Gas recovery factor from water drive gas reservoirs is very low compared to recovery made from depletion drive gas reservoirs. Other problems associated with gas recovery from water drive mechanism include high residual gas saturation in the water invaded zone of the reservoir, high volume of produced water, abandonment at high reservoir pressures and high possibility of hydrate formation in pipe lines. The use of carbon dioxide (CO2) in displacing natural gas from volumetric gas reservoirs has been studied, practised and is successful. In this paper, it is proposed that extending this practice to gas reservoirs under strong water drive mechanism can improve recovery and control water influx.
CO2 is denser than natural gas and water is denser than CO2. The different densities of these fluids can be taken advantage of to boost natural gas recovery from water drive gas reservoirs. The continuous CO2 injection process at the gas water (g/w) contact can partially prevent water encroachment into the system. The technique can change the water drive mechanism to full or partial depletion drive where CO2 will separate the natural gas zone from direct contact with the water zone. Any eventual water invasion into the reservoir affects the CO2 zone, not the upward moving natural gas zone.
This technique was studied by simulation using data from a lean gas reservoir under strong water drive. Two cases were considered. In the first case, which is the reference case, gas production under water drive was allowed for 30years. In the second case, CO2 was injected at the initial gas water contact for the same number of years. Simulation results showed that water production from the reservoir was drastically reduced to about 60% in the second case because the rate of water influx into the reservoir was controlled. Gas recovery from two producer wells out of three that were considered improved above 10% and gas condensate recovery was improved to about 4% over the period of production that CO2 was injected.