A new simplified surge and swab pressure model for yield-power-law drilling fluids

• Accurate prediction of surge and swab pressures is very important.
• At present, there is no exact analytical model to predict surge pressure for YPL fluids.
• Available models are developed based on narrow-slot approximation.
• A new and accurate regression-based surge-pressure model has been developed.
• The new model predictions are verified using experimental data.

sSurge and swab pressures have been known as common phenomena to cause wellbore pressure control problems such as lost circulation, formation fracture, fluid influx, kicks, and even blowouts. Accurate prediction of these pressures is very important to avoid associated drilling problems. To date, there is no exact analytical model to predict surge pressure developed in concentric annulus with yield-power-law (YPL) fluids. Most of the available models (analytical and regression models) are developed based on narrow-slot approximation of the annular flow. The models provide prediction for diameter ratio ranging from 0.4 to 0.85 with discrepancy of up to 20%. This paper presents a new regression-based surge-pressure model, which makes accurate predictions (maximum error of ±3%) for wide range of diameter ratios (0.4–0.85). To develop the regression model, an exact numerical model was formulated and extensive numerical simulations were performed. The results were analyzed to formulate a simplified regression model that predicts surge and swab pressures conveniently for YPL fluids without requiring iterative calculation procedures. To verify model predictions, laboratory experiments were conducted in small scale setup (50.8 × 33.5 mm annulus). Model predictions demonstrated reasonable agreement with experimental measurements and exact numerical solutions.