Hydraulic Fracturing of Naturally Fractured Reservoirs

A carefully engineered stimulation that makes use of opening mode fractures is an effective way to reduce injection and production impedance and provide better access for the fluid to enter and promote shearing in the main reservoir volume. We present results from field experiments that mapped the fracture geometry for opening mode hydraulic fractures formed in naturally fractured crystalline rock masses. The propped fractures were formed by water and crosslinked gel fluids injected at relatively low rates (300 to 400 Liters per minute), but into a short isolated zone. The fractures formed were mapped after mining. The fractures consisted of one dominant propped channel that was offset at some shear and vein structures. Numerical modeling indicates that a stepped fracture path requires higher pressure to extend and extends more slowly than a straight fracture path. The shear displacement that is generated on the fracture path leads to self propping. We conclude that it is practical to place unpropped and propped opening mode fractures into naturally fractured rock to reduce flow impedance of the well and improved stimulation of EGS reservoirs.