The Combined Effect of Temperature and Salinity on the Mechanical Behaviour of Well Cement

Carbondioxide Capture and Storage (CCS) has been identified as the best measure to reduce the concentration of carbon dioxide in the atmosphere. Carbon dioxide expedites global warming. The captured carbon dioxide is stored in deep underground reservoirs using injection wells. The integrity of these wells needs to be ensured to have a durable carbon dioxide sequestration. Generally, the well cements of these underground wells lose their integrity primarily due to their degradation caused by aggressive curing temperatures and also due to the salinity conditions prevalent in the earth’s down-hole. Therefore, the aim of this study was to ascertain the combined effect of the temperature and the salinity on the mechanical behavior of well cement. Sulphate resistant Class G cement samples were cured in various salinity concentrations (0 to 40% of NaCl of the weight of water) and at varying curing temperatures (25, 40, 60 and 80oC). The mechanical behavior of well cement under these varying salinity and temperature conditions was studied by analyzing its uniaxial compressive strength and the Young’s modulus. Scanning Electron Microscope (SEM) images of degraded samples showed microstructural variations caused during the degradation process. EDX (Energy Dispersive X-Ray Spectroscopy) tests were also carried out to find out the proportion of chemical ions in the degraded cement samples. The test results revealed that the uniaxial compressive strength of the samples initially increases up to an optimum salinity of 10% (by weight of water) and that it thereafter gradually decreases with increasing salinity. With the compressive strength varying with the curing temperature, the optimum temperature for 7 days of curing is found to be 40˚C and that for 28 days of curing is found to be 60˚C. On the whole, OPC sulphate resistant well cement shows its optimum strength at 60oC and at a NaCl concentration of 10%.