Stress path with depletion in coalbed methane reservoirs and stress based permeability modeling

That permeability is a critical parameter dictating the performance of naturally fractured reservoirs, like coalbed methane (CBM), is evident form the available field, experimental and permeability modeling information in the literature. Although modeling is often achieved at the expense of several input parameters, the exercise is typically unable to match sudden increases in coal permeability, encountered in deep coals after significant depletion. This paper is aimed at coupling stress and permeability in order to reduce the number of parameters required for modeling the permeability variation. Stresses in the reservoir are translated to invariants and stress path of coal is established in octahedral effective stress plane. Based on a detailed analysis of the stress path of three different coal types, a permeability model is presented in terms of stresses alone, that is applicable for elastic as well as inelastic deformations of coal. The model is validated using pressure-dependent-permeability experimental data for three coal types along with the geomechanical testing data used to develop the failure envelope. The primary implication of the study is improved capability to predict permeability of deep coal deposits, given that they are likely to undergo inelastic deformation or shear failure with continued depletion, using one parameter only. Finally, realistic constraints on the values of the parameter are provided to enable operators with the necessary tools to use the model for field applications, particularly in the new and upcoming CBM fields.