A numerical model to predict surge and swab pressures for yield power law fluid in concentric annuli with open-ended pipe

Excessive surge pressure can cause many downhole problems such as loss circulation, leakage, fracture formation, and blowout. Accurately predicting surge and swab pressures can provide a guarantee of safety in a downhole. Multiple research endeavors have been conducted based on close-ended pipes (CEP); however, no exact or approximate models to predict surge and swab pressures have been reported for yield power law (YPL) fluid in an open-ended pipe (OEP). In this paper, an exact numerical model was established to predict surge and swab pressures for YPL fluid in concentric annuli with OEP. Additionally, existing regression models for Bingham plastic (BP) and power law (PL) fluids were modified based on the exact numerical solutions in concentric annuli with OEP. The accuracy of the modified regression model is ±5% and ±15% for BP and PL fluids, respectively. In addition, an accurate (to within 5% error) new regression model was developed to predict surge and swab pressures for YPL fluid in a concentric annulus with OEP on the basis of exact numerical solutions. The effects of fluid properties, annular geometries and drilling parameters on surge and swab pressures in OEP were investigated using an exact numerical model.