Interpretation of Field Experiments and Observation of Fracturing Process

Abstract

Proposal Detailed experiments were performed in four instrumented boreholes to determine the growth pattern of an induced hydraulic fracture and its impact on flow through an ultra low permeability formation. The instrumentation allowed measurement of fracture pressure and width in three boreholes which were intersected by a fracture induced from a central well. The fracturing fluid was synthetic pore water to prevent formation damage. The experiments showed that because of the difference between the instantaneous fracture velocity and fluid velocity inside the fracture the fluid front lags behind fracture front. There was a large pressure drop along the length of the fracture which caused rapid narrowing of the fracture width, poor hydraulic communication along the fracture length, and localized growths and fluid movements. The data confirmed an off-balance fracture growth pattern, consisting of tensile and shear fractures and many branches. At the end of the treatment, even though the fracture contained some residual opening, it had very little conductivity along its length. At the same time, because of its off-balance growth the fracture had the capacity to transmit fluid locally and trap and hold part of the treatment fluid inside the ultra low permeability formation. These experiments have raised new questions about some of our common assumptions in hydraulic fracturing. These include the pattern of fracture propagation, pressure distribution inside the fracture, fracture closure and re-opening, in-situ stress measurement, etc.