Hydraulic fracturing mechanisms in coal: A review

Hydraulic fracturing mechanisms in coal differ in many aspects from those in inorganic rocks. Due to the lower Young’s modulus and higher Poisson’s ratio, the stress state in coal is higher than in adjacent formations. This leads to wider and shorter fractures than in inorganic rocks. Fractures are affected by the cleat system in coal, so that multiple fractures may follow in tortuous paths the coal cleats. Coal often has a fracture treatment pressure in excess of 22.6 kPa/m (1 psi/ft.). Several hydraulic fracturing mechanisms may account for that, namely fracture blocking by coal chips, fracture tip plugging by coal or the development of a complex multi fracture system. The orientation of the hydraulic fractures is predominantly horizontal at shallow depths (< 200 m) and mostly vertical at depths > 600 m. In between these depths, fracture orientation may be either horizontal or vertical. This paper gives a comprehensive overview of the proposed hydraulic fracturing mechanisms in coal. [Received: July 28, 2015; Accepted: January 14, 2016].