Permeability Reduction and Oil Recovery Improvement from In-Situ Precipitation of Calcium Carbonate from Calcium Hydroxide in Heterogeneous Cores

Summary Conformance control is an important part of improved oil recovery/enhanced oil recovery (EOR) operations to prevent injected fluid from cycling through high-permeability structures. The focus of this paper is on studying in-situ precipitation of calcium carbonate (CaCO3) via reaction between carbon dioxide (CO2) and calcium hydroxide (Ca(OH)2) to block high-permeability structures in coreflood experiments. Two sets of experiments were performed: two-phase experiments with CO2 and brine that primarily measured permeability reduction and three-phase experiments with CO2, brine, and oil that primarily measured improvement in oil recovery in artificially fractured cores. Experiments utilized ~2.9-in.-diameter × 11-in.-length Berea sandstone, Texas Cream limestone, and Indiana limestone outcrop cores, some containing planar artificial fractures. It was found that precipitation was most effective in reducing permeability in dual-permeability (artificially fractured and composite) cores than in single-permeability cores. Significant improvements in oil recovery were observed when precipitation blocked fractures and redirected flow into the oil-filled matrix. Producing oil cut increased from 0–8% to 25–50%. However, precipitation was found to be unstable (dislodged during injection) when higher pressures built up behind it. Additionally, larger fractures required multiple precipitation cycles to be blocked, as the width to bridge the fracture was too much for a single precipitation cycle to overcome. Overall, this conformance control method may be best for reservoirs with narrow natural fractures or thin high-permeability streaks that do not connect directly to production wells.