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Remedial cementing requires as much technical, engineering, and operational experience, as primary cementing but is often done when wellbore conditions are unknown or out of control, and when wasted rig time and escalating costs force poor decisions and high risk. Squeeze cementing is a "correction" process that is usually only necessary to correct a problem in the wellbore. Before using a squeeze application, a series of decisions must be made to determine (1) if a problem exists, (2) the magnitude of the problem, (3) if squeeze cementing will correct it, (4) the risk factors present, and (5) if economics will support it. Most squeeze applications are unnecessary because they result from poor primary-cement-job evaluations or job diagnostics. Squeeze cementing is a dehydration process.
Abstract Lost circulation is a very common and expensive problem during drilling and cementing operations in the oil and gas industry. The lost circulation problems encountered during drilling or cementing are a result of one of two factors. These factors are the presence of zones of weak fracture gradients and the presence of high permeability or thief zones downhole. Light weight cements are an effective solution for curbing lost circulation caused by the breakdown of weak zones by conventional cement slurries. This paper presents a discussion of the different methods, additives and technologies that have been and are currently employed in the formulation of lightweight and ultra-lightweight cement slurries for cementing oil wells as well as recent developments based on an extensive literature review. This paper also discusses the mechanical performance, cost effectiveness and field logistical considerations of lightweight slurries formulated using different methods as these are important factors that impact decision making on what slurry extension method to choose for any given scenario. The information presented in this paper is derived from an extensive review of information contained in papers, journals and books spanning the last 50 years of well cementing and is summarized in such a way that the paper serves as a quick guide to cement slurry extension technology and techniques. An extensive review of the literature regarding lightweight cements showed that there are three general methods of obtaining lightweight cement slurries. These methods include increasing cement slurry water content (water extension) with the aid of viscocifying agents such as bentonite and sodium metasilicate, adding lightweight materials like glass microspheres and incorporating foam into slurries. Reported test data shows that apart from cement slurries containing glass microspheres and foamed cements, lightweight slurries exhibit lower compressive strength and slower compressive strength development than heavy slurries. Foamed cements pose the greatest design and field logistics challenge while cement slurries with glass microspheres have gained more popularity due to the excellent compressive strength values achievable at ultra-low densities despite their higher cost compared to water-extended slurries and lightweight slurries containing other lightweight additives like fly ash. The literature review presented also indicates a need for more research into improving lab mixing and testing methods that replicate field applications of foam cement. There is also a need for research into more cost-effective slurry extension additives and technology that exhibit acceptable mechanical performance for well integrity assurance and rheological properties favorable to proper cement placement in the annulus.
Abstract Cement must be designed in a way to ensure acceptable properties such as mix ability, stability, rheology, fluid loss, and adequate thickening time. Different chemicals are used when designing cement slurries. These chemicals are used as retarders, fluid loss additives, dispersants, gas migration additives and expansion additives. Typical examples of compounds used as retarders include: calcium lignosulfonate, sodium lignosulfonate, sodium tetra borate decahydrate (borax), starch derivatives, hydroxyethyl cellulose and weak organic acids. Examples of dispersants are ferrous lignosulfonate, acetone, and polyxythylene sulfonate. Many additives for fluid loss are water soluble polymers such as Vinyl sulfonate based on the 2-acrylamido-2-methyl-propane sulfonic acid. To the best of the author's knowledge, there is no study that compares the performance of different chemicals in cement designs. The objective of this paper is to detail some of the cement chemistry and to go over the chemicals used in cementing oil and gas wells and their mechanisms of actions.
Micropozzolan spheres, condensed from the vapors of a metallic silicon/ferrosilicon liquid, possess multifunctional applications in oilwell cements. Commonly available and cost-effective, this amorphous substance imparts significant improvements in the physical and mechanical properties of Portland cements. The practical utility of silica fume as an admixture is practical utility of silica fume as an admixture is primarily attributable to its particle size, chemical composition, primarily attributable to its particle size, chemical composition, and reactivity. Composed largely of silicon dioxide (SiO2), silica fume has an average particle diameter of 0.1 pm (100 times finer than cement or fly ash) and yields a surface area on the order of 20 m2/g.
silica fume was added to cements or cement/fly ash mixtures, several beneficial effects were noted: (1) the water consumptive nature of silica fume allows it to function as an extender and a pozzolan substitute for lightweight cements; (2) high water adsorption combined with an increased pozzolanic reactivity promotes enhanced compressive strengths; (3) the purity and solubility of the material makes it suitable for combating strength retrogression in cements at temperatures above 230 deg. F (110 deg. C); (4) permeability and alkali content of the set product we reduced, both desirous properties in a carbon dioxide (CO2) environment. properties in a carbon dioxide (CO2) environment
As a special function admixture for construction concretes, silica fume has been shown to provide such beneficial properties as very high strength, impermeability', high electrical resistivity, superior abrasion endurance, freeze-thaw durability, and sulfate resistance. First used commercially in 1969, silica fume is being used increasingly for oilwell cementing applications.
Silica fume is a waste product generated during the production of silicon/ferrosilicon metals. In the production of silicon/ferrosilicon metals. In the presence of carbon, quartzitic silica is reduced in the presence of carbon, quartzitic silica is reduced in the 2000 deg. C (3600 deg. F temperatures of electric arc furnaces. A portion of the produced silicon reacts with air to form silicon dioxide (SiO2). As the resultant SiO2 fumes cool, they condense into tiny vitreous particles composed primarily of amorphous silica. The condensed silica fume is then processed to remove impurities and control particle size.
Also known in the construction industry as ferrosilicon dust, silica dust, microsilica, amorphous silica, or volatilized silica, the chemical composition of this material varies somewhat depending on its source. A typical analysis shows silica fume to be composed of 91-95% SiO2 0.5-3% Fe2O3, 1-3% mixed alkalis and 12% carton. The surface area of silica fume is generally in the range of 19-21 m2/g. Portland cements and fly ash have specific surface areas ranging from 0.3 to 0.7 m2/g. Even tobacco smoke with a specific surface area of 10 m2/g, has only half the surface area of silica fume.
This paper describes development of a cement slurry composition designed to cope successfully with gas migration problems. The gas blocking effect is obtained by addition of microsilica, a pozzolanic material of extremely fine particle size. Numerous tests have been performed under laboratory conditions to investigate the phenomena of gas migration through cements, and the corresponding amount of specially selected microsilica needed to effectively prevent gas flow.
Gas flow behind casing after cementing has been a common problem in the industry since the inception of oil well cementing. After 1970 a comprehensive understanding of the mechanism involved in gas migration has been obtained through laboratory testing and field trials. Different concepts pertaining to solution of the problem have been suggested. Extensive reviews of literature on gas migration mechanism and methods to prevent gas migration up to 1985 have been given by Sutton and Faul and Cheung and Beirute.
In Statoil's operation the need for gas tight cement was dictated by two frequently occurring phenomena in exploration and appraisal drilling in the Gullfaks field area, block 34/10 in the Norwegian North Sea: Gas migration through cement from a shallow gas zone outside 20" casing, and migration of gas from the paleocene and cretaceous formations outside 13 3/8" and 9 5/8" casings during the setting process of the cement. Both problems have presented serious, costly and time consuming consequences. In 1983 a research program was initiated to develop a lightweight gas tight cement for shallow gas problems. During the research period microsilica was found to have a suppressing effect on gas migration. Further study showed that provided a certain minimum dosage of microsilica was added, the slurry would exhibit gas blocking properties.
The first field test with microsilica was performed in 1985, when 20" casing was cemented through a shallow gas zone. No gas migration occurred. Since this initial test, some seventy casing strings have been cemented with microsilica based slurries with density variations from 1.54 g/cm 3 to 1.95 g/cm3. A total success has been achieved by tailoring each slurry composition to adapt to existing field conditions.