Experimental study of permeability behaviour for proppant supported coal fracture

Abstract

Proppants are typically added in the hydraulic fracture to maintain the fracture aperture and increase conductivity for gas production from coal seams. However, the presence of proppants complicate the permeability behaviour of the coal. Understanding permeability evolution of proppant supported fractures under dynamic stress conditions are necessary to predict the production of coalbed methane. In this work, a series of laboratory experiments were conducted on a cylindrical coal core from Chongqing, China, under hydrostatic condition. Permeabilities at various gas pressures and confining stresses were measured on the original sample, as well as the sample with a proppant supported fracture. Gas adsorption, matrix swelling behaviour were also investigated by injecting non-adsorbing (He) and adsorbing (CH4) gases. The results show that proppant supported fracture has little effect on adsorption capacity, as well as the swelling behaviour due to gas adsorption. However, the proppant supported fracture can significantly enhance permeability about 2-3 orders of magnitude higher than original sample depending on proppant type and its distribution in the fracture. Fracture compressibility may be decreased by 1 order of magnitude, suggesting that the permeability for the proppant supported fracture is less sensitive to stress. It was also found that sparsely placed monolayer proppant can have comparable permeability with densely packed multilayer proppant.