Economic assessment of strategies for CO₂-EOR and storage in brownfield residual oil zones: A case study from the seminole san andres unit

Brownfield residual oil zones (ROZ) may benefit from specific strategies to maximize production. We evaluated several strategies for producing from the Seminole ROZ. This ROZ lies below the main pay zone (MPZ) of the field. Such brownfield ROZs occur in the Permian Basin and elsewhere, formed by the action of regional aquifers over geologic time. CO₂ can be injected into these zones to enhance oil recovery and carbon storage. Since brownfield ROZs are hydraulically connected to the MPZs, development sequences and schemes should influence oil production, CO₂ storage, and net present value (NPV). We conducted economic assessments of various CO₂ injection/production schemes in the Seminole stacked ROZ-MPZ reservoir based on flow simulations. First, we constructed a high-resolution geocellular model from a seismic survey, wireline logs and core data. To calibrate the geological model and constrain the interface between the ROZ and the MPZ, we performed a comprehensive production-pressure history matching of primary depletion and secondary waterflooding. After this, we conducted flow simulations of water alternating gas (WAG) injection into the reservoir while considering several injection/productions schemes (e.g., switching injection from the MPZ to the ROZ, commingled production). For each scheme, various WAG ratios (i.e., reservoir volume ratio between injected water and CO₂) were tested to find the maximum oil production and maximum CO₂ storage. We assessed the economic results for each WAG ratio case on NPV. The results from simulating various injection/production schemes showed that simultaneous CO₂ injection into the MPZ and ROZ favors oil production. If instead, CO₂ is injected into the MPZ and ROZ, then into the ROZ alone, this leads to increased CO₂ storage. Storage performance is influenced by the interplay between the crossflow from the MPZ to ROZ and WAG ratios. As the WAG ratio increases, the amount of CO₂ stored decreases more for commingled injection cases than for separated ROZ injection cases. Also, the WAG ratio leading to maximum oil production does not necessarily yield the largest NPV, because of the complicated interactions among CO₂ consumption, reservoir heterogeneity, and oil recovery. Brownfield ROZs are common below San Andres reservoirs in the Permian Basin, and they can be exploited to increase oilfields' NPV and carbon storage potential. Our case study on the Seminole MPZ-ROZ is an analog for other similar reservoirs. We demonstrate that development sequences and WAG ratios influence the performance of CO₂ EOR and storage. Thus, this work provides valuable insights into the further optimization of brownfield ROZ development and helps operators to plan flexible storage goals for stacked ROZ-MPZ reservoirs.