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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. [ABSTRACT FROM AUTHOR]

The process of hydraulic fracturing makes use of a liquid to fracture reservoir rocks for the exploitation of unconventional resources. Hence, it is vital to understand the processes that produce the fracture networks that occur during hydraulic fracturing. A shale reservoir is one of the largest unconventional resources and it displays obvious anisotropic characteristics due to its inherent sedimentary structures. The viscosity and flow ability of the fracturing fluid plays an important role in this process. We conducted a series of hydraulic fracturing tests on shale cores (from the southern Sichuan Basin) using freshwater and supercritical CO2 (SCO2) as fracturing fluids to investigate the different modes of fracture propagation. The pump pressure curves that we obtained during the fracturing experiment show how the shale responded to each of the fracturing fluids. We examined the influence of the anisotropic characteristics on the propagation of hydraulic fractures by conducting a series of hydraulic fracturing experiments on the shale cores using different bedding orientations. The bedding orientation of the shale had a profound influence on the fracture propagation when using either freshwater or a SCO2 fluid. The breakdown pressure of the shale core was affected not only by the bedding orientation but also by the fracturing fluid. A macroscopic observation of the fractures revealed different fracture geometries and propagation patterns. The results demonstrated that the anisotropic structures and the fracturing fluids could influence the path of the hydraulic fracture. [ABSTRACT FROM AUTHOR]

The process of hydraulic fracturing makes use of a liquid to fracture reservoir rocks for the exploitation of unconventional resources. Hence, it is vital to understand the processes that produce the fracture networks that occur during hydraulic fracturing. A shale reservoir is one of the largest unconventional resources and it displays obvious anisotropic characteristics due to its inherent sedimentary structures. The viscosity and flow ability of the fracturing fluid plays an important role in this process. We conducted a series of hydraulic fracturing tests on shale cores (from the southern Sichuan Basin) using freshwater and supercritical CO2 (SCO2) as fracturing fluids to investigate the different modes of fracture propagation. The pump pressure curves that we obtained during the fracturing experiment show how the shale responded to each of the fracturing fluids. We examined the influence of the anisotropic characteristics on the propagation of hydraulic fractures by conducting a series of hydraulic fracturing experiments on the shale cores using different bedding orientations. The bedding orientation of the shale had a profound influence on the fracture propagation when using either freshwater or a SCO2 fluid. The breakdown pressure of the shale core was affected not only by the bedding orientation but also by the fracturing fluid. A macroscopic observation of the fractures revealed different fracture geometries and propagation patterns. The results demonstrated that the anisotropic structures and the fracturing fluids could influence the path of the hydraulic fracture. [ABSTRACT FROM AUTHOR]