The Fracability Evaluation of Deep Coal Reservoirs in the Ordos Basin Based on Well Logging and Rock Mechanics Experiments

The Ordos Basin contains abundant deep coalbed methane (CBM) resources, whose efficient development largely depends on the effective implementation of large-scale volumetric fracturing technologies. To comprehensively evaluate the fracability of deep coal reservoirs in this basin, this study focuses on the No. 8 coal seam of the Benxi Formation. Based on rock mechanical experiments and well-logging data, multivariate linear regression models were established to predict Young’s modulus (E) and Poisson’s ratio (μ). The Huang model was applied to determine the three principal in situ stresses of the coal seam. Furthermore, a comprehensive fracability evaluation model was constructed by integrating three key indicators, namely brittleness index (BI), horizontal stress difference (Δσh), and tensile strength (St). The entropy evaluation method was used to determine the weights of these indicators, and the fracability index (F) of deep coal reservoirs was calculated. The results show that the weights of the factors controlling fracability decrease in the following order: tensile strength (0.434), brittleness index (0.332), and horizontal stress difference (0.234). The No. 8 coal seam in the northern and southern parts of the basin, including the Daning–Jixian, Shenfu, Jiaxian, northern Yulin, and southern Yanchuan areas, exhibits relatively favorable fracability, whereas northern Liulin and southern Yulin show comparatively poor fracability. In addition, the fracability index shows a clear positive correlation with the peak gas production of vertical CBM wells. Based on this relationship, the deep coal reservoirs were classified into three categories: Class I reservoirs (F > 0.55), characterized by high fracability and high production potential; Class II reservoirs (0.50 ≤ F ≤ 0.55), characterized by moderate fracability and moderate production potential; and Class III reservoirs (F < 0.50), characterized by low fracability and low production potential. These findings provide a scientific basis for identifying fracturing sweet spots and for the classification evaluation of deep CBM resources in the Ordos Basin.