Assessment of sudden permeability uptick with depletion in coalbed reservoirs.

摘要

The cleat permeability of coal, critical to success of a coalbed methane (CBM) operation, is an area of significant interest. However, this is a dynamic parameter that is influenced by changes in effective stress and matrix shrinkage associated with desorption. In the San Juan basin, permeability increases with depletion are typically exponential. In addition, at low reservoir pressures, sudden flattening, increase or decrease in the permeability trend has been observed. This is attributed to possible failure of coal associated with declining reservoir pressure. It is critical to understand the dynamic permeability behavior in order to model and project long-term production from CBM reservoirs. Most previous studies replicated field conditions at shallower depths and lower initial in situ reservoir pressures when establishing the pressure-dependent-permeability (PdK) of coal. The objective of this study was to carry out an experimental study replicating the conditions prevailing in deeper coals and establish the PdK.;The experimental results showed that, under uniaxial strain conditions, decreasing pore pressure resulted in significant decrease in horizontal stress and increased permeability. The permeability increased non-linearly with pore pressure, with little increase in the high pressure range, followed by an uptick when pore pressure fell below a certain level and continued permeability increase at lower pressures. The sudden uptick in permeability was attributed to coal failure which was further investigated by geomechanical testing and failure analysis. Analysis of geomechanical results using Mohr-Coulomb failure criterion and stress invariant path plotted using Drucker-Prager failure criterion showed coal failure with methane depletion. This is attributed to decrease in the effective horizontal stress, instigated by matrix shrinkage, and increased differential stress with methane depletion. The state of stress when the failure occurred matched with the pressure when the sudden uptick in permeability was observed.