A depleted natural gas reservoir can store significantly more gas than a depleted oil reservoir (with the same initial hydrocarbon pore volume) for two reasons. First, ultimate gas recovery (about 65% of gas initially-in-place) is typically about twice that of oil (average 35% of oil initially-in-place). Second, gas is some 30 times more compressible than oil or water. CO2 has a critical temperature of 31°C (88°F) and a critical pressure of 7.38 MPa (1070 psia). Consequently, at pressures and temperatures typically encountered in the field, carbon dioxide will behave as a supercritical fluid. However, displacement of natural gas by supercritical CO2 has not been done in the field and is not well understood.

As part of a CO2 sequestration study, we have conducted experimental and analytical studies to evaluate the feasibility of displacing natural gas with supercritical CO2. The experiments involved injecting supercritical CO2 into an aluminum Hassler core-holder containing a 1-ft long by 1-in. diameter carbonate core initially saturated with methane. Rate and composition of the produced gas were measured, enabling determination of mole fraction of produced CO2 as a function of time. In addition, the core holder was CT scanned to determine core porosity, and CO2 and methane saturation. Experiments have been conducted for core pressures in the range, 500-3000 psig at 70°-140°F. Results indicate that some 73%-87% of the gas initially-in-place is recovered at CO2 breakthrough, with a low CO2 dispersion coefficient of 0.01-0.12 cm2/min.

We can tentatively conclude that CO2 injection into depleted or abandoned gas reservoirs would not only sequester CO2 but would also re-pressurize the reservoir, and most likely result in effective displacement of the gas. Thus, production of hitherto unrecoverable gas reserves could help defray the cost of CO2 sequestration: in short, a possible win-win technology.

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