Propagation of pressure drop in coalbed methane reservoir during drainage stage

Abstract

Numerical simulation was employed to investigate the propagation speed of pressure drop at the drainage stage in coalbed methane (CBM) reservoirs. A seepage model of singlephase water in CBM reservoirs was generated with the parameter from CBM well ZS39 in the Zhengzhuang block of the southern Qinshui Basin. The effects of stress sensitivity and reservoir properties on the pressure drop propagation process were analysed. Moreover the pressure drop funnel scale index was introduced to quantitatively characterize the propagation process. The results indicate that stress sensitivity cause the permeability form the permeability drop funnel, which is consistent with the shape of the pressure drop funnel. Under the constant bottom pressure, the propagation speed of the funnel will gradually decrease in both longitudinal and lateral direction. And the overall propagation speed rapidly increases first and then gradually decreases. In the scenario of steady decrease in the bottomhole pressure, the pressure drop speed shows an increasing trend in the longitudinal direction, and a decreasing trend in the lateral direction. The overall propagation speed of the pressure drop funnel increases all along. The reservoir pressure drop is positively correlated with the initial porosity, the initial permeability and the elastic modulus. For Poisson ratio, when the ratio is small, the reservoir pressure drop has a negative correlation. As Poisson ratio increases over 0.35, a positive correlation exists. It was found from the sensitivity analysis of reservoir pressure drop that petrophysical parameters have strong sensitivity to pressure drop, especially for permeability. Therefore, this work may provide insights into the CBM reservoir properties, and thus will be favorable for improving CBM recovery.

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