3.4 Enhanced gas recovery

CO2 injection into natural gas (predominantly methane) reservoirs has been proposed but not attempted. However, research suggests that injecting CO2 into mature natural gas reservoirs for enhanced gas recovery and CO2 storage is feasible [28, 33]. The average recovery of natural gas from gas discoveries worldwide is approximately 75% of the original gas-in-place in the reservoir. Of the original gas in place, water-drive gas reservoirs trap almost 35% of the original gas in place, while the depletion-drive reservoirs trap about 15%. These trapped volumes of natural gas are the potential targets for enhanced gas recovery using CO2 injection (CO2 EGR). In addition, for high CO2 content gas reservoirs, the requirement to purify the sales gas to meet sales specifications offers an opportunity for carbon capture and storage. The In Salah and Gorgon projects are good examples of this.

CO2 EGR mechanisms include CO2-methane mixing, displacing methane with CO2 and pressurising the reservoir. CO2 is more viscous and dense than methane, but well-to-well flow will still be dominated by reservoir heterogeneity. In reservoirs with good vertical communication, it might be possible to take advantage of the higher density of CO2 to design injection strategies and well completions that place CO2 low in the reservoir, with production taken from the top. Methane and CO2 need to be separated when the injected CO2 breaks through into the natural gas stream. Slower diffusional mixing also may be a concern in long-life projects, although diffusion is very slow compared to typical reservoir-flow velocities. However, in reservoirs that have experienced a pressure decline before CO2 injection starts, CO2 injection for storage can continue after reservoir pressure maintenance has ended. This is the case provided proper attention is paid to the accompanying stress changes and their effects on seal integrity.

CO2 storage in a gas reservoir would have the advantage that all CO2 from oxidising the methane produced from the reservoir could be stored in the same reservoir, at the same temperature and pressure, with additional volume available for storage of more CO2. One mole of CO2 is produced for each mole of methane oxidized. Furthermore, the molar density of CO2 is larger than that of methane at a given temperature and pressure.

The higher molar density of CO2 means that the volume of methane produced from a gas reservoir could be replaced by a mixture of nitrogen and CO2. For example, matching injection and withdrawal volumes would not require separating all the nitrogen from a flue gas. However, compared to the costs of compressing and transporting CO2 alone, there would be an additional cost associated with the nitrogen/CO2 mixture.

In discoveries that contain some condensate saturation, CO2 can vaporise quite efficiently the light hydrocarbons that make up the condensate. It is also possible for CO2 to develop multi-contact miscibility with two-phase gas and condensate mixtures [28]. If CO2 capture and storage becomes widespread, gas reservoirs would be candidates for permanent CO2 storage.

We are not aware of any projects in South-East Asia or world-wide in which the primary purpose of CO2 injection is to improve natural gas recovery (CO2 EGR). CO2 injection and storage in saline formations beneath gas reservoirs might have an additional advantage in enhancing gas recovery. However, the study of CO2 EGR is still in its early stages and it is difficult to assess its potential at the present time.

From the perspective of reducing CO2 emissions, enhanced gas recovery is not strictly comparable to CO2 storage in saline formations or depleted oil or gas fields. Enhanced recovery produces additional hydrocarbons, which are ultimately burned and therefore cause additional CO2 emissions unless they are captured and stored.