Gas slippage and matrix shrinkage effects on permeability of coal.

Abstract

The permeability of coal is one of the most important basic parameters in the simulation of gas transport in coalbeds and in the evaluation of the commercial feasibility of coalbed gas reservoirs. However, the permeability of coal and its variation as gas is produced are still not well understood. Unlike that in conventional gas reservoirs, the gas permeability of a coalbed is influenced during gas production, not only by the simultaneous changes in effective stress and gas slippage, but also by the matrix shrinkage associated with gas desorption. The objective of this work was to investigate experimentally the matrix shrinkage and gas slippage effects on the permeability of coal. Since these effects occur simultaneously during gas production, a theory to separate these effects was first developed. This dissertation presents a technique to conduct laboratory experiments to estimate their individual contribution, along with the results obtained for quantitative relationships of the gas slippage and matrix shrinkage effects with gas pressure. The results show that the total permeability of the coal sample increased dramatically due to gas slippage and matrix shrinkage effects with decrease in pressure. When the gas pressure is above 250 psi, the effect of matrix shrinkage dominates. As gas pressure falls below 250 psi, both the gas slippage and matrix shrinkage effects play important role in influencing the permeability. Finally, the change in permeability of coal sample resulting from gas slippage was found to be proportional to the reciprocal of the gas pressure. The change in permeability due to matrix shrinkage was found to be linearly proportional to the volumetric strain associated with desorption. Since the latter is linearly proportional to the amount of gas desorbed, the change in permeability is a linear function of the amount of desorbing gas.