Study on Fracture Propagation of Hydraulic Fracturing with Cyclic Shock Wave Pre-Cracking for CBM Horizontal Wells
Authors: Bi Yansen , Gao Deli , Xian Baoan , Li Guichuan 
Published: September 2023

ABSTRACT
A novel stimulation method of the hydraulic fracturing with cyclic shock wave pre-cracking for CBM horizontal wells is proposed to extend the control range of artificial fractures. The results of rock mechanics and physical tests show that the permeability, porosity and strength of the coal in Fukang of Xinjiang are very low. Based on rock fatigue damage mechanism, maximum tensile stress criterion and Mohr-Coulomb criterion, the formation of radial fractures in near-hole coal under cyclic shock is simulated by numerical method. The results show that 2-6 obvious radial cracks were formed by cyclic shock wave, and the rest were microcracks. And the pulse width, peak strength and frequency of cyclic shock wave have the best values for the crack propagation in coal. The fracture propagation of hydraulic fracturing with cyclic shock wave pre-cracking is simulated by numerical method. The results show pre-cracks weaken the influence of the in-situ stress difference on the fracture propagation. In the process of hydraulic fracturing, only 2-3 pre-cracks are propagated obviously. The hydraulic fracture propagation with pre-crack azimuth of 60°-90° is less affected by the in-situ stress. The hydraulic fracture turns to the maximum stress direction earlier with the increase of pre-crack length and width.

INTRODUCTION
Horizontal well completion and stimulation have gradually become the key technologies for efficient development of low permeability coalbed methane (CBM) reservoir. (Gao et al., 2022). Connection of the natural cracks and articular fractures is the key to the CBM horizontal well stimulation (Cao, M., 2022). At present, scholars have proposed many technologies such as directional perforation, cluster perforation, volume fracturing and fracture steering for CBM horizontal wells to form complex artificial fractures (Cao et al., 2020; Huang et al., 2022; Wang et al.,2018; Cao, M., 2022). The in-situ stress difference between vertical and horizontal is an important factor affecting the propagation of artificial fractures during hydraulic fracturing of CBM horizontal wells. When the vertical stress is greater than the horizontal stress, the artificial fractures propagate vertically to the roof and floor of coal seam, which will form T-shaped fractures and reduce the control range of artificial fractures (Li et al.,2020). Excessive in-situ stress difference between vertical and horizontal even causes hydraulic fractures penetrating into adjacent aquifers, which leads to high water production and low gas output (Baskoro I. K. et al., 2015). A pulse technology (Qin et al., 2012; Zhang et al., 2016) for unconventional reservoir stimulation has been proposed to form fractures of different scales in coal seam and promote coalbed methane production (Shi et al., 2016; Zhang et al., 2017; Zhang et al., 2019). Qin et al. (2014; 2021) and Li et al. (2015) conducted experiments and numerical simulations on cyclic shock wave fracturing coal, and analyzed the laws of crack propagation in coal. The new test results show that the shock wave can fracture the rock without the completion string failure (Li et al., 2022). Xian (2022) and Bi (2016) proposed an integrated technology with screen completion and coal seam permeability enhancement to maintain wellbore and increase gas production. The author proposes a horizontal well completion method with composite strings in coal seam, which provides conditions to implement multi-type reservoir stimulation technologies for single well (Bi et al., 2023). Gao et al. (2020) proposed a fracturing method of hydraulic pre-splitting blasting in coal seam. The effective influence radius of the hole with the new method can reach 6.98 m after, and its gas emission quantity is 3.18 times that of conventional blasting hole. Based on the finite element software, Li analyzed the cumulative effect of rock damage under cyclic blasting, and demonstrated the feasibility of rock cyclic blasting failure (Li et al., 2019).