Structure-rheological properties relationships in model drilling fluids formulated in aqueous solutions of carrageenan with varying chemical structures

Abstract Water-based drilling fluids (WDF) formulated with hybrid carrageenans (HY) recently showed attractive rheological and filtration properties, but the role of HY chemical structure on such WDF characteristics could not be established. Here, model drilling fluids were prepared by suspending sodium activated bentonite in two HY aqueous solutions, in order to assess relationships between the polysaccharide chemistry and the rheological-structural properties of the fluids. The rheological analysis of HY solutions indicates that the more sulfated HY is in coil conformation, whereas the other forms helices. The less sulfated HY promotes the slow build-up of a more elastic bentonite gel showing a larger yield stress, due to a less flocculated structure. A similar microscopic structure is achieved with the more sulfated HY, but a weaker gel is formed. In this suspension, the strain hardening behavior indicates that elasticity is ruled by flocs internal structure. Wide angle X-ray diffraction suggests that the weaker bentonite floc elasticity is related with a smaller number of bentonite lamellae in the aggregating stacks within the flocs. Overall, the set of results suggests that the carrageenan chemistry modulates the gel viscoelasticity by tuning the bentonite suspension structure at different length scales, from inter bentonite stacks to inter flocs distances.