A rock physics simulator and its application for CO2 sequestration process

Abstract

Injection of CO2 into underground saline formations, due to their large storage capacity, is probably the most promising approach for the reduction of CO2 emissions into the atmosphere. CO2 storage must be carefully planned and monitored to ensure that the CO2 is safely retained in the formation for periods of at least thousands of years. Seismic methods, particularly for offshore reservoirs, are the primary tool for monitoring the injection process and distribution of CO2 in the reservoir over time provided that reservoir properties are favourable. Seismic methods are equally essential for the characterisation of a potential trap, determining the reservoir properties, and estimating its capacity. Hence, an assessment of the change in seismic response to CO2 storage needs to be carried out at a very early stage. This must be revisited at later stages, to assess potential changes in seismic response arising from changes in fluid properties or mineral composition that may arise from chemical interactions between the host rock and the CO2. Thus, carefully structured modelling of the seismic response changes caused by injection of CO2 into a reservoir over time helps in the design of a long-term monitoring program. For that purpose we have developed a Graphical User Interface (GUI) driven rock physics simulator, designed to model both short and long-term 4D seismic responses to injected CO2. The application incorporates CO2 phase changes, local pressure and temperature changes, chemical reactions and mineral precipitation. By incorporating anisotropic Gassmann equations into the simulator, the seismic response of faults and fractures reactivated by CO2 can also be predicted. We show field examples (potential CO2 sequestration sites offshore and onshore) where we have tested our rock physics simulator. 4D seismic responses are modelled to help design the monitoring program.