Impact of CO2 Injection on Flow Behavior of Coalbed Methane Reservoirs

On the basis of observations at four enhanced coalbed methane (ECBM)/CO2 sequestration pilots, a laboratory-scale study was conducted to understand the flow behavior of coal in a methane/CO2 environment. Sorption-induced volumetric strain was first measured by flooding fresh coal samples with adsorptive gases (methane and CO2). In order to replicate the CO2-ECBM process, CO2 was then injected into a methane-saturated core to measure the incremental swelling. As a separate effort, the permeability of a coal core, held under triaxial stress, was measured using methane. This was followed by CO2 flooding to replace the methane. In order to best replicate the conditions in situ, the core was held under uniaxial strain, that is, no horizontal strain was permitted during CO2 flooding. Instead, the horizontal stress was adjusted to ensure zero strain. The results showed that the relative strain ratio for CO2/methane was between 2 and 3.5. The measured volumetric strains were also fitted using a Langmuir-type model, thus enabling calculation of the strain at any gas pressure and using the analytical permeability models. For permeability work, effort was made to increase the horizontal stress to achieve the desired zero horizontal strain condition expected under in situ condition, but this became impossible because the excess stress required to maintain this condition was very large, resulting in sample failure. Finally, when CO2 was introduced and horizontal strain was permitted, permeability reduction was an order of magnitude greater, suggesting that the excess stress would have reduced it significantly further. The positive finding of the work was that the excess stresses associated with injection of CO2 are large. The excess stresses generated might be sufficient to cause microfracturing and increased permeability, and improved injectivity. Also, there might be a weakening effect resulting from repeated CO2 injection, as has been found to be the case with thermal cycling of rocks.