A real-time monitoring system was established for 10-channel acoustic emission (AE) in hydraulic fracturing in the laboratory, with a hydraulic fracturing test conducted on two different sizes of natural outcrop cores under unconfined pressure and confining pressure respectively. Then, a discussion was carried out on the microscopic response in rock to the formation and growth of hydraulic fracture under confining pressure and unconfined pressure. The study of AE parameters shows that the change of RA (Rise Angle) in hydraulic fracturing process is always opposite to that of AF (Average Frequency). Also, AE parameters have higher dispersion under unconfined pressure. With breakdown pressure as a dividing point, as pumping pressure changes under unconfined pressure, the dominant internal fracture mechanism of rock will turn from tensile failure to shear failure. But under confining pressure, the transformation of the dominance is reversed, namely from shear failure to tensile failure. This phenomenon indicates that despite the same hydraulic fractures in macro scale under both confining pressure and unconfined pressure, there may be a large difference in microstructure between the hydraulic fractures formed under both pressure conditions respectively.

1. Introduction

Hydraulic fracturing technology has become the primary means of reservoir stimulation since its first application in North America in 1949 [1]. At present, as the conventional oil and gas resources are increasingly exhausted, and the yield of unconventional oil and gas resources rises gradually, hydraulic fracturing has grown into a more critical factor to artificial production's increase [2]. From the perspective of theoretical research, with the constant development of fracture mechanics and its application to rock engineering, scholars have come to realize that the nonlinear areas have an important influence on the mechanical behavior of a hydraulic fracture before and after its formation, while the occurrence of micro cracks is an important sign of nonlinear rock failure [3].

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