Stimulation of coal seam permeability by micro-sized graded proppant placement using selective fluid properties

•Repulsion between particles and coal is a favourable condition for graded proppant injection.•No permeability increase is observed after high salinity water injection with proppant.•High efficiency of graded particle injection using low salinity water is observed.•Graded proppant placement yields threefold increase in coal core permeability.•Graded proppant injection results in six-times well productivity increase.

During initial drawdown in coal bed methane (CBM) production, the coal permeability declines due to partial fracture closure. The recently proposed technique for stimulation of natural coal fractures and cleats by the graded proppant injection targets a uniform cleat filling by the proppant. It provides maximum opening and conductivity during pressure depletion. The technique for graded particle injection below the fracturing pressure has been experimentally evaluated using injection of micro-sized ultra-light high-strength particles into coal cores. The laboratory tests on one-dimensional injection of different size particles into coal cores have been conducted under different effective stress conditions. Calculation of electrostatic interactions results in determining physico-chemical conditions that are favourable for particle–particle and particle–coal repulsion. The repulsion condition prevents particle attachment to the coal surface, particle agglomeration and also formation damage due to external and internal cake formation on the fractures’ surface. Particle placement in repulsion condition with low-salinity water results in almost three-time increase in coal permeability. Implementation of an empirical permeability shape factor allows matching the laboratory data by the mathematical model. The laboratory-based mathematical modelling as performed for the field conditions shows that the proposed method can yield up to six-time increase in productivity index.

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