David L. Sutton

The Relationship of Thickening Time, Gel Strength, and Compressive Strength of Oilwell Cements

A previous investigation of the effect of thickening times on early compressive-strength and gel-strength development has been expanded to include a wider range of well conditions, a greater variety of slurry types, and a more thorough evaluation of static gel strength. No positive relationship between thickening time and the onset or the rate of static-gel-strength development could be found. With few exceptions, all slurries tested developed values greater than 48 Pa (100lbf/100 sq ft) gel strength is less than 20 minutes. Data presented show that times needed to develop a specific static gel strength are more closely related to the type of slurry than to thickening time. A calculation method is given for estimating shutdown factor from static-gel-strength data. New data confirm that 12- and 24-hour compressive strengths are not significantly reduced by reasonable increases in thickening time. The maximum single-stage cement interval without an unreasonable waiting-on-cement (WOC) time for specified compressive strength has been redefined in terms of interval temperature differences.

An Ultrasonic Device for Nondestructive Testing of Oilwell Cements at Elevated Temperatures and Pressures

This paper describes a measuring technique and a hardware system that automatically monitors and records the complete history of compressive strength development and initial set of oil well cement slurries under high pressures and high temperatures (HP/HT). The design is based on the transmission characteristics of ultrasonic compressional waves through cement slurries. In principle, an analyzer measures the transit time (reciprocal of velocity) of an ultrasonic wave pulse through a slurry sample, converts it to apparent compressive strength, and records the results continuously. Compressive strengths, determined from ultrasonic transit time data, have shown deviations equal to or better than those obtained from mechanical tests. Since specimens are not removed and exposed to atmospheric pressure, the results should be more representative of actual downhole conditions. Also, the process promises to be well-suited to strength retrogression studies since there are no time gaps in the test results. Further experimental and analytical work is being carried out to evaluate the potential of this technique for measuring other slurry properties.

New friction correlation for cements from pipe and rotational-viscometer data

This paper presents the results of 24 discrete slurries tested on a pipe flow loop while data were taken simultaneously with a flow-through rotational viscometer. Each slurry was pumped through four different pipe sizes at rates from low laminar to high turbulent flow regimes. Test slurries varied from very thixotropic to highly dispersed and from lightweight to dense. Analysis of pipe-flow data showed a correlation best described by a Bingham-plastic model. Standard flow properties calculated from the rotational-viscometer data were correlated to flow properties derived from the pipe-flow data. These results could be used to calculate pipe pressure losses with improved accuracy. Correlations for estimating friction losses of cement slurries in laminar and turbulent flow regimes and for predicting laminar/turbulent transition were developed. Results of these correlations are compared with the findings of previous investigators. An example illustrates the use of these correlations.

Effect of Excessive Retardation on the Physical Properties of Cement Slurries

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).