Characteristics of Hydraulic Fracture Height Growth in Deeply Layered Coalbed-Mudstone Reservoir
Authors: Wang Zhiwei , Sheng Mao , Ren Lejia , Deng Chao , Wang Bo , Wang Jia 
Published: August 2024

ABSTRACT:
The deep coalbed methane (CBM) is a prominent area for the exploration and development of unconventional gas resources. However, there is insufficient propagation of hydraulic fractures within the coal-mudstone interbed, resulting in the restricted stimulated volume. The impacts of pumping rate and fluid viscosity on fracture propagation in the Junggar Basin's deep coal was present by the true triaxial fracturing experiments. The outcrops were cut to form a five-layer structure consisting of mudstone-coal-mudstone-coal-mudstone bonded with marble adhesive. The experimental parameters were determined based on similarity criteria to establish correspondence between fracture propagation modes at field scale and laboratory scale. Results indicate that hydraulic fracture can propagate through the layer when the viscosity of the fracturing fluid utilized in the field reaches 150 mPa·s. Increasing pump rate from 2 m3/min to 6 m3/min per cluster can reduce fracture tortuosity but has minimal impacts on fracture penetration. This work was used to optimize the fracturing design of an exploration well in Junggar Basin, and the results have been verified from the field data.

1. INTRODUCTION
Hydraulic fracturing is a widely utilized technology in the field of coalbed methane reservoir reform. By injecting high-pressure fluid and proppant into the formation, one or more fractures with high conductivity are created to increase the volume of reform, relieve pressure, enhance permeability, and ultimately boost gas production. However, unlike conventional rock formations, coal possesses complex mechanical properties and an unclear understanding of fracture propagation mechanisms along with immature fracturing technologies. Consequently, numerous laboratory experiments have been conducted to explore the influence of different geological and engineering parameters on fracture propagation in coal.
It has been found that certain conditions such as high elastic modulus, low injection rate, low viscosity, and low stress difference coefficient facilitate the formation of complex fracture networks. Conversely, conditions like low elastic modulus, high injection rate, high viscosity, and high stress difference coefficient tend to form single fractures (Ai et al., 2018; Tan et al., 2017; Tan et al., 2019; Wan et al., 2019; Wang et al., 2022; Zhang et al., 2018). Additionally, the presence of weak surfaces such as bedding planes, cleats, and natural fractures significantly affects fracture initiation and propagation behavior. When cleats intersect with boreholes, hydraulic fractures tend to initiate through cleats rather than along the maximum horizontal principal stress direction. The presence of bedding planes influences hydraulic fracture height, and when intersecting with these planes, the hydraulic fracture may extend along them. Furthermore, natural fractures can locally guide hydraulic fracture extension while overall following the direction set by maximum horizontal principal stress (Fan et al., 2014).