Characteristics of apparent resistivity of coals with different particle sizes

The theory and technology of stress released CBM development is a new theory and technology for CBM development of tectonically deformed coals. The research on the apparent resistivity of tectonically deformed coal can provide technical support for the identification of tectonically deformed coal for the stress release applied extraction of CBM. In this paper, the apparent resistivity and stress-strain monitoring experiments of tectonically deformed coals with different particle sizes were carried out under uniaxial loading, and the change law and response mechanism of the apparent resistivity in the stress-strain process were discussed. The results show that when the axial load is greater than the uniaxial compressive strength, the apparent resistivity of tectonically deformed coal decreases negatively exponentially with the increase of the axial load. When the axial load ≥10 MPa, the sensitivity of the apparent resistivity to the load decreases. With the particle size decreasing, the apparent resistivity of tectonically deformed coal decreases. Under the same axial load, the smaller the particle size is, the more sensitive the apparent resistivity of tectonically deformed coal is to the axial load. Affected by the accumulation mode and cementation type of coal particles, the structural coal of 5~200 mesh（4.000~0.075 mm） is special, with small apparent resistivity and relatively sensitive to load. The changes of apparent resistivity of tectonically deformed coal with axial load can be divided into four stages. In the compaction stage, the apparent resistivity decline of tectonically deformed coal depends on the accumulation mode and cementation type of coal particles. In elastic stage, particle size determines the apparent resistivity decline of tectonically deformed coal. The larger the grain size is, the better the original macropores and microcracks of structural coal remain, the stronger the compressibility is, and the higher the apparent resistivity reduction is. In the dilatation stage, radial deformation and micro-fracturing lead to the increase of apparent resistivity of tectonically deformed coal, while the mechanical strength of tectonically deformed coal determines the change characteristics of apparent resistivity. With the decrease of particle size, the cementation strength and uniaxial compressive strength of tectonically deformed coal increase, while the increase of apparent resistivity decreases. In the continuous elastic stage, crushing and compaction make the particle size distribution of tectonically deformed coal more uniform, and the stacking more compact, which enhances the electronic conductivity and decreases the apparent resistivity.