Abstract
Using the dense linear multihole to control the directional hydraulic fracturing is a significant technical method to realize roof control in mining engineering. By combining the large-scale true triaxial directional hydraulic fracturing experiment with the discrete element numerical simulation experiment, the basic law of dense linear holes controlling directional hydraulic fracturing was studied. The results show the following: (1) Using the dense linear holes to control directional hydraulic fracturing can effectively form directional hydraulic fractures extending along the borehole line. (2) The hydraulic fracturing simulation program is very suitable for studying the basic law of directional hydraulic fracturing. (3) The reason why the hydraulic fracture can be controlled and oriented is that firstly, due to the mutual compression between the dense holes, the maximum effective tangential tensile stress appears on the connecting line of the drilling hole, where the hydraulic fracture is easy to be initiated. Secondly, due to the effect of pore water pressure, the disturbed stress zone appears at the tip of the hydraulic fracture, and the stress concentration zone overlaps with each other to form the stress guiding strip, which controls the propagation and formation of directional hydraulic fractures. (4) The angle between the drilling line and the direction of the maximum principal stress, the in situ stress, and the hole spacing has significant effects on the directional hydraulic fracturing effect. The smaller the angle, the difference of the in situ stress, and the hole spacing, the better the directional hydraulic fracturing effect. (5) The directional effect of synchronous hydraulic fracturing is better than that of sequential hydraulic fracturing. (6) According to the multihole linear codirectional hydraulic fracturing experiments, five typical directional hydraulic fracture propagation modes are summarized.
Highlights
Conventional hydraulic fracturing can effectively change the internal structure of coal rock reservoirs to significantly improve the permeability of coal rock
When directional hydraulic fracturing controlled by dense linear multihole is carried out in the coal mine [66,67,68,69,70,71,72], the in situ stress conditions, borehole arrangement angle, borehole spacing, and borehole pumping mode will significantly affect the shape and orientation accuracy of the directional hydraulic fracture, so this section will carry out sensitivity analysis for the above factors
When the value of Δσ was 4 MPa, the hydraulic fractures on both sides of H3 extended to the edge of the test block along the direction of the maximum principal stress, and the hydraulic fractures on both sides of H1 and H2 extended along the direction of the borehole line and approached the adjacent boreholes to form directional hydraulic fractures
Summary
Conventional hydraulic fracturing can effectively change the internal structure of coal rock reservoirs to significantly improve the permeability of coal rock. When the stress field is fixed, the orientation of the maximum tangential effective tensile stress on the borehole wall is fixed [12, 13], which makes the hydraulic fractures usually initiate and propagate perpendicular to the direction of the minimum principal stress [14,15,16,17] This provides a foundation for the mechanism and application of hydraulic fracturing. The hydraulic fracture can overcome the limitation of the far-field in situ stress, and the hydraulic fractures can be initiated and expanded along the wellbore line to achieve the directional fracturing of rock eventually This method is superior, and the engineering quantity is small with high efficiency. The research results can provide theoretical guidance for the implementation of directional hydraulic fracturing in coal mines
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