Multi-scale multivariate detection method for the effective impact range of hydraulic fracturing in coal seam

Abstract

Hydraulic fracturing enhances coal seam permeability and reduces original gas content. Determining the effective range of hydraulic fracturing is essential in evaluating its effectiveness. This paper analyzes physical field changes during hydraulic fracturing and proposes a multi-scale multivariate detection method for determining the effective range of hydraulic fracturing in coal seams based on changes in wave velocity and resistivity due to fracture expansion.The method was validated through field experiments. The result shows that vibration wave computed tomography (VWCT) is a passive, large-scale area detection method. In contrast, the direct current (DC) method is a small-scale, local active detection method. In the hydraulic fracturing process, the microseismic (MS) events increase. After fracturing, the wave velocity of coal seam decreases significantly, indicating the formation of fractures around the fractured holes and a fracture zone. The fracturing expands the pre-existing fractures adjacent to fractured holes, forming a fracture network between them and natural fractures, leading to a decrease in wave velocity around the coal seam. The DC detection results indicate a noticeable development of pores around the fractured holes following hydraulic fracturing. The increased resistance value area is mainly concentrated on both sides of the 9–4 drilling, with an influence range of approximately 30 m, and an increased range of 100–400 Ω·m, indicating a hydraulic fracturing pore expansion range of 30 m in coal seams. Comprehensively, the maximum impact area of fracturing is the area surrounding the fractured holes, and the impact area is not uniform. The DC method test results reflect the extension of fracturing around fractured holes, while the VWCT test results represent the extension of fracturing around fractured holes and the connectivity of primary fractures, both of which include each other.