A Visualization Study of Fracture Propagation in Tight Sandstone Under Triaxial Loading

ABSTRACT: Hydraulic fracturing is the key technique in the development of unconventional resources. Many optical experimental studies have been made to qualitatively study the hydraulic fracturing process. However, due to the time of fracture propagation being extremely short (ms), real-time quantitative description of the fracturing process is challenging. To clarify the process of fracture propagation, a novel triaxial optical hydraulic fracturing experimental apparatus is developed which mainly includes an ISCO displacement pump, triaxial servo control devices, high-intensity illumination devices, high-speed camera, and data acquisition systems. The fracturing process could be captured visually in real-time by using a high-resolution camera. This study also proposes an accurate and effective analysis method, Digital Image Correlation (DIC), for characterizing fracture and measuring the strain field. The results show that the pressure-time curve of hydraulic fracturing can be divided into three stages: pressure rise stage, the crack initiation and propagation stage, and the pressure plateau stage; the initiation pressures of water and CO2 fracturing in tight sandstone are 10.1 MPa and 5.2 MPa respectively; hydraulic fractures active the natural fractures, resulting in a complex fracture network structure; hydraulic fractures will turn and gradually become vertical to the minimum principal stress during the extension process. The results of this paper can provide guidance for the field application of hydraulic fracturing in tight sandstone resources. 1. INTRODUCTION With the rapid development of the economy, the energy demand has increased rapidly, and unconventional oil and gas resources such as tight gas, coalbed methane, and shale gas have become an important guarantee for the national energy strategy1. Hydraulic fracturing technology is an important technology for the exploitation of unconventional oil and gas resources2, among which the geometry of hydraulic fractures is the key to evaluating hydraulic fracturing operations, still needs further research3-7.