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

Abstract

Summary 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 [100 lbf/100 sq ft] gel strength in 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. Introduction Many aspects of oilwell cementing are important enough to warrant study. One area that needs attention is the procedures used in designing cement to be placed in the procedures used in designing cement to be placed in the well and the specific physical requirement of the cement for adequate downhole performance. The cement formulation is one of the few items that can be changed easily to suit existing well conditions, but only within limitations. Although the cement slurry is one of the most tested items pumped into a well, it is still one of the most variable. The basic design of a cement slurry starts with the determination of what general properties are needed for predicted well conditions. With the basic slurry formulation predicted well conditions. With the basic slurry formulation established, two physical properties need to be considered. The cement must remain fluid long enough to be pumped to its desired location downhole. Also, once the pumped to its desired location downhole. Also, once the cement is in place, it must set and develop an adequate compressive-strength value within a specific time period. Obviously, if a slurry cannot be pumped into place, period. Obviously, if a slurry cannot be pumped into place, its intended purpose would be, at most, academic. For this reason, the majority of laboratory time spent on testing oilwell cements is for testing thickening time to determine retarder concentration needed and to compare actual bulk-blend samples to laboratory blends. The thickening-time and compressive-strength tests account for most tests conducted on cement in field laboratories. Unfortunately, thickening-time and compressive-strength tests do not tell the whole story. The imperfect thickening-time test only simulates actual job conditions up to the predicted placement time. After test accuracy variation is allowed for, a thickening time longer than the placement time allows for some margin of safety, but only placement time allows for some margin of safety, but only for continuous pumping at a lower-than-predicted rate. Thickening-time safety factors do not relate directly to how long a slurry can remain static and still be moved after an inadvertent or intentional shutdown during placement. placement. Sabins et al. discussed the changes taking place during this time period. Tinsley et al. showed how these changes could contribute to annular gas flow. The physical property measured for this transition from the fluid physical property measured for this transition from the fluid to the solid phase was the static gel strength, and a static-gel-strength device for its measurement under high-temperature/high-pressure (HTHP) conditions was described. Rao et al. Showed the development of an ultrasonic test device that continuously measured compressive strength from initial set to any time of interest on a single sample maintained at HTHP conditions.