Influence of effective stress on mechanical and chemical alteration processes at the cement-caprock interface: application to CO2-brine leakage at the wellbore

Carbon storage sites aim to retain at least 99% of injected CO2 over timescales of at least hundreds of years. Abandoned wells have been considered as major potential leakage pathways because of the reactive nature of cement and potential defects that render it sensitive to mechanical and chemical degradation. Several previous experimental studies have showed that CO2-brine-cement exposure results in distinct zones, the formation of which can be influenced by factors such as temperature, CO2 concentration, pH, flowrates, cement composition, and transport conditions. However, few studies to date have considered the impact of effective stress on mechanical alteration of cement under geological sequestration conditions. Building on previous experimental work, CO2-rich brine was introduced into artificially fractured wellbore cement and sandstone cores at 0.1 mL/min for 8 days. Experiments were conducted at five confining pressures to assess the influence of effective stress (3, 7, 9 , 12.4, 16 MPa) on both chemical and mechanical degradation of the cement phase. We examined the mechanical and chemical properties of the cement by a combination of X-ray computed tomography analysis and image processing before and after each experiment; solution chemistry analysis; and micro-indentation analysis to determine changes to cement strength. Current results showed that ionic concentration did not show systematic trend with the effective stress. In these experiments, permeability changed only slightly (maximum decrease of 2 md) in comparison to the two order-of-magnitude decreases observed in previous experiments conducted at constant effective pressure. XRCT analysis shows the creation of the same reaction regions described by other studies. However, differences in the reacted volumes suggest that the residence time of the CO2-rich brine decreases with effective stress, allowing less reactivity at the cement-caprock interface. We use these new experimental data to extend the chemo-mechanical degradation model to assess relationships between cement degradation and evolution of mechanical properties. We also discuss the long-term behavior of the wellbore-cement interface.