Analysis of drilling fluid rheology and tool joint effect to reduce errors in hydraulics calculations



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Texas A&M University


This study presents a simplified and accurate procedure for selecting the rheological model which best fits the rheological properties of a given non- Newtonian fluid and introduces five new approaches to correct for tool joint losses from expansion and contraction when hydraulics is calculated. The new approaches are enlargement and contraction (E&C), equivalent diameter (ED), two different (2IDs), enlargement and contraction plus equivalent diameter (E&C+ED), and enlargement and contraction plus two different IDs (E&C+2IDs). In addition to the Newtonian model, seven major non-Newtonian rheological models (Bingham plastic, Power law, API, Herschel-Bulkley, Unified, Robertson and Stiff, and Casson) provide alternatives for selecting the model that most accurately represents the shear-stress/shear-rate relationship for a given non- Newtonian fluid. The project assumes that the model which gives the lowest absolute average percent error (EAAP) between the measured and calculated shear stresses is the best one for a given non-Newtonian fluid. The results are of great importance in achieving correct results for pressure drop and hydraulics calculations and the results are that the API rheological model (RP 13D) provides, in general, the best prediction of rheological behavior for the mud samples considered (EAAP=1.51), followed by the Herschel-Bulkley, Robertson and Stiff, and Unified models. Results also show that corrections with E&C+2IDs and API hydraulics calculation give a good approximation to measured pump pressure with 9% of difference between measured and calculated data.