An internal swelling factor model to examine the influence of permeability anisotropy on coalbed methane extraction

Abstract

Abstract Owing to the characteristics of coal reservoirs, the gas flow capacity and permeability exhibit strong anisotropy. The anisotropy in terms of the magnitude, which corresponds to the permeability in the horizontal direction being several orders of magnitude larger than that in the vertical direction, has been investigated. However, the anisotropy in terms of the mechanical boundaries, specifically, the presence of constant volume and stress boundaries in the horizontal and vertical directions, respectively, has not been examined. Therefore, a coupling model of the gas flow and coal seam deformation was developed, and a model to reflect the permeabilities in the horizontal and vertical directions was established considering the internal swelling factor. The models were verified considering field production data, and a numerical analysis was performed. In the early and later stages of gas production, the gas appeared from the fracture and matrix systems, and it was extracted in different regions in the timescale and synchronously, respectively. Owing to the constant stress boundary condition in the vertical direction of the coal seam, the reduction in the gas pressure in the fractures decreased the horizontal fracture opening, thereby decreasing the permeability in the horizontal direction. Because the horizontal direction exhibited a constant volume boundary condition, the desorption of the adsorbed gas resulted in volumetric shrinkage of the matrix, thereby increasing the permeability in the vertical direction. Non‐Darcy effects reduced the gas flow rate and exerted considerable influence in the early stage of the extraction. Moreover, this effect exhibited anisotropy, which was more pronounced in the horizontal direction. The surface diffusion coefficient continuously increased, which promoted the flow of the adsorbed gas in the matrix. The proposed model can be used to estimate the impact of the permeability anisotropy on the coalbed methane and underground gas extraction.

Keywords

• constant stress
• constant volume
• non‐Darcy flow
• permeability model
• surface diffusion