Experimental investigation on the feasibility and efficiency of shear-fracturing stimulation for enhancing coal seam permeability

摘要

The extraction of coalbed methane can supply clean energy to society and improve the safety of subsurface coal mining. The low and ultra-low permeability of coal seams has resulted in the significantly low efficiency of methane production in surface and subsurface extraction systems. Fluid stimulation has been extensively applied for the enhancement of formation permeability. However, the response of coal seams to fluid stimulation may be different from other formations due to their unique structure and mechanical properties. Moreover, the presence of shear-fracturing stimulation and its effect on permeability enhancement is more significant in coal seams during fluid stimulation. In this study, a fluid injection-induced hydrofracturing treatment and shear-fracturing stimulation were separately investigated through laboratory experiments. The permeability of both intact and fractured specimens was used to directly evaluate the efficiency of these two types of fluid stimulations in different types of reservoirs. The experimental results indicated that the closure of hydraulic (i.e., tensile) fractures significantly inhibited the permeability enhancement of the hydrofracturing treatment in coal seams, and resulted in the permeability of fractured coal specimens were considerably lower than that of hard rocks (e. g., shale and sandstone). The results also demonstrated that fluid injection-induced shear stimulation in the coal specimens resulted in dilation behavior primarily associated with permeability enhancement. This was attributed to the fact that the exfoliated particles and masses prop shear fractures and increase their stiffness. It was found that, because of the weak mechanical and discontinuous properties of coal seams, an increase in the deviator stress improved the feasibility of shear-fracturing stimulation. Finally, fluid stimulation can be achieved using low viscosity fluids, such as L/Sc-CO2, as the fracturing fluid tended to form a shear dilation zone with high permeability in the coal seams.