Petroleum Reservoir Evaluation and Development ›› 2022, Vol. 12 ›› Issue (4): 572-579.doi: 10.13809/j.cnki.cn32-1825/te.2022.04.004

• Methodological and Theory • Previous Articles     Next Articles

Experimental study on variation of apparent resistivity in CH4-coal adsorption/desorption process

SHI Liyan1(),LI Weibo1,KANG Qinqin2,LI Fei1,QI Jiaxin3   

  1. 1. Shaanxi Geological Science and Technology Center, Xi’an, Shaanxi 710043, China
    2. Geoanalysis Center, Shaanxi Geological Survey, Xi’an, Shaanxi 710043, China
    3. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China
  • Received:2022-04-12 Online:2022-08-26 Published:2022-09-02

Abstract:

In order to reveal the change law of apparent resistivity in the process of CH4-coal adsorption/desorption of different macroscopic coal petrology compositions, the No.4 coal seam of the Jurassic Yan'an Formation in Dafosi coal mine is taken as the experimental object. By the proximate analysis and apparent resistivity measurement experiments of different coal samples (the durain, vitrain and mixed samples) with different water content (air dry basis sample, natural water absorption sample and samples dried at 378.15 K) of coal, the relationship between the apparent resistivity, methane adsorption and pressures is studied. The experimental results found that when compared to the vitrain, the durain has higher fixed carbon content, but lower ash content and moisture. The initial resistivity of the samples dried at 378.15 K of the durain is the largest. The influence of the adsorption of methane on the resistivity of the coal body is manifested in that with the adsorption of methane, the apparent resistivity of coal is significantly reduced. During the pressure increasing process that is the process of adsorption, the coal resistivity has a quadratic function relationship with the pressure and the amount of adsorption. The reason is that methane adsorption heat release, coal body expansion, and the spread of water on the inner wall of the pore throat causes the apparent resistivity to decrease. Due to methane adsorption, gas-water replacement occurs on the surface of the coal matrix, free water dissolves soluble minerals, and the ion concentration increases. In the process of adsorption, the apparent resistivity of coal decreases, but due to the limited soluble minerals and the hydrophobicity of coal, the resistivity of coal tends to be stable in the later stage of adsorption. During the depressurization process, that is the process of desorption, since the shrinkage of the coal matrix after expansion is irreversible (deformation), the apparent resistivity of the coal body cannot be restored to its initial value. Compared to the durain, the pore connectivity of vitrain is poor, but the pore throat curvature is larger, when the number of charged particles in the double electric layer is high and aggregated in piles. This eventually leads to better electrical conductivity of vitrain than durain under the same conditions, i.e., the apparent resistivity of vitrain is smaller than that of durain.

Key words: Dafosi coal mine, apparent resistivity of coal, macroscopic coal petrology composition, adsorption/desorption, water content

CLC Number: 

  • TE122