Hydraulic Fracturing Fracture Initiation Model for a Vertical CBM Well

There are many cleats, fractures and other structural weak planes in coal seams. Fractures may origin from coal body or cleat cracks during hydraulic fracturing. Consequently, mechanisms related to fracture initiation may be significantly different from those observed in conventional reservoir formations. In this regard, a new calculation model of fracture initiation pressure suitable for gas wells in coal-bed methane formations should be established. Considering the delivery network distribution characteristics of the coal seam cleat system and change of cleats in their spatial positions, the stress distribution around the perforated holes and cleats walls was determined based on the rock mechanics and fracture mechanics theory. According to conditions related to tensile and shear failures, the calculation model for fracture initiation pressure of coal was established under different well completion methods. The calculation model was used for two fractured wells and the fracture initiation pressure difference between the calculated value and the measured value in the conditions of open hole completion and perforated completion were 3.96% and 4.72%, respectively. It could be seen that the calculated results coincided well with measured values. The results showed that a seam fracture could be generated from cleats, and the fracture initiation pressures were closely related to coal bed cleat angle, coefficient of internal friction of cleat walls, coal bed horizontal principle stress differences and other factors.