The Effect of Constant and Variable Eccentricity on the Spacer Performance During Primary Well Cementing

Abstract

Spacers are the intermediate fluids in the primary cementing process, preceded by the mud and followed by the cement. They help in mud removal, keep mud and cement separated and prepare the annulus walls for a good cement bond. The focus of this study is to analyze the spacer performance in keeping mud and cement separated in the vertical and horizontal well sections with eccentricity. Several combinations of eccentricity variation and displacement rates are simulated using Computational Fluid Dynamics (CFD) tool to analyze the temporal and spatial fluid volume fraction distributions in the annulus and validated against multi-fluid displacement experiments. A 50 ft vertical section with annular gap of 1.5″ and casing standoff from 5% to 100% and a horizontal section with variable eccentricity are studied. For initial conditions the annulus is modeled as filled with mud and subsequently swept by one annular volume of spacer and cement each respectively. Mud and cement are treated as Herschel Bulkley fluids and spacer as a Newtonian fluid. Qualitative results from simulations are shown in the form of volume fraction contour plots at different sections of the annulus and quantitative results in the form of temporal and spatial volume fraction of each fluid at specific planes in the entire annulus. For a vertical annular section with constant eccentricity, some unswept mud is observed on the narrow side and eventually becomes trapped after the eccentricity is increased above a threshold value. It is also observed that increasing displacement rate helps in displacing some of this trapped mud. After a threshold eccentricity value, the trapped mud cannot be displaced and the spacer and cement follow the path of least resistance and flow occurs mainly on the wider side of the annulus. For a horizontal section with variable eccentricity (maximum at the mid distance between centralizers), the flow is observed to move towards wider side around the center and then returning to the other side (narrow part) after passing through middle section. Some trapped mud is also observed in the vicinity of the middle section due to local maximum eccentricity.