Chemical‐Mechanical Impacts of CO 2 Intrusion Into Heterogeneous Caprock

The potential economic benefits offered by CO 2 ‐enhanced oil recovery (CO 2 ‐EOR) and storage, including increasing oil production and mitigating CO 2 storage cost, make it an attractive approach for reducing CO 2 emissions. Sealing formation (caprock) integrity is considered a key risk factor, because of the potential for leaked CO 2 or brine migrating into shallow groundwater formations. The primary purpose of this research is to evaluate general caprock sealing efficiency and integrity under typical CO 2 ‐EOR conditions, by assessing the influence of hydrological and mineralogical heterogeneity, possible mineralogical alteration, and potential failure of rock due to hydrological and mineralogical changes. An active CO 2 ‐EOR project at the Farnsworth Unit (FWU) in the northern Texas is selected as a case study. A coupled reactive‐transport‐geomechanics model of the FWU caprock (the Morrow Shale and the Thirteen Fingers Limestone) was developed based on site‐specific geological data. Key results suggest that the Thirteen Fingers Limestone is an effective caprock. After 5,000 years, effectively no supercritical CO 2 penetrates this formation, and the penetration depth of dissolved CO 2 in aqueous phase does not exceed 10 m. Because of mineral precipitation in the Morrow Shale, maximum porosity decreases ~25% at the reservoir‐caprock interface, suggesting increased caprock sealing efficiency. Geomechanical response of the caprock due to CO 2 intrusion and mineral alteration suggests low risk of induced fractures. This study provides a refined evaluation of long‐term caprock integrity as a function of coupled hydrological, chemical, and geomechanical processes and is intended to support future assessment of feasibility and safety of geologic CO 2 sequestration. Geologic CO 2 sequestration (GCS) is considered a viable solution for reducing greenhouse gas emissions. Particularly, as a widely used technology, injecting CO 2 into oil reservoirs or CO 2 ‐enhanced oil recovery (CO 2 ‐EOR), can increase oil production with associated profits. One concern of GCS is the possible CO 2 leakage through sealing formation into shallow groundwater aquifers and impact drinking water quality. Therefore, our study focuses on evaluating caprock sealing efficiency and integrity under typical CO 2 ‐EOR conditions. Once CO 2 enters the caprock, chemical reactions occur, and the minerals might change by dissolution and precipitation. This process would cause changes of CO 2 flow