Hydraulic Fracturing of Deep Wells

An experimental investigation has been conducted in open-hole instability due to either too low or too high well pressures under conditions simulating depths of 0 to 30,000 ft. The significant results were (1) the danger of well collapse due to shear crushing is very realistic in deep wells if well pressure is zero or very low. The Mohr failure envelope appears to be a conservative estimate of the differential stresses needed to induce cave-in at the well walls. (2) Through well pressurization a critical pressure is reached at which a different type of well failure is brought about. It is a vertical tensile rupture that initiates at the well and emanates into the formation. This so called hydraulic fracture appears unaffected by the depth of the well. (3) The critical pressures required to bring about hydraulic fractures in deep wells are considerably lower than those expected, and follow the values predicted by a failure criterion that incorporates the changes in the stress field due to well fluid penetration. These results are obtained under high field stresses even in rocks of extremely low permeability using very high viscosity fluids.