Chemical‐Mechanical Impacts of CO₂ Intrusion Into Heterogeneous Caprock

Abstract

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. Plain Language Summary 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 and strength of the caprock. In this study, we analyzed coupling effects of flow, chemical reactions, and mechanical changes of caprock under CO 2-rich condition by numerical simulations. The Farnsworth Unit (FWU) undergoing active CO 2-EOR is selected as a case study. Key results suggest that the selected caprock at the FWU is effective for preventing CO 2 leakage for at least 5,000 years. Geomechanical response of the caprock due to mineral changes is not significant and suggests low risk of induced fractures. This study provides benefits for f...

Multiscale Assessment of Caprock Integrity for Geologic Carbon Storage in the Pennsylvanian Farnsworth Unit, Texas, USA

“…Assessments of capillary heterogeneity and geomechanical behavior from small-scale core-plug measurements may not be representative of heterogeneity across formation scales and can reflect a sampling bias from the core recovery. However, both methods are used to infer large-scale behavior, which may include modeling to integrate to the reservoir and caprock properties made at different locations and different scales [41]. To build confidence in and understanding of caprock integrity at FWU, this study approaches caprock integrity by systematically assessing processes that govern the sealing quality at different scales following the framework of Figure 1.…”

Impact of Mineral Reactive Surface Area on Forecasting Geological Carbon Sequestration in a CO2-EOR Field

“…Following the primary and secondary production that started in the 1950s and 1960s, the FWU has been currently undergoing CO 2 -EOR since December 2010. As of 2020, more than one million metric tons of CO 2 has been sequestered at the FWU [31]. The CO 2 storage system at the FWU consists of late Pennsylvanian Morrow B sandstone as the storage formation and overlying Atokan Thirteen Finger limestone as the sealing layer.…”

“…Each grid block is 200 ft (or 60.96 m) by 200 ft (or 60.96 m) in x and y directions and about 8.67 ft (or 2.64 m) in the z-direction. This model only includes the Morrow B sandstone, as the integrity of the overlying Thirteen Finger limestone will not be compromised for at least 5000 years [31]. A total of 55 wells (23 injectors and 32 producers) were distributed across the model domain in 5-spot patterns (Figure 1b).…”

Chemical Impacts of Potential CO2 and Brine Leakage on Groundwater Quality with Quantitative Risk Assessment: A Case Study of the Farnsworth Unit

“…The primary goal of this project is to exhibit and evaluate an active commercial-scale CCUS operation, and demonstrate effective site characterization, monitoring, verification, accounting, and risk assessment for long-term CO 2 sequestration. To date, over one million metric tons of net CO 2 from anthropogenic sources (one fertilizer plant and one ethanol plant) is stored in the subsurface reservoir (the Morrow B Formation), with CO 2 injection and production volumes tracked at the FWU [40]. The SWP acquired significant near-surface monitoring data for potential CO 2 leakage, including soil CO 2 flux (to identify any potential point-source leakage to the surface), borehole CO 2 movement (to monitor subsurface CO 2 movement), and the overlying drinkable groundwater chemistry in the Ogallala aquifer (to identify any potential leakage into the USDW and drinking water quality change).…”

“…Energies 2020, 13, x FOR PEER REVIEW 3 of 15 fertilizer plant and one ethanol plant) is stored in the subsurface reservoir (the Morrow B Formation), with CO2 injection and production volumes tracked at the FWU [40]. The SWP acquired significant near-surface monitoring data for potential CO2 leakage, including soil CO2 flux (to identify any potential point-source leakage to the surface), borehole CO2 movement (to monitor subsurface CO2 movement), and the overlying drinkable groundwater chemistry in the Ogallala aquifer (to identify any potential leakage into the USDW and drinking water quality change).…”