Abstract
Due to the influences of various factors, such as temperature, stress, and composition, the research regarding rock permeability has been complicated. This study examined the variation laws of sandstone specimens under changing rates of confining and pore pressures after high-temperature heat treatments. The results showed that the free water in the rock volatilized during the low-temperature heat treatments in the range of 100°C to 300°C, with the increase of the heat-treatment temperature above 500°C; the crystal water in the rock is gradually separated out; and the particles in the samples had undergone phase transformations resulting in increased permeability. According to ultrasonic wave velocity test results, the internal cracks of the samples expanded with the increases in the heat-treatment temperatures. In addition, the high-temperature heat treatments were found to improve the accuracy of the direction of the Earth’s stress when using circumferential wave velocity anisotropy methods. Under the influence of slippage effects, as the pore pressure increased, the measured permeability of the samples decreased and the slippage effect occurs in the rock samples with the permeability of 10-3 μm2~10-6 μm2. The experimental results showed that the contribution rate of the slippage effect decreases with the increase of the heat-treatment temperature of the specimen, and the contribution rates of the sandstone slippage effect were generally higher than 5%. Therefore, the impact effects on the permeability of sandstone slippage should be considered in practical engineering processes.
Highlights
In recent years, the characteristics of rock permeability have become hot issues in rock physical and mechanical property research
Combined with the theory of slippage effects, the change rule of rock permeability under low pore pressure was analyzed, and the main conclusions were as follows: (1) During the process of the heat-treatment temperatures rising from 100°C to 700°C, the water volatilization and structural changes in the rock samples had led to increases in the permeability levels
In accordance with the corresponding relationships between the pore pressure and permeability, macroporous sandstone with an order of permeability of 10-3 μm2 will experience the phenomenon in which the measured permeability results will deviate from the absolute permeability
Summary
The characteristics of rock permeability have become hot issues in rock physical and mechanical property research. Due to many factors, such as temperature, stress states, and pore pressure changes, the permeability levels measured by the gas and the permeability levels measured by the liquid are often inconsistent. These differences become increasingly significant as the rock permeability decreases [1]. In order to more clearly understand the change law of permeability characteristics of sandstone under complex conditions, this paper took multistage confining pressure and variable pore pressure experiments of sandstone after heat treatment on the background of underground coal gasification engineering. The research results provide some theoretical guidance for seepage issues which may be encountered during future practical engineering projects
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