4-D Permeability Characterisation of Coal Enhances Exploration and Production of CBM

摘要

Reliable knowledge of the long-term dynamic behaviour of the in-situ permeability of coal is highly desirable for effective exploration of coalbed methane (CBM) reservoirs, de-methanating coal mines (CMM) and maximising gas reserves. Laboratory and field data shows coal permeability decreasing exponentially with increased net stress, creating concerns for dramatic reductions of permeability over time and loss of reserves as reservoir pressure is depleted. However, there are exceptions to this observed behaviour, indicating that a favourable effect on permeability probably occurs when the maximum tectonic stress vector is parallel to the face cleats. Of importance, there are three additional mechanisms to consider to define dynamic coal permeability. First, coal permeability is very anisotropic: the face to butt cleat permeability anisotropy ratio, Kh_(max) to Kh_(min), has been found to range from 2:1 to 17:1. In situ stresses are also quite directional, with a vertical lithostatic, a horizontal tectonic maximum and a corresponding minimum usually at 90° Second, in areas with a sound overburden or partially-elastic bulk modulus, depressurisation reduces the vertical stress on the coal matrix. Third, methane desorption shrinks the matrix; because of lateral confinement, the coal seam reacts by widening the apertures of the cleats, increasing horizontal permeability. With these dynamics, a 3-D permeability character becomes 4-D over time. A true triaxial stress coal permeameter, capable of testing at in-situ levels of stress and pore pressure, and with a constant-volume operating option, is presented. It is designed to define this 4-D behaviour for the benefit of geologists and engineers in CBM and CMM projects, leading to successful exploration and maximum profitability. Derivations are presented of skewed well pattern spacings for various anisotropy ratios, and of theoretical scale-up multipliers to adjust laboratory determinations from small specimens to full in-situ permeability levels.