Optimizing enhanced coalbed methane recovery for unhindered production and CO₂ injectivity

We explore the effect of gas pressure and stress on the permeability evolution of coalbed methane (CBM) reservoirs infiltrated by carbon dioxide (CO₂). Typically the recovery of methane induces shrinkage and the injection of CO₂ induces swelling respectively increasing or decreasing permeability for constrained coals. Permeability evolution was quantified for moisture equilibrated and partially dried bituminous coal samples together with the transitions caused by sequential exposure to different gases. We report experimental measurements of permeability evolution in a coal from the Uinta basin infiltrated by helium (He), methane (CH₄) and CO₂ under varying gas pressure (1-8MPa) and moisture content (1-9% by mass) while subjected to constant applied stresses (10MPa). Permeability decreases with increased moisture content for all the gases (He, CH₄ and CO₂). The decrease in He permeability may be as high as ∼100 fold if the moisture content is increased from 1 to 9%. Swelling induced by sorption of CH₄ and CO₂ in the coal matrix reduces permeability by 5-10 fold depending on the gas injected and the moisture content. Swelling increases with gas pressure to a maximum (strain based estimation 5%) at a critical pressure (∼4.1MPa) corresponding to maximum adsorption capacity. Beyond this threshold effective stress effects dominate. We use permeability evolution in bituminous coal for various moisture contents, effective stresses, and gas pressures to propose a mechanistic model. Also, we showcase this model to explain the published data for permeability evolution on water saturated Pennsylvanian anthracite coal. We use this model to investigate the performance of prototypical ECBM projects. In particular we examine the effect of the permeability loss with injection of CO₂. We define response in terms of two conditions: reservoirs either below (under) or above (over) the saturation pressure that defines the permeability minima in the reservoir. For oversaturated reservoirs withdrawal will always result in decreased permeability at the withdrawal well unless the critical pressure is transited. Similarly permeability will decrease at the CO₂ injection well unless the pressure increase is sufficiently large to overcome the reduction in permeability due to CO₂ - typically of order of one to a few MPa. For undersaturated reservoirs the permeability will always increase at the withdrawal well and can only increase at the injection well if the critical pressure is transited and further exceeded by one to a few MPa. These observations provide a rational method to design injection and recovery strategies for ECBM that account for the complex behavior of the reservoir including the important effects of moisture content, gas composition and effective stress.