Numerical and experimental comparisons of the coplanar coalescence of two fluid-driven fractures

The coalescence of two coplanar fractures growing under the symmetric injection of a Newtonian fluid from two point sources provides a unique data set to validate theoretical predictions of hydraulic fracture (HF) growth. We test the theoretical predictions resulting from the combination of linear elastic fracture mechanics and lubrication flow in the growing fracture. We use a numerical solver based on an implicit level set algorithm which notably combines a finite discretization with the asymptotic solution for a steadily moving HF locally near the crack front. For the first time, we compare blindly numerical predictions with data from coalescence experiments performed in hydrogel in the toughness dominated growth regime. Initially, the two fractures propagate radially independently until they start to interact and coalesce. The fracture front then exhibits a transition from a locally concave shape back toward a radial geometry at later times. Apart from material heterogeneities and resolution of the experimental techniques, the time evolution of the fracture footprint before and after coalescence is captured by the numerical predictions. The fluid velocity field and fracture opening at different times obtained experimentally and numerically are also in quantitative agreement.