Carbon dioxide (CO2) is commonly used for enhanced oil recovery (EOR) in the Permian Basin and is gaining interest for Carbon Capture, Utilization & Storage. A study was conducted to develop candidate selection criteria, pilot test the design, and optimize CO2 gas lift to stabilize production on intermittently flowing wells in one of these EOR fields.

The initial CO2 gas lift design was installed in 2019 using a capillary string, downhole check valve, gas lift mandrel, and packer. A 34-day bottomhole pressure and temperature survey was evaluated to assess the success of the pilot and improve the equipment design for future installations. The phase changes of CO2 were accounted for when evaluating the pilot, modeling gas lift, and improving equipment design.

Carbon dioxide is a complex fluid at the bottomhole pressures (BHP) and temperatures (BHT) observed during the pilot. These pressures and temperatures were plotted on the CO2 phase diagram, which showed phase changes between vapor and liquid at higher gas lift injection rates. Further analysis revealed the CO2 changed phase from a liquid to a vapor across the downhole check valve. The Joule-Thompson (JT) effect across the check valve at the tubing entry point dropped the temperature of the produced fluids so much that the CO2 changed phase from a vapor back to a liquid. This increased the hydrostatic pressure and therefore, the bottomhole flowing pressure.

These CO2 phase changes in the tubing occurred in cycles comprising five distinct stages: (1) BHT cooling forced CO2 from the vapor to liquid phase and increased BHP; (2) BHT remained fairly steady as BHP increased due to liquid loading; (3) BHT started warming at a faster rate as BHP rose due to the decreasing pressure drop across the downhole check valve; (4) the tubing unloaded as CO2 flashed in a chain reaction down the tubing, resulting in an influx of warmer reservoir fluid; and (5) BHT remained steady as BHP decreased and the annular packer fluid restarted the cooling process. Results from this initial pilot were used successfully to optimize CO2 gas lift for subsequent installations.

CO2 gas lift can be an effective artificial lift method to stabilize production if the equipment is designed correctly to maximize the CO2 gas fraction at the tubing entry point. A poorly designed CO2 gas lift installation may result in unstable production from liquid loading events caused by the cyclic JT effect. CO2 gas lift is a valuable artificial lift method to reduce failure frequency and operating costs in EOR fields with readily available CO2.

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