Impact of Stress History on the Breakthrough Response of Mudrocks as Caprocks for CO2 Storage

ABSTRACT: This study investigates the geomechanical and hydraulic behavior of synthetic mudrocks under varying stress histories for their application as caprocks for geologic carbon storage (GCS). We conduct high-pressure consolidation and fluid injection tests and examine the relationships between stress history, porosity, permeability, and CO2 breakthrough pressure. The findings reveal that higher overconsolidation ratios (OCR) lead to reduced porosity and permeability, significantly increasing CO2 breakthrough pressures. During brine injection into the brine-saturated specimen, the fluid freely permeated, resulting in elevated pore pressure and volumetric expansion as effective stress decreased. In contrast, before breakthrough, CO2 injection had no impact on the pore pressure and served merely as an external pressure, inducing specimen settlement. However, after breakthrough, expansion was observed as the pore pressure increased from the migration of CO2 through the specimen. Samples with higher OCR exhibited lower strain rates prior to breakthrough, attributed to smaller pore size distributions and decreased compressibility, which delayed pore pressure dissipation. These findings highlight that the stress history, particularly the maximum past stress, controls the microstructural development and sealing capacity of clay-rich caprocks. This indicates that accurate assessment of caprock integrity in GCS applications must account for the complete stress history evolution rather than solely considering the current stress state.