A prediction model of coal structure based on logging parameters in Liupanshui Coalfield, Guizhou, China

The distribution of coal structure plays an important role in controlling the optimization of favorable areas for coal mining and coalbed methane development and the evaluation of reservoirs. For further clarifying coal reservoir physical property and its distribution pattern, based on the existing research of many scholars, according to the degree of structural deformation, the coal seams of Liupanshui Coalfield in Guizhou are divided into three types, which are type I primary structure coal (primary structure or normal structure coal), type II transitional coal (initial cataclastic coal and transitional structure coal), and type III structural coal (porphyroclast coal, cataclastic coal, and mylongite coal). The logging response characteristics of different coal structures were analyzed and summarized. On the basis of qualitative division of coal structure by logging curves, four parameters that have better correlation with coal structure were optimized: deep lateral resistivity (LLD), acoustic time (AC), density (DEN), and natural gamma ray (GR). The T value is defined as the coal structure index, which indicates the degree of broken coal structure. The larger the T value is, the more broken the coal is. Based on the existing data of multiple coalbed methane exploration parameter wells in the study area, it is found that there is a linear relationship with the above four logging parameters in the logarithmic domain through Cartesian coordinate projection. Accordingly, the multiple linear regression equation between the logarithm of coal structure index and the logarithm of four logging parameters is established and the coal structure prediction model equation is obtained. By verifying the physical property parameters of the actual coal reservoir in the study coal field, the quantitative identification of the coal structure by this equation is consistent with the actual cored coal structure; this equation is also with the results of the coal mine underground lithology catalog and with high accuracy. Using methods such as continuous well analysis and ternary phase diagrams in the five coal-bearing blocks of the Liupanshui Coalfield, it can realize the fine division of coal structures and reveal the spatial distribution of different coal structures. Stratum burial depth and tectonic movement are related factors that affect the degree of coal seam deformation, and the regularity is obvious.