Fracture morphology of variable pumping-rate hydraulic fracturing in middle and deep coal reservoirs

Outcrop coal samples from the Shizhuang South Block of the Qinshui Basin, Shanxi Province, China, were subjected to true triaxial hydraulic fracturing experiments to simulate frature propagation. Combined with CT scanning and three-dimensional fracture reconstruction, the study examined fracture propagation patterns and bedding activation behaviors under variable pumping-rate fracturing in coal reservoirs. Results indicate that the variable pumping-rate fracturing technique effectively overcomes the strong trapping effect of coal bedding. Micro-fractures are initiated at multiple weak points along bedding planes, leading to multi-point fracture initiation and competitive propagation of fractures toward the far field, thereby generating a more complex three-dimensional fracture network. The geometry and aperture of the induced fracture network are primarily controlled by the ramp-up rate of injection flowrate. A gradual ramp-up favors the development of a more complex fracture network, though at the expense of lower breakdown pressure, insufficient initiation, and narrower apertures. In contrast, a rapid ramp-up produces wider fractures and larger propped lengths, but results in more pronounced aperture fluctuations. For coal reservoirs with relatively high rock strength, a moderately higher ramp-up rate is recommended to avoid excessively narrow fractures and potential proppant bridging. Different coal lithotypes necessitate tailored ramp-up strategies to optimize fracture morphology and stimulation effectiveness.