An Experimental Study on the Seepage Characteristics of Rough Fractures in Coal under Stress Loading

Abstract

Fracture and stress environments significantly affect the flow of coalbed methane. Under stress, fracture deformation and damage occur, which change the original fracture characteristics and lead to changes in gas flow characteristics. The change in gas pressure gradient makes the fluid flow obviously nonlinear. Using linear flow theory to describe the fracture flow leads to a large error in predicting coalbed methane productivity. In this study, seepage tests on fractured coal are carried out under different stresses and gas pressure gradients, the nonlinear flow and changes in related parameters are analyzed, and the applicability of the nonlinear flow equation is evaluated. The resulting seepage of the gas flow in the fracture under stress is obviously nonlinear, which gradually increases with increasing effective stress and gas pressure gradient. When the Forchheimer equation is used to characterize the nonlinear seepage in fractures, the coefficients increase with increasing effective stress. The permeability, nonlinear factor, and critical Reynolds number decrease with increasing effective stress. When the Izbash equation is used for this case, the linear coefficient ranges from 1015 to 1016, and the nonlinear coefficient ranges from 1.064 to 1.795. The coefficients are related to the effective stress through a power function. Both the Forchheimer and Izbash equations can characterize the flow in rough fractures in coal during stress loading. However, the Forchheimer equation better reveals the mechanism of flow transformation from linear to nonlinear in fractures.