Reservoir Data Pays Off: West Seminole San Andres Unit, Gaines County, Texas

Abstract

Abstract At this point in time the oil industry is under considerable pressure to maximize recoveries from existing reservoirs. In order to obtain this goal it is necessary that a reservoir be accurately described as to its internal structure and producing mechanisms. Improper definition of a reservoir producing mechanisms. Improper definition of a reservoir can result in a very inefficient operation and thus a lower recoverable reserve value. This paper is being presented as an example of how the interpretation of internal structure and producing characteristics of a particular reservoir has changed as a result of an extensive reservoir data gathering program. The new information has drastically program. The new information has drastically changed the concept as to how this reservoir should be managed. Introduction The West Seminole Field is located in west central Gaines County, Texas (Figure 1). The field has a productive area which covers 2782 surface acres, and consists of a large main dome with smaller domes to the southeast and northwest. A structure contour map and 3-D perspective view of the reservoir are shown in Figure 2. The West Seminole reservoir is a San Andres Dolomite producing from an average depth of 5,112 feet. A primary gas cap overlies the oil zone over the major portion of the field. The large dome and the adjoining dome to the southeast are connected through the oil zone and share a common gas-oil contact. The gas-oil and water-oil contacts are essentially flat lying, with the water-oil contact rising somewhat near the productive limits of the field. A schematic cross section through the reservoir is shown in Figure 3a. The average oil zone thickness in the main structure is 140 feet and the average gas cap thickness is 111 feet. The original volume of oil-in-place is estimated to be 172 MMSTB and the original volume of gas cap gas-in-place is estimated to be 137 BCF. Throughout the history of the field, the approach to operations and reservoir management at West Seminole has been guided by an underlying conception of reservoir structure. Major operational decisions have been delayed because of disagreement as to the correct reservoir description. The approach to operations and reservoir management has changed as the concept of reservoir structure has evolved. The development of current reservoir description is the result of an extensive combined geological and engineering effort. EARLY FIELD HISTORY The field was discovered in June, 1948. The initial field development consisted of 54 wells developed on approximately 40-acre spacing (Figure 4). Most of the wells were completed as open holes with casing set to just below the gas-oil contact. The early approach to reservoir operations was based on the concept of the reservoir as a relatively homogeneous dolomitic limestone. This reservoir description was based on the observation that relatively flat, horizontal gas-oil water-oil contacts exist across the field in spite of the large structural relief in the reservoir. Vertical continuity was supported When a number of wells began producing large volumes of gas; this was interpreted as gas coning and/or downward movement of the gas-oil contact. Production from the reservoir was thought to be primarily by solution gas drive assisted by an expanding gas cap, along with some limited water influx. The field was unitized in December, 1962. By this time reservoir pressure had declined from the initial discovery pressure of 2020 psi to about 1600 psi after having produced only about 6% of the original oil-in-place. It was decided that some action must be taken to reduce the rate of decline in reservoir pressure and to allow producing rates to rise to meet increased allowables. A program to inject produced gas back into the gas cap was initiated in 1963–64.