Waterflood Performance Modeling for the South Cowden Grayburg Reservoir, Ector County, Texas

RI0247. Waterflood Performance Modeling for the South Cowden Grayburg Reservoir, Ector County, Texas, by J. W. Jennings, F. J. Lucia, and S. C. Ruppel. 46 p., 37 figs., 4 tables, 1998. Print. 


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About This Publication
This Report of Investigations describes a geological, petrophysical, and reservoir engineering study of the South Cowden field, Ector County, in West Texas.

ABSTRACT
A geological, petrophysical, and reservoir engineering study was conducted of waterflood performance in the South Cowden field, an Upper Permian Grayburg reservoir on the Central Basin Platform in West Texas. The study was undertaken to understand the historically poor waterflood performance, obtain insight on the location of remaining mobile oil, determine the parameters having the most influence on waterflood performance predictions, evaluate three techniques for incorporating petrophysical measurements and geological interpretation into heterogeneous reservoir models, and identify issues in fluid-flow scaleup and heterogeneity modeling that require further research. The approach included geological description of the field stratigraphy and diagenesis, petrophysical modeling, analysis of relative permeability data, analysis of injection and production data, heterogeneity modeling, and waterflood simulation.

Partial completions, vertical permeability, and relative permeability were the most important parameters affecting waterflood performance predictions. The results were also influenced by the heterogeneity modeling procedure.

The poor waterflood recovery at South Cowden is due, in part, to completion of production and injection wells in only the top half of the formation; some of the remaining mobile oil is located in the poorly swept bottom half of the reservoir. Recompletion of wells through the entire formation is estimated to improve recovery in 10 yr by 6 percent of the original oil in place in some areas of the field.

All the flow simulations required a vertical-to-horizontal permeability ratio (k_v l k_h) smaller than that measured in whole core to explain waterflood behavior, suggesting the presence of barriers to vertical flow not explicitly accounted for in any of the heterogeneity models. The flow simulations also required modifications to the measured steady-state relative permeabilities, suggesting that small-scale heterogeneities and scaleup are significant. Vertical flow barriers, small-scale heterogeneity modeling, and relative permeability scaleup require additional research for waterflood performance prediction in reservoirs like South Cowden.

One of the heterogeneity models used in the study was based upon stochastic three-dimensional sequential-Gaussian simulations conditioned to well-log data. This model produced the best fit to waterflood performance and injectivity, but a simpler model based on two-dimensional smooth mapping of vertically averaged properties within each high-frequency cycle was almost as good. The results reaffirm the importance of large-scale heterogeneities in waterflood modeling but demonstrate only a slight advantage for stochastic modeling at this scale.

Keywords: Ector County, Grayburg Formation, heterogeneity modeling, scaleup, South Cowden field, Texas, waterflood performance prediction

CONTENTS

Abstract

Introduction

     Geologic Framework and Diagenesis

     Geologic framework

     Diagenesis

Petrophysical Modeling

     Porosity

     Initial water saturation

     Permeability

Relative Permeability Modeling

     Relative permeability measurements and models

     Choosing kr models for flow simulation

     Initial-residual oil saturation models

Production and Injection History

     Pressure history and fluid properties

     Injection and production balance

     Waterflood displacement performance

     Displacement of initially mobile water

     Injectivity, injection patterns, and well completions

Heterogeneity Modeling

     Smooth interpolation of HFC averages

     Stochastic simulation of HFC averages

     Three-dimensional stochastic simulation

     Constant-HFC and uniform models

Waterflood Simulations

     Model parameters

     Waterflood displacement results

     lnjectivity predictions    

Remaining Mobile Oil    

Conclusions and Recommendations

Acknowledgments

References

Figures

1. Shallow-water carbonate platform reservoirs of West Texas and New Mexico, including those producing from the Grayburg Formation

2. Unit boundaries and structure on top of the Grayburg Formation in the South Cowden field

3. Stratigraphic cross section A-A' illustrating high-frequency sequences and cycles in the South Cowden field

4. Map of the South Cowden field showing the location of altered dolomite intervals

5. Map of the South Cowden field showing the location of altered and removed anhydrite

6. Map of anhydrite alteration and dissolution based on cross plots of sonic travel time and neutron-density porosity  

7. Steady-state and unsteady-state kr measurements with increasing water saturation  

8. Steady-state and unsteady-state kr models with increasing water saturation  

9. Initial-residual oil saturation models used to set residual oil saturations for kr endpoint scaling  

10. Cumulative total fluid production and water injection volumes corrected to subsurface conditions for the South Cowden field, 1965-1993  

11. Cumulative total fluid production and water injection corrected to subsurface conditions for sections 5-8, 1965-1993  

12. Waterflood displacement performance for sections 5-8 compared with four laboratory unsteady-state displacements

13. Total waterflood displacement performance for sections 5-8, using the three residual saturation models, compared with Buckley-Leverett predictions from the two relative permeability models

14. Cumulative water injection for sections 5-8 in the South Cowden field
15.The number of active injectors in sections 5-8 of the South Cowden field
16. The injector-producer ratio in sections 5-8 of the South Cowden field

17. Location of heterogeneity and flow-simulation models

18. HFC-averaged porosities, initial water saturations, and permeabilities, compared with well-log models

19. Porosities, initial water saturations, and permeabilities generated by smooth interpolation of HFC-averaged well-log measurements, compared with well-log models

20. Porosity grid generated by smooth interpolation of HFC-averaged well-log measurements

21. Horizontal auto- and cross-variograms for Gaussian transforms of vertically averaged porosity, permeability, and initial water saturation from well logs

22. Vertical auto- and cross-variograms for Gaussian transforms of vertically averaged porosity, permeability, and initial water saturation from well logs

23. Porosities, initial water saturations, and permeabilities generated by stochastic simulation of HFC-averaged well-log measurements. compared with well-log models

24. Two realizations of porosity for the waterflood simulation model generated by stochastic simulation of HFC-averaged well-log measurements

25. Well-log porosity data for ail wells in the South Cowden field, using a stratigraphically normalized vertical coordinate

26. Well-log porosity data for all wells in the South Cowden field and a model for Gaussian transformation

27. Vertical and lateral variograms of Gaussian-transformed well-log porosity
28. Simulated porosities for 16 wells displayed on a stratigraphic coordinate system

29. Porosity grid generated by 3-D-stochastic simulation of foot-by-foot well-log measurements

30. Well arrangement used in flow simulations

31. Field performance in sections 5-8, one-dimensional Buckley-Leverett predictions, and three-dimensional waterflood simulations using fully completed and partially completed wells

32. Field performance in sections 5-8, one-dimensional Buckley-Leverett predictions, and three-dimensional waterflood simulations using partially completed wells

33. Field performance in sections 5-8, one-dimensional Buckley-Leverett predictions, and three-dimensional waterflood simulations using the 3-D-stochastic heterogeneity model 38

34. Field performance in sections 5-8, one-dimensional Buckley-Leverett predictions, and three-dimensional waterflood simulations using partially completed wells

35. Field performance in sections 5-8, one-dimensional Buckley-Leverett predictions, and three-dimensional waterflood simulations using the smooth-HFC model and one realization of the stochastic-HFC model

36. Field performance and 3-D waterflood simulations using the 3-Dstochastic model and the unsteady-state kr model

37. Estimated incremental oil recovery in the South Cowden field from recompletion of wells to sweep the bottom half of the formation

Tables

1. Properties of the eight core samples used in relative permeability measurement  

2. Summary of the five heterogeneity-modeling approaches used and the five sets of waterflood-simulation models produced  
3. Predicted injectivities from waterflood simulations with partially completed wells and a kvlkh ratio of 0.01  

4. Predicted injectivities from waterflood simulations with partially completed wells and the 3-D-stochastic heterogeneity model 


Citation
Jennings, J. W., Lucia, F. J., and Ruppel, S. C., 1998, Waterflood Performance Modeling for the South Cowden Grayburg Reservoir, Ector County, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 247, 46 p.