Experimental study on relationship between fracture propagation and pumping parameters under constant pressure injection conditions

•Constant pressure injection condition prolongs hydraulic fracture extension period.•Undulated and complex fractures are favored with the increase of output pressure.•Fracture behaviors are closely correlated with pumping parameter variation.•The fracturing process is characterized by cyclical variation in pumping parameters.•A model is presented to correlate the fracturing process with parameter variations. Field monitoring of hydraulic fractures is currently expensive, and the recorded data is often limited to reflect fracture networks, implying the necessity of utilizing additional evidence to document the fracture propagation state. This paper investigated the relationship between fracture behavior and pumping parameters (pressure and injection rate) by performing laboratory fracturing experiments under constant pressure injection mode (Pcon = 17, 19, and 21 MPa). In addition, microscope observation and CT scanning imaging were employed to visually investigate the morphological characteristics of fractures on the surfaces and the internal structures of the tested specimens. The results demonstrate that compared to constant flow injection, the constant pressure injection condition prolongs the fracture extension period after crack initiation and results in more significant changes in the pumping parameters and more tortuous morphology in hydraulic fractures. As the constantly injected pressure increases, more complex pumping pressure curves and undulated fractures with dendritic branches were observed. Combining the pumping pressure curves with the fracture behavior during the fracturing process, we found that fracture initiation, arrest, stable propagation, and unstable propagation are closely correlated to the variations of pumping pressure, injection rate, and pressure decay rate, which indicates the reliability of using pumping parameters to reflect the hydraulic fracturing process. Finally, a conceptual model based on wave-like theory was proposed to characterize the constant pressure injection fracturing process and correlate it with the cyclic variation of pumping parameters. The findings are instructive to control the formation of complex fracture networks and provide an essential reference for field treatment design.