Experimental Study of Hydraulic Fracturing in Sand as a Function of Stress and Fluid Rheology

Abstract

Abstract We present an experimental and numerical study of the mechanism of hydraulic fracturing in unconsolidated rocks. We performed hydraulic fracturing experiments on loose sand samples under various confining pressures. We have developed a large biaxial set up in which cylindrical sand samples of dimensions 0.4×0.51 m can be tested under confining stress up to 40 MPa. We prepare a sand sample by pouring the sand into the sample vessel, which is filled with water to obtain a fully saturated sample. There is a borehole (injection system) with an open section in the middle of the vessel. The open section is closed during the sample preparation. After compacting the sample and applying the confining pressures, the injection system is opened and fluid is injected. For modeling of the injection process we used a distinct element model in which we incorporated fluid flow that is coupled to the flow of the grains. The injection tests resulted in either infiltration or generating fractures. Injecting viscous Newtonian fluid and cross-linked gel yielded in most cases only infiltration and wellbore expansion. Clear fractures were induced when injecting cross-linked gel with quartz flour and bentonite mud. Tests were run at a stress level between 0.2 and 20 MPa. At high confining stress we observed a strong tendency for shear fracturing during fracture initiation. The fracture initiation pressure was about 2.5–3.5 times the confining pressure. The stress level had a strong influence on the ratio of initiation pressure to confining stress. Also, it appeared that much higher pressures were obtained on dense sand samples, compared with less compacted sand. Introduction In competent rock hydraulic fracture initiation is dominated by tensile failure at the borehole wall. Fracturing sand may induce different modes of failure because the natural tendency of rock to fail in tension will be suppressed in favor of shear failure. Because shear failure is a function of stress level, the confining stress will be an important experimental parameter for investigating hydraulic fracturing of sand. Moreover, we can expect that fluid rheology plays an important role because sand is more permeable than competent rock. In order to determine the influence of rheology we used different fluids: a Newtonian fluid (Viscasil with a viscosity of 500 Pas), Bentonite slurry (that represents drilling mud or slurried waste) and cross-linked gel. The pressure of fracture initiation is of interest in situations where fracturing is undesired, such as drilling or tunneling. For stimulation, it is important to predict the fracture pressure to ensure that a fracture can be propagated and also for predicting the fracture width based on pressure measurements. Our objective is to establish the fracture pressure as a function of effective stress and to determine the fracture geometry for different fluids and stress levels. Experiments Figure 1 shows the set-up of the sample in the biaxial cell. We use fully saturated samples and control the axial pressure on the top block and the radial pressure on the sleeve. We measure also the movement of the top block with an LVDT. Weighing the pore water that flows out of the sample monitors the volume change of the sample. The injection system consists of a long tube that is connected to the injection line. The tube goes through the bottom plate and this tube is free to move when the sample compacts. We increase the stress to the lowest level that ensures a stable sample (about 2 MPa). Then we open the injection system by pulling out the injection tube over a distance of 80 mm. During the opening of the borehole, we keep the injection pressure constant, by adjusting the flow rate. After opening the borehole, we increase the stress isotropically to the desired confining stress (with a maximum of 40 MPa); then we increase the axial stress to 1.5 times the radial stress. After stabilizing the external stresses on the sample, we perform one or more injection cycles.