Laboratory Investigation of Lightweight, Low-Viscosity Cementing Spacer Fluids
Authors: Haut Richard Carl , Crook Ronald James 
Published: December 2007

Summary
The use of lightweight, low-viscosity cementing spacer fluids was investigated with a large-scale apparatus that simulates actual field conditions. Results show that various fluids can be effective when used as cementing spacer fluids. Lightweight, low-viscosity cementing spacer fluids appear to improve the total mud-removal process by eroding the mud filter cake and increasing the mobility of the drilling fluid. To maximize total mud displacement, displacing fluids (both spacer fluids and cementing slurries) had to be pumped as fast as possible. Even when turbulent flow could not be achieved by the cementing slurry, displacing at a maximum flow rate was more effective than plug flow displacement. Laboratory results were independent of spacer volume. However, for long intervals of interest, such as those under field conditions, the volume of spacer fluid probably should be considered.

Introduction
Effective isolation is a critical factor in successful completion of oil and gas wells. Effective isolation can be obtained only through effective displacement of drilling fluid by cement. Zone isolation failures are caused by incomplete displacement of drilling fluid by cement. This paper demonstrates how lightweight, low-viscosity fluids used as cementing spacer fluids can improve displacement. This work is a continuation of work reported in Refs. 1 and 2.
An experimental approach was used to study the effects of various spacer fluids in the primary cementing of a vertical wellbore. This paper demonstrates the effectiveness of various fluids as cementing spacer fluids under actual field conditions in the absence of pipe movement. By optimizing the use of lightweight, lowviscosity spacer fluids, the probability of obtaining a satisfactory primary cement job may be improved.

Previous Investigations
Research in displacement mechanics was performed first by Jones and Berdine in 1940. Their work and most of the subsequent research in the area of displacement mechanics focused on basic factors concerning displacement and did not involve evaluating cementing spacer fluids.
Camey, in the mid-1970's, performed research on cementing spacer fluids that could be used to separate cement and drilling fluid. His work involved developing a fluid compatible with various drilling fluids and cements. The main goal of his work was to develop a fluid that effectively would separate and prevent the mixing of drilling fluid and cement. Other work involved developing similar spacer fluids to help eliminate viscous cement-mud mixtures created at the interface between mud and cement.
Some field studies on the use of spacers have been performed. These field results indicated that improved cement bonding and zone isolation is accomplished when water-base spacers are used ahead of cement during oil or gas well cementing operations.
McLean et al. reported that a water preflush was very effective in breaking the gel structure of a mud inside a plastic model. They reported that a water preflush removed gelled mud even for mud densities higher than 1.438×10-3 kg/m3 (12 Ibm/gal). They further stated that water preflushes could remove heavy muds from areas where all other means may be ineffective. Their tests involved a testing apparatus that did not simulate field conditions accurately. They stated, however, that heavy mud may slough and fall into less dense flowing fluid, provided that the shear stress of displacing fluid is low enough to prevent any support to the mud. Although some work has been done to study the effects of various spacer fluids, an experimental study with actual field conditions never has been undertaken.

The Mud Mobility Concept
Earlier work introduced the mud mobility concept.
JPT
P. 1828^