Quantifying the yield stress of bentonite muds mixed with other clays during drilling operations

In civil engineering, bentonite-water mixtures, commonly referred to as drilling muds, are intensively used in order to lubricate tools, consolidate walls, and help extracting cuttings during drilling operations. The efficiency of drilling muds in each of these tasks lies in their rheological properties depending mainly on the amount of clay materials. During the field works, drilling muds are mixed with the excavated soil materials (e.g., sand, clay, organic matter) that may change drastically the rheological properties of mixtures. With the aim of understanding better the rheology of field drilling muds mixed with other clays, rheological measurements on monoand binary-clay suspensions were performed using a rotational rheometer equipped with coaxial cylinders, for which the type of clay materials (i.e., bentonite, kaolin and illite), the total clay volume fraction $\varphi_t$ and the bentonite to clay volume ratio $R_b$ were varied. The contribution of this work is twofold: (i) to highlight the major role of $\varphi_t$ and $R_b$ independently on the rheology of binary-clay suspensions and (ii) to provide phenomenological models to quantify the dependency of the yield stress on both $\varphi_t$ and $R_b$ that would be particularly useful for industrial applications.