Effects of coal bedding dip angle on hydraulic fracturing crack propagation

Abstract

As a commonly used and effective technology for increasing the permeability of coal–rock reservoirs, hydraulic fracturing has been widely used in engineering sites to realize the efficient exploitation and utilization of gas resources in coal–rock reservoirs. The core of hydraulic fracturing is the initiation, propagation, and path of hydraulic cracks. In this paper, the combination of true triaxial physical test and numerical simulation is used to study the influence of coal bedding characteristics on the crack propagation of hydraulic fracturing and to discuss the important role of bedding in hydraulic crack formation. Results show that the control effect of the coal bedding dip angle on the hydraulic crack propagation under the same stress conditions is stronger than that of the maximum principal stress, and the control effect of the bedding on the crack propagation is weaker under the bedding dip angles of 0° and 90°. Reasonable fracturing fluid displacement setting is conducive to the formation of complex hydraulic fracture network structure, small displacement is conducive to the opening of primary natural fractures, and large displacement is conducive to hydraulic cracks that pass through the structural surface and the coal–rock interface. Global and local methods of finite element mesh embedding zero-thickness cohesion element and a pore-pressure node merging method to simulate fracturing are established using Python language and ABAQUS numerical analysis platform, respectively. The numerical simulation results suggest that the main fractures are formed along the principal stress direction, and the secondary branch fractures are formed along the bedding direction under the condition wherein the coal bedding dip angle is 30°. Under the conditions of different stress fields and fracturing fluid discharges, the controlling effect of bedding on hydraulic fracture is closely related to the fracturing parameters.

Analysis of geological factors affecting propagation behavior of fracture during hydraulic fracturing shale formation

“…Shales located deep underground are subjected to high pressure at depth, resulting in a series of reactions and changes. As the depth of burial increases, the pressure on the rock increases, leading to a corresponding increase in its geostress (Aliha et al 2012;Isaza et al 2021;Yang et al 2021). It is found that the complexity of the generated fracture network decreases gradually with the increase of the principal stress difference during the fracturing process.…”

A comprehensive review of numerical simulation methods for hydraulic fracturing

“…The contact of hydraulic fracture with the reservoir discontinuities can be reviewed by ABAQUS 131 . Huang et al 132 . used the ABAQUS to study coal reservoir attributes on the hydraulic fracturing propagation characteristics.…”

Current Status of Research on the Coal and Gas Outburst Control Technology of Hydration and Anhydrous

“…36 Qing et al conducted a theoretical analysis based on the mechanisms and process of hydraulic fracturing to derive pressure parameters applied to high methane low permeability outburst coal seams, and finally came up with a reasonable water injection pressure equation. 37 Huang et al 38 used true triaxial hydraulic fracturing experiments combined with numerical simulation to study the effect of coal seam geological characteristics on hydraulic fracturing crack extension and found that under the same stress conditions, the maximum principal stress has a weaker control on hydraulic fracturing crack extension, while the coal seam inclination angle has a greater control on the crack extension, and moreover, the reasonable layout is more conducive to the development and extension of the cracks. Kang et al 39 used similar materials to simulate coal seams, combined with theoretical analysis and field applications, to study the effect of ground stress on the fracture expansion law of hydraulic fracturing in coal seams (as shown in Figure 5), proposed the optimal arrangement of gas extraction boreholes based on the fracture expansion law under the effect of ground stress, combined hydraulic fracturing and gas extraction according to the characteristics of ground stress and geological structure, and proposed a set of integrated hydraulic fracturing and gas extraction process systems of "selection−judgment− In view of the large uncertainty of the risk of coal and gas outburst, the current outburst risk assessment is still mainly based on gas dynamic accident phenomena or empirical indicators of coal and gas outburst, while the question of whether stress perturbation and structural changes to the coal rock body by pressure during hydraulic fracturing will induce outburst has not been fully confirmed; therefore, further observation and confirmation are needed.…”