Effect of adsorption-induced matrix deformation on coalbed methane transport analyzed using fractal theory

• Changes in porosity of coal samples was described by fractal dimension.
• Relationship between absolute permeability and fractal dimension was explored.
• The method was introduced in analyzing experimental data of coal sample.

It is well-known that adsorption-induced deformation of coal matrices influences the prediction of the estimated coalbed methane (CBM) yield. Additionally, changes in porosity contribute to changes in matrix permeability. In this paper, changes in porosity and absolute permeability are studied on the basis of fractal theory, and the reliability of equations are verified by experimental data from coal samples from the San Juan Basin. Furthermore, two permeability-affecting stages (i.e., rapid and slow deformation stages) are identified, and the result also illustrates that its applicability varies with different gas pressures that arise from the decay of the Klinkenberg effect when the gas pressure is increasing. However, its applicability is limited when the gas pressure remains high because of the same decay effect; therefore, using a reasonable value for the Klinkenberg constant plays a crucial role in formulating a sound permeability-change model for high pressure environments. Fractal theory can clearly explain permeability change patterns due to the adsorption-induced matrix deformation, which can be applied to understand the mechanism underlying methane seepage during CBM production.