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Coal Geology & Exploration

Abstract

Purpose Multi-stage, multi-cluster fracturing in horizontal wells represents a key technique for deep coalbed methane (CBM) recovery and the sustainable development of the industry. However, during multi-cluster fracturing in deep CBM horizontal wells, the stress shadow effect tends to induce non-uniform fracture propagation and a limited stimulated reservoir volume (SRV). Methods To address these challenges. this study established a two-dimensional (2D) numerical model for multi-cluster hydraulic fracturing in horizontal wells using the finite element method. Then, it systematically analyzed the impacts of in situ stress and cluster spacing on both the stress shadow effect and the competitive propagation of hydraulic fractures during multi-cluster fracturing. In combination with field microseismic monitoring, the mechanisms underlying the impacts were verified, and suggestions regarding the optimization of multi-cluster fracturing schemes were put forward. Results and Conclusions The fracture network formed during multi-cluster fracturing is characterized by the unilateral propagation of individual fractures and the symmetrical development of fracture networks. A lower stress level or larger cluster spacing corresponds to a weaker stress shadow effect and a more pronounced symmetrical morphology of individual fractures. During hydraulic fracturing, the distribution of the perturbed stress field is jointly determined by reservoir fluid pressure and the superimposition of fracture opening-induced compressive stress on the initial in situ stress field. The included angle between the direction of the maximum principal stress and its initial direction decreases as the initial in situ stress difference increases. The fractures formed under multi-cluster hydraulic fracturing generally exhibit the typical propagation characteristics of mutual constraint in the early stage and fracture network coalescence in the late stage under engineering interventions. Overall, the multi-cluster fracturing technique generally demonstrates a higher fracture generation efficiency in deep high-stress environments than in shallow low-stress conditions. With the single-cluster fracture growth efficiency and fracture aperture as assessment criteria, the optimal cluster spacings for fracturing in horizontal wells are preliminarily considered to be 8 m, 6 m, and 4‒6 m, respectively, under in situ stress differences of 4 MPa, 8 MPa, and 12 MPa. Comprehensive analysis of an engineering case reveals that fracture propagation between clusters within the same stage and between adjacent stages is significantly affected by the stress shadow effect. Therefore, the stage and cluster combinations and the principle behind the stage and cluster layouts should fully account for the progressive distributions of the fracturing scale, the injection rate of fracturing fluids, and the perturbed stress field. It is recommended to adopt fracturing schemes with variable spacing and variable scale tailored to different stages and clusters. Such design, combined with targeted engineering interventions, will help maximize the area of the effectively propped fracture network.

Keywords

deep coalbed methane (CBM), horizontal well, multi-cluster fracturing, stress shadow effect

DOI

10.12363/issn.1001-1986.26.07.0438

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