Fracture swarm formation during shut-in driven by pore pressure waves

Abstract

Hydraulic fracturing involves injecting a high-pressure fluid mixture to create fractures in underground rock formations, thereby enhancing hydrocarbon flow. Recent field observations, such as those from the Hydraulic Fracture Test Site (HFTS) project and tests in the Eagle Ford, have revealed surprising complexity of hydraulic fractures, including the presence of densely distributed fracture swarms. These findings challenge conventional expectations and necessitate a deeper understanding of fracture mechanisms. Existing studies have explored the propagation, connectivity, and implications of multiple fractures, but questions remain about the mechanisms behind the formation of fracture swarms, particularly the distribution of these secondary fractures. Our research introduces an alternative mechanism, proposing that secondary fractures result from a pore-pressure wave in which the pore pressure exceeds the compressive stress. This hypothesis suggests that pore-pressure variations within the rock and fluid exchange between the fracture and rock after shut-in can initiate secondary fractures. Through theoretical modeling and scaling, we have identified the governing dimensionless parameters that determine the number and distribution of secondary fractures. Numerical simulations enabled us to construct the parametric space for these parameters. Finally, we propose a procedure to interpret field observations of fracture swarms. Our approach provides new insights into predicting fracture swarms and using the observed fracture swarms to constrain some parameters that are relevant to hydraulic fracturing.