Relations Between Coal Permeability And Directional Strains And Their Application to San Juan Basin

In this study a permeability model is developed to define the evolution of gas sorption-induced permeability anisotropy under the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement. In the model, gas sorption-induced coal directional permeabilities are linked into directional strains through an elastic modulus reduction ratio, Rm. This defines the ratio of coal mass elastic modulus to coal matrix modulus (0<Rm<1) and represents the partitioning of total strain for an equivalent porous coal medium between the fracture system and the matrix. Where bulk coal permeability is dominated by the cleat system, the portioned fracture strains may be used to define the evolution of the fracture permeability, and hence the response of the bulk aggregate. The coal modulus reduction ratio provides a straightforward index to link anisotropy in deformability characteristics to the evolution of directional permeabilities. The validity of the model is evaluated against results for special cases representing uniaxial swelling, constant volume reservoirs, and for the case of ten coalbed methane production wells in San Juan Basin. 1. INTRODUCTION Knowledge of the transient and anisotropic characteristics of permeability evolution in fractured coals is of fundamental importance to the recovery of methane from CBM reservoirs and equally important for CO2 storage suing ECBM [1], which have been investigated through experimental, field-scale, and numerical studies. Experiments have investigated the sorption and related swelling characteristics of coals. The effects of water content on swelling and sorption have been explored for CO2 uptake at 298 K [2] using a gas-flame coal, a gascoking coal and an anthracite and indicate a reduction in swelling strain for “dry” coal versus “pre-wetted” samples [3]. Rates of swelling are controlled largely by diffusive length scales imparted by the cleats. A surrogate of this case is powdered coals where for powdered high volatile bituminous Pennsylvanian coals the adsorption rate decreases with increasing grain size for all experimental conditions [4]. Similarly, coal type and rank influences the preferential sorption behavior [5- 6] and the evolution of permeability with these changes is linked to macromolecular structure [7]. Adsorption kinetics may also be determined for various gases (e.g. for CO2 and CH4) using confining cells to apply desired pressures and temperatures [8] and using X-ray CT methods to determine the resulting sorption isotherms [9]. These experiments have focused on the isotropic characteristics of intact or powdered coals. Conversely, some experiments have focused on the anisotropic characteristic of coal. Water transmission characteristics have been shown to be significantly different [10-11] under confining pressures when measured perpendicular to either face cleats, butt cleats, or bedding planes. These results are congruent with optical measurements of coal swelling under in CO2 and other gases where swelling in the plane perpendicular to the bedding plane was always substantially higher than parallel to the bedding plane [13]. This phenomenon has also been observed in the field well tests in the Warrior Basin (USA) where the anisotropy ratio of permeability in the direction of the bedding plane was as high as 17:1 [14].