Quick Look Assessment of Thermal Stress Effect on Caprock Integrity for Carbon Capture and Storage Projects

Abstract Carbon capture and storage (CCS) projects are of paramount importance globally to achieve the targets set during COP21 in Paris. Depleted hydrocarbon reservoirs and saline aquifers qualifies were identified as potential candidates for such storage projects. However, containment is a major risk as severe temperature changes downhole and cooling of the formations during various stages of injections can breach the caprock. This paper presents a quick look analysis to quantify the effect of thermal stresses on cap-rock integrity. Based on wellbore-scale temperature modelling, a range of temperature difference close to the injection wellbore were assumed for the analysis. Using this temperature changes, the possible range of stress reduction induced by the thermal expansion/contraction of the rocks in the reservoir storage (sandstones) and caprock layers (shales) were calculated from the thermoelastic equations. The estimation considers the elastic properties of rocks (poisson’s ratio and young’s modulus as derived from 3D static geomechanical modelling), linear expansion coefficient of storage and caprock as derived from core tests and the temperature difference between reservoir and bottom hole temperature. Assuming the cap rock will experience the same level of cooling effect as the reservoir, the reservoir and caprock fracture pressure incorporating the thermal stress effect at the well location at the start of the injection stage/end of history matching were estimated. The predicted temperatures were modelled from well modelling for the start of injection and end of prediction stage. At the start of the injection stage, the results show that in the depleted reservoirs, the reservoir fracture pressure with thermal stress effect were reduced by ~400 psi as compared to the deepest storage layer which were reduced by ~2000 psi. The caprock fracture pressure incorporating the thermal stress effect at the well location reduced by 300 psi for the shallowest storage layer as compared to the deepest storage layer which were reduced by ~2400 psi. Similarly, the stress reductions for the end of the prediction stage were estimated for the reservoir and caprock layers. This paper presents an integrated approach for a quick look assessment of caprock integrity risks and identify a feasible operating window for CCS operations. It provides a firsthand information on caprock integrity risks based on 3D static geomechanical and wellbore-scale temperature modeling even before moving to 4D dynamic simulation modeling. The methodology established in this case study can be referred for storage projects to ensure the wellbore integrity and formation sealing integrity.