Impact of additives on well cement setting temperature and on casing design

Cement is composition of several raw materials such alumina, silica, iron and magnesium oxide that burned together in the kiln and as a result of whole process the powder is obtained. This raw material is widely used in many industries, including petroleum industry as well. In drilling operations, cement is used to provide isolation between casing and formation. It helps to hold casing string in the well properly and eliminate fluid migration from the formation in to the wellbore. In the petroleum industry, cement slurries are prepared at the surface where mainly cement is combined with water and some other additives for various purposes. According to the problems raised in the well, cement slurries are designed and pumped behind the casing throughout the well. Whereas these additives help to eliminate encountered problems in the well but meanwhile inverse effect of these components change other properties as well. For instance to additives, accelerators help to increase the rate of hydration at the early ages while retarders prevent hydration and slow hardening or having extenders in the slurries are resulted to an increase at the rate of pumpability. Particularly, in permafrost zones and in extremely deep wells designing of slurries at the surface become more challenged. Abnormal pressures and thermal loads directly affect the cement slurries or reverse, added agents in cement composition at the surface may change the physical properties of the slurry in these particular cases. In other words, internal and external alterations may affect the wellbore cement stability throughout the setting period. Natively, combination of the cement with water is an exothermic chemical reaction. However, in this study, the effects of some additives on the extra heat generation of the cement slurry has been investigated experimentally. Two different broadly used additives, friction reducer, CFR–3 and fluid – loss additive, Halad9 had been used throughout tests. Approximately 120 different samples were prepared to observe temperature track during 24 hours. Samples divided into 12 different sets where in each set of experiment, concentration of friction reducer was kept constant while concentrations of fluid – loss agent looped from 0.1% by weight of cement up to 1.0%. It would be relevant to note that all of samples are performed in the laboratory according to the API 10 B – Recommended Practice for Testing Well Cements. Obtained results from the experiments illustrated the direct effect of additives on the heat generation of cement during setting. At the different concentrations of agents, variety of temperature readings have been recorded. Maximum temperatures were recorded above 70 0C at the friction reducer concentrations of 0.3% by weight of cement. Furthermore, it was also monitored that addition of fluid – loss additive may cause to reach maximum temperatures lately and Halad9 did not let to raise temperatures in the experiments. The role of CFR–3 in the temperature contributions is higher than Halad9. After all, obtained maximum temperature readings were assessed and statistical approach applied to them. Hence variety of readings existed among the samples, a reasonable results could not be yielded from them. Nonetheless, using statistical evaluation for individual sets triggered to figure out some statistical relationship between them. Specially, the values of R - squared at the CFR-3 concentrations of 0.4% and 0.5% BWOC were high enough to set mathematical relationship between them. The value of R – squared was 0.955 for CFR-3 compositions at the 0.4 per cent concentration and 0.956 for 0.5 per cent by weight of cement. Thermal loads due to additives in the cement slurries were taken under consideration as well, particularly in the casing design and/or formation during the time period of first 24 hours. Thermal loads regarding to maximum temperatures at the early times of hydration also was studied and the magnitude of the stresses were computed. In general, these effects are not so much to hazard the casing or formation, but shall not be neglected during the designing.