Abstract
Hydraulic fracturing is the most effective reservoirstimulation techniques in the coalbed methane. However, the polymer in the fracturing fluid has a strong effect on the surface of the coal, causing the water lock damage as high as 70% to 90%. It is important to develop an efficient method for releasing coal seam water lock. In this paper, adsorption experiment, SEM, particle size experiment, core flow experiment, wettability and surface tension experiment are used to study the cause of coal seam water lock damage during fracturing and the effect of nano-emulsion on releasing water lock damage in coal seams. Experimental results show that after coal fracturing, the adsorption amount of polymer on the surface of coal is 14.81 mg/g. The large amount of hydrophilic polymer adsorption causes the pore radius of the coal to narrow. And the surface wettability changes from weak hydrophilic to strong hydrophilic, which increase the water lock damage. Compared with conventional slick water, fracturing fluid, the composite of nano-emulsion and fracturing fluid forms mixed micelles, which reduces the polymer adsorption capacity from 14.81 mg/g to 7.42 mg/g. After scanning by electron microscope, it is observed that the surface roughness of the rock sample is restored; The size of the nano-emulsion is about 10nm, and the very small volume can act deep in the pores of the coal seam; After using nano-emulsion, the gas/water interfacial tension is reduced by 45.1mN/m, and the wettability of coal is improved from hydrophilic to neutral, which reduces the capillary pressure in the pores of the coal and reduces the breakthrough pressure of coalbed methane by 11.1KPa; The water lock release rate is as high as 53.09%. The Nano-emulsion is an ideal choice to remove water lock damage.