A New Technique for Enhancing Hydraulic Fracturing Treatment in Unconventional Reservoirs

Abstract

Abstract A physical model and experimental data supporting the technology and benefits of graded proppant injection into naturally fractured reservoirs to stimulate natural fracture permeability has been developed recently. In this study, the graded proppant injection has been coupled with hydraulic fracturing treatment 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 the coal cleats open during water-gas production and consequently increases the efficiency of hydraulic fracturing treatment. Experimental studies and mathematical modelling for stimulation of natural cleat system around the main hydraulic fracture are conducted. In the laboratory study, core flooding tests are performed to inject flow of suspended particles inside the natural fractures of a coal sample. By placing different particle sizes and evaluating concentration of placed particles, an experimental coefficient is found for optimum proppant placement in which the maximum permeability is achieved after proppant placement. In the mathematical modelling study, a laboratory based mathematical model for graded peroppant 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. 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 productivity index.