Effect of Bulk-Cement Storage Temperature Investigated
Authors: Dealy S. Thomas , Carpenter Bob 
Published: December 2007

Reports from the North Slope of Alaska indicated that cement stored in arctic conditions appeared to set faster than the same blends in cementing operations conducted elsewhere. Laboratory personnel on the North Slope observed that thickening times decreased as storage times increased. To test these findings, personnel selected one Class G and two Class H oilfield cements to use as test media. The cement samples were ground during the same month and were obtained from three different cement plants. These samples were opened on the same day and weighed into 750-g aliquots. Each aliquot was placed in a heavy polyethylene bag and vacuum-sealed to exclude any external moisture. The bags were then placed in a freezer, refrigerator, or oven and stored for a specified time (21-, 42-, and 63-day samples were stored at temperatures of -20°, 28°, and 110°F). One set of samples was cycled from the freezer to ambient conditions randomly over the test period.

Polished sections were then examined according to established cement microscopy techniques, and thickening times were determined based on API RP10 procedures performed on high-pressure, high-temperature (HPHT) consistometers. These tests were performed at 21-day intervals to gauge the effects of storage as a function of time.

Cement microscopy was performed with an Olympus BX-60 microscope with a polarized light attachment. Micrographs were captured with a Sony UP-5000 video printer. Particle size was determined with a Malvern Model S Mastersizer. Samples were first allowed to acclimate to ambient temperature and were then mixed with 309.2 g of deionized water before thickening-time tests were performed. The slurries were placed in a HPHT consistometer and tested according to Schedule 9.5 in the API Recommended Practice 10B. A mild temperature schedule was chosen to yield a lower standard deviation. Small aliquots of each sample were also taken at the specified times so that particle-size distribution could be determined and polished-section microscopy could be performed on the clinker phase.

All three cements exhibited shorter thickening times when stored at just below freezing, especially the two finer-ground cements (Class G and the north Class H), which are very similar chemically. As expected, the two Class H cements had longer thickening times when stored at 110°F (43°C), but the Class G cements did not appear to be affected at that temperature. All three cements had considerably longer thickening times when cycled between −20° and 72°F (−29° and 22°C). At all temperatures, the average particle size increased with age for the Class G cement and decreased for both Class H samples. The smallest particle size present also increased for the Class G sample at all temperatures, but it remained fairly constant for both Class H samples. The largest particle size present tended to increase for all three cements at all temperatures.

API cooperative test data over many years have shown a standard deviation of 15% for thickening-time tests at 125°F (52°C) with neat cements mixed at 38% water. Considering this standard deviation of 15%, our test data does not show a significant change in any of the cements stored at below-freezing temperatures, which fails to support the observations of North Slope personnel. Longer thickening times are observed after 21 days of cycling from below freezing to moderate ambient temperatures. This effect is probably the result of in-situ moisture being released and reacting with C3A during storage.