Injectivity Evaluation for Offshore CO2 Sequestration in Marine Sediments
- Zhenxue Daia, Ye Zhangb, +6 authors Jeffrey M. Bielickig
The J1 and J2 reservoirs of the Bullwinkle field in Green Canyon 65 contain a combination of interconnected sheet and channel sands. Well log analysis shows that rock properties are facies dependent and vary across the field. We used the depositional model to break out the facies of the J1 and J2 into separate flow units, each with its own rock properties. The thick, clean sheet sand facies has the most favorable rock properties, with an average porosity and permeability of 0.33 and 2400 mD, respectively. The depositional model also sheds some insight into the nature of the connectivity between the J1 and J2 reservoirs. The J1 and J2 hydraulically communicate because channel facies have cut through the shale separating both reservoirs. Hydrocarbon production from the J1 and J2 reservoirs resulted in dynamic changes which are resolvable with time-lapse seismic data. Between 1989 and 1997, the oil-water contact (OWC) had moved vertically by as much as 284 m. We track the movement of the OWC using production and pulsed neutron logs and we show that the its position in 1997 was not horizontal. The drainage scenario we develop from these data predict the actual produced volumes within 8%. The seismic properties of the J1 and J2 were effected by production because of changes in effective stress and saturation. We found using Gassmann theory that water-swept areas exhibit an increase in acoustic impedance by as much as 30%. This 30% increase in acoustic impedance resulted in a 70% decrease in the reflection coefficient at the top of the reservoirs. Areas in the reservoir which had experienced an increase in gas saturation due to the reservoir pressure falling below the bubble point did not exhibit a noticeable change in acoustic impedance and reflection coefficient between 1989 and 1997.