In a wellbore, loss of zonal isolation can be caused by the mechanical failure of the cement or by the generation of a microannulus. However, the behavior of the sealant is driven by the specific boundary conditions like the rock properties. Large-scale laboratory testing of the cement sheath in an annular geometry and in a confined situation was performed to simulate various well conditions and to evaluate the behavior of several sealants under simulated downhole stress conditions. The failure modes of the cement sheath were determined as a function of the cement mechanical properties, loading parameters, and boundary conditions. The results were used to validate an analytical model that predicts cement sheath failure. Introduction Interzonal communication in a wellbore may lead to loss of reserves, contamination of zones, production of unwanted fluids, or safety and environmental issues. Remedial solutions exist to repair the problems, but for technical or economical reasons, the well may be shut in or abandoned. To improve the lifetime of the well, the cement sheath must be chemically and mechanically durable. Sealants resistant to aggressive formation fluids are designed when required. In the same way, sealants should be designed to withstand the stresses experienced during production and well operations - e.g., casing pressure tests, stimulation treatments, or temperature changes during production cycles-throughout the well life. To achieve this, a better understanding of the mechanical behavior of different sealants under downhole conditions is required to design fit-for-purpose materials.1,2 Several papers have been written on the subject. According to Thiercelin et al.,3 changes in downhole conditions can cause mechanical damage to the cemented annulus (mechanical failure or creation of microannuli) that may lead to a loss of zonal isolation. The key conclusion of that paper was that instead of considering the strength of the sealant as the main property, one should rather look at the complete mechanical system formed by the steel casing, the cemented annulus, and the formation. Indeed, increase of pressure and/or temperature in the wellbore firstly expands the inner steel casing, which instantly imposes this deformation on the neighboring cement sheath. As a consequence, imposed displacements rather than imposed stresses are applied to the cement inner diameter (ID). At a greater time scale (the lifetime of the well), the cement sheath must withstand multiple displacement cycles. Several authors have proposed numerical models4,5 to simulate the sealant mechanical behavior and predict initiation of failures according to known mechanical properties of the complete system (steel, cement, and rock). A large-scale laboratory test for sealants in an annular geometry has been developed. Changes in the well conditions resulting in either the contraction or the expansion of the inner casing can be simulated. Furthermore, the confining role of the formation or outer casing can be evaluated. Such an experiment allows the evaluation of the sealant mechanical response under wellbore conditions. Indeed, the nature of stresses generated in the annulus (tensile and/or compressive) is similar to those the sealant must withstand in a real wellbore. The loading scenario simulated in the full-scale annular sealing test is close to reality. Several cement systems exhibiting different mechanical behaviors have been tested, and the experimental results have been compared with the predictions of a numerical model. Laboratory experimentation The experiments are designed to compare different cement formulations at room conditions in a large-scale annular geometry and determine the effect of cement mechanical properties and boundary conditions (rock stiffness) on cement cracking and permeability to air. Imposed deformations can be applied on the cement ID to simulate changes in wellbore conditions caused by variations of temperature and/or pressure. Equipment The equipment developed for the study is shown in Figs. 1 and 2. There are two main components.

Summary Loss of zonal isolation in a wellbore can be caused by mechanical failure of the cement or by development of a microannulus. However, behavior of the sealant is driven by specific boundary conditions such as rock properties. Large-scale laboratory testing of the cement sheath in an annular geometry and a confined situation was performed to simulate various downhole stress conditions and evaluate the behavior of several sealants. Failure modes of the cement sheath were determined as a function of cement mechanical properties, loading parameters, and boundary conditions. Results were used to validate an analytical model that predicts cement-sheath failure. Introduction Interzonal communication in a wellbore may lead to loss of reserves, contamination of zones, production of unwanted fluids, or safety and environmental issues. Remedial solutions exist to repair the problems, but for technical or economic reasons, the well may be shut in or abandoned. To maximize well life, the cement sheath must be chemically and mechanically durable. Sealants resistant to aggressive formation fluids should also be designed to withstand stresses exerted during production and well operations, such as casing-pressure tests, stimulation treatments, or temperature changes during production cycles. To achieve this design goal, a better understanding of the mechanical behavior of different sealants under downhole conditions is required.1,2 According to Thiercelin et al.,3 changes in downhole conditions can cause mechanical damage (e.g., mechanical failure or creation of microannuli) to the cemented annulus, which may lead to loss of zonal isolation. Thiercelin et al.'s paper3 concludes that the complete mechanical system formed by the steel casing, cemented annulus, and formation should be considered, rather than sealant strength alone. Increase of pressure and temperature in the wellbore first expands the inner steel casing, which instantly imposes this deformation on the surrounding cement sheath. This applies imposed displacements, rather than imposed stresses, to the cement inner diameter (ID). Over the lifetime of the well, the cement sheath must withstand multiple displacement cycles. Several authors4,5have proposed numerical models to simulate sealant mechanical behavior and predict initiation of failures according to known mechanical properties of the complete system (i.e., steel, cement, and rock). A large-scale laboratory test for sealants in an annular geometry has been developed. This test simulates changes in well conditions that cause contraction or expansion of the inner casing. It can also evaluate the confining role of the formation or outer casing. Such an experiment enables the evaluation of sealant mechanical responses under wellbore conditions. The tensile and compressive stresses generated in the annulus are similar to those the sealant must withstand in a real wellbore. Loading simulated in the full-scale annular sealing test is close to real field conditions. Several cement systems exhibiting different mechanical behaviors have been tested, and the experimental results have been compared with predictions of a numerical model.

Placing the cement slurry in the entire annulus, in both the wide and narrow sides, is essential to achieving effective zonal isolation. Well conditions, such as deviated wellbore geometry, eccentric annulus, and gelled drilling fluid, pose unique challenge in achieving this objective. It is known in the industry that pipe movement helps improve hole cleaning and cement-slurry placement. However, the quantitative effect of pipe rotation on hole cleaning and cement-slurry placement for a given eccentricity, flow rate, and geometry was not well studied. Hence, it was difficult to design a primary cement job where the effects of all these parameters were considered at the same time and optimized for best results. The effective velocity and pressure drop in an eccentric annulus with and without pipe rotation is modeled for Herschel-Bulkley fluid. This is then extended to evaluate the effect of pipe rotation on hole cleaning and cement-slurry placement. The modeling clearly demonstrates the improvement in hole cleaning from pipe rotation in an eccentric annulus. Now there is a tool available to study the interaction of various factors on hole cleaning and optimize them for the well in question. The results from the modeling study have been compared with field data. The comparisons show a good match between the improvement in cement-slurry placement predicted that was inferred from the cement bond logs. The study clearly indicated the importance of using representative rheological model and input values. The modeling results and field validation are presented and discussed. The work presented in this paper should help the industry optimize the various factors during a cement job to help maximize hole cleaning and cement-slurry placement in the wide and narrow sections of the annulus. This should help in achieving zonal isolation for the life of the well and reduce operating expenses by reducing remedial jobs. Introduction In recent years, the study of the hydraulics effects of drillpipe rotation on annular pressure drop and equivalent circulating density (ECD) has been receiving a great deal of attention. Initially, researchers focused on theoretical studies and laboratory measurements of pressure drop (?P) (Luo and Peden 1987; Walker and Al-Rawi 1970). In these studies, the authors observed that, under laminar-flow conditions, drillstring rotation served to lower pressure drop and ECD. Several studies were later conducted in support of slimhole drilling efforts where the Di/Do ratios are quite high (0.75 to 0.85). In one of the slimhole drilling studies (Bode et al. 1991), data showed increased ?P with increasing drillpipe rotation speed (up to 200 rev/min) for fluids with Reynolds numbers < 2000. In the second slimhole study (Delwiche et al. 1992), field measurements of drillpipe-rotation effects on pressure drop were made, and the results showed increasing pressure drop with drillpipe rotation speed. In a third slimhole drilling study (McCann et al. 1995), the authors concluded ?P decreased with increasing drillpipe rotation speed for power-law fluids in laminar flow. In all of these early studies, no attempt was made to model the results into a coherent calculation scheme. In Marken et al. (1996), the authors reported increasing measurements of ?P with increasing drillpipe rotation speed (18 to 67% increase over the ?P reported at 0 rev/min). They cited the occurrence of centrifugal instabilities (Taylor vortices) and uncertainties in annular eccentricity and drillstring motion and vibration as reasons for industry models to incorrectly predict annular pressure drops with drillstring rotation. Later, others studied the effects of drillstring rotation in fluids in laminar flow from a theoretical perspective and concluded rotation had a significant effect on pressure drop, especially in smaller-diameter gaps through which fluid was moving (Ooms and Kampman-Reinhartz 2000). However, the modeling in this work was valid for Newtonian fluids only.

Cement systems that can survive in the CO2 environment are needed in various applications. Examples of these applications include:producers: CO2 in the reservoir is produced along with the hydrocarbons,injectors: CO2 is injected for sequestration and/or enhanced oil recovery, andproducers and injectors: produced CO2 can subsequently be injected for the purpose of enhanced oil recovery/sequestration. However, the carbonation of Portland cement is a well-documented, thermodynamically favorable process. When CO2 or carbonic acid comes in contact with Portland cement, it initially reacts with it to form water-insoluble calcium carbonate. Longer term, the presence of water dissolved with CO2 (or carbonic acid), if allowed to contact the cement sheath, can dissolve the calcium carbonate to bi-carbonate, which then could be displaced if a flow channel were to be present or formed during the life of the well. This can threaten long-term effective zonal isolation. A dual level solution is required to effectively address this challenge. As a first level, the potential for CO2 to enter the cemented annulus and contact the cement sheath is minimized, by placing the cement slurry in the entire annulus, reducing the permeability and endowing the set sheath with the properties necessary to withstand the well events. The second level involves reducing the amount of material in the set cement sheath that is reactive to CO2. This holistic approach has worked well in practice. Both the physical and chemical integrity of the cement sheath is addressed using this approach. Following the design logic described above, a cement system with improved resistance to CO2 environments was created by 1) designing a reduced-permeability cement sheath to withstand well operations with low cement hydration volume shrinkage and 2) optimizing the cement slurry formulation so that its hydration products have a lower amount of materials that are reactive to CO2. This cement system was then tested in the laboratory under expected in situ conditions and optimized for different well situations before placement in the field. The cement systems have been successfully placed in anumber of wells and these wells are all operating as required with no loss of zonal isolation reported. The design approach, laboratory test procedure and results from laboratory and field are presented and discussed in this paper. Introduction Technologies associated with carbon capture and storage (CCS) are coming more and more to the forefront as the world tackles long-term trends for increasing global energy demand coupled with the need to address the challenge of associated CO2 emissions (Fig. 1). In addition, it has been estimated that 40% of the world's remaining gas reserves contain more than 2% CO2. In considering how to deal with CO2 emissions, the scale of the task ahead cannot be underestimated. Global CO2 emissions are projected to rise from less than 20 thousand million tons in 1980, to wells in excess of 30 thousand million tons by the year 2030. CCS is one way in which it is hoped CO2 emissions to the atmosphere can be reduced. Currently four largescale CCS projects are operating around the world, each separating around 1 million tons of carbon dioxide per year from produced natural gas: Sleipner and Snohvit in Norway, Weyburn in Canada (with the carbon dioxide sourced in the United States), and In Salah in Algeria. In considering these numbers it is easy to see that as many as 10,000 Sleipner-sized projects might be required by the year 2030 if CCS were the only method selected to reduce CO2 emissions to 1980 levels.

Carbonate reservoirs are often characterized by high pressure and high content of H2S and CO2. For these reasons, drilling the reservoir is the most challenging activity of such fields and long-term zonal isolation across the reservoir section is one of the primary requirements. In the example well considered for this study, the production liner is set at a depth of approximately 4,500 meters and the mud density is 16.2 lb/gal (1.95 g/cc). After a production liner is cemented, the well undergoes several operations such as fluid displacement, casing/liner pressure tests, stimulations, production, and injection; these operations create load on the cement sheath. Carbonation of neat Portland cement systems in CO2 environments is well known in the industry. The carbonation is of significant concern if the CO2 can enter the cemented annulus. The surface area of the cement sheath that contacts CO2 should be minimized to help prevent carbonation. This can be achieved by reducing the permeability, preventing the formation of cracks and micro-annulus, and reducing the components in the cement sheath prone to attack from CO2. To assure long-term sealing properties of the cement sheath during the life of the well, a cement formulation has been developed to be mechanically durable and chemically resistant to aggressive environments. The cement system discussed in this paper was designed to withstand the stresses imposed by changes of pressure regime during the well life by improving elasticity and thus helping prevent damage to the cement sheath. In addition, the potential for carbonation was limited by reducing the components in the slurry formulation that could react with CO2. Mechanical properties and resistance to CO2 environments of the cement system were tested in the laboratory. The cement system was successfully evaluated in the yard test. Cement sheath analysis, slurry design and testing are discussed. The results presented in this work should help in the design and implementation of solutions to contain reservoir fluid and injected fluids, including in the presence of H2S and CO2. Introduction The main objectives for cementing are to:support the casing.protect the casing from shock loads (tubular collapse).provide a pressure-tight seal between zones containing either different pressure regimes or fluid content for the entire well life.protect the casing from corrosion.seal off zones of lost-circulation or thief zones. To meet these objectives, the properties required in the cement slurry and the set cement sheath includes the following:Stable at the given density—no free water and no settling.Easily mixed and pumped.Provide adequate thickening time, fluid loss, and gel strength.Meet the optimum rheological properties required for mud removal.Impermeable to annular fluids while curing.Develop strength quickly after placement in the annulus.Develop mechanical properties to help ensure well integrity for well life.Bond to casing and formation.Have low permeability to resist reservoir fluid migration and attack.Stable under downhole conditions of temperature, pressure, and chemical exposure.