Dynamic-Geomechanics Coupled Modelling Helps Improve Understanding of the Reservoir and Improve Storage Capacity Estimate of Depleted Carbonate Gas Reservoir for Bulk Carbon Dioxide Sequestration

A depleted gas reservoir in Central Luconia field, located offshore Sarawak, was evaluated for future carbon dioxide (CO2) storage. This carbonate reservoir has experienced seafloor subsidence during producing life. When coupled dynamic-geomechanics model was history matched, it was discovered that earlier understanding of reservoir performance along with aquifer extent and size were incomplete. This new understanding has led to a significant change in forecasted CO2 storage capacity of the reservoir. In this reservoir, as production rate declined, reservoir pressure decline started reversing. During the producing life of the reservoir, standalone history matched dynamic model was used to forecast reservoir performance. This model sufficiently explained production performance and in fact production forecasts made from the model were reliable. After the reservoir had ceased to produce, it was evaluated for CO2 storage. As part of the CO2 storage studies, a coupled dynamic-geomechanics model was built. This was when it was noted that seafloor continued to subside even as reservoir pressure was increasing post cessation of production. This highlighted the fact that something other than hydrocarbon reservoir section was compacting. Further studies revealed that regional aquifer that was not included in the standalone dynamic model was undergoing compaction post-production. This reservoir has a bottom acting aquifer, which has a barrier/baffle zone at the bottom of the aquifer that separates it from an extended/regional aquifer. Standalone dynamic model was truncated at the barrier/baffle zone. This aquifer was sufficient to explain the pressure support received but was insufficient to explain sea floor subsidence post-production. Further studies revealed that the hydrocarbon section was in communication with extended aquifer. And the baffle zone has a major role to play in understanding CO2 storage. Using dynamic-geomechanics coupled model with extended aquifer, it was discovered that baffle zone undergoes pore collapse during production. This translates to significantly lower aquifer efflux across the baffle zone during CO2 injection compared to aquifer influx during production. This directly impacts the storage capacity of the reservoir since the invaded aquifer could not be efficiently displaced by injected CO2. Storage capacity estimate from coupled dynamic-geomechanics model is about 50% lower compared to standalone dynamic model.