Brittleness Evaluation and Favorable Development Zone Prediction of the No.8 Coal Reservoir in the Carboniferous Benxi Formation, Jiaxian Area, Eastern Margin of the Ordos Basin
The deep coalbed methane (CBM) resources in the Jiaxian area of the Ordos Basin are abundant; however, studies on coal brittleness characteristics remain limited. Clarifying brittleness features is of great significance for the development of deep CBM. Based on array acoustic logging, conventional logging data, and triaxial rock mechanical experiments, a logging-based coal brittleness evaluation method was established by comprehensively considering the effects of mechanical parameters and mineral composition. The controlling effects of brittleness on the mechanical properties and coal petrological characteristics were analyzed, and the spatial distribution of brittleness in the study area was clarified. The results indicate that the No.8 coal seam in the Jiaxian area exhibits a vertical pattern of lower brittleness in the upper part and higher brittleness in the lower part. The brittleness index shows a negative correlation with compressive strength and displays clear relationships with macroscopic coal lithotypes and coal structure: brittleness increases progressively from semi-dull coal to semi-bright coal and bright coal, and from primary coal to fractured coal and granular coal. This suggests that lower clay mineral content corresponds to higher coal brittleness. In the study area, the length of the main hydraulic fracturing fractures increases initially and then decreases with increasing coal brittleness index, with a moderate brittleness index (45-60) being most favorable for fracture propagation. Based on the relationship between brittleness index and gas content, three types of favorable CBM development zones in the No. 8 coal seam were predicted. Among them, Type I favorable zones exhibit good gas production performance, with single-well daily gas production generally exceeding 30,000 m³, effectively guiding the prediction of favorable zones for deep CBM development.
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