East Vacuum Grayburg San Andres Unit, 30 Years of CO2 Flooding: Accomplishments, Challenges and Opportunities

Abstract

Abstract The East Vacuum Grayburg San Andres Unit (EVGSAU) began CO2 injection in 1985 which is still continuing today. Performance of CO2 injection has been very favorable in the main pay zone with oil recoveries exceeding 45 percent overall in the unit. Technical challenges remain in maintaining and increasing oil recovery but future opportunities also exist such as CO2 flooding the transition/residual oil zone and improving overall volumetric sweep efficiency. Waterflooding in EVGSAU began in 1980 to pressure up the reservoir in preparation for CO2 flooding. CO2 injection began in 1985 with a 2:1 water: alternating gas (WAG) ratio to help provide conformance control. Overall oil recovery from the main portion of the reservoir in the CO2 flooding area alone is above 55 % OOIP. Approximately 12.5 % OOIP recovery has been due to CO2 flooding. This paper will detail some of the challenges experienced during CO2 flooding which include maintaining reservoir pressure above minimum miscibility pressure, maintaining high injection rates in WAG injection wells, and handling large volumes of produced gas. During the early 2000's, 19 laterals were drilled out of existing vertical wells which assisted the unit in increasing oil production rates and reducing gas coning. Future opportunities include expansion of the current gas plant to handle the increasing gas-oil ratio of produced oil as the CO2 flood continues to mature, CO2 flooding the transition/residual oil zone, infill drilling in the main pay zone to increase areal sweep efficiency and polymer gel treatments to address poor vertical conformance. A foam treatment is also being planned for EVGSAU. CO2 flooding in the EVGSAU has been impacted during its history by changing oil and CO2 prices and many technical challenges. Despite these challenges, oil recovery by CO2 flooding has been favorable and further steps are being taken to increase long-term oil recovery.