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Fluid-Loss-Control Additives (FLAs) are used to maintain a consistent fluid volume within a cement slurry to ensure that the slurry performance properties remain within an acceptable range. The variability of each of these parameters (slurry performance properties) is dependent upon the water content of the slurry. If the water content is less than intended, the opposite will normally occur. The magnitude of change is directly related to the amount of fluid lost from the slurry. Because predictability of performance is typically the most important parameter in a cementing operation, considerable attention has been paid to mechanical control of slurry density during the mixing of the slurry to assure reproducibility.
Abstract Commonly used fluid loss additives (FLAs) in today's invert emulsion drilling fluids include materials with various attributes. The unmet needs of existing materials may include: Environmental restrictions due to ecotoxicity or biodegradability concerns Performance issues at high temperatures Overdosing at high temperatures High costs Formation damage To address these challenges, a FLA was developed for invert emulsion drilling fluids that is made from a renewable raw material and performs at high temperature and high pressure. The renewable raw material used to make this novel FLA is a biopolymer byproduct of the paper pulping process, and was chemically modified under controlled conditions to create a high-performing FLA. Detailed testing was done to determine the additive's performance in different base oils (mineral and diesel), at various mud weights (12 to 16 ppg), at elevated temperatures and in different fluid systems characterized by rheology and high-pressure, high-temperature (HPHT) fluid loss. The novel FLA was compared to other commercially available FLAs for fluid loss performance. The novel FLA outperformed or was on par with the industry available FLAs tested in this study. The novel FLA realized comparable fluid loss performance of less than 10 ml at 375 F at lower concentrations as compared to the industry FLAs. In some cases, the novel FLA performed at higher temperatures, whereas some of the industry available FLAs did not. The novel FLA also boosted the electrical stability (ES) of the emulsion in certain fluid systems. The novel FLA showed minimum change in the rheology of the oil-based fluids as compared to the industry available FLAs. The novel FLA demonstrated reasonable performance in different mud weights, base oils and fluid systems. Since this novel FLA is derived from a renewable raw material, it may have less of an environmental impact compared to other FLAs utilized today. The novel FLA: Was developed from a renewable raw material for invert emulsion drilling fluids; Performed on par or outperformed industry available FLAs; and Boosted the ES of the emulsion for certain fluid systems.
Abstract Fluid loss additives (FLAs) based on 2-acrylamido-2-tert.-butyl sulfonic acid (ATBS) provide fluid loss control by reducing filtercake permeability through polymer adsorption. However, when highly anionic dispersants or retarders are present in the slurry, adsorption of the ATBS polymer is hindered resulting in high fluid loss. To overcome this problem, ATBS - N,N-dimethyl acrylamide (NNDMA) copolymers were modified with phosphate groups to enhance their affinity to the surface of cement and make them more robust in the presence of other anionic additives. Fluid loss tests revealed that modification of the ATBS-NNDMA copolymer with phosphate groups greatly improves its fluid loss performance in both fresh and sea water cement slurries. Additionally, it extends its temperature stability up to 150 °C (300 °F), as was observed in stirred fluid loss tests. Furthermore, the excellent fluid loss performance of the phosphate-modified FLA remained unaffected in the presence of AFS dispersant or ATBS-co-acrylic acid retarder, while the effectiveness of the conventional, non-phosphated FLA was severely impeded by these additives. Adsorption measurements illustrated that the superior fluid loss performance can be attributed to stronger adsorption of the phosphated FLA on cement. Adsorbed layer thickness measurements elucidated that the phosphated FLA adsorbs in a "train" like conformation on cement as compared to the conventional ATBS-NNDMA copolymer which attains a "loop"-type conformation.
Summary Oilwell-cement slurries commonly incorporate several admixtures such as retarder, dispersant, fluid-loss additive (FLA), antifreewater agent, and defoamer. Between them, additive/additive interactions may occur that can result in incompatibilities and reduced performances (the most frequent case) or, oppositely, in improved effectiveness. Here, an overview of some synergistic and antagonistic effects between selected cement additives is presented. Four combinations of additives were tested and studied. First, the interaction between 2-Acrylamido-tertiary-butyl sulfonic acid-co-N,N-dimethylacrylamide (CaATBS-co-NNDMA) FLA and an NaATBS-co-itaconic acid retarder as well as welan gum, an anionic biopolymer applied as an antifree-water additive, was investigated. It was found that the retarder, which possesses a particularly high-anionic charge, reduces the effectiveness of the CaATBS-co-NNDMA FLA by decreasing its amount adsorbed on cement. Similarly, the anionic biopolymer can also negatively affect the effectiveness of the FLA through competitive adsorption, in which the biopolymer hinders the sufficient adsorption of the FLA on cement. The incorporation of stronger anchor groups (e.g., dicarboxylates or phosphonates) into the CaATBS-co-NNDMA FLA enhances its affinity for the surface of cement and thus renders it more robust against the negative impact from other admixtures. Second, the compatibility between an Na lignosulfonate (Na–LS) retarder and the CaATBS-co-NNDMA FLA was investigated. Here, surprisingly, a dual synergistic effect was found. Na–LS improves the fluid-loss performance of CaATBS-co-NNDMA, whereas the latter greatly enhances the retarding effectiveness of lignosulfonate. The experiments demonstrate exceptionally high compatibility of both admixtures. The positive effect is based on coprecipitation of both polymers, which enhances FLA adsorption on cement. At the same time, because of the thick adsorbed polymer layer, the dissolution of the clinker phases is hindered, resulting in the retardation of cement hydration. Finally, it was found that hydroxyethyl cellulose (HEC) and sulfonated formaldehyde polycondensate-based dispersants - such as poly melamine sulfonate (PMS) or acetone formaldehyde sulfite (AFS) - act synergistically; thus, the fluid-loss control provided by HEC is considerably improved. Dynamic light-scattering measurements revealed that, in the presence of those dispersants, the association of HEC molecules into large hydrocolloidal assemblies was greatly enhanced. Obviously, the increased ionic strength resulting from the polycondensate dispersants renders the nonionic HEC molecules less water-soluble and initiates their aggregation at an earlier stage. The larger hydrocolloidal polymer associates can plug filter-cake pores more effectively, thus reducing cement fluid loss. The study suggests that multiple additive/additive interactions can occur in oilwell cement. Understanding the underlying mechanisms can help both to avoid unwanted incompatibilities and to develop mitigation strategies.
Abstract Water-soluble acrylamide tert-butylsulfonic acid (ATBS)-based copolymers are commonly used to provide fluid loss control for oil well cement slurries. In our study, we investigated the behavior of a CaATBS-NNDMA copolymer synthesized by aqueous radical polymerization with respect to its fluid loss performance in presence of sulfate, chloride and Welan gum and at high temperature (80°C). First, we found that CaATBS-co-NNDMA adsorbs in high amounts on both cement and silica flour. Its effectiveness relies exclusively on high adsorption onto these mineral surfaces. FLA adsorption may, however, be perturbed by several physico-chemical effects. Inorganic anions present in high concentrations such as e.g. chloride, as well as organic anionic admixture molecules such as Welan gum, can negatively impact the effectiveness of CaATBS-co-NNDMA. They were found to compete with the polymer for adsorption sites on the surfaces of cement and silica. This way, they reduced the adsorbed amount of the ATBS copolymer. Their impact on the ATBS copolymer generally depends on their anionic charge density, the quality of their anchor group to the cement/silica surface and their concentration. Elevated temperature (80°C) causes a significant increase in the concentration of solved sulfate present in cement pore solution. As a result of the higher ionic strength, the ATBS copolymer changes its solved conformation from stretched to coiled. Coiling of the macromolecule, however, results in lower adsorption. Through this mechanism, higher temperature reduces effectiveness of the FLA.
Abstract Hydraulic fracturing injection experiments were performed in unconsolidated sand under stress to delineate the mechanisms controlling fracture propagation and to determine the effect of these mechanisms on potential formation damage. The tests included injection of cross-linked guar and visco-elastic surfactant into 3-Darcy sand samples subjected to different overburden stresses. The following observations were made: The experimental data indicate that fracture propagation in unconsolidated sand is primarily a result of fluid invasion and shear failure in a process zone ahead of the fracture tip. The shear failure is caused by large tip stresses or by pore pressure increase within the process zone. Three different invasion/damage zones were observed, including the external filtercake, the gel-invaded zone (or the internal filtercake), and the filtrate-invaded zone. Sub-parallel "micro fracturing" and complex fracture geometry was encountered. The sub-parallel fractures may be initiated at the tip or at the fracture wall due to shear failure and is dependent on the fluid efficiency and the type of leakoff, i.e., wall building or viscous. Field consequences of micro fracturing during stimulation may include early screenout, short fracture length and extensive formation damage as the fracturing fluid invades the sheared interfaces. Typically, lower efficiency fluids were associated with increased net propagation pressure and higher density of micro fracturing. These findings suggest that injection of low efficiency fluids in weak, poorly consolidated formations results in a different type of formation damage, namely creation of sub-parallel micro fractures enveloping the main propped fracture, that could severely undermine post-stimulation productivity. Introduction Frac pack completions are performed in high-permeability and poorly consolidated sand formations to bypass drilling induced formation damage, enhance productivity and control fines production. Generally, this stimulation involves injecting a small pad of clean fluid, followed by slurry with as much as 12 ppa proppant concentration, depending on the sand carrying capacity of the fluid. As much as 200,000 lbs of sand may be injected in one zone and fracture lengths over 40 ft may be obtained. Many fracpack designs include tip screenout (TSO) to induce an inflated, highly conductive fracture that is packed by proppant as the fluid leaks off. A variety of water-based fracturing fluids is in use for fracpacking. These include linear guar-based gels such as hydroxypropyl guar, HPG, synthetic polymers such as HEC, borate cross-linked guar, and viscoelastic surfactants (VES). The physical and mechanical parameters influencing the fluid selection process includethe formation permeability, in situ temperature, the required net injection pressure, proppant carrying capability of the fluid, the formation mechanical properties, and the expected fracture length. Because of their wall building capability, cross-linked polymers are used in a vast majority of frac pack jobs in high permeability formations. These fluids, however, are known to result in excessive formation damage because of incomplete breakage and removal of the polymer. Typical post-stimulation skin values in excess of +5 are common. Linear gels and VES-based fluids are expected to promote less formation damage with the drawback that the resulting fracture is typically short and narrow due to large leakoff. Therefore these fluids may not be used in high-permeability zones, in excess of 200 md, unless they contain a sufficient quantity of fluid loss additives (FLA). An accepted practice for frac pack fluid selection is to minimize the polymer concentration needed to accomplish the designed fracture geometry. Aside from the cost benefits of this practice, the smaller polymer concentration is expected to help in achieving TSO. Moreover, it is generally believed that formation damage increases with increasing polymer concentration in the fracturing fluid.
Previous work on the vibrations of infinitely long cylindrical shells submerged in an infinite acoustic medium is extended to the case where the medium is bounded by a plane surface, either rigid or free. The solution leads to an infinite set of simultaneous, linear, algebraic equations, the coefficents of which are obtainable in closed form. If this set is replaced by a finite number of equations, a procedure is given for estimating the accompanying error. Typical numerical examples indicate that significant results can be obtained, in certain cases at least, using only a small number of equations. For steel shells in water, the results show that the presence of a fluid boundary has surprisingly little effect on either the response of the shell, in the case of forced vibrations, or the frequencies of free vibration, even when the shell is in close proximity to the boundary.