Study on the Influence of Liquid Nitrogen Cold Soaking on the Temperature Variations and Seepage Characteristics of Coal Samples with Different Moisture Contents

Abstract

Due to its advantages such as environmental friendliness and remarkable permeability enhancement effect, the technology of liquid nitrogen cold soaking (LNCS) cracking coal has become a hot spot in the research on coal seam permeability enhancement in recent years. The frost heave force generated by water-ice phase transformation and the temperature stress are the main mechanisms of LNCS cracking water-containing coal. This paper focuses on the effect of LNCS on the temperature variations and seepage characteristics of coal. To further this purpose, the temperature measurement test and the permeability test were conducted on coal samples with different moisture contents under LNCS, respectively. In addition, by comparing the computer tomography test results of coal samples before and after LNCS, the internal pore structure changes of coal samples were further analyzed from a three-dimensional perspective. The test results show that the coal sample with a higher moisture content consumes a shorter time to reach internal temperature equilibrium and experiences faster temperature changes. LNCS can enhance coal permeability, and the growth rate of permeability increases exponentially with the increase of moisture content. After the LNCS treatment, the dried coal sample is mainly sprouting new pores on the basis of primary pores; in contrast, for water-containing coal samples, new pores are sprouted while primary pores will penetrate each other spatially to form a fracture network. In the process of LNCS, moisture has a significant effect on the seepage characteristics of coal, so appropriately increasing the moisture content of the coal seam conduces to achieving a better permeability enhancement effect.

Mechanism and Model Analysis of Ultralow-Temperature Fluid Fracturing in Low-Permeability Reservoir: Insights from Liquid Nitrogen Fracturing

“…Wang et al [66] found that, in the process of LNFT, the coal water content was positively correlated with its surface frost heaving force and porosity, and the coal permeability showed stage characteristics. Li et al [67] found that, in the process of LNFT, the coal permeability increased exponentially with the increase in the water content. Dry coal samples mainly germinate new fractures based on primary pores, whereas water-bearing coal samples germinate new fractures, and, at the same time, the primary fractures are spatially connected to each other to form dense fracture networks.…”

Study on the Influencing Factors on the Effectiveness of Liquid Nitrogen Fracturing Technology

“…The larger the initial water content, the greater the frost heave force formed by water freezing and expansion, and the more significant the fracturing effect. Wang et al [32] found that in the process of LNF, the water content of coal was positively correlated with its surface frost heaving force and porosity, and the permeability of coal showed stage characteristics; Li et al [33] found that in the process of LNF, the permeability of coal increases exponentially with the increase of water content. Dry coal samples mainly germinate new fractures on the basis of primary pores, while water-bearing coal samples germinate new fractures.…”

Selection Effect of Liquid Nitrogen Freeze–Thaw Cycles on Full Pore Size Distribution of Different Rank Coals

“…Similar rules have been also found in the liquid N 2 fracturing test, 36,37 which is comparable to the supercritical CO 2 fracturing process. 48 laboratory experiments, the penetration of low-temperature liquid N 2 into coal results in various fractures and damages, which can increase crack length and density. 50 The temperature gradient generated by LNCFT can sharply increase fracture density.…”