Coal Permeability Variation during the Heating Process considering Thermal Expansion and Desorption Shrinkage

Abstract

In order to explore the influence of coal deformation caused by temperature and desorption on seepage characteristics in the process of heat injection mining of coalbed methane, the permeability test, thermal expansion, and constant temperature adsorption desorption of coal samples under different temperature and stress states were carried out using the high temperature multifunctional triaxial test system, and the influence of thermal expansion and desorption deformation effect on coal permeability in the process of temperature increase is studied. The results show that (1) with the increase of temperature, the sensitivity of coal thermal expansion deformation to temperature decreases gradually. The thermal expansion deformation makes the coal matrix expand, and the seepage channel is squeezed and the permeability decreases. (2) The effect of thermal expansion deformation is related to the porosity of coal. When the porosity of coal is high, the thermal expansion deformation reduces the permeability; on the contrary, the inward expansion of thermal expansion deformation is limited, and the effect on permeability is weakened. (3) The desorption of coal cause matrix shrinkage. The higher the desorption amount, the more obvious the shrinkage and the higher the permeability. Increasing temperature promotes desorption deformation of coal and increases permeability. (4) In the process of increasing temperature, the change of coal permeability is affected by thermal expansion deformation and desorption deformation. With the increase of temperature, when the influence of thermal expansion deformation on coal permeability is dominant, the permeability decreases gradually, and when desorption deformation is dominant on coal permeability, the permeability increases gradually. (5) With the increase of axial pressure, confining pressure, and pore pressure, the decrease of coal porosity is smaller. When the temperature increases, the temperature corresponding to the minimum permeability point is smaller. The research conclusion provides a basis for the technology of heat injection mining coalbed methane.

Thermal Expansion and Adsorption Characteristics of Coal Based on Temperature Change and Its Influence on the Slippage Effect

“…Combined with Figure , after 90 °C, the intercept between the fitting line of permeability and average pore pressure and the ordinate is negative. This is because, after 90 °C, the elastic modulus enhances rapidly . The lower the elastic modulus is, the greater the deformation will be induced by the variety of effective stress with the pore pressure enhancement.…”

“…High-temperature multifunctional triaxial test system: (a) diagram of the experimental device; (b) schematic of the high-temperature triaxial percolation meter.…”

“…The adsorption and desorption deformation caused by the variety of pore pressure and the deformation caused by the variety of effective stress will lead to the change of coal cracks. 18 With the pore pressure increase, the coal adsorbs nitrogen, as well as the matrix expands which makes decrease of cracks and increases the slippage factor. As the effective stress enhance, the crack becomes larger, and the slippage factor decreases gradually.…”

Experimental research on adsorption characteristics of N2, CH4, and CO2 in coal under different temperatures and gas pressures

“…As the temperature rose, the elastic modulus of coal decreased, 53 and due to the constraints of external stress (axial pressure and confining pressure), the coal was squeezed and shrunk. As a result of the compression effect of external stress on coal, the thermal expansion of coal was limited, and the inward expansion is more obvious 54 …”

“…As a result of the compression effect of external stress on coal, the thermal expansion of coal was limited, and the inward expansion is more obvious. 54 During the increase in gas pressure, the number of gas molecules per unit volume increased, and the compression and expansion of free gas on coal were enhanced. In the initial stage, the adsorption-induced deformation was slight and the effect of free gas was noticeable.…”