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 Mossor area of Northern Brazil.
Recent failures in a number of cyclically steamed, Huff and Puff, production wells in several fields in Northern Brazil, resulted in more a detailed study of the cementing slurries and techniques utilized in the area. The nature of the failure was noticed after steam vapor was observed at the surface after one or more cycles in several wells. Failure was also observed when increased water production was encountered and determined to be originating from nearby water-saturated zones that were thought to be isolated behind cemented casing. Costly and remedial treatments have been performed to shutoff this unwanted water production with limited success. After a remedial job, when a new steam cycle is commenced, the cement fails again (water cut increases) because the remedial slurry properties are similar to the original slurry.
The objective of this study is defined as follows:
Develop a cement slurry or cementing technique with the capacity to completely seal the annulus between casing and formation where steam injection and oil production cycles will be occuring and guarantee hydraulic isolation of the annulus after several cycles of steam injection.
Evaluate the cement-casing bond after various cycles of steam injection.
Incorporate new slurry technology into remedial cementing operations.
In order to develop a cement slurry that will withstand the conditions encountered in these fields a two step approach was incorporated. First, slurries were identified and evaluated with the use of a computer simulator. Second, the slurries were tested in the lab utilizing temperature cycles that simulate the extreme temperature changes of steam injection and oil production.