Abstract
Abstract Directional fracturing is fundamental to weakening the hard roof in the mine. However, due to the significant stress disturbance in the mine, principal stresses present complicated and unmeasurable. Consequently, the designed hydraulic fracture (HF) extension path is always oblique to principal stresses. Then, the HF will present deflecting propagation, which will restrict the weakness of the hard roof. In this work, we proposed an approach to drive the HF to propagate directionally in the hard roof, utilizing a set of hydraulic fractures and their stress disturbance. In this approach, directional fracturing in the hard roof is conducted via the sequential fracturing of three linear distribution slots. The disturbed stresses produced by the first fracturing (in the middle) are utilized to restrict the HF deflecting extension of the subsequent fracturing. Then, the combined hydraulic fractures constitute a roughly directional fracturing trajectory in rock, i.e., the directional fracturing. To validate the directional fracturing approach, the cohesive crack (representing rock fracture process zone (FPZ)) model coupled with the extended finite element method (XFEM) was employed to simulate the 2D hydraulic fracturing process. The benchmark of the above fracturing simulation method was firstly conducted, which presents the high consistency between simulation results and the fracturing experiments. Then, the published geological data of the hard roof in Datong coal mine (in Shanxi, China) was employed in the fracturing simulation model, with various principal stress differences (2~6 MPa) and designed fracturing directions (30°~60°). The simulation results show that the disturbing stress of the first fracturing significantly inhibits the deflecting propagation of the subsequent fractures. More specifically, along the direction parallel to the initial minimum principal stress, the extension distance of the subsequent hydraulic fractures is 2~3 times higher than that of the deflecting HF in the first fracturing. The fracturing trajectory of the proposed direction fracturing method deviates from the designed fracturing path by only 2°~14°, reduced by 76%~93% compared with the traditional fracturing method utilizing a single hydraulic fracture. This newly proposed method can enhance the HF directional propagation ability more effectively and conveniently in the complex and unmeasurable stress field. Besides, this directional fracturing method can also provide references for the directional fracturing in the oil-gas and geothermal reservoir.
Highlights
Hydraulic fracturing is a fluid-solid coupled process of injecting high-pressure fluid to fracture rock and drive the fracture to propagate
We proposed an approach to control the HF directional propagation in the hard roof, utilizing a set of hydraulic fractures and their stress disturbance
(2) Due to the stress disturbance of the first HF, the direction of the minimum principal stress deflects perpendicular to the designed fracturing path, enhancing the directional propagation of the subsequent hydraulic fractures
Summary
Hydraulic fracturing is a fluid-solid coupled process of injecting high-pressure fluid to fracture rock and drive the fracture to propagate. Directional hydraulic fracturing is operative to weaken the roof structure in the mine, reducing dynamic disasters due to the hard roof sudden caving [1, 2]. The disturbed stress due to roof bending and breaking will generate dynamic disasters [1,2,3] such as rock burst and coal-gas outburst. To avoid the above disasters, controlling the directional extension of hydraulic fracture (HF) in rock can break the hard roof along a reasonable path and transfer stress in the roadway (Figure 1). Controlling the HF directional extension in rocks bearing complex stresses is fundamental to avoiding dynamic disasters in mine and stimulating geoenergy (such as oil-gas and geothermal energy) reservoirs
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