Results from a detailed program undertaken to characterize the properties of both a curing and a cured cement sheath are presented. These properties include (a) hydration volume change during curing, (b) mechanical properties and (c) endurance limit, and are used to design cement systems to withstand stresses resulting from well operations.
Industry evidence indicates a significant fraction of wells are exhibiting annulus pressure during the life cycle of the field. Ideally, the drilling fluid in which the casing is run should be displaced, and the cement slurry placed in the annulus covering the critical zones for zonal isolation. The cement sheath placed in the annulus should have optimum properties to withstand stresses during well operations. However, inadequate post-curing sheath properties can negate a successful placement operation.
The hydration volume change during cement curing is measured for different cement systems. Both the so-called internal and external volume changes are measured. Procedures are discussed to measure these properties under downhole conditions. The implications of these values on downhole performance of cement sheath are also discussed.
Small-scale tests were conducted to determine the effect of changes in pressure and temperature on cement sheath characterization. The results from these tests, along with the relation to the properties measured are presented and discussed.
Endurance limit is defined, as with a metal, as that percentage of the yield to which the cement sheath can be subjected to and still withstand a large number of load cycles. Test procedures and results for different cement systems will be presented and discussed along with the implications on downhole performance.
The work presented in this paper can be used to characterize cement sheath properties under downhole conditions and design cement sheath to withstand stresses during the life cycle of the well.
The main purpose of a primary cementing job is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically. To achieve this objective, the drilling fluid should be removed from both the wide and narrow annulus, and the entire annulus should be filled with competent cement. The cement should meet both the short-term and long-term requirements imposed by the operational regime of the well. Typical short- and long-term properties required from the cement are listed in Table 1.
(Table in full paper)
Traditionally, the industry has concentrated on the short-term properties that are applicable during the cement slurry stage. This is necessary and important for effective cement slurry mixing and placement. However, the long-term integrity of cement sheath depends on its mechanical properties such as Young's modulus, tensile strength, resistance to downhole chemical attack, etc.
After placing the cement slurry in the annulus, if there is no immediate migration of fluid to the surface, it is likely that short-term properties such as density, rate-of-strength development, and fluid loss of the cement slurry have been designed satisfactorily.
However, recent experience has shown that after operations such as completing, pressure testing, stimulating, and producing, the cement sheath could lose its ability to provide zonal isolation.
