Variations of shale's pore characteristics and hydraulic properties after long-term imbibition in hydraulic fracturing fluids

During hydraulic fracturing, the interaction of fluids with shale modifies the pore hydraulic characteristics, altering hydrocarbon recovery efficiency and CO2 storage of the shale. We investigated the interaction of fracturing fluid-shale with varied pH (pH = 5,7,9) and different durations (short-term, long-term, 0-12 months). Pore structure characteristics, hydraulic characteristics, Mineralogy, and surface morphology of shale were measured before and after fracturing fluid treatment. Time dependence of fluid-shale reactions was found in this study. In the short-term experiment (0-1 months), pyrite and carbonate minerals dissolved rapidly, and the pH value of the solution decreased quickly (by 1.7-5.1). The main dissolved minerals during the long-term experiment (1-12 months) were feldspar and clay minerals, and the pH value rose slowly (by 0.5). The dissolution of minerals can increasing porosity and gas adsorption capacity of shale (by 14.1%). The pore surface became rough in the short-term experiment, while the pore surface became smooth in the long-term experiment, as shown by the alteration in fractal dimension D-1. Through calculation, it was found that the absolute permeability of shale increased (by 60.0-129.1%), and the pore tortuosity decreased (by 26.1-57.8%). However, secondary precipitation, including gypsum and iron hydroxide, blocks fractures and pores, reducing porosity, hydraulic characteristics, and adsorption capacity under neutral and alkaline conditions in the long-term experiment. At the same time, acidic circumstances can prevent secondary precipitation from forming. In hydraulic fracturing, monitoring the change in pH value is critical to boosting hydrocarbon recovery and increasing the reservoir's geological storage capacity.

Automated summary

This study investigates the interaction between fracturing fluid and shale during hydraulic fracturing. The results show that the fluid-shale reaction is time-dependent, and the dissolved minerals and pH values vary with time. The dissolution of minerals can enhance the porosity and gas adsorption capacity of shale, but secondary precipitation during long-term experiments can block pores and decrease the shale's porosity and hydraulic characteristics.