Enhanced Gas Recovery from Watered-Out Reservoirs: Port Arthur Field, Jefferson County, Texas. Digital Download

RI0142D. Enhanced Gas Recovery from Watered-Out Reservoirs--Port Arthur Field, Jefferson County, Texas, by A. R. Gregory, Z. S. Lin, R. S. Reed, R. A. Morton, and T. E. Ewing. 58 p., 45 figs., 7 tables, 2 appendices, 1984. doi.org/10.23867/RI0142D. Downloadable PDF.



To purchase this publication in book format, please order RI0142.



ABSTRACT
Gas reservoirs that water out under moderate to strong water drives are normally abandoned when the expenses associated with salt-water disposal make continued operations uneconomical. Under favorable conditions, however, watered-out reservoirs can continue to produce substantial quantities of gas at competitive prices if operators are prepared to dispose of large volumes of water. Enhanced gas recovery (EGR) techniques can extend production from many reservoirs that are now watering out and will soon be abandoned if conventional practices are followed.

The EGR method involves co-production of gas and water. If large volumes of water are produced, the reduced reservoir pressure causes expansion of free gas formerly trapped in the water-invaded zone during the primary production stage. Some of this free gas becomes mobilized and producible. Also, pressure reduction at the surface releases additional but minor amounts of gas dissolved in the formation water.

The Port Arthur field, Jefferson County, Texas, contains several watered-out gas reservoirs, thick sandstone aquifers, and gas stringers that collectively make the field ideal for testing the co-production technology. The objective sandstones occur in the lower Hackberry (Oligocene) interval at depths of 10,850 to 11,700 ft. The field covers about 1,900 acres (3 mi²) and originally produced gas condensate from an anticlinal closure on the downthrown side of a major fault that separates the Port Arthur field from the Port Acres field.


Some of the lower Hackberry sandstones, interpreted as submarine channel and fan deposits, are laterally extensive and have excellent physical characteristics for producing gas and water. Net-sandstone thickness averages 350 ft. Core data and well log analyses show that porosities average 30 percent, permeabilities average 60 md, salinities average 67,900 ppm sodium chloride, and methane solubility averages 25.7 scf/bbl. Abundant shallow Miocene sands in the area could be used for salt-water disposal. Available well logs were analyzed to determine porosity and other formation characteristics of reservoirs being studied. Water saturations were also calculated from logs to help locate gas-water contacts.

Seismic data were acquired and reprocessed to (1) provide structural information to supplement geological interpretations, (2) locate boundaries of aquifers and gas reservoirs, and (3) evaluate seismic response to low saturations of free gas dispersed in water-invaded zones of watered-out gas reservoirs. Attaining these objectives was severely limited by the poor signal-to-noise ratio in the seismic data. The quality of data was adequate for structural interpretation but was not suitable for reservoir delineation or detection of gas zones. Modeling studies showed what kind of seismic response should be expected from known subsurface geology and suggested that better reservoir delineation could be achieved with increased bandwidth, improved signal-to-noise ratio, and a better knowledge of reservoir acoustic impedances; but it remains unclear whether dispersed free gas in a watered-out reservoir can be detected with seismic data.

The Hackberry C reservoir was selected for numerical modeling because of its high productivity, high average abandonment pressure gradient (0.67 psi/ft), and excellent physical properties. The original gas in place (OGIP) was estimated to be 56.2 Bcf, 35 percent of which was recovered during primary production. A three-dimensional, two-phase model was used to perform history matches and to predict the amount of fluid that might be produced under natural flow conditions if a new well were drilled. During a projected 8-year production period, the predicted reservoir bottom-hole flowing pressure would decline from 6,600 to 4,200 psi. Predicted production was 5.1 Bcf of gas, 51,000 bbl of condensate, and about 9 million bbl of water. An additional 10 percent of the OGIP was predicted as recoverable if the EGR co-production method were used. These results of the simulation study predicted the reservoir performance if a new well were drilled to a depth of 11,650 ft and located on a site near the Meredith No. 2 Doornbos (well 14). Because the reservoir is still geopressured, artificial lift methods would not be required to produce from this test well.

Cash-flow calculations show that the break-even gas price is $2.40/Mcf for a 15-percent rate of return after payment of Federal income tax. The net present worth of the investment is about $968,000 for a gas price of $3.00/Mcf, and it increases substantially at higher gas prices. The economic outlook for the prospect would be even better if production from the C reservoir were commingled with production from other reservoirs in the field.

Keywords: gas production, gas reservoirs, reservoir performance, well logging, computerized simulation, economic analysis, Jefferson County, Texas


CONTENTS
ABSTRACT

INTRODUCTION

SELECTION OF TEST AREA

STUDIES OF THE PORT ARTHUR AREA

REGIONAL GEOLOGICAL SETTING

FRIO STRATIGRAPHY

PORT ARTHUR FIELD

GEOLOGY

POTENTIAL SALT-WATER DISPOSAL SANDS

WELL LOCATIONS, STATUS OF WELLS, AND RESERVOIR PROPERTIES

RESERVOIR FLUID PROPERTIES

Methane solubility

Temperature and pressure gradients

WELL LOG ANALYSES

SEISMIC DATA ACQUISITION AND PROCESSING

Seismic modeling

PRODUCTION HISTORY

The C reservoir

Other reservoirs

PREQICTED RESERVOIR PERFORMANCE AND ECONOMIC ANALYSIS

Reservoir simulation studies

Model description

Model data and history matches

Predictions

Economic analysis

CONCLUSIONS AND RECOMMENDATIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX A: Metric conversion factors

APPENDIX B: Nomenclature

FIGURES

1. Fluid saturation zones within a hypothetical watered-out reservoir

2. Typical well log response for thick aquifers with gas caps and thin gas stringers

3. Stratigraphic diagram of Tertiary strata, paleomarkers, sand-body distribution, and marker horizons, Jefferson County area

4. Map showing location of Port Arthur field with respect to major faults, other nearby fields, and points of interest

5. Well locations, lines of cross sections, and structural configuration on top of the lower Hackberry sequence, Port Arthur - Port Acres area

6. Net-sandstone distribution in the lower Hackberry sequence, Port Arthur - Port Acres area.

7. Structural dip cross section Z-Z', Port Arthur field

8. Structure map contoured on the pre-Hackberry unconformity, Port Arthur field

9. Stratigraphic strike cross section X-X', Port Arthur field

10. Type log showing reservoir intervals, Port Arthur field

11. Submarine-fan facies model showing SP curves from the Hackberry C sandstone, Port Arthur field

12. lsopach map showing distribution and log character of the Hackberry G sandstone, Port Arthur field

13. lsopach map showing distribution and log character of the Hackberry F sandstone, Port Arthur field

14. lsopach map showing distribution and log character of the Hackberry C sandstone, Port Arthur field

15. Structure map contoured on top of the C sandstone, Port Arthur field

16. Onlapping submarine-fan depositional model of lower Hackberry sandstones, Port Arthur field

. 17. Cross section T-T' showing thickness of shallow Miocene sands suitable for disposal of waste salt water

18. Distribution of initial pressure gradients, Hackberry C sandstone, Port Arthur field

19. Distribution of temperature, Hackberry C sandstone, Port Arthur field

20. Distribution of salinity, Hackberry C sandstone, Port Arthur field

21. Distribution of initial methane solubility, Hackberry C sandstone, Port Arthur field

22. Pressure, temperature, salinity, and methane solubility versus depth, well 14, Port Arthur field

23. Geothermal gradients, Port Arthur field

24. Bottom-hole shut-in pressure versus depth for 13 wells, Port Arthur field

25. Porosity distribution, lower Hackberry sandstones, Port Arthur field

26. Location of seismic lines in the Port Arthur area

27. Model of the Hackberry sands along a cross section coincident with seismic line 3

28. Line 3 showing interpreted and modeled top of Hackberry C sandstone

29. Spike synthetic seismic section with gas sands shaded, Port Arthur field

30. Synthetic seismic section with wavelet bandpass = 15 to 45 Hz, Port Arthur field

31. Synthetic seismic section with wavelet bandpass = 15 to 65 Hz, Port Arthur field

32. Synthetic seismic section with signal-to-noise ratio = 25.1

33. Synthetic seismic section with no. gas, bandpass = 15 to 65 Hz, Port Arthur field

34. Stratigraphic strike cross section A-A' showing lower Hackberry sandstone intervals and perforated gas production zones, Port Arthur field

35. Reservoir production rates and bottom-hole flowing pressure versus time, Hackberry C sandstone, well 14, Port Arthur field

36. Reservoir production rates and bottom-hole flowing pressure versus time, Hackberry C sandstone, well 23, Port Arthur field

37. Simulator grid used for reservoir simulation of Hackberry C sandstone

38. Distribution of sandstone thickness used for reservoir simulation of Hackberry C sandstone

39. Permeability distribution used for reservoir simulation of Hackberry C sandstone

40. Gas/ condensate ratio versus time

41. Relative permeability curves (Corey-type equation) used in reservoir simulation

42. History matches for pressure and water production rates, Hackberry C sandstone, well 14, Port Arthur field

43. Predicted gas and water flow rates in Hackberry C sandstone

44. Break-even gas price versus rate of return before and after payment of Federal income tax

45. Net present worth versus rate of return after Federal income tax

TABLES

1. Identification, location, and status of wells, Port Arthur field

2. Pressure gradients and production history by reservoir and well, Port Arthur field

3. Salinity, temperature, pressure, and methane solubility at initial reservoir conditions, lower Hackberry reservoirs, Port Arthur field

4. Cumulative production from Hackberry C reservoir, Port Arthur field

5. Model data used in simulation studies

6. Past and predicted production from Hackberry C sandstone (natural flow conditions)

7. Cost data used in economic analysis



Citation
Gregory, A. R., Lin, Z. S., Reed, R. S., Morton, R. A., and Ewing, T. E., 1984, Enhanced Gas Recovery from Watered-Out Reservoirs--Port Arthur Field, Jefferson County, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 142, 58 p.