Issue 47

Z.-Y. Han et alii, Frattura ed Integrità Strutturale, 47 (2019) 74-81; DOI: 10.3221/IGF-ESIS.47.07 75 tight sand layer, and achieved good results without experimental verification. Mcdaniel et al. [7, 8] conducted a liquid nitrogen immersion test on coal rock in the laboratory. The results showed that low temperature fracturing might increase the production of coalbed methane and conduct field tests. Because of the existed natural cracks and obvious layering, the damage mechanism of shale after low-temperature impact is more complicated. In fully understand the failure mechanism of shale after low-temperature impact, many experts and scholars have conducted a lot of basic experimental research [9- 11]. Cai et al. [12, 13] conducted an experimental study on the damage of shale pore structure under the action of liquid nitrogen. In recent years, to meet the needs of low-temperature fracturing technology, many scholars began to analyze the physical and mechanical properties of shale after liquid nitrogen treatment through indoor experiments, as well as considered the influence of shale bedding orientation [14-16]. In the physical simulation of hydraulic fracturing, the researchers [17, 18] used artificial cement samples to simulate low-temperature cracking of shale, but there were also some differences, such as randomly distributed natural cracks and bedding, between artificial and natural samples. In this paper, the crack propagation characteristics of shale after low-temperature fracturing are analyzed by the hydraulic fracturing experiment of natural shale samples, and the factors affecting crack propagation are discussed. E XPERIMENTAL PROGRAM he sample was taken from the outcrop shale of Longmaxi Formation in a certain area. The outcrop shale is a natural extension of the Silurian Longmaxi Formation shale reservoir in the southeast of Guizhou, China. The shale mineral composition analysis was carried out by an X-ray diffractometer, and the results are shown in Fig. 1. The main component of the experimental sample is quartz, accounting for 52%, followed by clay minerals, accounting for 17%. Among the clay minerals, Illite is the main component, accounting for 60%. Figure 1: Shale mineral composition and its content. Shale tensile strength test Hydraulic fracturing is one of the prerequisites for the development of shale gas. The principle of shale hydraulic fracturing is to pump the fluid into the well by the ground high-pressure pump set with a displacement that greatly exceeds the absorption capacity of the formation, and then cause the high pressure near the bottom of the well. When the pressure is greater than the in-situ stress near the borehole wall and the tensile strength of the shale, cracks forms in the formation near the bottom of the well. The tensile strength of the shale characterizes the ultimate bearing capacity of the shale during tensile failure. The tensile strengths of the shale before and after freezing were determined before the hydraulic fracturing experiment. The results are listed in Fig. 2. It can be seen that the tensile strength of shale decreases by about 20% before and after freezing of liquid nitrogen, and the reduction of tensile strength is more conducive to the formation of cracks during fracturing. Shale fracturing experiment assisted by liquid nitrogen A true triaxial experimental system was used for liquid nitrogen fracturing test. It mainly includes liquid nitrogen self- pressurizing system, intermediate container, and true triaxial experimental equipment, as shown in Fig. 3. The experimental sample was cut from a cuboid shale rock to a dimension of 100mm  100mm  100mm, with the surfaces smoothed by grinding. The length of the open hole of simulated wellbore was 30 mm, as shown in Fig. 4.