Evaluation of Coal Reservoir Fracturability Based on Mechanical Stratigraphy

Abstract

The difficulty of coal reservoir stimulation is a key bottleneck restricting coalbed methane (CBM) development. Scientific and accurate evaluation of coal reservoir fracturability is essential for achieving commercial CBM production. Given the complexity and uncertainty of coal reservoir geological conditions, the concept of mechanical stratigraphy was introduced in this study. Seven quantitative indicators were selected to establish a fracturability evaluation system, including the geological strength index (GSI), in-situ stress difference coefficient, critical strain energy release rate, brittleness index, fracture pressure, mosaic index, and water inrush coefficient. The analytic hierarchy process (AHP) was adopted to determine the weight of each indicator. Membership functions and membership degrees were defined according to relevant industry standards, and an evaluation model together with a grading set was constructed. Reservoir fracturability was divided into three grades: Class I reservoirs with strong fracturability (index ≥ 0.8), Class II reservoirs with moderate fracturability (0.6–0.8), and Class III reservoirs with weak fracturability (≤ 0.6). A case study was carried out in a block on the eastern margin of the Ordos Basin, and the evaluation results were verified by actual well gas production performance. The results show that coal reservoir fracturability presents a vertical distribution pattern from top to bottom: Class I, Class II, Class III, Class II. Class I engineering sweet spots are widely developed in gentle slope zones and structural platforms where the formation dip angle is less than 3°, with weak Yanshanian–Himalayan tectonic deformation. In such areas, Class I mechanical layers are thick and laterally continuous, the coal reservoir structure remains relatively intact, the in-situ stress difference coefficient is low, and the fracture pressure is relatively high. These structural locations are favorable for the formation and deployment of CBM engineering sweet spots. The coal reservoir fracturability evaluation system based on mechanical stratigraphy exhibits good practicability and reliability. It can provide a scientific basis for CBM favorable area optimization and hydraulic fracturing design.