Gravity monitoring of CO2 storage in a depleted gas field: A sensitivity study

In 2006, the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) plans to undertake (subject to receiving the necessary approvals) a Pilot program for CO2 storage within a depleted gas reservoir. The Otway Basin Pilot Program (OBPP) aims to demonstrate that subsurface CO2 storage is both economically and environmentally sustainable in Australia. This will be the first CO2 storage program in the world to utilise a depleted gas reservoir and, hence, the experience gained will be a valuable addition to the range of international CO2 storage programs that are underway or being planned. A key component of the OBPP is the design of an appropriate geophysical monitoring strategy that will allow the subsurface migration of the CO2 plume to be tracked and to verify that containment has been successful. This paper presents the results from modelling the predicted gravity response to CO2 injection into the Otway Basin reservoir, where the goal was to determine minimum volumes of CO2 that may be detectable using non-seismic geophysical techniques. Modelling results indicate that gravity measurements at 10 m spacing within the existing observation well and the planned CO2 injection well would provide excellent vertical resolution, even for the smallest CO2 volume modelled (10 000 tonnes), but resolving the lateral extent of the plume would not be possible without additional wells at closer spacing.