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.