TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe investigation of cement integrity over life of well conditions continues to be a high priority within the well cementing industry. Increasing awareness of problems associated with cement sheath failure and subsequent loss of zonal isolation or sustained casing pressure have demanded that set cement material behavior and the coupled behavior of casing, cement and formation be more fully understood in order to make rational engineering decisions. Recent advances in wellbore stress modeling can now provide a probabilistic determination of the suitability of a particular cement design for the expected range of induced well stresses. This paper describes the cement evaluation and wellboremodeling methodologies specifically developed to predict the magnitude of tensile or compressive forces created by changing wellbore or reservoir conditions.

Ongoing problems with sustained annular gas pressure on producing wells wordwide is a prime indicator that more work is needed in understanding how cement system design can impact long-term annular isolation. One important area is the relative expansion or shrinkage of cement systems as they hydrate in the annulus of a well. Most Portland cement systems undergo chemical and bulk shrinkage as they set. A number of chemical materials have been developed that can be added to cementing systems to overcome this shrinkage, and in some cases, even impart a degree of expansion. However, the process whereby the degree of shrinkage or expansion is measured has almost universally been some type of a single point test, performed at ambient conditions. This is typically the case even if the cement sample was first hydrated under downhole conditions. The authors detail new methodology and test equipment that allows for the tracking of cement shrinkage and/or expansion real-time, under downhole conditions of pressure and temperature. The system allows for the application of curing pressures in either an aqueous or non-aqueous environment. Data acquisition is totally computerized and can be maintained for whatever time interval is required, without exposing the hydrating cement to surface conditions. With this new technology, the authors will detail how cement expansion and shrinkage can be significantly different under downhole conditions of pressure and temperature than when measured at ambient conditions. It has also been observed by the authors that cement expansion or shrinkage is a dynamic process that may take many hours or even days before any significant degree of volumetric stability is achieved. Finally, a successful field application of the new testing methodology will be presented as validation for the transfer of this new technology from the laboratory to the real world of oil and gas well cementing. Introduction Service companies and operators alike have become more aware of the many factors that can influence long-term zonal isolation in a cemented annulus. It is now apparent that a significant change in the physical volume of the set cement in an annulus, be it an excessive reduction (shrinkage), or even excessive increase (expansion), can play a major role in the determination of success or failure for long-term annular isolation. In order to better quantify these cement hydration volume changes, the authors have seen the industry progress from days when positive expansion was tested simply by observing if the setting cement would expand enough to break a glass bottle, to their newest micro-processor controlled expansion testor. Over the years, many skilled in the art of oil and gas well cementation have extensively researched the shrinkage that typically occurs when a Portland cement-based annular sealant hydrates, and undergoes a chemical transition from a liquid slurry state to the final set state, which will hopefully effect a long-term seal of the well's annulus. The fact that most typical Portland cement systems will exhibit some degree of shrinkage during this "setting", or hydration process, has been well documented in the literature, and for the most part is now accepted within the industry. The deleterious effects of cement shrinkage on annular isolation have also been well documented. This acceptance is in spite of the fact that there have been a wide variety of tests and devices developed in an effort to quantify both the type, and amount of shrinkage that occurs, without the oil and gas industry ever standardizing testing for cement volumetric changes during hydration.