Integrating hydrodynamic analysis of flow systems and induced-pressure decline at the Otway CO2 storage site to improve reservoir history matching

Abstract

The CO2CRC Otway Project is the first demonstration scale project for geosequestration of CO2 in Australia. The storage site is located in the depleted Waarre reservoir of the Naylor gas field contained within a single fault block, in the onshore Otway Basin of Victoria. During 2009, approximately 65,000 tonnes of a mixture of CO2 and CH4 (∼80%/20%) was injected into the reservoir, accumulating at the top of the structure.To fully understand the pressure response of a depleted reservoir to CO2 injection all of the mechanisms that may impact the reservoir pressure, prior to injection, must be identified. In the case of the Otway Project there were five possible mechanisms with potential for impacting the Naylor Field pressure. These are (1) the rate of recovery due to gas production from the Naylor Field; (2) depletion from the nearby Boggy Creek CO2 production field; (3) depletion from the nearby Buttress CO2 Field, the source of the CO2 for injection into the Naylor Field; (4) depletion from the regional Waarre Formation reservoir which has been producing gas (and water) since 1986; and (5) hydraulic potentiometric disequilibrium via connection to other active aquifers.These mechanisms were examined through a regional conceptual hydrodynamic model as part of the pre-injection site characterisation. The reservoir pressure measured at the Naylor-1 well, prior to production, was lower than predicted by the hydrodynamic model. Examination of regional drawdown demonstrated that the reservoir is experiencing pressure decline which could have extended to the Naylor Field at this time.Ongoing monitoring of the Naylor-1 well, prior to injection, showed the depleted reservoir recovering faster than predicted by the reservoir simulation model matched to the production history. Connecting the target fault block to the regional aquifer using a dual aquifer model supported by the hydrodynamic model significantly improved the predicted recovery of the depleted reservoir.