Experimental and numerical simulation study on hydraulic fracture propagation law of coal seam

In this article, the expansion law of hydraulic fractures in coal seams was explored using a combination of hydraulic fracturing experiments and numerical simulations with coal samples. The results show that (1) the minimum horizontal stress difference is high in the coal seam with undeveloped laminae, which will confine the fractures entirely within the coal reservoir, thus affecting the effect of multilayer fracturing. The high elastic modulus of the spacer is favorable for vertical fracture expansion. Critical displacement is essential in determining the vertical fracture breakthrough from the formation. The concentration of fracturing fluid is proportional to the height of the fracture and inversely proportional to the length of the fracture. (2) The fracture pressure decreases significantly during the development of natural laminae in coal. The fracture prefers to expand along the direction of the laminae with a smaller angle between the laminae and the maximum horizontal principal stress. The fracture passes directly through the interface to form a vertical fracture when the adhesive strength of the interface is high. When the interfacial cementation strength is low, the cracks extend along the interface and then turn to produce new vertical cracks and finally form "Z" cracks.