The physical model and experimental data support the beneficial technology of graded proppant injection into naturally fractured reservoirs to stimulate natural fracture permeability. Injection of particles with increasing size, at poroelastic and hydraulic fracturing conditions, yield deeper penetration and gradual filling of natural fractures with a resulting increase in permeability. This work expands on the concepts and outlines steps to maximize the benefit of graded proppant injection to enhance coal seam gas stimulation by focusing on the effect of the chemistry of injected fluid on the overall performance and the use in conjunction with hydraulic fracturing.

Low productivity indices can be observed in many moderate- to low-permeability coal bed methane (CSG) reservoirs due to low aperture and poor connectivity of natural cleats. Graded proppant injection in CSG environments can: stimulate a stress sensitive cleat system below the fracturing pressure as well as enhance a fracturing treatment by invading cleats, lowering fluid leakoff, and maintaining aperture during production. Further, periodic or remedial treatments could to counter effective stress on the cleats improving production by maintaining cleat aperture.

Laboratory tests on coal core flooding by water under increasing pore pressure with proppant injection at the maximum pressure have been carried out under different salinities of the injected water. Proppant-proppant and proppant-coal Derjaguin-Landau-Verwey-Overbeek (DLVO) total interaction energies were calculated to optimise the condition for successful proppant placement. Results on the DLVO total energy of interaction showed that conditions favourable for successful proppant placement in coal cleats are suspension ionic strengths between 0.05 M and 0.1 M NaCl. At these conditions no proppant agglomeration and proppant-coal attachment are observed, allowing deeper penetration of proppant into the natural coal cleat system. Lower suspension ionic strengths can lead to natural coal fines migration, cleat plugging and coal permeability reduction.

Based on the experimental results and previously developed model a case study has been conducted to evaluate the productivity enhancement using this technique. The results show about four-fold increase in well productivity index at injections below fracturing pressures and may further improve the stimulated reservoir volume when used in conjunction with low permeability coal hydraulic fracturing treatments.

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