Carbon Capture Utilization and Storage (CCUS) has been widely accepted to be an effective technology to control anthropogenic greenhouse gas emissions globally. For CO2 geo-sequestration, the key is to understand the caprock integrity to assure the safety of the long term sealing effect. However, many caprock integrity studies do not consider the effects of geochemical reactions among CO2, formation brine and caprock.

In this work, we used the Lloydminster heavy oil region as the target area and studied the effects of geochemical reactions on the caprock integrity. We collected caprock core samples from both Waseca heavy oil layer and the Deadwood saline aquifer in this area. Then, static tests (Caprock submerged into formation brine over-saturated with CO2 for 40 days) were conducted in the autoclave system under the reservoir pressure (Waseca: 5.4MPa, Deadwood: 11.5MPa) and temperature (Waseca: 25⁰C Deadwood: 35⁰C) to mimic the process of CO2 storage in two candidate formations. Finally, triaxial tests were conducted to compare the change in rock strength between core samples before and after CO2 treatment. Meanwhile, XRD analysis has also been conducted to provide the information of mineral composition change on caprock samples before and after CO2 treatment.

Triaxial test results showed that caprock strength has increased (higher axial stress and lateral stress) and fracture pressure increased by 15.84% and 5.45% on average in Waseca and Deadwood formation respectively. Mineralogy analysis indicated that a large amount of carbonate minerals reacted with CO2-saturated brine and stable minerals were generated which help tighten the caprock structure, triggering the self-sealing effect thus caprock strength was enhanced. The research outcomes indicate that in the future site selection for CCS projects, reservoirs with caprocks containing carbonated minerals can be competitive candidates.

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