Theoretical method for calculating porosity and permeability under self-regulating effect

In order to study the effect of stress-sensitivity and matrix shrinkage on coalbed methane production, equivalent matrix particle model is proposed considering the process of adsorption and desorption. Calculating mathematical models for calculating porosity and permeability which considered matrix shrinkage by combining diameter model and desorption were established. The calculations of porosity and permeability under self-regulating effect were obtained by combining traditional stress-sensitivity equations. The changes of porosity and permeability in different reservoirs were calculated and analyzed through a variety of basic parameters. The results show that high coal rank reservoir has the biggest range ability of porosity and permeability under the same pressure difference conditions, followed by the middle rank and the low rank. The research observed the positive relationship between stress-sensitivity and declining period of porosity and permeability in low rank coal reservoir, and the inverse relationship between matrix shrinkage and declining period of porosity and permeability. The stronger the matrix shrinkage is, the earlier declining period and rise period will occur.