Quantitative characterization of micro forces in shale hydration and field applications

Shales (illite was the dominant clay mineral) of Silurian Longmaxi Formation in Sichuan Basin and Triassic Yanchang Formation in Ordos Basin were taken as subjects to examine the mechanisms of shale-water interaction, quantitative characterization of hydration force and potential field applications based on micro forces analyses. Mica sheet with composition and property very similar to illite was tested for micro forces between the crystal layers. In electrolyte solution, micro forces between mica-solution-mica system include DLVO (Derjaguin-Landau-Verwey-Overbeek) force and hydration force; when the electrolyte concentration was low, the tested curve agreed with the theoretical DLVO curve; when the electrolyte concentration was higher than the critical value and the distance between mica sheets was less than 5 nm, the tested curve deviated from the DLVO curve completely, and the hydration force became dominant. Quantitative analysis indicated that the hydration force decayed in a rapid double-exponential type with the growth of distance. Field applications indicate that strict control of water invasion and reducing the strength of hydration force are the keys in designing collapse-preventing drilling fluids; meanwhile, during the shut-in period of shale gas wells, shale-water interaction can induce and extend micro-cracks, further improving the stimulation effect of shale reservoirs.