Cooling-induced geomechanical response of North Sea reservoirs, and relevance for CO2 storage monitoring

Many studies on CO₂ injection monitoring focus on fault stability risks due to increased pore pressure. However, the temperature of the injected fluid can also significantly impact the near-wellbore region through cooling-induced strain, stress changes, and fracturing, necessitating tailored monitoring strategies. This study evaluates the potential for near-well monitoring by examining the magnitude of strain and associated failure scenarios in reservoir formations down to approximately 2.6 km, where injected CO₂ may cause temperature decreases of around 80 °C. The combined effects of pore pressure and cooling on stress path and elastic-inelastic strain are assessed using laboratory triaxial test data and selected well logs. The impact of cooling was simulated directly via uniaxial strain tests or indirectly through multistage tests with Acoustic Emission (AE) monitoring. An analytical approach based on the Mohr-Coulomb failure criterion is employed to evaluate the stress path relative to the failure criteria. Results indicate that shallow, uncemented sands and weakly cemented sandstones predominantly exhibit elastic expansion in response to injection. In contrast, deep, stiff sandstones can experience cooling-induced contraction, inelastic damage, and even fracturing. The findings are further discussed in relation to well-based monitoring techniques, including fibre optics and other methods.