Abstract
In the process of shale gas development, different drilling, hydro-fracturing, and pumping speeds will produce different productivity effects. This reflects the effect of the loading rate of rock mechanics. Therefore, it is of great scientific significance to study the deformation and failure characteristics of shale under different loading rates. In order to reveal the loading rate effect of the Longmaxi shale, a series of laboratory experiments were carried out, including scanning electron microscope, XRD, and uniaxial compression tests at different loading rates. The results show that 1) the peak strength and elastic modulus of shale increase with the increase in the loading rate, but Poisson’s ratio has no obvious rule with the increase in the loading rate. In contrast, the loading rate causes the peak strength to vary by about 20%, which is larger than that of the elastic modulus. 2) The loading rate has a significant influence on shale failure. A higher loading rate will lead to severe damage but with simple cracks, whereas a lower loading rate will lead to complex damage of cracks. With the decrease in the strain rate, the length of the failure crack increases greatly. Therefore, a lower strain rate is helpful to form more broken fragments in the shale matrix. 3) By analyzing the relationship between elastic strain energy/dissipated energy and the loading rate, it is found that the elastic strain rate has a preliminary increasing and then a decreasing trend with the loading rate, but the dissipated energy has a decreasing trend with the loading rate. A higher loading rate is helpful to increase the brittleness of the shale, but a lower loading rate is beneficial to forming more cracks and a greater degree of fracture in the shale matrix. The effect of the loading rate on the mechanical properties and fracture properties of the shale is discussed. It is suggested that the lower hydro-fracturing rate is useful for generating more complex crack networks in the shale reservoirs.
Highlights
Natural gas is a type of clean and quality energy (Qiao et al, 2020a), and the consumption of natural gas in China has continuously increased in the past 2 decades (Zhang X et al, 2021; Qiao et al, 2020b)
Different loading rate is an important index to reflect the response of elastic modulus, peak strength, and Poisson’s ratio of the rock samples
It can be seen that the response characteristics of loading rate to peak strength, elastic modulus, CONCLUSION
Summary
Natural gas is a type of clean and quality energy (Qiao et al, 2020a), and the consumption of natural gas in China has continuously increased in the past 2 decades (Zhang X et al, 2021; Qiao et al, 2020b). The study on the influence of the loading rate on mechanical properties of shale is of great guiding significance for shale gas development. Commercial development of shale gas in China is limited to marine strata in the Sichuan Basin and its periphery; the exploration and development of marine–terrestrial transitional shale have not made great progress and are still in the early evaluation stage (Liu et al, 2014) This fully illustrates the influence and restriction of the sedimentary environment on mining. Different reservoir characteristics, corresponding brittleness characteristics, bedding and microfracture development characteristics, and hydraulic characteristics will have great differences (Xiao et al, 2021), which will have a significant impact on the fracturing effect of FIGURE 4 | Testing device of MTS 815 (1 shale sample; 2 pressure head; 3 circumferential strain gauge). Where BI is the brittleness index, and BMC refers to the brittle mineral contents of the shale cores
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