3D Displacement Simulator Realistically Predicts Free Fall during Cementing

Abstract

Understanding equivalent circulating density (ECD)/downhole pressure is important for a successful cement job design and execution. One of the critical parameters to control ECD is the pumping rate, which, in turn, controls the wellbore fluid rates. But, in some cases, specifically when pumping heavier fluids, the well might go into a scenario called "free fall." During free fall, the fluid in the wellbore loses physical contact with wellhead/pump and flows on its own under the influence of gravity, making control of ECD difficult from the surface. For a well under free fall, the dynamics of wellbore rate depend not only on the densities but also on the movement of the wellbore fluids. Traditionally, job design simulators assumed flat fluid interfaces, thereby compromising on the accuracy of wellbore rate estimation. A more accurate estimation of the fluid movement/flow patterns would help operators better predict the pressures. In the present work, a free-fall algorithm is implemented to complement a three-dimensional (3D) displacement simulator. The displacement simulator solves momentum balances, continuity equations, and scalar convection-diffusion equations on top of a 3D grid to provide fluid flow profiles in three dimensions. The simulator is capable of simulating intermixing of fluids and the resultant frictional pressure drops. This paper presents, for a model wellbore configuration, the differences in free-fall estimation and the resultant differences in ECD estimation between a traditional hydrostatic job design simulator and a 3D simulator. Traditional hydraulics is observed to be conservative for estimating wellbore rates compared to the 3D simulator during free fall, which can result in predicting lower than actual ECDs.