Counterion-specific shale hydration inhibiting performance of vinylimdazolium ionic liquids

Abstract

The role of cation and the influence of the cation alkyl chain length of vinylimdazolium-based ionic liquid (IL) were studied previously to develop high-performance shale inhibitor for application in water-based drilling fluids (WBDFs). Results demonstrate that they have a considerable effect on the inhibition performance. However, the physicochemical properties can be tuned by changing the cation/anion combination. Therefore, the influence of anion type on the inhibiting performance must be investigated. In this study, we selected several vinylimdazolium-based ILs with different anions, such as bromide (Br−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), and bis(trifluoromethylsulfonyl) (TFSI−). The size, solubility, hydrophilicity, and interaction in-between were tuned accordingly by changing the anion species. Results from linear swelling height, hot rolling recovery, and rheological measurements show that the IL with bromide anion has the most outstanding shale inhibiting performance. The anions influence the inhibitory performance of ILs in a distinct manner from cations by measuring the properties of Na–Bent dispersions added with different anion-based ILs. The former produces different abilities to suppress electric double layers and decreases the interlayer distance, whereas the latter mainly affects the interlayer distance. Despite different hydrophilic/hydrophobic properties, they have similar performance to inhibit the crystalline hydration and swelling. This condition is probably because they have a common cation that dominates the process of entering the interlayer void and narrowing interlayer spacing. Many hydrophilic ILs with anion Br− or BF4− have the capacity to strongly resist the repel interaction between clay particles and prompt aggregation, leading to better inhibitory performance. Combined with the previous result, cations and anions affect the IL inhibiting ability although their underlying mechanism is different. Thus, this work explored the relationship between inhibiting performance and IL moiety. The findings provide consolidate theoretical foundation for developing shale inhibitors for WBDFs.