Numerical simulation study on hydraulic fracture propagation of multi-cluster fracturing of horizontal well in deep fractured coal seams

Abstract

In recent years, deep coalbed methane has emerged as a focal point for exploration and development, achieving significant success in extraction through large-scale fracturing measures. However, the widespread development of cleat/fracture systems within coal seams has a significant impact on the propagation of hydraulic fractures, and the increasing stress differences caused by deeper burial depths make hydraulic fracturing more challenging. As a result, this study establishes a hydraulic fracturing model for fractured coal seam using a three-dimensional lattice method, and investigates the impact of cleat/ natural fractures, deep geological stress environments, and engineering intervention measures on the propagation of hydraulic fractures. The results indicate that the existence of cleat/ natural fractures in coal seams induces the formation of a complex fracture network system for hydraulic fractures, however the difference in stimulation volume among various perforation clusters increases. When natural fractures near the wellbore are aligned with the direction of hydraulic fractures propagation, the initiation pressure can be significantly lowered. Under different stress regimes, the reverse stress regime exhibits poor stimulation effects, tending to form horizontal fractures, manifesting as “T” and “Z” shaped fractures; the normal stress regime and strike-slip stress regime can form a considerable scale of primary fractures and branch fractures, which is more favorable for the formation of fracture networks. However, an increase in horizontal stress difference promotes hydraulic fractures length growth. When the horizontal stress difference is 12 MPa, hydraulic fractures can develop uniformly and the dynamic distribution of fracturing fluid tends to stabilize. Increasing the fracturing fluid viscosity reduces the reconstruction volume and shortens the fracture length, which is detrimental to the development of long fractures. However, the rapid increase of pumping volume of fracture fluid is most beneficial to the development of fracture length and improves the efficiency of fracturing fluid, making it the most effective for the transformation of fractured coal seams. The results provide a better understanding of the hydraulic fractures propagation mechanism in deep fractured coal seams, offering a reference for the engineering transformation of deep fractured coal seams.