Principles for Best Practice Geomechanics for CO2 Sequestration in Depleted Reservoirs

Abstract

As the world is facing challenging climate targets, one of the initiatives is to reduce carbon dioxide (CO2) emission through carbon capture and storage (CCS). However, to understand the viability of CCS study, field scale geomechanical risk assessment is key to determine short- and long-term injection and storage capacity. A comprehensive study was carried out to investigate the geomechanical risks associated with injection in the depleted reservoirs. This was done by preparing well-centric (1D) geomechanical models using inputs from petrophysical, drilling and production data of historical wells. This was then extended to a 3D geomechanical model for the entire area of interest in the field. The model was then used to investigate the caprock integrity (threshold of maximum injection pressure), reactivation of the existing faults (caprock & reservoir) and thermal stress effect on caprock & reservoir, A caprock integrity analysis was carried out for all the storage layers, and it was found that the storage layer pressures (reservoir pressure increase during injection) did not exceed the fracture pressure values of the caprock that will cause tensile failure. A fault stability analysis was carried out for the modelled faults and the Tau ratio for the maximum reservoir pressure (close or greater than initial reservoir pressure) was calculated. Results shows that there is no risk of fault stability using the current injection design (injection pressures close to the initial reservoir pressure). The stress changes induced by the thermal expansion/contraction of the rocks are calculated from the thermoelastic equations for the start of injection stage for both the reservoir and caprock. The caprock fracture pressure incorporating the thermal stress effect at the well location reduced significantly for the shallowest storage layer. With the focus of managing carbon emissions, this is one of the best principal practices and fit for purpose methodology which can be adopted for field scale geomechanical risk assessment to evaluate the short- and long-term CO2 injection risk and storage capacity in any field of interest.