The injectivity of coalbed CO₂ injection wells

Though it may be possible to enhance methane production from coal by injecting CO₂, because coal is poorly permeable it is usually necessary to inject under fracturing conditions to achieve acceptable injectivity. Concomitantly, the process of replacing methane by the injected CO₂ causes the matrix to swell. These two processes-the fracturing of the coal and the swelling-have opposite effect on the injectivity. TNO-NITG have pressure data from a CO₂ injection test in a coalbed methane field. We used the SIMED II coalbed methane simulator to history match the test behaviour and to find the most sensitive parameters affecting the injectivity of the CO₂ injection well. An analysis of the pressure records revealed both the occurrence of fracturing and the reduction in permeability that swelling induced. When applied to an extended injection simulation, the simulator showed that the most sensitive parameters influencing the injectivity were the permeability, the fracture conductivity, and the cleat system porosity. Unfortunately, due to the adsorption of the CO₂ and the fluctuations in pressure during injection tests, all these vary over time.