The technology of injecting micro-sized proppant particles along with fracturing fluid is proposed to improve the conductivity of naturally fracture systems (e.g., cleats, natural fractures) in stress sensitive reservoirs, by placing graded particles in a larger, preserved stimulated reservoir volume around the induced hydraulical fracture. One of the main parameters determining the efficiency of the proposed technology is the concentration of placed proppant particles in the fracture systems. A laboratory study has been conducted to evaluate the effect of placed proppant concentration on coal permeability enhancement using a one-dimensional linear injection of micro-sized proppant into coal core and varying effective stress. Permeability values are measured for different concentrations of placed particles as a function of effective stress. The results show that there is an optimum concentration of placed particles for which the cleat system permeability reaches a maximum and permeability enhancement is more sensitive to concentration of placed proppant at higher than lower effective stress. The experimental results show maximum permeability enhancement of about 20% for an optimum concentration of placed particles at 490 psi effective stress. Permeability enhancement by 3.2 folds is observed at elevated effective stress of 950 psi. Finally, the paper proposes a field application strategy to apply graded particle injection in field case study.