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Abstract Recent advances in oil and gas cementing technology allow for the modeling and prediction of both compressive and tensile stresses upon an annular cement sheath, throughout the life of a well. Given the knowledge of the type and magnitude of stresses likely to be encountered in a specific location in a wells annulus gives designers target parameters for designing the mechanical properties necessary in the set cement to be able to sustain those stresses without failing. Such a mechanical failure in a cement sheath can cause a loss of annular isolation. However, the authors feel the ability to model these stresses is only one-half of the information necessary to design cement systems for long-term zonal isolation. While some good work has been done on certain lower density cement systems in an attempt to develop fit-for-purpose designs with improved tensile and flexural strengths, the authors have found that some wells requiring higher density cement systems, also need cements with "enhanced" mechanical properties. Towards this end, the authors have conducted mechanical properties research of several relatively common cement additives. These included organic materials as well as non-organic materials. For this study, these materials were added to oilfield cements with water contents averaging from 50 to 66 % by weight of cement (bwoc). Besides the more common unconfined compressive strength tests, the samples are also subjected to tensile and/or flexural strength testing. While the API has long ago established procedures for running unconfined compressive strength tests, there are currently no API standards in place covering the testing methodology for tensile and/or flexural strengths of oilfield cements. Accordingly, the authors present not only the mechanical properties achieved with the use of the various materials tested, but also the methodology used to achieve their data. In an effort to more closely scrutinize the effect each individual material has on the mechanical properties of the set cement, each additive is examined independently. Armed with this information, design engineers should be equipped to propose cement systems that produce effective long-term zonal isolation at the induced annular stresses of their own wells. Introduction In the process of oil and gas well drilling various types of cement systems are being placed into the annular space between the casing and the formation. The purpose of this cement is to structurally support the casing string and prevent casing corrosion, as well as to create a competent hydraulic seal for long-term zonal isolation during the entire operational life of the well. As mentioned by Ravi, the cement should meet a wide range of short-term criteria such as free water, thickening time, filtrate loss, gelling, strength development, shrinkage, etc., as well as certain long-term requirements like resistance to chemical attack, thermal stability and mechanical integrity of the cement sheath. In today's oil and gas fields, it is common to find design engineers who understand that changes throughout the life of a well can significantly impact induced stresses on the annular cement sheath responsible for maintaining annular isolation. Changes in wellbore stresses can affect the mechanical integrity of the cement sheath and can be caused by a variety of different factors such as:production rate changes depleting reservoirs formation compaction workovers stimulation treatments pressure and temperature changes secondary and tertiary recovery methods
Abstract Assurance of well integrity is critical and important throughout the entire well's life cycle. Pressure build-up between cemented casings annuli has been a major challenge all around the world. Cement is the main element that provides isolation and protection for the well. The cause for pressure build-up in most cases is a compromise of cement sheath integrity that allows fluids to migrate through micro-channels from the formation all the way to the surface. These problems prompt cementing technologists to explore new cementing solutions, to achieve reliable long-term zonal isolation in these extreme conditions by elevating shear bond strength along-with minimal shrinkage. The resin-cement system can be regarded as a novel technology to assure long term zonal isolation. This paper presents case histories to support the efficiency and reliability of the resin-cement system to avoid casing to casing annulus (CCA) pressure build-up. This paper presents lab testing and application of the resin-cement system, where potential high-pressure influx was expected across a water-bearing formation. The resin-cement system was designed to be placed as a tail slurry to provide a better set of mechanical properties in comparison to a conventional slurry. The combined mixture of resin and cement slurry provided all the necessary properties of the desired product. The slurry was batch-mixed to ensure the homogeneity of resin-cement slurry mixture. The cement treatment was performed as designed and met all zonal isolation objectives. Resin-cement’s increased compressive strength, ductility, and enhanced shear bond strength helped to provide a dependable barrier that would help prevent future sustained casing pressure (SCP). The producing performance of a well depends in great part on a good primary cementing job. The success of achieving zonal isolation, which is the main objective of cementing, is mainly attributed to the cement design. The resin-cement system is evolving as a new solution within the industry, replacing conventional cement in many crucial primary cementing applications. This paper highlights the necessary laboratory testing, field execution procedures, and treatment evaluation methods so that this technology can be a key resource for such operations in the future. The paper describes the process used to design the resin-cement system and how its application was significant to the success of the jobs. By keeping adequate strength and flexibility, this new cement system mitigates the risk of cement sheath failure throughout the life of well. It provides a long-term well integrity solution for any well exposed to a high-pressure environment.
Abstract Cement plugs play a central role in providing hydraulic isolation for oil and gas well integrity. They are routinely required for abandonment purposes, drilling sidetracks and wellbore remedial operations. Despite extensive industry experience from around the world, there are many cases in high-pressure, high-temperature (HPHT) wells where an otherwise straightforward cement plug operation has led to major non-productive time (NPT) resulting in escalation of overall well costs. There are a number of issues that increase risks especially when it involves placement of high-density cement slurries in HPHT wells. Downhole conditions present additional challenges, which make it difficult to do the job right the first time. Whenever a job goes wrong in these conditions there is often an impact apart from the immediate non-productive rig time. In addition to the increase in costs there are other associated impact, e.g. potential loss of downhole barrier with negative implications for safety and the environment. Many studies and publications have highlighted the risk of unmitigated fluids contamination during placement as one of the most common causes of cement plug failure. One service company with extensive experience operating in the North Sea has used a model that integrates design and planning combined with a structured, detail-oriented process workflow to reduce surface execution and downhole placement risks thereby increasing the chances of success. The model relies heavily on close cooperation between the service company and the operator. Following the same strategy, this model can be applied in other geographical environments with the core objective of improving quality ensuring the job is always done right the first time. Some case histories, which inspire confidence in the ability to sustain the success rate, are described in this paper.
Abstract Maintaining zonal isolation is vital to well economics and productive life. Well integrity is becoming more challenging with the drilling of deeper, highly deviated, and horizontal wells worldwide. Oil companies are focused on to enhance the well productivity during drilling long horizontal wells in a harsh environment by achieving maximum accessible reservoir contact. These wellbore geometries incorporate additional challenges to design and deliver a dependable barrier. In this paper, a case study about cementing the longest liner across Khuff-C reservoir has been presented discussing the main challenges, engineering considerations, field implementation, results, and conclusions. The well was drilled horizontally across Khuff-C carbonates using oil-based drilling fluid. The 5-7/8-in open hole section was planned to be cemented in single stage, utilizing 8370 ft of a 4-1/2-in liner. Careful attention was paid to estimate the bottom hole circulating temperature, using the temperature modeling simulator. A 118-lbm/ft3 slurry was designed to keep the equivalent circulation density intact. Gas migration control additives were included in the slurry design to lower the slurry's transition time, in order to reduce the chances of gas migration through the cement slurry. The slurry was batch-mixed to ensure the homogeneity of the final slurry mixture. A reactive spacer was designed to improve the cement bonding from long term zonal isolation perspective. Additionally, the spacer was loaded with optimum amounts of surfactant package to serve as an aid to remove the mud and to water-wet the formation and pipe for better cement bonding. Centralizers placement plan was optimized to allow around 63% average standoff around the pipe, staying within the torque and drag (T&D) limits. The cement treatment was performed as designed and met all zonal isolation objectives. The process of cementing horizontal liners comes with unique procedures. There are several challenges associated with carrying out wellbore zonal isolation for primary cementing of horizontal liners, therefore, a unique level of attention is required during the design and execution stages. The slurry design requires careful formulation to achieve the desired specifications while ensuring its easy deployment and placement in the liner annulus. By planning in advance and following proven techniques, many of the problems associated with the running and cementing of deep and long horizontal liners can be alleviated. This paper highlights the necessary laboratory testing, field execution procedures, and treatment evaluation methods so that this technique can be a key resource for such operations in the future. The paper describes the process used to design the liner cement job and how its application was significant to the success of the job.
Abstract In recent years, a project was initiated to identify a safe and practical alternative to conventional silica-based cement slurries. The objective was to eliminate exposure to respirable crystalline silica (RCS) in oilfield cementing operations. RCS, as well as being linked to various respiratory illnesses (1), has now been identified as a human lung carcinogen (2), implying there is an established link between exposure to RCS and lung cancer. When cementing well sections with a bottomhole static temperature (BHST) of over 110°C [230°F], a cement powder containing silica flour must be used to ensure the set cement matrix is stabilized to be able to withstand persistent exposure to these temperatures over time (mitigating against strength retrogression). A certain proportion of the silica within these conventional cement blends is classified as RCS. The newly developed system, known as zero respirable crystalline silica blend or ‘ZRCS-blend’, was successfully applied on numerous occasions on a Norwegian rig. Interestingly, the system found successful application in a batch-drilled conductor campaign on a high-pressure/high-temperature (HPHT) field where the potential exposure during production exceeded 110°C [230°F]. Whilst being a safer alternative to conventional cement systems, the ZRCS-blend was able to deliver a top of cement (TOC) to the seabed on each and every occasion during the campaign, thus eliminating the need for any remedial operations. As shown by proven success of the ZRCS-blend offshore on the Norwegian continental shelf, the system provides an alternative to silica-based cement for operations worldwide, to comply with regulations and health, safety, and environmental guidelines.