Cleat and Fracture Systems in Coal with Implications for Strength, Hydrofracking, and Gas-Water Flow Mechanisms: A Critical Review.

The cleat is a characteristic structural feature found within coal lithotypes, often in a regular pattern. The presence of cleats and fractures in coal is advantageous for miners, as they provide weak planes that aid in extraction. However, the stability of pillars and roofs relies on factors such as cleat/fracture orientation, spacing, and intensity. These aspects are critical in rock mechanics for the safe mining of coal. Recently, the extraction of methane from coal seams has led to investigations into cleats as they directly influence gas and water flow mechanisms. The study of cleats and fractures dates back to the early days of coal mining. Nonetheless, the exact origin of cleats remains unclear, and various hypotheses exist. It is generally believed that cleats result from either tectonic forces or nontectonic processes like compaction, devolatilization, and dehydration. In bituminous coal seams of the Gondwana region, cleat formation is attributed to local structural forces rather than internal forces within the coal due to diagenesis, catagenesis, and metagenesis through compaction. Permian-age Gondwana coal seams generally feature two primary types of fractures: face and butt cleats. These fractures are generally orthogonal to the bedding plane and each other. The orientation of face cleats, perpendicular to the axial trend of folds, suggests formation through extension fractures. In contrast, the direction of butt cleats, parallel to the axial trend, indicates formation after the release of compressional forces. Vitrain, characterized by bright coal bands, exhibits a higher cleat intensity due to its brittle nature. Dark bands lack face and butt cleats, with only master cleats spanning from one vitrain layer to another through dull layers. Cleat intensity is observed to be directly proportional to coal maturity and vitrinite content. In multiple vitrain layers, different layers are stacked with thin laminae between them, showing frequent cleat spacing with short persistence. Cleats in durain layers are more widely spaced compared to bright vitrain lithotype, hence greater integral strength is found in dull coal. The permeability in coal depends on the orientation and magnitude of cleat planes, controlling the water and gas flow through the coalbed. This paper provides a comprehensive overview of cleats and fractures in coal, along with various parameters influencing coal strength, coal composition, maturity, lithotype, and the requirements of the hydrofracturing process to enhance porosity and permeability for the desired gas flow mechanism. The variations in face cleats, butt cleats, tertiary cleats, joints, etc. in coals around the different coalfields in India are also explored, highlighting their significance in coal mining and coalbed methane extraction.