Two different methods of evaluating cement hydration kinetics, namely chemical shrinkage and isothermal calorimetry tests, are used to investigate the early stage hydration of different classes of oilwell cement at various temperatures. For a given cement paste, the hydration kinetics curves measured by the two methods are proportional to each other at the same curing temperature. The ratio of heat of hydration to chemical shrinkage for different cements used in this study ranges from 7500 J/mL to 8000 J/mL at 25 °C and increases almost linearly with increasing curing temperature at a rate that varies only slightly with cement composition (approximately 58 J/mL per °C). A previously proposed scale factor model for simulating the effect of curing temperature and pressure on cement hydration kinetics is further validated in this study for its temperature aspect. The model is shown to be particularly helpful in correcting for slight temperature errors in the experiments.

In completion of oil and gas wells, cementing operations are employed to provide zonal isolation, a means to prevent wellbore fluids from contaminating sensitive zones such as freshwater aquifers. Perhaps the most important factor engineers and operators should consider for successful cementing is adequate drilling-fluid removal, or "mud displacement." To help optimize mud removal, the primary technique used is to pump a spacer fluid with modified rheology that creates a favorable fluid-fluid interface to enhance mud displacement. In many instances, it is highly desirable to monitor how this interface evolves over time. Fluid intermingling may inhibit the ability of a fluid to perform its intended purpose, for example, intermixing of spacer fluid with cement slurry may lead to contamination of the cement. This contamination may cause an undesirable failure of the setting of the cement and, consequently, a significant increase in cost because of increased wait time or remedial repair. Therefore, a three-dimensional (3-D) simulator modeling the intermixing of wellbore fluids in a highly eccentric annulus with casing reciprocation and rotation has been developed. The computational system is formulated on a general curvilinear coordinate system whose boundaries can conform to irregular boreholes such as those with washouts. Unlike existing models limited to weakly eccentric annuli without casing movement, the present simulator handles multiple real-world effects and efficiently performs trade-off studies that can enable more economical and effective cementing jobs. The finite difference model provides visual output useful in prejob design and post-job analysis. Among these outputs are 3-D color plots illustrating axial velocity, concentration, viscosity, and density evolution. Introduction Efficient mud displacement is perhaps the most important factor in providing a successful cement job. The primary technique used today is to pump a spacer fluid ahead of the cement slurry. Several other factors that directly impact mud displacement are also known, including wellbore geometry, mud conditioning, casing movement via reciprocation and rotation, casing centralization, and optimizing the pump rate.1,2 However, it is often unknown the extent to which these variables affect mud displacement, especially when applied in combination with one another. Even a relatively straightforward cementing operation can quickly become a challenging scenario with multiple variables. The industry has conducted numerous large-scale physical studies3–8 over the last half-century to empirically evaluate the importance of these factors on displacement efficiency. More recently, however, a number of studies have taken advantage of computational numerical methods to describe the different aspects of the mud displacement process in annular geometries. Tehrani et al.9 discuss combined theoretical and experimental studies of laminar displacement in inclined eccentric annuli. The authors appropriately couple the momentum equation with the concentration equation suggested earlier by Landau and Lifshitz.10 Cui and Liu11 address helical flow in eccentric annuli based on the bipolar coordinate system. Pelipenko and Frigaard12 examine fluidfluid displacement in a two-dimensional (2-D) "narrow annuli" without casing reciprocation or rotation. The well known model discussed by Escudier et al.13,14 considers non- Newtonian viscous helical flow in eccentric annuli for a single fluid.

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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. Introduction 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).