More and more cement compositions used for oil and gas wells, such as resilient cement with post expansion additives, are designed to obtain elastic and strength properties as well as pre-stresses in the cement sheath that allow the cement sheath to resist tensile or shear stresses induced by pressure and thermal loads emanating from the well during its full life cycle.
The computation of stress changes induced by well loads once the cement is hardened is straightforward. Those stresses will depend on geometrical parameters such as casing and hole sizes on one side and on elastic properties of casing, cement and formation on the other side. The difficulty stems from the need to compute the pre-stress in the cement sheath which is the result of complex hydraulic, chemical, thermal and mechanical interactions during hydration. Another difficulty is to measure the evolution of relevant hydro-thermo-mechanical (HTM) parameters that affect the transition from hydrostatic pressure after placement to stress with shear components after hardening.
In the recent years, Total has developed both modeling and laboratory capabilities that allow to characterize the HTM behavior of cement under in-situ pressure and temperature conditions and to use the measured parameters to analyze the mechanical integrity of cement sheath from placement on. Those tools are now routinely used to determine the best suited ranges of mechanical properties on one side and to QC and adjust cement systems proposed by service companies. This approach allowed achieving successful cementation of wells in harsh environment such as HP-HT wells.
The development of the software and laboratory facilities with internal resources proved beneficial from many perspectives. A great advantage is to be able to know how the various parameters associated with different cement compositions are affecting the cement integrity under specific down hole conditions. More importantly, it allows Total as an operating company to perform sound validation through confrontation to real field cases.
The aim of this paper is to present the fundamental concepts used in the Total's proprietary cement integrity model called T-CemInt and to describe the associated parameters and the ways of measuring them in the laboratory. Also, the paper will show a real case study that included lab tests, modeling and post cement sheath assessment with standard logging tools.