Experimental investigation on crack competitive extension during hydraulic fracturing in coal measures strata

Hydraulic fracturing is an effective method to improve the permeability of coal seams and enhance extraction of coalbed methane (CBM). Evolution of cracks in coal seam becomes increasingly complex when one borehole is drilled through multiple coal seams. In this work, physical models with two coal seams having different uniaxial compressive strength (UCS) ratios and height ratios were prepared to study the competitive initiation and extension of cracks based on pump pressure, acoustic emission (AE) source location, and crack morphology. The results showed that, a competitive relationship exists between the two coal seams in terms of crack extension. The pump pressure curves of all specimens presented four stages: (I) fluid-filling stage, (II) pressure rise stage, (III) pressure drop stage, and (IV) pressure stabilization stage. Compared with the pure specimen, the crack initiation pressure of specimens with two coal seams reduced generally. The AE source locations were first generated and mainly distributed in the softer coal seam, resulting in the formation of a complex crack network, whereas only a few cracks were generated in the harder coal seam, as observed after cutting the specimens. In seams with different height ratios, the AE source locations and tracers indicated that cracks preferentially developed and extended in the thinner coal seam. The higher the height ratio of the two coal seams, the more difficult it was for cracks to extend simultaneously in two coal seams. The research results will help improve the permeability of CBM reservoirs when subjecting multiple coal seams to hydraulic fracturing.