Numerical simulation of the laws of fracture propagation of multi-hole linear co-directional hydraulic fracturing

Weiyong Lu,Changchun He

doi:10.1177/0144598721988993

Abstract

Directional rupture is one of the most important and most common problems related to rock breaking. The goal of directional rock breaking can be effectively achieved via multi-hole linear co-directional hydraulic fracturing. In this paper, the XSite software was utilized to verify the experimental results of multi-hole linear co-directional hydraulic fracturing., and its basic law is studied. The results indicate that the process of multi-hole linear co-directional hydraulic fracturing can be divided into four stages: water injection boost, hydraulic fracture initiation, and the unstable and stable propagation of hydraulic fracture. The stable expansion stage lasts longer and produces more microcracks than the unstable expansion stage. Due to the existence of the borehole-sealing device, the three-dimensional hydraulic fracture first initiates and expands along the axial direction in the bare borehole section, then extends along the axial direction in the non-bare hole section and finally expands along the axial direction in the rock mass without the borehole. The network formed by hydraulic fracture in rock is not a pure plane, but rather a curved spatial surface. The curved spatial surface passes through both the centre of the borehole and the axial direction relative to the borehole. Due to the boundary effect, the curved spatial surface goes toward the plane in which the maximum principal stress occurs. The local ground stress field is changed due to the initiation and propagation of hydraulic fractures. The propagation direction of the fractures between the fracturing boreholes will be deflected. A fracture propagation pressure that is greater than the minimum principle stress and a tension field that is induced in the leading edge of the fracture end, will aid to fracture intersection; as a result, the possibility of connecting the boreholes will increase.

Highlights

Fossil fuel, such as coal, oil and natural gas, plays an important role in the existing energy system
Directional hydraulic fracturing is necessary during the directional pre-cracking of the hard roof of a coal seam (Huang et al, 2017; Huang et al, 2018a; Lin et al, 2016; Wang et al, 2019; Yu et al, 2019), the weakening and caving of hard roof coal (Huang et al, 2015, 2018b), the prevention and control of rock bursts (Dou et al, 2009; He et al, 2012; You et al, 2017), roadway retention along the void, roadway formation using the top-cutting method, permeability improvement of methane-bearing coal seams and shale oil and gas reservoirs (Huang and Lu, 2018; Li et al, 2019; Lu et al, 2019)
For (4) directional hydraulic fracturing guided by a non-uniform pore pressure, the pore water pressure gradient guides the propagation of the tip of hydraulic fracture

Summary

Introduction

Fossil fuel, such as coal, oil and natural gas, plays an important role in the existing energy system. When the energy has difficult accumulating in large quantities, rupture termination is reached, and the number of micro-fractures does not continue to increase over time It can be seen from the results of the numerical simulation of multi-hole linear co-directional hydraulic fracturing (Figures 6 and 7) that, (1) before the of initiation of hydraulic fracture, the water pressure continues to increase. In this stage, there is no acoustic emission signal and there are no microcracks (Figures 6(a) and 7(a)). According to the elastic theory of porosity, the calculation formula of rock fracture pressure is as follows pb

À 2 1 À a 1 À 2

Conclusions