Numerical Simulation of Crack Initiation and Propagation Evolution Law of Hydraulic Fracturing Holes in Coal Seams Considering Permeability Anisotropy and Damage

Abstract

Hydraulic fracturing has been widely used in practical engineering as an essential means to prevent coal seam gas outburst, increase coal seam permeability and improve gas drainage efficiency. Accurate prediction of fracture propagation law is an important basis for optimizing fracturing parameters to achieve high-efficiency gas drainage in coal seams. In this paper, a new seepage–stress–damage coupling model considering permeability anisotropy is first established and then used to study the evolution laws of crack initiation pressure (σci), fracture pressure (σcd), AE behavior and pore water pressure with the lateral pressure coefficient (ξ) and permeability anisotropy coefficient (λ) in the process of hydraulic fracturing. Finally, the influence of initial pore water pressure on σci is discussed, and an efficient gas drainage method is proposed. Research results indicate that: the in situ stress still plays a leading role in the approach of crack propagation whether the permeability is isotropic or anisotropic; the non-uniform pressure condition is favorable for the crack growth compared with uniform pressure under the isotropic permeability condition; when the direction of maximum permeability is consistent with the direction of maximum principal stress (ξ = 0.5, λ < 0), the coal seams are easily fractured; AE behavior of fracturing holes can be divided into three stages: initiation stage, fracture smooth expansion stage and the breakdown stage for any λ or ξ; and the more complex the crack distribution, the more the area of the gas pressure release zone (GPRZ) increases, which is very beneficial to achieve high-efficiency gas drainage. This study can provide a basis for optimizing fracturing parameters and technology in improving the efficiency of coal seam gas drainage using the hydraulic fracturing method.

Determination of Effective Radius under the Condition of Nonuniform Gas Occurrence and Borehole Plastic Failure

“…Liu 20 quantitatively studied the nonuniform distribution of gas pressure and coal seam permeability caused by the landform of the Loess Plateau. Chen 21 established a seepage–stress–damage coupling model considering permeability anisotropy and studied the evolution of key parameters of hydraulic fracturing to strengthen gas extraction under the condition of in situ stress and permeability anisotropy. In order to improve the efficiency of gas extraction, it is necessary to start from the root cause, such as improving the permeability of coal seams and optimizing the layout of gas predrainage boreholes.…”

Seismic Response Models and the AVO Simulation of Coal-Bearing Reservoirs

“…The reflectivity method, which is a numerical transformation technique, is one of the most effective methods for simulating the full wave field in a layered half-space. Its accuracy is higher than those of the finite difference method and the ray method, and its computational cost is intermediate to those of these two approaches [12,13]. The advantage of the reflectivity method is that it considers multiple reflections and converted waves in a system comprising numerous reflectors; accordingly, the synthetic records are highly accurate [14].…”