A New Coal-Permeability Model: Internal Swelling Stress and Fracture–Matrix Interaction

Abstract

We have developed a new coal-permeability model for uniaxial strain and constant confining-stress conditions. The model is unique in that it explicitly considers fracture-matrix interaction during coal-deformation processes and is based on a newly proposed internal swelling stress concept. This concept is used to account for the impact of matrix swelling (or shrinkage) on fracture-aperture changes resulting from partial separation of matrix blocks by fractures that do not completely cut through the whole matrix. The proposed permeability model is evaluated using data from three Valencia Canyon coalbed wells in the San Juan Basin, where increased permeability has been observed during CH 4 gas production, as well as using published data from laboratory tests. Model results are generally in good agreement with observed permeability changes. The importance of fracture-matrix interaction in determining coal permeability, demonstrated in this study using relatively simple stress conditions, underscores the need for a dual-continuum (fracture and matrix) mechanical approach to rigorously capture coal-deformation processes under complex stress conditions, as well as the coupled flow and transport processes in coal seams.

Coal permeability model on the effect of gas extraction within effective influence zone

“…4. Uncertain parameters: during the model development, the fracture compressibility C f is a calibrated value quoted from the model proposed by Liu and Rutqvist (2010). It is an alterable parameter affected by numerous factors, such as pore volume and effective stress.…”

“…As a result, a relationship between normal stress (effective stress) and coal-mass permeability is easily developed. Based on the cubic law, the relationship between coal-mass permeability and effective stress is given as follows (Liu and Rutqvist 2010):…”

“…Considering no residual fracture aperture remains, Eq. (1) can be simplified as follows (Liu and Rutqvist 2010):…”

“…More detailed description regarding the abovementioned equations can be found in Liu and Rutqvist (2010). r 0 and r 1 are often regarded as two different stress states.…”

“…Connell et al (2010) proposed a coal permeability model under a triaxial stress state. Liu and Rutqvist (2010) explicitly considered the interaction of fracture matrix and proposed the concept of swelling stress based on the effect of gas pressure change and matrix swelling or shrinkage on coal permeability. Wu et al (2011) represented heterogeneous swelling processes, which proved that coal swelling is a heterogeneous process that depends on the distribution of coal voids.…”