Experimental study on propagation of hydraulic fracture in volcanic rocks using industrial CT technology

Abstract To understand the planar and three-dimensional geometry of hydraulic fractures in volcanic rocks, a hydraulic fracturing simulation experiment was conducted using the large-scale triaxial hydraulic fracturing test system in laboratory. The industrial CT technology was employed to scan the volcanic rock sample before and after fracturing experiment to observe hydraulic fractures. The 3D tomograms and visualized images were obtained from the original CT images. The original CT images indicate that the hydraulic fractures in volcanic rock cross natural pores and/or microcracks, but the fractures don't propagate in the area where natural pores are abundant. The fractures propagate along the maximum horizontal stress direction. The 3D tomograms and visualized images show that it is difficult to create hydraulic fractures in volcanic rocks due to a high fracture pressure gradient. The ability of fracturing in volcanic rock was analyzed, which indicates that the rock mechanical properties near the borehole are the key factors controlling the initiation and propagation of hydraulic fractures. With relatively high Young's modulus and compressive strength, volcanic rocks are generally low in fracturing ability. The key in hydraulic fracturing of volcanic rocks is to bring down the fracture pressure gradient.