Complex Fracture Propagation Behavior in Volcanic Rocks: An Experimental Study

Abstract

Hydraulic fracturing is a key technology for the development of tight reservoirs. Affected by factors such as volcanic activity and tectonic movements, volcanic rock reservoirs have characteristics such as diverse lithofacies, complex mineral compositions, and well-developed natural fractures. The effectiveness of reservoir stimulation largely depends on its ability to connect with natural fractures to form a complex fracture network. In this study, in order to clarify the fracture propagation behavior in volcanic rock reservoirs, experiment was performed on specimens prepared from the outcrops of Emeishan based on a true triaxial fracturing simulation system. The influences of natural fracture, fluid viscosity, and fracturing fluid type were mainly analyzed through post-fracturing specimen splitting, pressure curve analysis, and acoustic emission test results. The experimental results show that volcanic reservoirs have strong heterogeneity, natural fractures are developed, and multiple natural fractures can be connected after fracturing to form a complex fracture network. The fracture morphology is dominated by the main fracture, secondary vertical fracture, and secondary horizontal fracture, and the horizontal fracture has a certain control over the fracture height. The natural fracture shows different failure modes; under a high stress difference, the failure mode of the matrix type with mainly horizontal fracture shows a shear-dilation failure mode. The failure mode of the natural fracture well-developed type shows a shear slip failure mode. Lower-viscosity slickwater is more likely to leak through natural fractures, increasing the likelihood of the formation of complex fractures. The use of supercritical carbon dioxide can increase fracture complexity, and this fracturing fluid is more likely to form a complex fracture network due to its physical properties that allow it to efficiently penetrate microfractures in the reservoir. Based on a true triaxial fracturing simulation experiment, this paper simulates the field parameters and analyzes the fracture propagation behavior for a volcanic reservoir. The research results provide a theoretical basis for optimizing fracturing construction parameters in volcanic rock reservoirs.