Bo Ren, Frank Male , Ian J Duncan

Residual oil zones (ROZ) undergoing CO2-EOR may benefit from specific strategies to maximize their value. We evaluated several strategies for producing from a Permian Basin, West Texas, USA field’s ROZ. This ROZ lies below the main pay zone (MPZ) of the field. Such brownfield ROZs occur in the Permian Basin and elsewhere. Since brownfield ROZs are hydraulically connected to the MPZs, development sequences and schemes influence oil production, CO2 storage, and net present value (NPV). We conducted economic assessments of various CO2 injection/production schemes in the stacked ROZ-MPZ reservoir based on flow simulations of a high-resolution geocellular model built from wireline logs and core data and calibrated through production history matching. Flow simulations of water alternating gas (WAG) injection, such as switching injection from the MPZ to the ROZ, commingled production was studied. The simulation results showed that simultaneous CO2 injection into the MPZ and ROZ lead to the highest oil production and, generally, the highest NPV. If instead, CO2 was simultaneously injected into the MPZ and ROZ, then into the ROZ alone, this maximized CO2 storage. CO2 storage can be used as a tax credit under the Internal Revenue Code Section 45Q. Storage performance depended on the development approach and WAG ratio. Developing the ROZ increased storage compared to remaining in the MPZ. The WAG ratio to maximize oil production did not always yield the largest NPV. These findings can be applied to other Brownfield ROZs, which are common below San Andres reservoirs in the Permian Basin and other basins. ROZ development can increase oilfields’ NPV and carbon storage potential. Our study is an analog for similar reservoirs. This work provides valuable insights into the further optimization of brownfield ROZ development and information for operators to plan to develop stacked ROZ-MPZ reservoirs.