Experimental study on gas adsorption and drainage of gas-bearing coal subjected to tree-type hydraulic fracturing

Abstract

Exploiting coalbed methane can optimize the energy mix and improve safety in coal mines. Coalbed methane production commonly requires that the coal’s permeability be enhanced. However, conventional permeability enhancement techniques commonly used at present have their own shortcomings, such as a limited range of permeability enhancement or high fracture initiation pressures Tree-type fracturing is one of the new techniques that are expected to eliminate the above-mentioned shortcomings and allow the efficient exploitation of coalbed methane reserves. The unclear of gas migration mechanism in tree-type fracturing limits the development of this technology. For this paper, a fluid–solid–gas coupling experimental system was assembled to study the changes in gas pressure and gas flow in gas-bearing coal after hydraulic fracturing. In the experiments, tree-type branch boreholes can effectively connect hydraulic fractures and surrounding natural cracks, increasing the scope and rate of gas extraction. The degree of connectivity of fractures (cracks and pores) will affect the gas desorption rate, which makes the gas flow trace “U” shaped lines. Tree-type hydraulic fracturing can increase the coal’s permeability by factors of 3.1 to 3.9, increase the effective drainage rate by factors of 1.23 to 1.41, and increase the proportion of gas drained by 3.7% to 5.5%. These results provide a reliable basis for additional research on tree-type fracturing and can aid in its engineering application.