Micro–Macro Behavior of CBM Extraction in Multi-well Mining Projects

Abstract

Multi-well extraction is a prevalent technique in coalbed methane (CBM) recovery projects. Although numerous studies have extensively explored aspects such as well spacing, the degree of multi-well pumping, and well count, the dynamics of fracture microstructure evolution in proximity to wells—particularly in inter-well regions—remain inadequately understood in relation to the effects of multi-well mining project. This research delved into the multi-well extraction methodology employed in CBM recovery endeavors, aiming to elucidate the development of the fracture microstructure network. We introduce a novel, interdisciplinary, and integrative research framework that amalgamates the multi-field coupling effects observed during the multi-well extraction process with fractal theory. This model has been validated, and it facilitates the examination of changes in fracture micro-evolution subjected to multi-well extraction. Additionally, this study investigated alterations in fracture characteristics, seam stress, and CBM pressure within sensitive zones (i.e., inter-well spaces and adjacent areas) under varying extraction pressures. Following a 180-day extraction period, the findings indicate a significant reduction in gas pressure by 83.9% for the extraction wells and the nearby areas, alongside a decrease in fracture network length by 10.94% and density by 5.04%. Compared to existing models for assessing multi-well CBM extraction, our interdisciplinary model demonstrates considerable analytical superiority. Notably, when the fractal parameters D_f and D_Tf, which characterize fracture density and tortuosity quantitatively, increase from 1.2 to 1.8, the residual gas pressure is reduced further by 11.6% and increased further by 3.9%, respectively.