The effect of aquifer condition on methane recovery for CO2 injection in coalbed

Abstract

To estimate methane recovery from an enhanced coal bed at the field scale, it is important to understand CO2 movement.Since coal beds are generally connected with aquifers, CO2 movement is affected by aquifer position and strength. Under conditions of no aquifer, CO2 initially has a tendency to move to the top layer of the coal seam due to the buoyancy effect.However, the permeability of the upper layer is decreased due to the swelling effect. The viscosity flow effectively improves CO2 movement due to high permeability in the bottom layer. Because of the offsetting effects of viscosity and gravitational flow, the vertical sweep efficiency of injected CO2 is very effective. Thus, methane recovery is highly calculated. Under conditions of an upper aquifer, CO2 flow in the bottom layer decreased as the injected CO2 leaked to the aquifer. As the hydraulic connection between the overlying aquifer and the coal seam is strong, the vertical sweep efficiency is weakened due to the high gas leakage rate. Therefore, methane recovery decreases. However, under conditions in which an aquifer is at the bottom, methane recovery should be high because the gas-leakage rate is almost negligible. Under conditions with an aquifer edge, injected CO2 in the coal bed has an asymmetric flow because pressure is only supported in the flow direction of the aquifer. As the pressure is greater at the aquifer, the asymmetric flow is gradually strengthened by changes in the equilibrium caused by viscosity and gravity. Consequently, enhanced coalbed methane recovery has irregular efficiency, in part due to the CO2 movement. However, by suitably adjusting the CO2 injection site and production well perforation, total methane recovery can be made more efficient, despite producing an asymmetric flow of injected CO2. These findings clearly indicate that production or injection plans for enhanced coalbed methane must be designed to consider CO2 movement.