Successful development of shale gas in North America triggered great enthusiasm of developing shale gas in China. Experiments for hydraulic fracturing shale Outcrops sample in 762ϫ762ϫ914mm(height) were performed by using large-scale true tri-axial testing system to study what kind of hydraulic fracture will be for South China marine shale and how the formation condition or the human-controllable parameter influence the fracture complexity. Investigations of fracture geometry and fracture complexity are conducted with different natural fracture distribution, different main stress difference, different fracturing fluid viscosity, different injection rate, using methods such as acoustic emission monitoring and cutting the rock sample into pieces. The testing results show the hydraulic fracturing in shale generates more complex fractures than that of in sandstone, where both tensile and shear cracks are observed. Natural fractures, shale bedding and small principal stress differences are in helping to form complex fractures. When a low viscosity fluid is used, complex fractures are easier achieved than that with high viscosity fluid. This work explores a way of conducting hydraulic fracturing large-scale shale test. The testing results provide important basis for numerical simulation of fracture propagation, pre-frac layer or stage choosing and fracturing design optimization for hydraulic fracturing in shale.

In order to determine the effective stress factor of proppants during the multi-stage hydraulic fracturing operation in horizontal wells and select the appropriate proppant type, the analysis of the effective stress characteristics of proppants is carried out. This analysis considers the geomechanics and long-term production characteristics of multi-stage fractured horizontal wells in reservoirs like Xinjiang Mahu shale oil and Southwest China shale gas. The analysis of influencing factors is also carried out. The results show that, during the multi-stage hydraulic fracturing operation in horizontal wells, the stress on proppants is not only related to geological factors such as reservoir closure stress, but also closely related to total injection volume, cluster spacing, liquid type, injection displacement and post fracture management. First, increasing injection intensity, reducing fracture spacing, using low viscosity fracturing fluid, adopting high injection displacement and utilizing reasonable flowback system can effectively supplement reservoir energy, postpone the effective stress peak of proppants, and increase the effective conductivity of proppants. Second, the production performance analysis results of nearly 300 horizontal wells (from Xinjiang oil field, Erdos tight oil reservoir, Sichuan shale gas reservoir, etc.) shows that: the effective stress of proppants in horizontal wells is only 50–60% of that in vertical wells, which results in a different proppant selection criterion in the volume stimulation of horizontal wells and provides the geomechanics basis of replacing ceramsite proppant with quartz sand to reduce cost and increase efficiency. Based on the above conclusions, the field test of replacing ceramsite proppant with quartz sand was carried out. The proportion of quartz sand increased from less than 30% in 2014 to 69% in 2019. Without any impact on production, the annual investment cost was decreased by more than 1 billion yuan, which set a great example for the promoted low-cost development of unconventional oil and gas reservoirs under low oil price background.

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