Stimulation of unconventional reservoirs using graded proppant injection

A method is proposed for enhancing conductivity of micro-fractures and cleats around the hydraulically induced fractures in coal bed methane reservoirs. In this technique, placing ultra-fine proppant particles in natural fractures and cleats around hydraulically induced fractures at leak-off conditions keeps coal cleats open during water-gas production and, consequently, increases the efficiency of hydraulic fracturing treatment. Experimental and mathematical studies for stimulation of natural cleat system around the main hydraulic fracture are conducted. In the experimental part, core flooding tests are performed to inject suspended particles inside natural fractures of a coal sample. By placing different particle sizes and calculating concentration of placed particles, an experimental coefficient for optimum proppant placement is evaluated. This coefficient corresponds to the maximum permeability achieved after proppant placement. In the mathematical modelling study, a laboratory-based mathematical model for graded proppant placement in naturally fractured rocks around a hydraulically induced fracture is proposed. Derivations of the model include exponential form of the pressure-permeability dependence and accounts for permeability variation in the non-stimulated zone. The explicit formulae are derived for well productivity index by including the experimentally found coefficient for optimum proppant placement. Particle placement tests result in almost three times increase in coal permeability. The laboratory-based mathematical modelling as performed for the field conditions shows that the proposed method yields around 6-times increase in well productivity index