizontal flow barriers are few, and pillars of impermeable anhydritic dolomite are scattered throughout the area.

The presence of a 100-ft residual-oil zone (ROZ) at the base of the reservoir downdip indicates that the downdip part of the reservoir is in imbibition rather than drainage capillary-pressure mode. Imbibition curves have lower water saturations in the productive interval and significantly smaller transition zones than predicted by drainage curves.

The initial water-saturation profile can be approximated by assuming that the base of the ROZ is the zero capillary-pressure level (zcpl). However, making this assumption will overestimate the volume of oil in the ROZ and transition zone. If the zcpl is placed above the base of the ROZ, the original oil in place will be understated. The ROZ is thought to have formed by remigration of oil as a result of eastward tilting during the Tertiary because a ROZ does not appear to be in the updip part of the field.

The three-dimensional distribution of remaining oil can be described by integrating permeability maps, vertical conformance of waterflooding from log analysis, and operational history. Intervals that have been invaded by floodwater are identified by comparing original-water-saturation estimates from capillary-pressure models with water saturation calculated from resistivity logs according to the Archie equation. Flooded intervals have more than 1 md permeability, and considerable volumes of movable hydrocarbons are located in zones having 1 to 0.1 md permeability. Not all of the greater-than-1-md rock is flooded, however. Unflooded high permeability is found (1) below perforated intervals, (2) between injection wells when injection and producing wells are located within a large area of high permeability, and (3) between producing wells and low-kh areas, even if an injector is in the low-kh area.

Keywords: capillary pressure, cyclicity, Ector County, Grayburg, imbibition, permeability, petrophysics, remaining oil, reservoir modeling, rock fabrics, South Cowden, Texas, water-saturation models

CONTENTS

ABSTRACT

INTRODUCTION

FIELD LOCATION AND HISTORY

GEOLOGICAL FRAMEWORK AND CYCLE CORRELATION

ROCK FABRICS

             Description

             Origin of Rock Fabrics

POROSITY CALCULATIONS

             Neutron Logs

             Acoustic Logs

WATER-SATURATION EST1MATIONS

             Original Water Saturation from Capillary-Pressure Curves

             Current Water Saturation from Archie Equation

TRANSITION-ZONE CONSIDERATIONS-THE IMBIBITION RESERVOIR

PERMEABILITY ESTIMATIONS

ROCK-FABRIC AND PERMEABILITY DISTRIBUTION

             Small-Scale Variability

             Variability at the High-Frequency-Cycle Scale

                         Areas 1 and 2

                         Area 3

             Variability at the Reservoir Scale

LOCATION OF REMAINING-MOBILE-OIL SATURATION

             Simulation Results

             Kh maps compared with Production Maps

LOCATING UNTAPPED MOBILE OIL IN INFILL WELLS

             3-D Distribution of Remaining Oil

             Later lnfill Wells

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

Figures
1.Structure map on top of Grayburg. South Cowden field, showing production units and study area

2. lsopach map of cumulative production from original development wells in study area

3. West-east structural cross section A-A' of South Cowden reservoir, showing distribution of permeability and anhydrite alteration types

4. West-east stratigraphic cross section of South Cowden field, showing high-frequency sequences and cycles

5. Core descriptions from wells Moss Unit No. 16-14 and Emmons Unit No. 143, showing sequences and high-frequency cycles

6. Northwest-southeast cross section EB', showing examples of cycle correlations made by using gamma-ray logs that show good upward-shallowing cycles in cycles F1-F2 through F-Fx

7. Diagram illustrating South Cowden rock-fabric petrophysical classes

8. Photomicrographs of petrophysical rock fabrics

9. Photomicrographs of petrophysical rock fabrics

10. Photos of petrophysical rock fabrics

11. Comparison of average kh values obtained from cased-hole porosity logs run in several wells by five cased-hole logging companies

12. Cross plot of core porosity and open-hole neutron-log porosity values for seven wells

13. Cross plot of core porosity and cased-hole neutron porosity values for three wells

14. Cross plot of neutron-density porosity and transit time from acoustic logs, showing three groups of data related to pore type, anhydrite alteration, and porosity transform

15. Map showing distribution of interparticle, separate-vug, and touching-vug pore types on the basis of acoustic-porosity cross plots

16. Map of the distribution of altered sulfate and intervals of complete sulfate removal on the basis of core data from South Cowden field

17. Capillary-pressure graphs of three dolograinstones of varying porosity from the Moss Unit, compared with the generic class-1 capillary-pressure model

18. Petrophysical class-2 averaged capillary-pressure curves compared with curves calculated from the class-2 saturation model using identical porosity values

19. Comparison of class-3 averaged capillary-pressure curves with curves calculated from the class-3 saturation model

20. Comparison of capillary properties from the three petrophysical classes at similar porosity values

21. Comparison of model class9 capillary-pressure curve with two curves from class-2, touching-vug fabric, Emmons Unit No. 142

22. Cross plot of porosity- and log-calculated water saturation from Moss Unit No. 8-12, showing water-saturation-porosity transform used to calculate original water saturation above the transition zone

23. Comparison of capillary-pressure saturation model using a reservoir height of 350 ft with the saturation transform from Moss Unit No. 8-12

24. Calculated logs from MU 201 1, illustrating close agreement between class-2-model and Archie water saturation, where porosity is greater than 6 percent

25. Cross plots of porosity, water saturation, and rock fabric for the G interval

26. Part of Moss Unit No. 1722 well, illustrating method for estimating rock fabric by comparing Archie water saturation and model saturation

27. Moss Unit No. 6-20 log calculations, showing that Archie water-saturation values do not identify a transition zone between the producing oil-water contact at -1,850 ft and the base of oil saturation in the core at -1,950 ft

28. West-east structural cross section GC' of South Cowden reservoir, showing residual-oil zone and present and paleo-oil-water contact

29. Diagram illustrating difference in saturation values between drainage and imbibition capillary-pressure curves

30. Illustration of the effect of tilting and remigration on saturation profiles resulting from a change from drainage to imbibition model

31. Effect of imbibition on water saturation

32. Porosity and permeability data from Moss Unit No. 6-1 6 core-analysis data compared with the generic class-2 porosity-permeability transform of Lucia

33. Porosity-permeability cross plot of core data from Emmons Unit No. 135, Emmons Unit No. 136, and the 4,800- to 4,900-ft interval in Moss Unit No. 6-20

34. West-east cross section of Moss Unit tracts 16 and 20, illustrating general conformance of flow units with high-frequency cycles in the area of unaltered to altered anhydrite

35. Detailed kh maps in 640-acre study area-location map, cycles Eo-Ex, Ex-F, F-Fx, Fx-Fo, and Fo-Fox

36. Detailed kh maps in 640-acre study area-cycles Fox-Foo, Foo-F1, F1-F2, F2-F3, and F3-G

37. West-east cross section of the Emmons Unit, illustrating lack of conformance between flow units and high-frequency cycles in the area of anhydrite dissolution

38. Illustrations of anhydritic dolomite pillars within dissolved-anhydrite, touching-vug facies

39. lsopach map of permeability-feet in reservoir interval of study area

40. Map showing recovery efficiency in each 160-acre tract in 640-acre study area at 0.1-md cutoff

41. 3-D waterflood simulation results, showing incremental recovery from recompleting wells below the F1 marker

42. Comparison of kh and cumulative-production maps, showing that two areas are underproduced and are potential locations for remaining mobile oil

43. Examples of unflooded and flooded infill wells based on saturation comparisons

44. Illustration of flooded and unflooded intervals in Moss Unit No. 8-8w water well, a replacement well for the 8-8w water-injection well

45. Development history of 160-acre study area

46. West-east cross section F-F' of Moss Unit wells in 160-acre study area, showing flooded intervals interpreted from saturation comparisons, poor vertical waterflood conformance, and interpreted 2-D distribution of flooded areas and location of remaining floodable oil

47. Examples of unflooded cycles

48. Remaining floodable pay in cycle Fx-Fo

49. Remaining floodable pay in cycle Foo-F1

50. Remaining floodable pay in cycle F1-F2

51. Well status in 640-acre study area in 1996, showing underdeveloped area and location of four infill wells drilled in 1996

52. North-south cross section of recent infill wells MU 201 2 and 201 3, showing flooded andunflooded pay, which is based on comparison of model and Archie water saturation

53. East-west cross section of recent infill wells MU 1723 and 1722, showing flooded and unflooded pay, which is based on comparison of model and Archie water saturation

Tables

1. Wireline logs available in South Cowden study area

2. Parameters used in converting capillary-pressure data into reservoir height

3. Class-2 sample descriptions

4. Average porosity for class-2 porosity groups

5. Class-3 sample descriptions

6. Average porosity for class-9 porosity groups


Citation
Lucia, F. J., 2000, Petrophysical Characterization and Distribution of Remaining Mobile Oil: South Cowden Grayburg Reservoir, Ector County, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 260, 54 p.