Techniques for Accurate Characterisation of Aqueous High Temperature Drilling Fluids with Slip

Abstract

Drilling fluids are an important component of cost-efficient drilling of geothermal wells. Drilling fluids typically have a composition to position them on the boundary between solid-like and liquid-like behaviour. Operating at this boundary makes their behaviour sensitive to changes in fluid ionic strength, pH, solids content, and most importantly for geothermal drilling operations, temperature. Changes in these conditions can alter their yield stress and high-shear viscosity. Measurement techniques to accurately characterise their yield stress behaviour and high-shear viscosity are vital. Here we show that rheological characterisation is critically dependent on geometry selection. Measurements of an aqueous Hausmannite suspension showed evidence of slip when using a concentric cylinder geometry, and evidence of secondary flow when using a vane geometry. The concentric cylinder geometry – the most common in industrial use – can provide misleading yield stress or gel strength results due to slip at the smooth surfaces. Prediction of drilling fluids behaviour at in situ conditions requires considered use of multiple geometries to ensure that rheological characterisation is robust and not subject to confounding measurement errors. This work provides a foundation for accurate drilling fluids characterisation at elevated temperatures.