Abstract
Liquid nitrogen is a type of super-cryogenic fluid, which can cause the reservoir temperature to decrease significantly and thereby induce formation rock damage and cracking when it is injected into the wellbore as fracturing fluid. An experimental set-up was designed to monitor the acoustic emission signals of coal during its contact with cryogenic liquid nitrogen. Ultrasonic and tensile strength tests were then performed to investigate the effect of liquid nitrogen cooling on coal cracking and the changes in mechanical properties thereof. The results showed that acoustic emission phenomena occurred immediately as the coal sample came into contact with liquid nitrogen. This indicated that evident damage and cracking were induced by liquid nitrogen cooling. During liquid nitrogen injection, the ring-down count rate was high, and the cumulative ring-down counts also increased rapidly. Both the ring-down count rate and the cumulative ring-down counts during liquid nitrogen injection were much greater than those in the post-injection period. Liquid nitrogen cooling caused the micro-fissures inside the coal to expand, leading to a decrease in wave velocity and the deterioration in mechanical strength. The wave velocity, which was measured as soon as the sample was removed from the liquid nitrogen (i.e. the wave velocity was recorded in the cooling state), decreased by 14.46% on average. As the cryogenic samples recovered to room temperature, this value increased to 18.69%. In tensile strength tests, the tensile strengths of samples in cooling and cool-treated states were (on average) 17.39 and 31.43% less than those in initial state. These indicated that both during the cooling and heating processes, damage and cracking were generated within these coal samples, resulting in the acoustic emission phenomenon as well as the decrease in wave velocity and tensile strength.
Highlights
Natural gas is a significant fossil energy, which has played an important role in national industry and economy
We mainly focused on the cracking effect of coal due to liquid nitrogen cooling and the influence of cryogenic cracking on coal mechanical properties
The effect of liquid nitrogen cooling on coal cracking and mechanical properties was researched by the experimental methods, including acoustic emission (AE) tests, ultrasonic tests, and tensile strength tests
Summary
Natural gas is a significant fossil energy, which has played an important role in national industry and economy. As Rui et al (2017) stated that natural gas supplies have accounted for about one-fourth of all energy around the word. To further improve the natural gas supplies, the unconventional resources, such as coalbed methane (CBM), have attracted increasing attention (Vedachalam et al, 2015). The CBM is mainly stored in adsorption form within the coal matrix and the gas production is strongly dominated by the natural fractures (e.g. cleats). It is because that these natural fractures can provide high permeability pathways for the gas flowing from coal matrix to wellbore (Clarkson and Salmachi, 2017). Salmachi et al (2016) pointed out that the productivity of coalbed can be predicted by interpreting the cleat systems and the open natural fractures based on borehole image logs. Because of strong adsorption characteristics of coal, the simulation methods, such as hydraulic fracturing, have to be implemented to maintain CBM production at an economic rate (Aguilera et al, 2014; Salmachi and Yarmohammadtooski, 2015)
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