Field experiences suggest significant interaction between natural fractures and hydraulic fractures in unconventional reservoir and some tight gas sandstone. Microseismic monitoring is a widely used methodology to understand the real-time evolution of fracture geometry during hydraulic fracturing, which is essential for evaluating the SRV under certain treatment, especially in unconventional reservoir. Howerver, the correlation between the real fracture geometry and microseismic monitoring results is still open to discuss. Therefore, understanding the relationships between the induced fractures and acoustic emission in different laboratory conditions is of high importance for studying the mechanism of hydraulic fracturing and ultimately optimizing fracture complexity.

A large polyaxial testing system was built for investigating the factors of inducing complex fractures in sandstones, shale and coal for hydraulic fracturing treatment. This paper presents the experimental results of a study conducted to understand the mechanism of fracture initiation and propagation based on acoustic emission monitoring. Large-scale test was conducted in dry sandstone outcrop of 762×762×914mm to simulate the hydraulic fracturing. True triaxial stresses were applied to the specimen. The acoustic emission events associated with fracture propagation or failure process during injection were monitored to provide further data to characterize the fracture. The hydraulic fractures and oil penetration surface were observed after injection.

Results show that the acoustic emission monitoring is in good accordance with the split block surface dyed red. AE localizations clearly demonstrate fracture initiation and propagation dynamics in the sample which can be used to control the fracture. The AE rates increased with opening hydraulically induced fracture and its closure. The real-time fracture geometry can be calculated by integrating the borehole pressure, AE localizations, injection rate, fluid leakage and rock mechanical properties. We are currently investigating various ways for building the fracture propagation model. The work presented here a new hydraulic fracturing testing system integrated with geophysical monitoring and processing technologies that will improve laboratory study of hydraulic fracturing mechanism and provide a trial on new ideas and technology

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