Does Low-Viscosity Fracturing Fluid Always Create Complex Fractures?

Lower viscosity fluids are commonly believed to be able to create more complex fractures in hydraulic fracturing; however, the mechanism remains stubbornly unclear. We use a new grain-scale model with accurate coupling of hydrodynamic forces to simulate the propagation of fluid-driven fracturing. The results clarify that fracturing fluid with a lower viscosity does not always create more complex fractures. The heterogeneity in the rock exerts the principal control on systematic evolution of fracture complexity. In homogeneous rock, low viscosity fluids result in low breakdown pressure, but viscosity exerts little influence on fracture complexity. However, in heterogeneous rock, lower viscosity can lead to more complex network of fracturing. A regime map shows the dependence of fracture complexity on the degree of rock heterogeneity where low viscosity fracturing fluid more readily permeates weak defects and creates complex fracture networks. Plain Language Summary Hydro-fracturing is an important technique in petroleum industry to stimulate well production. Generally, it is believed that lower viscosity fluids will create more complex fractures with higher cost. The mechanism of induced more complex fracture is still not fully understood yet, which may result in uncertain expense on fluid treatments. This problem will be solved by a grain-scale numerical framework to reproduce and simulate this process. The results clarify that fracturing fluid with a lower viscosity does not always create more complex fractures. The heterogeneity in the rock exerts the principal control on systematic evolution of fracture complexity. In homogeneous rock, low viscosity fluids result in low breakdown pressure, but viscosity exerts little influence on fracture complexity. However, in heterogeneous rock, lower viscosity can lead to more complex network of fracturing. The results will improve understanding of mechanism of hydro-fracturing mechanics and therefore help to optimize the cost-benefit relationship in industrial applications.