Assessing the Feasibility of Geological CO2 Storage in a Depleted Gas Field with Multi-Scale Coupled Reservoir Geomechanics Simulation

ABSTRACT Korea National Oil Corporation (KNOC) is now leading the nation's 1st large-scale carbon capture and storage(CCS) demonstration project using a depleted gas reservoir in the East Sea of offshore Korea. The Donghae-1 gas field, first discovered in 1998, came online producing gas and condensate in 2004 putting the country in the list of hydrocarbon producers worldwide. The field shut down in 2021, and is now considered as one of the most immediate candidates for geological CO2 sequestration. As a candidate CO2 storage site, the geomechanical risks are vital to the injectivity, containment, and long-term sustainability which must be assessed and dealt with before field operations. To this end, cap rock integrity and well completion integrity evaluation of CO2 injection and storage are required for the feasibility study. In an effort to evaluate the integrity of CO2 storage in Donghae-1 gas field, a thorough geomechanics study workflow was established. Core laboratory tests were firstly carried out to delineate geomechanical characteristics for both reservoir and cap rocks. The degradation of rock stiffness and strength was taken into consideration by treating core samples with CO2. 1D mechanical earth model (MEM) for a few selected wells was constructed honoring core testing results and logging data, and then the 3D MEM modelling was performed in full-field scale and in near wellbore scale. For the full field scale, the gas production was numerically simulated and history-matched before coupled reservoir geomechanics simulation for CO2 injection and plume migration was performed. To elevate the study to a comprehensive level, different CO2 operation scenarios were considered, covering different potential injection wells and maximum allowable injection conditions. Cap rock integrity was assessed by the failure criteria derived from core laboratory tests. As significant physical processes occur near wellbore, a well-centric and fine-grid sector model was built to delve more into hydraulic-mechanical coupled behavior in terms of rock integrity and well completion integrity. This study indicates that it is less likely to damage the storage and cap rocks given the formation properties and the safe injection rate, and the potential risks of CO2 leakage are under control. Besides, multiple layers of shale above the target storage zones provide additional containment assurance. The workflow proposed in this study can be utilized to understand long-term behavior of CO2 storage, which is helpful to many similar CCS projects worldwide.