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 under Award No. DE-FC26-05NT42591, 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 Pennsylvanian Upper Morrow 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 trapping, dissolution in formation water and oil, supercritical CO₂ phase, and mineral 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.

Simulation results show that with over 80% of the CO₂ was sequestered, significant amount 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 was much larger than that dissolved in formation water. Mineral trapping was slow and does not contribute significantly to the storage until after several years of simulation run. Pressure, volume of reservoir fluid present, and cap rock integrity were significant parameters to determine long-term CO₂ storage capacity within FWU.

This work shows an improved strategy of maximizing CO₂ storage within a depleted oil reservoir. It was evident in this study that pressure changes within the reservoir have to be continuously monitored to enhance CO₂ storage. The FWU project will serve as a benchmark for future CO₂-EOR projects in the Anadarko basin or geologically similar basins throughout the world.