Hydraulics and Cuttings Transport Properties of Two Oil-Based Drilling Fluids: A Comparative Analysis

Abstract Selecting the right drilling fluid for an offshore well is a challenging exercise. Both because a drilling fluid needs to have a set of properties which must satisfy different and often conflicting requirements, and also because relating properties as measured in a lab to actual downhole performance can be quite challenging. Here we address the second issue, focusing on the hydraulics and cuttings transport properties of two oil-based drilling fluids. The paper presents results from flow loop experiments using these fluids, without and with cuttings injection in an eccentric annulus, as well as results from a rheological characterization of the fluids. Flow curve measurements using a scientific rheometer show that the two fluids have similar rheological performance, however one fluid having a slightly lower viscosity. Calculated pressure loss for flow without cuttings agree well with corresponding data from the flow loop experiments, both for eccentric annulus and for pipe flow in laminar regime. For turbulent flow we observe that a commonly accepted friction factor correlation consistently underpredicts the pressure loss in pipe, while the flow in annulus was laminar for all flow rates investigated. We also investigate the effect of inner pipe rotation on pressure loss both without and with cuttings, and on the hole cleaning performance. We find that pressure loss increases with increasing string rotation rate when cuttings are not present, while the trend is reversed during experiments with cuttings injection. The latter can be explained by the agitating action of the fully eccentric inner pipe, creating a larger effective flow area. The main message of the paper is that the two fluids, supplied by the same manufacturer and with similar densities and rheological properties, have different hole cleaning performance characteristics. At similar flow rates, steady state cuttings bed height was the same in the two fluids, but the corresponding pressure gradient was consistently smaller for one of the fluids. In the paper we discuss possible mechanisms for explaining this behavior. The results can be used for model verification, for model selection and for cost-benefit analysis with respect to hole cleaning.