Evaluation of Residual Oil Zone Development in a Mature West Texas Carbonate Reservoir

Abstract West Texas has long been known as a showcase for successful post-waterflood (tertiary) recovery by CO2 flooding of mature waterfloods in conventional oil reservoirs. However, a large oil resource exists in the form of Residual Oil Zones (ROZs) that have been more economically challenging to develop compared to the familiar Main Pay Zone (MPZ). The conventional MPZs are producible at a feasible cost. As these CO2 floods are becoming very mature and the production is continuously declining, ROZ is becoming attractive enough to be considered and evaluated as a development opportunity. The subject field (Means) has been producing for the last several decades with nearly half of Original Oil in Place (OOIP) residing in the untapped ROZ. This is a light oil for which CO2 has already been shown to be an effective miscible displacement injectant. Oil production from the MPZ has been in steady decline and therefore, a decade ago, several wells were drilled through the ROZ layers to gather additional data. Since the production was commingled with the MPZ and given the vertical heterogeneity, the ROZ performance was less than expected with the recovery being less than 1% after 10 years of CO2 injection. Hence the idea of utilizing dedicated wells in ROZ was evaluated by conducting this simulation study. This study was commenced by gathering all the recent log, core, and injection/production data and construction of a rigorous ROZ geologic model. Special consideration was given to the location and extent of vertical barriers as well as location of high-perm layers to adequately capture their effect. A section of the reservoir comprising of nine 5-spot sectors was selected for a scoping simulation study. Both vertical and horizontal wells (dedicated to ROZ only) were evaluated with various well spacings. Sensitivities were also conducted with different Water Alternating Gas (WAG) cycles, continuous initial gas injection followed by WAG cycles, and with gas injection only.