Review of the Means San Andres Unit CO2 Tertiary Project

Abstract

Summary A full-scale CO2 miscible tertiary project in the Means San Andres Unit, Andrews County, TX, is expected to result in the additional recovery of morethan 38 million STB [6.0 × 10(6) stock-tank m3]. Design, implementation, surveillance, and early performance of the CO2 project are discussed. Introduction The tertiary flood implemented in the Means San Andres Unit was part of anintegrated reservoir development plan that included apattern-modification/infill-drilling program and construction of newfacilities. The 10-acre [4-ha] infill program consisted of drilling 205producers and 158 injectors in the project area. Facility construction includedCO2 injection and production facilities and a gas compression and treatingfacility to recycle the produced CO2/gas stream. CO2 injection began in Nov.1983. Unique aspects providing engineering and operational challenges in thisproject include a more viscous crude than is found in most west Texas CO2projects, a relatively high miscibility pressure. and a narrow window betweenoperating pressure and the bottomhole formation parting pressure. Field Discovery and Development The Means (San Andres) field is about 50 miles [80 km] northwest of Midlandin Andrews County, TX. Geologically, the field lies along the eastern edge ofthe central basin platform and in a northwest/southeast trend of Permian SanAndres fields. Production is from the Grayburg and San Andres formations atdepths ranging from about 4,200 to 4,800 ft [1280 to 1460 m]. The Meanstertiary target is the San Andres. which consists of dolomite with minoramounts of shale and anhydrite. The field is a north/south-trending anticlineseparated into north and south domes by a dense structural saddle near thecenter of the field (Fig. 1). Table 1 gives reservoir and fluid properties forthe San Andres. Most significant is the oil viscosity of 6 cp [6 mPas], compared with 1 to 2 cp [1 to 2 mPas] for most San Andres reservoirs in westTexas. The primary producing mechanism was a combination of fluid expansion anda weak waterdrive. In 1963, the field was unitized and water injection beganinto a peripheral waterflood pattern. CO2 Project Planning/Design A review of other proposed CO2 projects indicated that, although Means wassimilar to other large San Andres fields in the area, some reservoir and fluidproperties appeared unique or at least substantially different. Programs weredesigned to evaluate the impact of high oil viscosity, relatively highmiscibility pressure, and low formation parting pressure. The initial screeningstudies indicated that the miscible CO2 process would work despite potentialproblems associated with these reservoir properties. Fieldwide implementationplans proceeded simultaneously with these investigations. Process Characterization. Investigations characterizing the CO2 floodprocess for Means were important in identifying the tertiary oil target, conducting miscible simulation studies, and designing the CO2 flood. Laboratorytest results, combined with actual field data, were used to determine minimummiscibility pressure (MMP), residual oil saturation (ROS) to waterflood, andthe ability of CO2 to mobilize the waterflood residual oil. Operating pressureis more important at Means than in many Permian Basin CO2 projects because ofthe small window between MMP and formation parting pressure. Slim-tubeexperiments indicated that the MMP was between 1,850 and 2,300 psi [12.8 and15.9 MPa], Because of the slim-tube data and the bottomhole injection pressurelimitation of 2,700 to 2,800 psi [18.6 to 19.3 MPa] caused by a low formationparting pressure, a 2,000-psi [13.79-MPa] operating pressure was selected. Anestimate of waterflood ROS was needed to determine the magnitude of thetertiary target. Relative permeability data, remobilization tests, and pressurecore results indicated that the 34% ROS determined from the pressure cores wasthe most representative. Remobilization tests to determine the ability of CO2to mobilize waterflood residual oil were run with Means crude and native-statecomposite cores. Many authors have reported observance of water blocking in thelaboratory. After the core was water-flooded, CO2 and water were injected at2,300 psi [15.9 MPa] at a water-alternating-gas (WAG) ratio of 1:1 until themiscible residual saturation of 9.1 % was reached. JPT P. 638⁁