Effect of Excessive Retardation on the Physical Properties of Cement Slurries
Authors: Sabins Fred L. , Sutton David L. , Cook Clyde 
Published: December 2007

Summary
As well depths become greater over-retardation becomes more and more commonplace. Precise retarder response is difficult to attain at high temperatures and many cementing slurries end up with thickening times far in excess of what is considered good practice. To avoid sensitive retarder concentration, field blends often show thickening times of more than 6 hours for jobs where 3 to 4 hours is sufficient.
Excessive thickening time has been commonly considered detrimental to good cement jobs, but the effect of over-retardation on cement properties was mostly derived from theoretical speculation. Through an extensive laboratory study, thickening time retardation is correlated to static gel strength development, initial set, and early compressive strength. The results indicate that moderate retardation has minimal effect on static cement properties. For deep wells, thickening times up to 10 hours do not appear to be detrimental to early compressive strength at bottomhole static temperature (BHST).

Introduction
The design of cement slurries and the actual techniques used for cementing hot, deep casings and liners becomes especially important under severe conditions. Most of these conditions complicate slurry design. Some of the more critical design parameters are high-pressure gas sands, lost circulation problems that limit amounts of hydrostatic pressure, and small hole-pipe annuli.
Previous work on cementing long, deep liners and casings has been directed at increasing the displacement efficiency of the gelled or immobile mud from the annulus. This displacement problem has been addressed in many papers and studied in great detail. Another very important but sometimes overlooked aspect of deep-well cementing that merits attention is the performance of the cement slurry in the static state after it is placed in the annulus.
The first basic design test that every proposed deepwell cement slurry undergoes is a thickening time determination. This value is a dynamic measurement of the hydration of the cement at bottomhole circulation temperature (BHCTI) and pressure (BHP). Many times, this is the only datum that is readily available to indicate how well the cement will perform after placement. It is important to be able to determine to what extent changes in a cement thickening time affects the cement's ability to support the pipe and to control gas flow as it develops compressive strength. The dynamic thickening time must be related to critical cement properties occurring after placement. Because temperature is the most important condition affecting hydration of cement, it is also critical to determine the effects of downhole temperature on the cement slurry properties.
To study these phenomena fully, the following subjects are analyzed on the basis of laboratory collected data.What is the relationship between the laboratory measured thickening time and the actual hydration (and set) of the cement under downhole conditions?What is the correlation of thickening time with gel strength development and the corresponding annular gas flow potential?At what point will we actually see over-retardation influence the properties of the cement slurry (gel strength development, set time, and compressive strength development)?In deep applications from 3048 m to 4877 m [10,000 to 16,000 ft] how do temperature differentials between the top of cement (TOC) and bottom of cement (BOC) affect static gel strength, bonding, sealing, and compressive strength development?What are some design criteria that could enhance cement performance in deep, hot applications and limit waiting-on-cement (WOC) time?

Scope and Testing Program
The mechanism of hydration that a cement undergoes is a complicated process. Some of the variables that influence this process are water content, admixes, stirring time, temperature, and pressure. The two conditions that cannot be modified by design are the downhole temperature and pressure, with temperature being the most critical. Thickening time measurements are conducted on cement slurries at the BHCT and BHP. From a design standpoint, thickening time should provide enough time to place the slurry with ample safety factor to allow for unexpected job difficulty. The safety factor desired for each job varies from company to company.
JPT
P. 1357^