Compositional Simulation of the Coyanosa Wolfcamp Field Gas Cycling Operation

Abstract

SPE Member Abstract The Coyanosa Wolfcamp Field, located in Pecos County, Texas was discovered in September 1962. The reservoir rock is comprised of poorly sorted, coarse-grained heterogeneous conglomerates with thinly interbedding shaly limestones. The dew point pressure of the Gas Unit reservoir (representing about 80% of the Coyanosa Wolfcamp Field pore volume) was 6,179 psia at a reservoir temperature of 175 degrees F. The initial reservoir pressure was 6,700 psia. Mobil initiated a gas cycling pressure was 6,700 psia. Mobil initiated a gas cycling operation in August 1964. The condensate yield has declined steadily during the lean gas cycling operation. A compositional simulation study was undertaken to simulate complex mass transfer and composition changes. The study's objectives were to estimate the remaining condensate reserves and to determine an efficient depletion scheme including an optimum reservoir blowdown timing. After matching the past 23 years of the production history, the model was then used to predict production history, the model was then used to predict future performance under the various operating scenarios. The sensitivity of liquid recovery to pressure, rate and degree of cycling was investigated. pressure, rate and degree of cycling was investigated. Depletion plans such as added compression, partial cycling and immediate blowdown were evaluated to devise an optimal operation plan. Introduction The Coyanosa Wolfcamp Field is located in Pecos County, Texas on the northeast flank of the Delaware Basin. Mobil discovered the field in September 1962 with the completion of the E. P. Sibley No. 1 well. This well flowed 2,206 BBL/D of condensate and 14.7 MMCF/D of gas. The Coyanosa Wolfcamp Field encompasses approximately 5,840 acres with sixteen producing wells and five injectors. The producing horizon is the lower Permian Wolfcamp and upper Pennsylvanian Strawn Permian Wolfcamp and upper Pennsylvanian Strawn formations at an average depth of 10,000 feet. Fluid analysis of the E. P. Sibley No. 1 well in 1963 indicated the reservoir fluid to be a rich condensate system with a dewpoint pressure of 6,179 psia at a reservoir temperature of 175 degrees F. The initial reservoir pressure was 6,700 psia. Due to severe retrograde condensate characteristics of the reservoir fluid, Mobil initiated a pressure maintenance gas cycling operation in August pressure maintenance gas cycling operation in August 1964. Except during a four well development drilling program in late 1984 to 1985, the condensate yield program in late 1984 to 1985, the condensate yield has been declining steadily throughout the past 23 years of gas cycling operations due to complex mass transfer. In order to devise an optimal depletion plan of the gas condensate reservoir, a reservoir simulation study was carried out. The study's objective was to optimize liquid recovery from the Unit by:recommending methods of improving the sweep efficiency of the cycling operation, anddetermining the optimum timing and scheme for the reservoir blowdown. A compositonal reservoir simulator can best describe complex mass transfer and compositional changes due to a gas cycling operation. To simulate the high pressure gas injection process, Mobil's compositional simulator was used. P. 29