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.
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.
In this paper we present new cement systems where the value of elastic parameters are specified to meet the requirement for long term mechanical durability. These systems can also contain expanding additives leading to an optimum configuration to prevent loss of zonal isolation due to change of down hole conditions. The advantages of these systems are demonstrated in field test cases where the cement sheath behaviour as a function of changes of down hole conditons is modeled to determine whether damage could occur or not. Introduction Cement in oil and gas wells is placed in the annular gap between the drilled formation and the steel casing. The main function of the cement is to prevent any fluid communication between these drilled formations to provide long term zonal isolation. Zonal isolation has to be achieved during the life of the well and after its abandonment. However, even if the slurry was properly placed during the cementing job, and initially fulfills its isolation role, changes in down hole conditions can induce sufficient stresses to destroy the integrity of the cement sheath. The consequence will be a loss of zonal isolation1,2,3 which can be detected, for example, by long term gas migration problems or, even worst, by casing collapse. The loss of zonal isolation, in absence of chemical attack, can be due to either mechanical failure of the cement itself, debonding of the casing from the cement or debonding of the cement from the formation. Mechanical failure leads to the formation of cracks while debonding leads to the formation of a micro-annulus. Both mechanisms create a high conductivity path for any fluid. To quantify the deformation mechanism and the amount of damage which are generated down hole, mathematical modeling of cased cemented wellbores was previously carried out3. This modeling determines the properties the cement must have to prevent loss of integrity. To avoid mechanical damage, it was determined that cements with high tensile strength to Young's modulus ratio, and with a low Young's modulus value compared to that of the rock, are the best cements in term of mechanical durability. These requirements are functions of the down hole specific well environment such as well geometry, casing properties, rock mechanical properties and expected loading history. Mechanical damage is either caused by a large increase of wellbore pressure (pressure integrity test, increase of mud weight, casing perforation, stimulation, gas production), a large increase of wellbore temperature (geothermal production, steam injection, HT/HP wells) or the formation loading (creep, faulting, compaction). The weaker the formation, the worse the condition since a weak formation is not able to mechanically support the cement deformation. In the case of temperature increase, the thermal properties of the steel, cement and rock, and the rate of temperature increase have also to be considered. Recently Bosma et al4 follow the same approach and agree that to attain effective zonal isolation a mathematical model based on solid mechanics has to be applied. They also recommend that the selection of the well sealant should be engineered by taking into account the mechanical properties of the "sealant". The compressive strength alone is not sufficient as the quality factor. Finally, mechanical damage can also follow excessive shrinkage of the cement, as demonstrated in Thiercelin et al7, and non shrinking cement is recommended.
Marcellus gas-shale trends have transformed the regional and national outlook for natural gas supply and are particularly attractive for their proximity to high demand markets and existing pipeline infrastructure. Marcellus shale plays offer unique operational and regulatory challenges during mud removal, cementing, and completion operations. Sustained casing pressure (SCP) is one of the greatest challenges encountered after completion. The Department of Environmental Protection (DEP) in Pennsylvania has enacted strict policies regulating cementing practices in Pennsylvanian Marcellus shale trends to reduce the risk of inter-zonal communication and SCP due to substandard annular cement sheath integrity. To ensure compliance with DEP cementing guidelines, a flexible, expanding cement system (FECS) was developed with fit-for-purpose mechanical properties. A further FECS blend modification included an additive to promote bulk cement expansion during hydration. Since implementation, this approach, coupled with good mud removal and cementing best practices, resulted in rapid static gel strength (SGS) development, acceptable compressive strength development and waiting on cement (WOC) time, and improved flexible and expansive properties. Since introduction in 2010, six jobs (two intermediate and four production strings) have been successfully cemented with FECS technology. A Marcellus shale trend case study will be presented in this paper that discusses the successful application of FECS during cement placement around a production casing. After completion of each job, a successful shoe test was performed. After stimulation/fracturing treatments, SCP was not reported by the client in Marcellus wells cemented with FECS. Since implementation in 2010, FECS technology has become a proven approach for cementing Marcellus horizontal tight-gas shale environments where long-term zonal isolation and minimal SCP are required. This approach has been applied to Marcellus shale and Permian Basin formations while other applications are currently being explored.
TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThere is a very large number of wells worldwide that leak or have sustained casing pressure (SCP). In Central Europe and the Middle East there are hundreds of wells with reports of trapped pressure that cannot be bled off. In the US and Canada there are thousands of wells leaking to surface, which may or may not be discharged to the atmosphere. Furthermore, 25% of all wells in the Gulf of Mexico have measurable sustained casing pressure. Additionally, remedial work fixing issues relating to cement failure has been estimated to be more than $50M a year in the US alone.Throughout the lifecycle of a well, planned cycle or operational changes can contribute to unknown damage to the cement sheath integrity that is hard to identify or locate, including the generation of a microannulus. Within flow paths, hydrocarbons can either migrate to surface, or become trapped below the wellhead leading to pressure build-up. Typical events occur during cementing, while perforating or stimulating, throughout the subsequent production, and even after abandonment. These can easily create this loss of cement integrity.This paper describes a novel isolation system that is activated only when a cement integrity problem occurs. The system will automatically and rapidly form a complete hydraulic barrier by swelling in the presence of hydrocarbon flow. Once activated, it will seal the damaged zone, and will even be able to be activated again, should further damage occur again during production or abandonment. The system has properties equivalent to conventional cement systems, and requires no modifications to standard surface equipment.High pressure static and dynamic laboratory tests highlight the ability of the system to rapidly shut off gas flows within 30 minutes. Field tests have also highlighted the robustness of the system, with a number of wells currently using the system remaining leak-free.
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