Stress induced permeability changes due to production from a Coal seam

Abstract

Gas production from coal seams has attracted a great deal of attention around the world and in particular in Australia with its rich resources of coal. In general the coal bed methane (CBM) resources are located at lower depths (i.e. less than 1000 m) compared to conventional gas reservoirs. This increases the chance of a horizontal fracture to be developed if hydraulic fracturing is used to enhance gas production from a coal seam by connecting the cleates more effectively to each other and to the wellbore. During gas production, due to changes in stress fields, fractures, from the very small scale of coal cleates to those extending a few meters and large scale fault planes may get highly stressed and slide to some extent and experience changes in their apertures. The potential for this depends on the magnitude of in-situ stresses and also the geometry of the fracture plane with respect to the direction of principal stresses. This is a dynamic process which results in continuous changes in productivity of a CBM reservoir. This paper aims at studying the potential for fracture reactivation during the production life of a CBM reservoir and how this may result in changing the permeability of the coalbed by communicating small fracture planes to each other. The Mohr-Coloumb criteria was used to investigate the sliding potential.Also, to demonstrate the significance of stress changes due to gas production from coal seams on changing the sliding status of small fracture planes (here the coal cleats) a number of fracture planes were generated randomly and the sliding potential was assessed for these planes with respect to depletion rate. It is seen that increased production will change the results to a large extent: this shows how the secondary porosity and permeability may change significantly as a result of communication of cleates on a random basis during the production life of the coal seam. Changing the stress regime in the field, e.g. from normal to strike-slip, also changes the results significantly. The results are presented in this paper and conclusions will be made.