Permeability changes in coal seams: The role of anisotropy

Abstract

The permeability of coal seams is pore pressure-dependent. A number of analytical models have been proposed to investigate this problem, but many disregard a crucial factor: the anisotropy of coal. This paper is devoted to investigating the role of anisotropy in modeling the change of horizontal permeability with pressure. Analysis is conducted using a fully anisotropic model that incorporates both the anisotropies of mechanical properties and of the permeability dependence on stresses. Analytical expressions of the pressure-permeability relationship are derived in oedometric and isochoric geomechanical conditions, and validations are conducted against both laboratory and field data. Then, the roles of the anisotropy of stiffness and of permeability dependence on stresses in the permeability-change model are explored. We demonstrate that the mechanical anisotropy can be simplified to an isotropic model without introducing significant errors in prediction of pressure-permeability relationship, while neglecting the anisotropy of permeability dependence on stresses leads to considerable errors. When both anisotropy sources are disregarded, the pressure-permeability curve can be exactly reproduced by a totally isotropic material in both oedometric and isotropic conditions. However, the material properties (e.g., bulk modulus) are skewed; moreover, such an equivalent material might lead to significant errors in other geomechanical conditions. Finally, the permeability change is investigated at the reservoir scale, and the reservoir simulation results confirm the conclusions obtained from the analytical analysis.coalbeds are conferred unique poromechanical properties by their two-scale porosity systems: macro-porosity and micro-porosity (Espinoza et al., 2014;Nikoosokhan et al., 2014). The macro-pores in coalbeds are commonly constituted by cleats, that are, natural fractures developed during diagenetic processes (Laubach et al., 1998). The cleats act as the major channel for methane to flow and thus govern the permeability of fractured coal seams. Between cleats, one finds a microporous organic continuum, routinely called as coal matrix. Methane is stored inside the micro-pores of coal matrix. In the micro-pores (typically sized in the order of 10 −9 to 10 −8 m), all fluid molecules interact with the atoms of solid matrix; they are therefore not in their bulk state as in the macro-pores but are in adsorbed state (Vandamme et al., 2010;Brochard et al., 2012). The adsorbed state of methane in micro-pores governs the adsorption/desorption phenomena and subsequent swelling/shrinking of coal matrix (Pan and Connell, 2007;Day et al., 2008).The permeability of fractured CBM reservoirs changes with depletion during production, and this process plays an important role during production and enhanced recovery operations. The permeability change with depletion (i.e., decrease of pore pressure) mainly stems from two mechanisms with opposing effects. The first mechanism involves the mechanical deformation due to pressure changes: w...

Evolution of Production and Transport Characteristics of Steeply-Dipping Ultra-Thick Coalbed Methane Reservoirs

“…The limitation of the existing isotropic permeability model, which is commonly adopted in numerical simulation, results in this failure. Actually, coal is one kind of typically anisotropic material, including permeability and mechanical properties, particularly in the cases of low-rank coal seams [41,42]. Figure 16 shows that the aperture sizes of the cleats (face and butt cleats) and structure of bedding planes, which potentially cause the permeability anisotropy.…”

Influence of geological factors on coal permeability in the Sihe coal mine

“…The relationship between coal permeability and pressure sensitivity was studied using coal samples collected from the Xinyuan coal mine in Yangquan city. The study showed the following: (1) permeability of coal exhibits anisotropy, (2) permeability along the strike of the coal seam is higher than that along the direction of propensity (Wang et al 2018). These observations indicate that coal permeability is highly sensitive to stress anisotropy.…”

Anisotropic Permeability Model for Coal Considering Stress Sensitivity, Matrix Anisotropic Internal Swelling/Shrinkage, and Gas Rarefaction Effects

“…Gas adsorption/desorption causes a surface energy change, which leads to coal matrix and bulk rock swelling/shrinkage . The magnitudes of swelling/shrinkage in different directions can be substantially different (anisotropic swelling/shrinkage). − When gas flows through coal, gas pressure variation (effective stress variation) changes the fracture (cleat) aperture, resulting in permeability evolution. , Meanwhile, gas adsorption/desorption induces swelling/shrinkage of coal matrices near the fracture (cleat) surface (internal surface), which narrows/opens the fractures (cleats) and reduces/increases coal permeability. − This swelling/shrinkage phenomenon is called internal swelling/shrinkage and is regarded as a key factor affecting coal permeability evolution. , In addition, the impact of the gas rarefaction phenomenon (gas slippage) on permeability has been widely observed in different experiments. − For the small flow channels within coal, the permeability is also influenced by gas rarefaction effects, ,,, which lead to invalid classical permeability effective stress laws . Due to the aforementioned factors, it is challenging to accurately describe the anisotropic coal permeability evolution in the process of gas injection/extraction under the combined effects of effective stress, anisotropic internal swelling/shrinkage, and the gas rarefaction phenomenon.…”

“…6−8 When gas flows through coal, gas pressure variation (effective stress variation) changes the fracture (cleat) aperture, resulting in permeability evolution. 9,10 Meanwhile, gas adsorption/desorption induces swelling/shrinkage of coal matrices near the fracture (cleat) surface (internal surface), which narrows/opens the fractures (cleats) and reduces/increases coal permeability. 9−11 This swelling/shrinkage phenomenon is called internal swelling/ shrinkage and is regarded as a key factor affecting coal permeability evolution.…”