Low-Toxicity Oil Muds: A Knowledge of Downhole Rheological Behavior Assists Successful Field Application

Summary The introduction of oil-based drilling fluids has led to significant improvements in drilling efficiency for many North Sea operations. The original formulations that were based on diesel are generally being replaced by the lower-toxicity mineral oils. One factor crucial to the successful application of mineral oils has been a knowledge of their downhole rheological characteristics. This paper describes a purpose-built viscometer capable of simulating realistic downhole temperature (300°C [572°F]) and pressure (100 MPa [1,000 bar]) conditions. Results from this study are used to indicate how surface-fluid rheologies can be tailored to provide adequate downhole viscosities, thus reducing total system pressure losses.