Granular Calcium Carbonate Reinforced the Cement Paste Cured by Elevated Temperatures.

In a heavy oil thermal recovery well, cement paste experiences the cyclic elevated temperature and steam of steam stimulation, the elevated temperature and steam of steam driving, and the high-concentration CO (HCC) of in situ combustion conditions in sequence. To understand the effects of different conditions of heavy oil thermal recovery wells on the properties and microstructure of the cement paste, this paper investigated the influence of the cyclic elevated temperature, elevated temperature, and high-concentration CO conditions on the compressive strength of the cement paste. Then, low-field nuclear magnetic resonance, scanning electron microscopy, and X-ray diffraction were used to test the pore structure, microstructure, and crystal type of the cement paste cured under different conditions. Experimental results showed that the elevated temperature curing loosened the microstructure of the cement paste and increased its pore size and porosity, resulting in reducing the compressive strength to 21.04 MPa, compared with that of the cement paste at cyclic elevated temperature. For the cement paste cured under high-concentration CO conditions, the calcium hydroxide and calcium-silicate-hydrate reacted with CO to generate granular vaterite, aragonite, and calcite in the pores and cracks, which repaired the cement paste by reducing the porosity and pore size of the cement paste and increasing its compressive strength. When the carbonation time increased to 28 days, the cement paste was completely carbonized, and the compressive strength of the cement paste increased by approximately 169%, compared with that of the cement paste cured at an elevated temperature.