Abstract
Artificially fracturing coal‐rock mass serves to form break lines therein, which is related to the distribution of cracked boreholes. For this reason, we use physical experiments and numerical simulations to study the crack initiation and propagation characteristics of dense linear multihole drilling of fractured coal‐rock mass. The results indicate that only in the area between the first and last boreholes can hydraulic fracturing be controlled by dense linear multihole expansion along the direction of the borehole line; in addition, no directional fracturing occurs outside the drilling section. Upon increasing parameters such as the included angle θ between the drilling arrangement line and the maximum principal stress σ1 direction, the drilling spacing D, the difference Δσ in principal stress, etc., the effect of directional fracture is gradually weakened, and the hydraulic fractures reveal three typical cracking modes: cracking along the borehole line, bidirectional cracking (along the borehole line and perpendicular to the minimum principal stress σ3), and cracking perpendicular to σ3. Five propagation modes also appear in sequence: propagating along borehole line, step‐like propagation, S‐shaped propagation, bidirectional propagation (along the borehole line and perpendicular to σ3), and propagation perpendicular to σ3. Based on these results, we report the typical characteristics of three‐dimensional crack propagation and discuss the influence of the gradient of pore water pressure. The results show clearly that crack initiation and propagation are affected by both the geostress field and the pore water pressure. The pore water pressure will exhibit a circular‐local contact‐to‐integral process during crack initiation and expansion. When multiple cracks approach, the superposition of pore water pressure at the tip of the two cracks increases the damage to the coal rock, which causes crack reorientation and intersection.
Highlights
Coal mining often encounters technical problems such as hard roofs, hard thick top coal, rock bursts, coal and gas outburst, etc
Directional hydraulic fracturing controlled by dense linear multihole drilling is used mainly to improve the directionality of hydraulic fracturing of radial perforations, enhance the penetration of cracks, and increase the production of petroleum [16,17,18] and coalbed methane [19, 20]
To study the crack initiation and propagation of directional hydraulic fracturing controlled by dense linear multihole drilling, we used RFPA2D-flow to simulate the hydraulic fracturing controlled by dense linear multihole drilling under different conditions
Summary
Received 23 July 2019; Revised 16 October 2019; Accepted 5 November 2019; Published 7 December 2019. Fracturing coal-rock mass serves to form break lines therein, which is related to the distribution of cracked boreholes For this reason, we use physical experiments and numerical simulations to study the crack initiation and propagation characteristics of dense linear multihole drilling of fractured coal-rock mass. We use physical experiments and numerical simulations to study the crack initiation and propagation characteristics of dense linear multihole drilling of fractured coal-rock mass. Five propagation modes appear in sequence: propagating along borehole line, steplike propagation, S-shaped propagation, bidirectional propagation (along the borehole line and perpendicular to σ3), and propagation perpendicular to σ3 Based on these results, we report the typical characteristics of three-dimensional crack propagation and discuss the influence of the gradient of pore water pressure. The superposition of pore water pressure at the tip of the two cracks increases the damage to the coal rock, which causes crack reorientation and intersection
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