Over the past several years much interest has been generated in the industry of late in the development of ductile cements, exhibiting high values of tensile and flexural strength. There has been a great deal of field information suggesting that hydraulic isolation from cement is not what it should be or is believed to be. In other words many cement jobs are failing. Conventional cement systems used in oil wells exhibit high compressive strengths, yet are limited in tensile and flexural strength development. Recent field experience and research has shown that in the majority of cases, the mechanism of cement sheath failure is radial tensile cracking, causing a loss of hydraulic isolation.
Flexible cements are more able to withstand the tensile forces generated by fluctuations in downhole pressure and temperature caused by drilling, production and workover operations. Also affecting the cement seal are factors such as hydraulic fracturing treatments, high drawdown pressures, steam injection, and subsequent drilling operations.
This paper reviews the application of a flexible cement designed to address problems with loss of hydraulic isolation encountered in steam injection wells. The paper also reviews the basic principles for estimating the cement tensile strength requirement and describes the process used in selecting the final cement slurry formulation, including the use of a mathematical simulator. Also described is the introduction of a novel mineral fiber, which imparts higher tensile strength to the set cement and results in slurry that is better able to withstand the high temperatures induced by steam injection operations. The new flexible cement is also able to provide the required characteristics at lower slurry densities. A summary is presented of the results of the first 4 wells cemented with this technique in the Mossoró area of Northern Brazil.