Impact of pressure-dependent diffusivity on transient pressure analysis of a dry Coalbed Methane (CBM) wells: A new approach

Coalbed Methane (CBM) reservoirs demonstrate sensitivity to in-situ stress conditions. The depletion of reservoir pressure due to production leads to changing stress conditions within the reservoir. The matrix shrinkage phenomenon due to the coal matrix gas desorption results in altering the pore volume of the cleat network. Consequently, the porosity and permeability of the coal cleat system change as pressure depletes. This phenomenon contradicts the assumption made in the derivation of the diffusivity equation. The inherent assumption of reservoir properties produces inaccurate results for CBM reservoirs during conventional pressure transient analysis under the stress-driven permeability effects. This study deals with the issues related to pressure-dependent diffusivity in dry CBM reservoirs. In this paper, the authors attempt to present a new equation that incorporates pressure-dependent cleat porosity and permeability in the transient state flow equation for dry CBM reservoirs. The concept of stress-dependent pseudo pressure (SDPP) and pseudo-time has been leveraged to derive a new solution to the diffusivity equation under transient flow conditions. The outcome of this study finds its application in accurately estimating the cleat permeability and skin in a dry CBM well that flows with reservoir conditions of pressure-dependent permeability.