Viscoelasticity and Drilling Fluid Performance

Viscoelastic properties of drilling fluids are not often measured due to a lack of understanding of their impact on fluid performance as well as a lack of field equipment suitable for such measurements. A study has been conducted recently to evaluate the viscoelastic properties of xanthan gum and invert drilling fluids and their impact on barite suspension quality and rheology. Both a Brookfield YR-1 rheometer and a Bohlin Gemini 150 rheometer were used to generate data for comparison. The impact of viscoelasticity on steady-state rheology, thixotropy and shear thinning was evaluated using a multi-speed rheometer. A soon-to-be-adopted API recommended procedure was used to measure the barite sag tendency under dynamic conditions. Aqueous solutions of xanthan gum showed that viscoelasticity, shear thinning and thixotropy increased with increasing polymer concentration. When the solutions were weighed up with barite, they became more viscoelastic, slightly more thixotropic, but less shear thinning. Proper suspension of barite was observed at a xanthan gum concentration of 2 lb/bbl without any other additives. Compared to xanthan-barite suspensions, invert drilling fluids of similar density exhibited a greater viscoelasticity but less thixotropy and shear thinning. Treatment of invert drilling fluids with viscoelastic polymers resulted in a further enhancement of viscoelasticity and thixotropy, but a slight deterioration in shear thinning. Barite suspension quality showed a certain degree of correlation with viscoelasticity as well as steady-state rheology; however, these properties were temperature dependent for invert drilling fluids. Hydraulic analyses indicated that viscoelastic additives can impact fluid viscosity thus affecting pressure loss, equivalent circulating density and hole cleaning. Viscoelasticity enhancement may be used to improve barite suspension quality under certain conditions, but its impact on hydraulics must be carefully considered.