Silica content influence on cement compressive strength in wells subjected to steam injection

High temperatures cause deleterious changes in hydrated products of Portland cement, compromising its physical properties, mainly, the compressive strength. This phenomenon is known as strength retrogression and occurs when the CaO/SiO2 ratio is close to 2.0. To deal with this condition, silicon dioxide (SiO2) rich materials are added to cement. In this study four slurries with 0%, 30%, 35% and 40% BWOC of silica flour were formulated and tested under conditions of 38 °C and 300 °C for 7 and 28 days to cure it. The highest temperature was chosen to represent the one employed in thermal cycles, when an oil well is subjected to steam injection, seeking to increase the production - Enhanced Oil Recovey (EOR). In order to investigate the influence of temperature on the behavior of the compressive strength on the samples, a uniaxial compression test and crystallographic characterization of the material tested through the technique of X-ray diffraction (XRD) were performed. Research has shown that the SiO2 reacted at high temperature for the two periods evaluated, forming the xonotlite phase and improving the mechanical behavior, especially in the slurries with 35% and 40% BWOC. The standard slurry, without silica, was heavily damaged by the strength retrogression phenomenon after the heat cycle of 300 °C for both 7 and 28 days. For the slurries that were submitted to 38 °C (low temperature) for both periods, the silica flour basically acted as a load in mixes with cement. Also, it was noticed some mechanical strength loss of the samples with silica when compared with the standard sample, without silica.