Experimental Investigation on the Fractures Induced by Hydraulic Fracturing Using Freshwater and Supercritical CO2 in Shale Under Uniaxial Stress

Abstract

The process of hydraulic fracturing makes use of fluid injection to conduct the fracturing treatment of reservoirs for the exploitation of unconventional resources. As is well known, hydraulic fracturing using water-based fracturing fluid is employed regularly in the commercial development of shale gas. The potential advantages of supercritical CO2 (SC-CO2) make it an ideal candidate of non-aqueous fluid to avoid problems associated with the hydraulic fracturing operations using water. The fracturing fluid plays such an important role in the fracturing process that it can directly affect the fracturing results of shale reservoir. Hydraulic fracturing experiments using freshwater and SC-CO2, under uniaxial stress state, were performed on the shale cores for study on the fracture propagation and micro-crack characteristics. Stereomicroscope was used for the measurement of the fracture to evaluate the fracture complexity. Scanning electron microscope (SEM) was used to reveal the micro-cracks densely developed along the two sides of the fracture surface. The investigation results show that the fractures and micro-cracks induced by hydraulic fracturing constitute the transportation pathways for shale gas, which can link the organic pores to the macro-fractures. Investigation of the micro-cracks was carried out to compare the characteristics induced by freshwater and SC-CO2. The formation mechanism of the micro-crack was analyzed based on the SEM micrographs. Small slippage occurred along the micro-crack surface due to the deviator stress, which could bring about the sustained width of micro-crack for gas transportation. Analysis of the fracture propagation and micro-crack characteristics of hydraulic fracturing shows that SC-CO2 can create the fractures and micro-cracks with more complexity.