Increased Gas Recovery From a Moderate Water Drive Reservoir

Abstract

Summary Accelerated gas withdrawals from a U.S. gulf coast water-drive gas reservoir resulted in a 20% increase in remaining gas recovery vs. continued low-rate depletion. Pressures measured in water-invaded areas showed significant reduction and indicated gas percolation from these areas to the producing wells. Introduction The Lovells Lake Frio 1 reservoir contained approximately 175 Bscf (5 × 109 std m3) of gas and 18 MMSTB (2.9 × 106 stock-tank m3) of oil upon discovery in 1938. This reservoir is generally a low-dip anticlinal structure with a natural moderate water drive as the primary depletion mechanism. The natural water influx maintained reservoir pressure at about 80% of the original pressure through 1976.Oil production operations continued at a very high water percentage, and gas cap cycling had been completed successfully in 1976. Both "tank-type" and three-dimensional simulations of the reservoir were performed to determine the optimum final gas-cap gas depletion program. These simulations indicated the potential for additional gas recovery through the reduction of reservoir abandonment pressure, with a highly accelerated gas production schedule.On Nov. 2, 1977, gas cap withdrawals were increased to 115 MMcf/D (3.3 × 103 m3/d) and averaged 84 MMcf/D (2.4 × 106 m3/d) during the next 12 months. Reservoir pressure declined by as much as 1,000 psi (6.9 MPa) in the center portions of the reservoir within 10 months. Blowdown at a reserves/production ratio (R/P) of 0.9 years provided an increase in gas recovery of some 21 Bcf (595 × 106 m3) or about 20% over blowdown at an R/P of 9 years. Gas-cap gas recovery from the water-invaded areas was increased from a historical 58% before blowdown to 70% upon final depletion of the gas cap. This was due to a reduction of the gas trapping pressure and percolation of previously trapped gas to the producing wells. Reservoir Description The Lovells Lake field is 14 miles south of Beaumont on the upper Texas gulf coast. The field was discovered in 1938 and was developed on 20-acre (80 × 103-m2) spacing with some 250 wells. The basic structure for the Lovells Lake reservoirs is a deep-seated, elliptical dome lying to the southeast of a northeast-southwest trending fault. The principal producing formations are the Anahuac and Frio sands.Of the productive reservoirs included at Lovells Lake, the Frio 1 is the largest, covering approximately 3,600 surface acres (14.5 km2). The Frio 1 Sand is encountered at an average depth of 7,650 ft (2332 m). It is treated as one reservoir for regulatory purposes and has been subdivided into two zones for the purpose of this study. These designations are shown on the type electric log in Fig. 1. The Frio stringer is a shaly discontinuous sand that is primarily gas productive. This sand had exhibited pressure depletion behavior and was not included in this study.The main Frio 1 is continuous over the entire field, is not faulted, and varies in thickness from 60 to 80 ft (18 to 24 m). Upon discovery, reservoir fluids consisted of a large gas cap and a thin oil rim as indicated by the original 6:1 ratio of gas cap to oil zone volume. The initial thickness of the oil zone averaged 9.5 ft (2.9 m), while the gas zone thickness averaged 30 ft (9.1 m). The reservoir was underlain by a water-bearing sand throughout most of the productive area.The regional aquifer for the Frio 1 is rectangular and bounded by faults on three of four sides. JPT P. 2475^