Rock Crack Propagation Mechanism of Oriented Perforation Hydraulic Fracture under Different Perforation Parameters
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RESEARCH ARTICLE-CIVIL ENGINEERING
Rock Crack Propagation Mechanism of Oriented Perforation Hydraulic Fracture under Different Perforation Parameters Songyong Liu1,2 · Deyi Zhang1,2 · Hao Liu1,2 Received: 5 March 2020 / Accepted: 21 July 2020 © King Fahd University of Petroleum & Minerals 2020
Abstract In order to obtain the mechanism of crack propagation of oriented perforation hydraulic fracture and its effect on mechanical properties of rock samples, an experimental study on hydraulic fracture of rock samples without confining pressure was carried out. The confining pressure ratio coefficient was defined, and oriented perforation hydraulic fracture was numerically simulated based on the extended finite element method so as to explore the comprehensive influence of different perforation parameters. The results demonstrated that the crack initiates and propagates along the direction of perforation azimuth angle, and the crack of oriented perforation is slightly wider than that of non-oriented perforation. The difficulty of crack initiation and turning increases with the perforation azimuth angle and decreases with the confining pressure ratio coefficient. Crack initiation and propagation are mainly driven by tensile stress, and the crack initiation pressure increases with the perforation azimuth angle and the confining pressure ratio coefficient. Keywords Crack propagation · Oriented perforation hydraulic fracture · Confining pressure ratio coefficient · Perforation azimuth angle
1 Introduction Hydraulic fracture is a technology for changing mechanical properties and stress distribution near the hole of rock samples under the water pressure, and inducing crack initiation and propagation at the holes’ edge, so as to weaken the overall mechanical properties of rock samples [1, 2]. Hydraulic fracture has been widely used in shale reservoir stimulation, coal seam permeability enhancement and hard coal roof weakening since 1985, and some significant results have been achieved [3, 4]. As shown in Fig. 1, hydraulic fracture goes through three stages: the fracture hole drilling stage, the sealing water injection stage and the crack initiation and propagation stage. A high-pressure area will * Deyi Zhang [email protected] 1
School of Mechatronic Engineering, Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, Xuzhou 221116, China
Jiangsu Collaborative Innovation Center of Intelligent Mining Equipment, China University of Mining and Technology, Xuzhou 221008, China
2
be formed at the bottom of the hole after the borehole sealing. When the water injection pressure reaches the ultimate tensile strength, the rock samples will be damaged and the cracks will be formed, which weaken the overall mechanical properties of the rock. According to the action modes of water jet pressure, hydraulic fracture can be divided into oriented and non-oriented perforations. For extremely hard rock (compressive strength σc ≥ 160 MPa) [5], the weakening effect of non-oriente
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