Impact of common assumptions on the accuracy of coal permeability models

When CO₂ is injected into coal seams, complex interactions of stress and chemistry have a strong influence on the properties of coal. These include influences on gas sorption and flow, coal deformation, porosity change and permeability modification. In this study, we define this chain of reactions as “coupled processes” implying that one physical process affects the initiation and progress of another. The individual process, in the absence of full consideration of cross couplings, forms the basis of very well-known disciplines such as elasticity, hydrology and heat transfer. Therefore, the inclusion of cross couplings is the key to rigorously formulate the full mechanics of CO₂ sequestration in coal seams. Among those cross-couplings, the coal permeability model is one of the most important ones. A variety of permeability models were developed to address how CO₂ can be injected into coal seams in a controlled and socially responsible manner. These models were derived normally under the condition of uniaxial strain and/or constant overburden stress. Our comprehensive review concluded that these models have so far failed to explain experimental results from controlled stress conditions, and only partially succeeded in explaining in situ data. We identified the absence of the effective stress transfer between matrix and fracture as the fundamental reason for these failures, and developed a rigorous approach to explicitly quantify this effect on permeability during CO₂ sequestration. We applied this approach to generate a series of permeability type curves under the full spectrum of boundary conditions spanning prescribed stresses through constrained displacement. We benchmarked the solutions generated by using the “industry-standard” permeability models against our “exact” solutions for the full spectrum of boundary conditions, and concluded that these “industry-standard” models could produce unacceptable errors.