The Influence of Crack Velocity During Brittle Fracture

Abstract

SUMMARY This paper investigates the effects of different commercially available processes for fracturing consolidated rock structures by applying the concept of Brittle Fracture. Brittle Fracture occurs when there is little or no plastic deformation of the material before the material fails. The available commercial fracturing processes are grouped into three categories, namely (1) Hydraulic fracturing treatment using pump trucks and fluids, (2) High Energy Gas Fracturing (HEGF) treatment utilizing propellent mixtures deflagrated in situ to generate a fast pressure surge of gases and (3) Explosive-based treatment that makes rubble of the rock in the wellbore vicinity. The fracturing processes vary in the duration of the energy-loading process and the maximum pressures achieved during the event (See Fig. 1 and Ref. 1). Figure 1 illustrates the order of magnitude of difference in time and pressures of the different fracturing processes. The "Mechanism of Brittle Fracture of Rock" (Ref. 2) as developed by Dr. Z. T. Bieniawski and widely accepted in the mining industry as a basis for understanding rock failure, is applied to oilfield wellbore and reservoir conditions to evaluate the fracture geometries and penetrations of the different fracturing processes. The concept of Brittle Fracture also is applied to the mine-back and pressure-time records of open-hole and cased-hole tests of the fracturing processes conducted by Sandia National Laboratories at the Nevada Test Site (Ref. 3, 4 and 5). The effect on wellbore and reservoir stimulation is investigated by applying Darcy's modified radial and linear flow equations (Ref. 6) to reservoir flow conditions. By applying the most applicable fracturing process for the wellbore and reservoir condition, one can maximize the evaluation and stimulation efforts of a well completion attempt. Radial flow enhancement is best achieved by fast fracturing processes such as High Energy Gas Fracturing and explosive-based treatments, while linear flow enhancement is best achieved by hydraulic fracturing treatments. Radial flow enhancement treatments may stimulate reservoir productivity by 1 to 3 times, while linear flow enhancement treatments may stimulate reservoir production by 6 to 10 times.