Fluid-driven cracks and backflow in a multi-crack system

We design a~two-crack experiment~to study~the effects of natural fractures on hydraulic fractures. Instead of~a single layer elastic matrix, we~inject liquid into a~multi-layered elastic matrix.~A similar system has been used to study~geological processes such as dike-sill formations. ~In our experiments, pressurized fluid fractures the matrix and pressures are quantified based on laser imaging of particle displacement.~ First, we report the opening of the cracks and subsequently, the direction of the fluid reverses upon release of~injection~pressure, driven by the elastic relaxation of the gelatin.~The volume remaining in the gelatin matrix is measured as a function of time for different gelatin Young's moduli and fluid viscosities. We explain our results with scaling arguments.