Fracturing Perforation Optimization for Commingling Gas Production in Coal Measure Strata
Authors: Cui Zhuang , Hou Bing , Chang Zhi 
Published: September 2023

ABSTRACT
The coal measure strata is rich in coalbed methane and sandstone gas. However, the difference in lithology between coal and sandstone makes it difficult to predict the hydraulic fracture height. Optimization of fracturing perforation is of great significance to the fracture height control in coal measure strata. In this paper, a three-dimensional model of the hydraulic fractures penetrating layers was established based on 3D lattice algorithm. The influence of perforation position and length on the propagation of hydraulic fractures was investigated. The results showed that (1) The closer the perforation was to the lithological interface, the easier it was for hydraulic fractures to propagate through the interface. (2) Direct fracturing of thick coal seams and indirect connection of thin coal seams is advantageous for multi-gas co-production. Shortening the perforation length was the main way to increase gas production, whether fracturing a single thin coal seam or a multi-layered medium. (3) The fracture width in a coal seam was significantly greater than in sandstone under the condition of simultaneous perforation. The decrease in fracture width was attributed to the increase in perforation length when the perforation position was all within the coal seam.

INTRODUCTION
Hydraulic fracturing has achieved great success in the exploration and development of unconventional oil and gas (Bing et al., 2022; Adeyeye et al., 2013; Warpinski et al., 2009). However, the fracturing effect of multi-lithology superimposed reservoirs is not ideal. In order to realize the combination development of different formations, it is necessary to promote the effective connection of hydraulic fractures to more formations. There has been a great amount of research on the prediction of fracture vertical propagation through the layers. It was found that hydraulic fracture had various propagation modes in the vertical plane of layered shale reservoir, such as passivation fracture, shaped fracture, and fish-like fracture (Zhang et al., 2019; Bennour et al., 2015). It was also shown that natural bedding surface, interbed stress difference, and reservoir thickness were the internal factors affecting the propagation of fractures (Bing et al., 2022; Xing et al., 2018; Zhang et al., 2017; Li et al., 2014; Anderson, 1981). Damjanac and Cundall (2016) used the discrete element method to study the influence of rock mechanical properties differences on fracture height propagation in shale reservoirs. Zhao et al. (2018) established a fracture propagation model based on the cohesive zone method of damage mechanics, studied the influence of construction parameters and geological conditions on the vertical propagation behavior of fractures, and clarified the vertical propagation law of fractures in sand-shale interbedding. Xie et al. (2020) studied the influence of bedding face on fracture height growth based on the 3D displacement discontinuity method and found that shear displacement along the bedding plane was the main mechanism controlling the vertical propagation of fractures. Dong et al. (2015) established a mud-shale fracture propagation model based on the finite difference method and studied the effects of horizontal stress difference, interface friction coefficient, and rock tensile strength on vertical fracture propagation. Li et al. (2016) studied fracture morphology when crossing multiple reservoirs in shale by combining finite element and discrete element methods. Most of these numerical simulations were only coupled with aspects of mechanical deformation, fluid flow, and fracture propagation, and the dynamic coupling analysis between fracture fluid loss and fracture opening angle is rarely considered. The synthetic rock mass (SRM) method could solve these problems perfectly (Pierce et al., 2007; Ivars et al., 2008). Therefore, the 3D lattice algorithm is suitable for studying the fracture initiation and propagation behavior under macroscopic size in multi-lithologic superimposed strata.