Abstract
Freeze-thaw induced fracturing coal by liquid nitrogen (LN2) injection exerts a significant positive effect on the fracture permeability enhancement of the coal reservoir. To evaluate the different freeze-thaw variables which modify the mechanical properties of treated coals, the effects of freezing time, number of freeze-thaw cycles, and the moisture content of coal were studied using combined uniaxial compression and acoustic emission testing systems. Freezing the samples with LN2 for increasing amounts of time degraded the strength of coal within a certain limit. Comparison to freezing time, freeze-thaw cycling caused much more damage to the coal strength. The third variable studied, freeze-thaw damage resulting from high moisture content, was restricted by the coal’s moisture saturation limit. Based on the experimental results, equations describing the amount of damage caused by each of the different freeze-thaw variables were empirically regressed. Additionally, by using the ultrasonic wave detection method and fractal dimension analyses, how freeze-thaw induced fractures in the coal was quantitatively analyzed. The results also showed that the velocity of ultrasonic waves had a negative correlation with coal permeability, and the freeze-thaw cycles significantly augment the permeability of frozen-thawed coal masses.
Highlights
For fracturing coal using an anhydrous fluid, liquid nitrogen (LN2) is attracting increasing attention[1,2,3]
The strength and deformation results were obtained by measuring the physical properties of frozen-thawed coal with different moisture contents and after coal samples had been subjected to the different freezing time durations and different numbers of freeze-thaw cycles as described in the experimental method section
The uniaxial compression experiments plus the acoustic emissions allowed the development of the induced fractures to be monitored[21]
Summary
For fracturing coal using an anhydrous fluid, liquid nitrogen (LN2) is attracting increasing attention[1,2,3] The feasibility of this fracturing technology has been studied for the exploitation of unconventional oil and gas[4,5,6]. Under the triple effects of LN2’s low-temperature, the vaporization and expansion of LN2, and the volume expansion due to the water-ice phase transition, strong freeze-thaw cycles occurred in the coal and these cycles both decreased the coal’s mechanical strength and produced a fracture network. This provided very favorable reservoir stimulations for extracting CBM
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