Numerical simulation of ground surface subsidence due to coal-bed methane extraction

Abstract

Coal bed methane (CBM) has gained significant attention as a source of natural gas. CBM recovery is achieved through either primary production or enhanced CBM production, the later of which remains at an infant stage. Primary CBM extraction involves production of CBM reservoir fluids using production wells to facilitate
pressure drawdown within the targeted formation. De-pressurization is required to release adsorbed methane within the interior surface of the coal matrix. However, de-pressurization can cause compaction within the CBM
reservoir, especially in the vicinity of production wells. This, in turn, can lead to ground surface subsidence.

The objective of this project is to develop a semi-analytical solution to explore ground surface subsidence above CBM extraction wells. To achieve this, an existing analytical solution, for ground surface subsidence above a cylindrical uniform pressure change, is extended to allow for a non-uniform pressure distribution using the principle of superposition. The non-uniform effective pressure to drive the semi-analytical solution for ground surface subsidence is derived from a numerical fluid flow model describing water and methane production from a CBM formation, also developed as part of this project.

The numerical fluid flow model describes two-phase fluid flow (gas and water) in porous media in conjunction with non-equilibrium gas adsorption and stress dependent porosity and permeability. The resulting set of partial differential equations is solved using the method of lines by discretising in space using finite difference and then solving the resulting set of coupled non-linear ordinary differential equations (ODE) using MATLAB's ODE solver, ODE15s. The numerical fluid flow model was verified by comparison with published modeling results from the literature. As a further verification, the model's ability to simulate field production and pressure data was demonstrated using field data from a CBM case study in the US.


The potential role of initial water saturation on ground surface subsidence was investigated by studying the associated spatial distributions of fluid pressure. It was found that, for a given time, the mean fluid pressure within the reservoir reduces with increasing initial water saturation. However, the spatial distribution of fluid pressure, for a given volume of produced gas, was found to be insensitive to initial water saturation. This can be attributed to the fact that the volume of water stored in the cleats of the coal-bed is very small as compared to the volume of gas stored within the coal matrix. Consequently, the presence of water in the cleats was found to have no influence on ground surface subsidence for a given gas production volume.

It was also found that ground surface subsidence for a given gas production volume is insensitive to initial coal permeability and cleat volume compressibility.
A simplified analytical solution for ground surface subsidence was derived assuming that the pressure distribution within the reservoir is uniform. Sensitivity analysis showed that the simplified analytical solution is effective at predicting ground surface subsidence for a given gas production volume, predicted by the numerical model, for all of the scenarios studied. This suggests that pressure distribution within a CBM reservoir is not important for determining ground surface subsidence in this context.