Compositional Simulation of CO2 Storage Capacity in Depleted Oil Reservoirs

The Farnsworth Unit (FWU) of Ochiltree County, Texas operated by Chaparral Energy L.L.C. is the site of a CO₂-EOR project using anthropogenic CO₂. The Southwest Regional Partnership on Carbon Sequestration (SWP), sponsored by the Department of Energy's National Energy Technology Laboratory, is using this project to monitor CO₂ injection and movement in the field to determine CO₂ storage potential in depleted oil reservoirs. The field was discovered in 1955 with estimated initial oil in place of about 120 MMBO. The target reservoir is the Pennsylvanian-aged Upper Morrow sandstone, locally termed the Morrow "B" sandstone. CO₂ flooding was initiated by Chaparral Energy in December 2010.

This paper describes a compositional simulation of CO₂ storage in a depleted oil reservoir. Mechanisms considered for CO₂ storage include structural/stratigraphic trapping, dissolution in formation water and oil, and residual trapping. A high resolution geological model constructed from geological, geophysical and engineering data from FWU was used for the study. FWU has no recorded oil-water contact. The model was first calibrated to the reservoir's primary and secondary recovery history performance as a benchmark for the study. Several models were constructed and the storage capacity was analyzed as a function of injection volume, time and pressure.

Numerical simulation results show that with over 25 years of WAG injection, 75% of the CO₂ was sequestered. Afterwards wells were shut-in to monitor the storage. Significant amount of stored CO₂ was dissolved in remaining oil, contributing to enhanced oil recovery from the tertiary stage of the field operations. Supercritical phase CO₂ mass within the reservoir compared to CO₂ dissolved in formation water was dependent on CO₂ injection rate. Residual trapping contribution was significant when hysteresis was modeled. Pressure, volume of reservoir fluid present, and cap rock integrity and optimized WAG injection strategies were significant parameters to determine long-term CO₂ storage capacity within FWU.