CO2 Storage Development Plan Evaluation of a High Porosity Depleted Carbonate Field Using Two-Way Coupled Geomechanics-Dynamic-Thermal Modelling Incorporating Rock Properties Degradation

ABSTRACT: A comprehensive CO2 storage development plan study was conducted on Field X in offshore East Malaysia. The study is aimed at addressing, among others, the numerous geomechanical challenges and risks associated with the CO2 injection and geological storage, injectivity and storage capacity. The key geomechanical challenges include maintaining the seal integrity throughout the injection operation and storage life and the impact of thermal cooling by the injected CO2 on hydraulic fracture initiation and propagation in the reservoir and caprock. Furthermore, due to the high porosity carbonate reservoir underwent pore collapse during production, the reduction in injectivity and storage capacity is required to be delineated. Two-way coupled geomechanics-dynamic-thermal modelling workflow with rock properties degradation due to CO2-rock interaction and simultaneous history match for reservoir simulation and geomechanical subsidence modelling was implemented. Recognising the pore collapse behaviour of the reservoir, Modified Cam Clay model was implemented in the geomechanical modelling. The rock properties degradations were based on triaxial tests conducted on untreated samples and samples dynamically aged with CO2 saturated brine, and the properties degradations ranged between 8 and 30%. Based on the evolution of total minimum horizontal stress during injection, caprock injection pressure limit is determined which corresponds to final caprock injection pressure limit of 3890 psi. The subsidence predicted in the model before CO2 injection shows good match with GPS measurement data over a period of 16 years. The mechanical properties degradation effects on fault stability, caprock integrity, expansion and uplift due to CO2-rock interaction are minimal. The proposed CO2 injection scenario showed low risk of shear and tensile failures in caprock, and all the faults remained stable. Based on the coupled thermal geomechanical simulation, the temperature cooling is localised at the injectors within 2200 ft lateral distance from the injectors. The cooling resulted in temperature drop by up to 103 °C in the reservoir and 30 °C in the caprock. The cooling resulted in fracture gradient reduction of up to 1400 psi in the reservoir and up to 700 psi in the caprock. The hydraulic fractures initiated will propagate in the reservoir but not in the caprock. The output from the coupled geomechanical modelling was subsequently used to develop recommendations from geomechanical perspective for the CO2 injection and storage operation in order to mature the field as a CO2 storage site, including the CO2 injection pressure and its respective storage capacity together with measurement, monitoring and validation programme using seabed and downhole monitoring as well as surface seismic methods and micro-seismic techniques.