Experimental Investigation on the Propagation of Hydraulic Fractures through Coal-Rock Interfaces

Although hydraulic fracturing has been one of the primary stimulation methods for coal-bed methane (CBM) exploration, it is difficult to be applied in soft and low-permeability coal seams due to the instability of wells in such geological structures. In order to solve the problem, an idea of indirect fracturing is proposed, that is, fractures are initiated in stable and hard rocks and then propagated to coal seams in which crack networks can be formed. To verify the feasibility of such an approach, the true triaxial hydraulic fracturing experiments were conducted using two-dimensional and three-dimensional coal-rock combination samples, respectively. This study investigates the fracture patterns, pressure variation, and fracture morphology. The results show that in the process of fracture propagation from sandy mudstones to coals, the strain energy release rate in the sandy mudstones is 10.69 similar to 25.53 times greater than that in the coal. When the fracture has a tendency to deflect toward the lower strength coal strata, under the condition of large K-2/K-1, the deflection criterion will be met first and the fracture will deflect and grow into the coal strata. In addition, the complex crack network can be generated when the hydrofracture intersects the coal-rock interface and the fracture pattern is analyzed.