CO2 Flood Performance Evaluation for the Cornell Unit, Wasson San Andres Field

Abstract

Summary A numerical reservoir simulator has been used to predict CO2 enhanced oil recovery (EOR) performance for two process designs, either of which could be implemented at the Cornell Unit, Wasson San Andres field. The two CO2 displacement processes examined are a straight CO2 slug followed by continuous water injection, and a water-alternating-gas (WAG) process with CO2, followed by continuous water injection. Predicted recovery performances were found sensitive to viscous, capillary, and gravity-driven crossflow, as well as to reservoir stratification. Hence, evaluation of alternative process designs was influenced strongly by assumed values of effective vertical permeability within continuous pay intervals, even though the vertical permeability between major pay intervals is effectively zero. Work involving a no-crossflow stratification model of the Wasson pay suggested that a straight CO2 slug process would be preferable to a WAG process. On the other hand, work with a stratification model that allowed intrazone crossflow indicated that recovery could be improved significantly for a given size CO2 slug by means of a WAG process. Introduction Cornell Oil Co. and Markland Oil Corp. operate the Cornell Unit in the Wasson San Andres field, Yoakum County, TX. This 1,923-acre (7.7 x10 M) unit originally contained about 185 million STB (29.4 x 10 stock-tank m) oil. The property was unitized for supplemental recovery, and water injection began in mid-1965. Cumulative oil production before water injection was 18 million STB (2.9 x 10-6 stock-tank m3). As of Jan. 1982, secondary production was an additional 34 million STB (5.4X 10-6 stock-tank m3). A recent infill drilling program has treated a 35-acre (141 640-M) five-spot well configuration over the entire unit in preparation for a CO2 EOR project. This paper describes a numerical simulation study of alternative operating policies for CO2 injection in the Cornell Unit. CO2 Flood Simulator A CO2 flood simulator, using a finite-difference-based compositional model, was used in this study. The simulator was designed to reproduce the effects of major mass-transfer and phase-transport phenomena associated with CO2 EOR. For immiscible conditions, phase equilibria may be input to the simulator to represent EOR mechanisms of oil- phase swelling with condensed CO2 and vaporization (or extraction) of hydrocarbon fractions into the CO2-rich phase. Multiple-contact miscible (MCM) displacement may be represented explicitly with the program by incorporating appropriate phase equilibrium data. However, the philosophy of the modeling technique used in the program is to maintain segregated CO2- and oil-rich regions. By controlling the degree of segregation of these regions through the mixing parameter approach, we may represent the important phenomenon of viscous fingering, characteristic of highly unfavorable mobility ratio displacements, without describing the detailed structure of the unstable frontal advance. Local miscibility is determined by comparing computed pressure with miscibility pressure input to the program as a function of composition. This modeling technique represents miscible displacement and miscible/immiscible transition without describing the detailed compositional path required for explicitly representing MCM displacement. JPT P. 2271^