The Use of Numerical Simulation To Design a Carbon Dioxide Miscible Displacement Project

Abstract

The prediction of a single-layered, two-dimensional areal model was compared with results from cross-sectional, two-dimensional vertical models. Effects of gravity segregation and existing reservoir heterogeneity were not significant enough to justify use of a three-dimensional model for predicting field behavior of the North Cross (Devonian) Unit CO2 miscible displacement project in West Texas. project in West Texas. Summary A numerical simulation method was used to design a carbon dioxide (CO2) miscible displacement project in a low-permeability carbonate reservoir. A field history match was obtained as a check on the model's validity. Two-dimensional, vertical (cross-section) models were used to determine the potential effects of a high gas saturation at the top of the formation on CO2 injection profiles and to evaluate the effect of reservoir heterogeneity on sweep efficiency. A twodimensional areal model was used to describe the field's response to CO2 injection. Using an areal model, various CO2 injection patterns were simulated and compared, and an inverted patterns were simulated and compared, and an inverted nine-spot was found to be the optimum pattern for this field. Further simulation predictions were conducted to evaluate various schemes of handling produced CO2-contaminated gas after CO2 breakthrough. produced CO2-contaminated gas after CO2 breakthrough. It was found that direct reinjection of the contaminated gas was preferable to separating the CO2 from the gas or venting the contaminated gas. The project - the North Cross (Devonian) Unit in West Texas - is unlike the classical miscible project in that continuous injection of CO2 and residue project in that continuous injection of CO2 and residue gas is planned rather than injection of miscible slugs followed by or alternating with water or dry gas. Injection of CO2 commenced in April 1972. Actual project performance to date has been encouraging. project performance to date has been encouraging. Reservoir Description The Crossett field is located at the southern edge of the central basin platform of West Texas. The reservoir is a chalky, siliceous carbonate having 21 percent porosity and 3 md permeability. Fig. 1 shows percent porosity and 3 md permeability. Fig. 1 shows the gross Devonian reservoir thickness above the water-oil contact. There is no significant water drive present, so the primary recovery mechanism is solution gas drive. The reservoir is limited on the west by truncation caused by the post-Devonian unconformity. The eastern limit of the reservoir is provided by a water-oil contact, and the north and provided by a water-oil contact, and the north and south boundaries are defined by permeability and porosity deterioration. The reservoir is unusually uniform for a carbonate. Fig. 2 shows a plot of core permeability and porosity vs well depth. Properties of the reservoir fluid are listed in Table 1. The field contained 53 million STB of oil in place when discovered in 1944. The Crossett field was unitized in 1964 and renamed the North Cross (Devonian) Unit. The purpose of unitization was to implement a gas-injection purpose of unitization was to implement a gas-injection pressure maintenance project. All residue casinghead pressure maintenance project. All residue casinghead gas was injected into the three updip gas injection wells shown on Fig. 1. Fig. 3 shows the Unit's production history since unitization. production history since unitization. Residue gas injection would only increase the field's recovery to 19 percent from the expected primary efficiency of 13 percent. With so much oil being left in the reservoir, further supplemental recovery schemes were considered. P. 1327