Cement sheath is a critical piece when constructing a well for long-term competence. The ability of the cement to block annular fluid flow has both economical and environmental implications. Examples of uncontrolled movement of formation fluids through a leaking cement sheath include unwanted water migrating to the perforations, hydrocarbons escaping to a lower-pressure reservoir, and hydrocarbon-based fluids flowing to environmentally sensitive water zones or to the surface.
Throughout the years, several advances in cement technology have been introduced to help ensure zonal isolation is maintained throughout the operational life of the well. Such advances include the design of cement with mechanical modifiers tailored for a specific well type and anticipated loads on the cement. Today, a new cement system is being introduced. This system aims to help assure zonal isolation, even if the cement is loaded beyond its capacity resulting in cracks, micro-annuli, or pathways for wellbore fluids to migrate. The new system works on the premise that the migrating fluids react with the damaged cement system resulting in the cement automatically sealing the cracks to help prevent further fluid migration.
The purpose of this paper is to illustrate simple evaluation techniques that allow for quantification of auto-sealing cements in a simulated static or dynamic-downhole environment. Experimental apparatus used consist of a standard annular ring mold used for expansion/shrinkage measurements as well as a cement fluid-loss test apparatus with a specialized insert designed for continuous flow past the cement test specimen. The capability of the cement system to react to the flowing fluid and ultimately reduce the flow is monitored. Results of various cement systems reacting with fluids under a range of simulated downhole conditions are presented.
The goal of the primary cement job is to establish competent zonal isolation for the life of the well. Competent cement will not only protect and support the casing strings but also allow for complete control of wellbore fluids by preventing them from migrating through the annulus. There are many factors that determine the effectiveness of the cement job.
The cement must be pumped in place, which requires the displacement of the drilling fluids already in the wellbore. Poor mud properties and decentralization of the casing string make the displacement process difficult, often resulting in contaminated cement that might not fill the full annular volume. Problems with the placement of the cement can yield a cement sheath that initially fails to provide the annular seal for which it is intended. There has been much work addressing the importance of mud removal and cement placement (Ravi et al. 2006; Tahmourpour et al. 2007).
Once in place, the hardened cement must be able to handle the thermally and mechanically induced stresses encountered during the wells functional life. Wellbore operations result in pressure and temperature changes on the inside of the casing string. These changes cause the casing to expand or contract. However, the cemented casing strings are not free to move so that the expansion/contraction results in stress changes in all the wellbore components: casing, cement, and formation. If the physical stress limits that the cement can withstand are exceeded, then cracks can develop within the cement. In addition, operational changes can cause the formation or casing to debond from the cemented annulus. If a cement sheath's hardened properties are not designed correctly for the functionality of the well, thus resulting in either cracked cement or gaps between the wellbore components, then flow paths can develop through the cemented annulus (Ravi et al. 2002a; Ravi et al. 2002b).