The influence of mineralogy and geomechanical properties on the development of natural fractures in coal-bearing strata and its implications: an example from Barakar Formation, Eastern India
摘要
Coalbed methane recovery requires hydraulic fracturing to maximize production. Consequently, understanding the relationships between bed lithology, geomechanical properties, and cleat intensity through rigorous subsurface or surface analogs is required. In this work, coal mine wall studies, followed by laboratory analyses, were conducted to uncover these relationships. Linear decrease in natural fracture abundance with increasing bed thickness was observed in all lithofacies, i.e., coal, parting sandstone, and shale beds. Gamma ray has a positive correlation with fracture abundance in coal/sandstone and negative correlation in shale. In sandstone, fracture abundance positively correlates with quartz and negatively correlates with both clay and feldspar contents. Coal has the highest fracture intensity, lowest gamma ray, and lowest Schmidt hammer rebound values among the three lithofacies. Multilinear regression analysis for coal indicates that bed thickness and gamma ray together are the strongest predictors of fracture abundance. Adding density, P-wave, and S-wave velocities does not substantially improve predictability. In contrast, multilinear regression for sandstone suggests that gamma ray, bed thickness, density, and P-wave velocity together provide the most accurate predictions of fracture abundance. Schmidt hammer measurements have weak correlations with fracture abundance in all lithologies. For higher coalbed methane recovery, coal beds with relatively higher gamma ray values should be targeted, as their abundant natural fractures are likely to promote multidirectional interconnected flow networks when subjected to hydraulic fracturing. Notably, face cleat orientations shift systematically clockwise (NNW to NNE) from the lower to middle, followed by anticlockwise (NNE to NNW) from the middle to upper coal seam.
Graphical abstract