ESTIMATION OF DIFFERENT COAL COMPRESSIBILITIES OF COALBED METHANE RESERVOIRS UNDER REPLICATED IN SITU CONDITION

Studies completed recently have shown that desorption of methane results in a change in the matrix volume of coal thus altering the permeability of, and production rates from, coalbed methane (CBM) reservoirs. An accurate estimation of different coal compressibilities is, therefore, critical in CBM operations in order to model and project gas production rates. Furthermore, a comprehensive knowledge of the dynamic permeability helps in understanding the unique feature of CBM production, an initial negative gas decline rate. In this study, different coal compressibility models were developed based on the assumption that the deformation of a depleting coalbed is limited to the vertical direction, that is, the reservoir is under uniaxial strain conditions. Simultaneously, experimental work was carried out replicating these conditions. The results showed that the matrix volumetric strain typically follows the Langmuir-type relationship. The agreement between the experimental results and those obtained using the proposed model was good. The proposed volumetric strain model successfully isolated the sorption-induced strain from the strain resulting from mechanical compression. It, therefore, provides a technique to integrate the sorption-induced strain alone into different analytical permeability models. The permeability variation of coal with a decrease in pore pressure under replicated in situ stress/strain conditions was measured. The results showed that decreasing pore pressure resulted in a significant decrease in horizontal stress and increased permeability. The permeability increased non-linearly with decreasing pore pressure, with a small increase in the high pressure range, increasing progressively as the pressure dropped below a certain value. The experimental results were also used to test the proposed coupled sorption-induced strain model and several analytical permeability models. One of the commonly used models overestimated the permeability increase between 200 and 900 psi. The other two models were able to predict the permeability trend with constant cleat compressibility although the values used for the two models were different. Finally, the coupled strain and permeability models were employed to validate the field observed permeability increase data. The results indicated that the coupled models can predict the permeability trend with accuracy as long as the input parameters used are reasonable. The technique can thus serve as a particularly powerful tool for new CBM regions with limited production data since it only requires the basic adsorption data and mechanical properties and both are typically available. However, the physical meaning of the cleat compressibility term used in the permeability models needs to be clarified to ensure that its effect is not counted twice