Experimental simulation of the hydraulic fracture propagation in an anthracite coal reservoir in the southern Qinshui basin, China

As coalbed methane (CBM) reservoirs have extremely low permeability, hydraulic fracturing is a common stimulation process for enhancing the CBM production. An in-depth understanding about the propagation mechanism of hydraulic fractures in coal is important for designing a hydraulic fracturing process and thus for improving the CBM production. This study performed a set of true tri-axial fracturing experiments on six block samples (300 mm × 300 mm × 300 mm, including raw coal and artificial roof/floor) with consideration of in situ conditions, aiming at simulating the propagation of hydraulic fractures in the CBM reservoir in the Changzhi field, southern Qinshui basin. Four groups of experiments were organized to evaluate the influences from the pre-existing natural fracture, in situ stresses and injection flow rates on the hydraulic fracture propagation. Meanwhile, five series of numerical simulations were constructed to model the relationship between in situ horizontal stresses and hydraulic fracture propagation. The results show that the hydraulic fracture propagates only along the pre-existing natural fracture direction under a small approaching angle, while it propagates along both the directions of the pre-existing natural fracture and the maximum horizontal principal stress (σ_H) under a large approaching angle. Whether pre-existing natural fractures exist or not can result in a distinct influence on hydraulic fracture propagation. Hydraulic fractures straightly propagate along the σ_H direction under a high value of horizontal stresses difference coefficient (K_h), while they tend to deviate from the σ_H direction under a low K_h value. The influence of K_h is greater than that of the horizontal stresses difference (Δσ) in determining fracture propagation to extend along the σ_H direction in the coal seam. Large approaching angle, high in situ stresses and a high injection flow rate are three major factors to cause the roof/floor broken by hydraulic fluids.