A number of reservoirs around the globe are in deep environments, and often it is necessary to drill and cement through salt sections to reach them, such as in offshore Brazil. These reservoirs are mainly in deep water and are commonly referred to as pre-salt zones. It can be very challenging to drill and cement the salt section. The salt zones could be a few hundred meters thick, creep at high rates, and when in contact with drilling fluid and cement slurry, their chemical properties can change. The effect of salt movement or creep is addressed in this work.Salt formations pose a unique challenge to the structural integrity of the wellbore by applying a time varying load on the cement sheath and casing. This load is caused by the existence of deviatoric stress and temperature variations. Deviatoric stress is the difference between formation in-situ stress and the wellbore fluid's hydrostatic stress. When a salt formation creeps during a well operation, it does so to achieve a hydrostatic state such that the deviatoric stress is zero.The present work is a study of cement sheath integrity under downhole thermo-mechanical loading conditions using finite element analysis (FEA). The effect of salt creep on the stresses experienced by the cement sheath at various stages in the life of the well, including drilling, cementing, shut-in, completion, and production, are simulated. A material model for salt creep is validated against experimental data reported in the literature. Multiple cement systems are compared for their effectiveness in providing zonal isolation for the life of the well. The effect of structural and thermal loads due to salt creep on cement are quantified through stresses in cement.The method discussed in this paper should help the industry select cement formulations that can withstand stresses caused by salt movement during the life of the well. Method and case studies pertaining to offshore Brazil are presented and discussed in this paper.

Annular pressure build up (APB) is caused by thermal expansion of wellbore fluid enclosed in an annulus experiencing cyclic thermal loads due to production and injection. This may lead to significant damage to the cement sheath causing zonal isolation issues and may also cause casing collapse/burst. This work focuses on using an engineered approach based on finite element modeling to analyze the cement sheath response to APB loads for the life of the well. Actual field data of pressure build up and bleeding rates are used in our analysis. The present study provides a methodology to analyze cement systems for their ability to survive APB conditions. A proper knowledge of response of wellbore materials (cement and casing) to APB can be helpful in design of better cement systems and in selection of appropriate casing materials which can withstand the APB conditions. It has been observed from our comparative study of two cement systems that the stresses in the elastic cement are less than those in conventional cement. A parametric study has also been performed to analyze the effect of different cement properties. The results are quantified in terms of stresses experienced by cement sheath and casing. This study is based on fundamental principles of structural mechanics, thus providing more reliable predictions. It helps in deciding the appropriate cement system for a predetermined production/injection rate or can give a recommendation of production/injection rates for a given cement system such that damage to the cement sheath can be avoided.

Expansion additives have been used in cement plugs to mitigate the potential risk of plug failure resulting from shrinkage. These additives are effective only when their amount is tailored for downhole boundary conditions, and their role should be well understood. This work discusses using an improved testing method that enhances the dependability of the volume change measurement and exhibits the impact of test boundary conditions on the shrinking and expanding behaviors of cement plugs. Boundary conditions investigated with this method include temperature, pressure, water access to the cement from the formation, and the role of mechanical constraints. Dependability is demonstrated by verifying the repeatability and reproducibility of the method at two different laboratories. Together with the noninvasive continuous volume change, supplementary measurements, such as ultrasonic compressive strength, tensile strength, and chemical composition analysis, have provided inferences about the mechanism of volume change. The new method embodies all attributes listed in API 10 TR2 (1997), including a constant external stress state in all measurements and a constant pore pressure during total volume change measurement. The results of percentage volume change from this test method present an extremely small variance, highlighting its repeatability; additionally, the measurement was reproducible between laboratories. Expansion value increased with a decrease in confining pressure, and excessive expansion in the absence of an effective confining pressure produced weak samples. The absence of outside water caused cement containing the expansion aid to shrink more than its neat equivalent; such observations highlight the importance of fluid boundary on the action of expansion additives. These observations were possible because the test method can capture temporal and boundary condition effects more aptly. Thus, the improved method provides a dependable measurement for tailoring plug properties.

Long-term zonal isolation requires effective mud displacement. One best practice for achieving this is to use the highest pumping rates allowable during cementing operations. However, in zones with a narrow pressure window, fracture pressure sometimes does not support higher flow rates. This is because high flow rates result in high friction pressures, which can exceed the fracture pressure of the formation. This is an issue particularly in long horizontal wells. In such cases, fluid rheology is important for achieving better displacement efficiency without inducing fractures. The current work discusses a procedure in which the rheology of cement slurry can be tuned such that good displacement efficiency is attained at sufficiently low flow rates. The tuning should be such that the shear stress is almost invariant for varying shear rates. This reduces the friction pressures considerably during placement. A hydraulics model was run on an example well configuration with a narrow margin situation. Pressure response and displacement characteristics of two types of slurries were analyzed. These include conventional and modified slurry, the latter of which was tuned for better mud displacement at low flow rates. The modified slurry showed an increase in yield point with a shear stress profile that was almost invariant for the range of shear rates analyzed. A decrease in equivalent circulating density (ECD) occurred as a result of rheology modification, and this resulted in avoiding ECDs exceeding the fracture gradient at critical locations. Displacement efficiency increased by 10% when using the modified slurry at the same pump rates as those used for the conventional slurry. On the other hand, when targeting the same displacement efficiency as the conventional slurry, the required pump rates for the modified slurry were lower. This study indicates that better displacement can be attained through rheology modification, even by using relatively low flow rates, thereby maintaining low ECDs to help ensure effective cementing operations.

Wells often require being drilled through and cemented across salt formations. In many parts of the world, salt sections consist of multiple salt types. These include Halite, Carnallite and Tachyhydrite. The last two salt types could move one hundred times faster than Halite and are chemically reactive. Typically, Carnallite and Tachyhydrite occur as streaks inserted between Halite. In these cases, cement sheath should withstand salt creep loading. Creep load is primarily compressive in cases of single salt types. For intercalated salts, varying creep rates can result in tensile loads, particularly at salt-salt junctions. Presalt exploration in offshore Brazil is an apt example displaying this behavior. In such a scenario, the cement sheath along a longitudinal well section should be analyzed for both tensile and compressive responses. It is well known that tensile strength of conventional cement systems is low, hence there is the need for detailed analysis and action to prevent damage to cement sheath.This work demonstrates cement design procedures by evaluating cement sheath mechanical integrity in intercalated salts. A typical presalt Brazilian well with different lengths of Tachyhydrite, Halite, and Anhydrite sections is used as an example in this work. A creep model for these salts is validated with data from triaxial creep testing. Responses of two cement systems, which are cured and tested, are compared. The analysis accounts for classical well loads during drilling and production etc., along with salt creep loading.This analysis shows that intercalated salts subject cement sheath to a series of tensile and compressive loads whose magnitude depends on the size and relative position of different salts. The salt-salt interface effects dominated the general tenet of increasing creep rate with increasing depth.This kind of detailed design procedure representing intercalated salt zones is the first of its kind known to the authors. Results from this analysis help fortify the importance of considering salt layering patterns during cement design for long term mechanical integrity. The design procedure discussed and presented in this study should help the industry design cement systems to withstand loads across single and intercalated salt zones and maintain well integrity.