Prediction of Coal Body Structure of Deep Coal Reservoirs Using Logging Curves: Principal Component Analysis and Evaluation of Factors Influencing Coal Body Structure Distribution

Deep coalbed methane (CBM) extraction shows that coal body structure (CBS) influences the original pore and permeability conditions of a coal reservoir and that it has a significant effect on CBM production. CBS prediction by using logging curves has become an important aspect in CBM reservoir engineering. In this study, we identified vertical development of CBS in coal seam 8 of the Benxi Formation in 23 wells in the Ordos Basin based on core observation. Moreover, logging curves of all coal seams were collected to study the correlation between different logging parameters and CBS, and the logging curve parameters were then optimized. Principal component analysis was used to make a comprehensive evaluation of CBS. Subsequently, factors such as structural curvature, coal seam depth, thickness and sedimentary environment were explored to investigate the main controlling factors of CBS in the Benxi Formation of Mizhi area. The results were as follows. (1) The CBS of the target coal reservoir includes primary structured coal, fragmented structured coal and mylonite coal. As the damage degree of coal structure became stronger, the volume change of micro-pores was significantly stronger than that of the meso-pore volume, and the methane adsorption capacity gradually enhanced, which is more conducive to methane adsorption. (2) Natural potential, natural gamma, acoustic time difference, compensated neutron and density logging curves of different coal structures were quite different. The identification of CBS by using dual logging parameters had poor performance. The accuracy of coal structure recognition based on principal component analysis was better. (3) The identification results of logging curves indicate that mylonite coal was widely developed in the northeast of the Mizhi area, which is related to the larger structural curvature, resulting in an increased degree of coal seam deformation. (4) The development of mylonite coal in the central and eastern regions is due to the widespread development of intertidal gray flat facies in the area. The top and bottom floors are mainly composed of limestone and mudstone, and there are two layers of interbedded gangue in the thick coal seams. Therefore, the strong heterogeneity inside the coal seams and the similar mechanical properties of the top and bottom rocks lead to the development of mylonite coal in this area.