Experimental Investigations of Stress-Dependent Petrophysical Properties and Reservoir Characterization of Coalbed Methane (CBM)

Abstract The increase of successful exploration and economical importance of coalbed methane makes the understanding of petrophysical properties-stress-dependent more essential. This paper presents the results of experimental investigations into the influence of net stress on porosity, permeability, and reservoir quality index of coalbed methane (CBM) formations in Australia. Two groups of actual CBM samples were used to perform this study: low permeability from 0.01 to 1.5 md and medium permeability from 5.21 to 13.22 md. The variation of porosity, permeability, permeability-porosity ratio (K/φ) and reservoir quality index (RQI) under different stress conditions is studied. Current permeability-porosity models under stress are tested using the newly-attained experimental results from this study. New correlations have been developed which can be used as simple tools for predicting changes of petrophysical properties with stress. Attained results also indicated that the reduction rate of porosity of low and medium permeability CBM samples can be represented by a straight-line equation form, whereas the permeability reduction of low and medium permeability CBM formations follows consistently an exponentially reduction mode. The increase of net stress causes a reduction in both (K/φ) and RQI values. The results also revealed that the effect of stress is more dominant for low permeability samples than that ones for medium permeability CBM.