Study of Injection Rate on Fracture Network Complexity during Hydraulic Fracturing: Experimental Insights from Concrete, Sandstone and Bituminous Coal

This study examines the influence of injection rate on fracture development during hydraulic fracturing using water as the fracturing fluid. Laboratory experiments were performed on concrete, low-permeability sandstone, and brittle bituminous coal over a broad range of injection rates. The results indicate that both low (<1.0 mL/min) and high (≥5.0 mL/min) injection rates predominantly produced simple, planar fractures, reflecting limited fracture branching. In contrast, an intermediate injection rate of 2.2 mL/min consistently resulted in the most complex fracture patterns. The potential for enhanced fracture complexity was inferred from fracture surface roughness obtained via 3D scanning and characterized using the Joint Roughness Coefficient (JRC), which exhibited its highest values at this intermediate injection rate. Although water is sometimes considered less effective than unconventional fluids, the findings demonstrate that it performs well in moderately permeable rocks with sufficient tensile strength. However, water was less effective in brittle coal and low-permeability sandstone, where more compressible, low-viscosity fluids may be advantageous. Overall, three fracture behavior regimes were identified: a low-rate regime producing simple fractures, a mid-rate optimal regime producing complex networks, and a high-rate regime where fracture complexity decreased. Building on earlier findings that associate higher fluid injection rates with increased fracture complexity, this study explicitly identifies a limiting injection rate beyond which fracture network complexity no longer exhibits significant growth. Thin fractures dominated because controlled injection conditions focused on observing fracture behavior rather than merely inducing fracture, as reflected by the relatively high post-fracture injection pressure.