Competitive adsorption of chemical admixtures onto cement is of critical importance in delivering bulk performance requirements of cement slurries employed in constructing high-performing structures, like oil wells. This challenge is complex to investigate, because of the many variables that include the heterogeneity, high pH, and ionic strength of cement fluids; the multiple crystalline phases present in unhydrated and set cement; and the high number of admixtures required to meet performance criteria in commercial operations. The purpose of this study is to relate chemical structures to relative adsorption behavior of admixtures onto cement when present together and classify such interactions as beneficial (synergistic) or detrimental (antagonistic). Adsorption characteristics of single admixtures were examined by total organic carbon analysis, FT infrared spectroscopy, scanning electron microscopy, calorimetry, and UV/vis spectrophotometry. Results show that the adsorption of single chemical admixtures follows the order of monomeric hydroxycarboxylate molecule > sulfonated linear polymer > sulfonated aromatic polymer > carboxylated/sulfonated linear polymer > carboxylated branched polyether polymers. The observed adsorption behavior of polymers correlates extremely well with the order for cement hydration retardation, with carboxylated polymers being the most powerful retarders. Results correlate closely with the proposed mechanism that sulfonated polymers adsorb onto aluminate phases, presumably the tricalcium aluminate phase; and the carboxylate polymers onto silicate phases, particularly the predominant tricalcium oxysilicate phase. The hydroxycarboxylic monomeric molecule was the strongest retarder of all and has the highest adsorption level, presumably on tricalcium oxysilicate. The competitive adsorption behavior in binary mixtures was studied by monitoring the displacement of a signaling polymer by a second admixture. Results indicate that, for similar functional groups, shorter polymers are competitively more strongly adsorbed than longer chain molecules and that the shorter chain polymers were not desorbed significantly by longer chain polymer molecules. Rheological measurements correlated admixture adsorption behavior to the observed slurry fluidity.

a presente divulgacao refere-se a aditivos de fluido de poco incluindo uma argila, um polimero hidroxilado, um cation e agua. a divulgacao refere-se ainda a fluidos de fundo de poco, incluindo fluidos de perfuracao, espacos, cimentos e fluidos de distribuicao de propantes contendo um aditivo de fluido de poco e metodos de uso desses fluidos. o aditivo fluido do poco pode ter qualquer uma das varias funcoes no fluido do poco e pode conferir qualquer uma das varias propriedades sobre ele, como a tolerância ao sal ou as viscosidades desejadas, mesmo em altas temperaturas do poco.

Abstract The stability of cement sheaths during well workover, production and intervention is an important element for well integrity and operational safety. Cement sheaths support casing, provide zonal isolation, seal off annuli, and protect casing from corrosive fluids. Understanding the interaction of the cement with the production and intervention fluids under reservoir conditions is essential to prevent unexpected operational problems. This paper provides a systematic study on the interaction between the cement and many oilfield elements including fresh acid, corrosion inhibitor, carbon steel, hydrogen sulfide, and carbon dioxide. A series of laboratory experiments were conducted by exposing set cement to various fluids at temperatures ranging from 50 up to 200 °F. Both fresh and aged cements were evaluated. Cement degradation during production in sour environments could be severe and consequently provide a source of iron- and calcium-related deposits. The severity of the problem depends on the level of degradation and production conditions. The degradation process can also maximize the exposure of casing surfaces to corrosive fluids leading to an increased concentration of dissolved cations in produced fluids, thus, increased potential for inorganic deposits and water-oil emulsion formation. Severe cement degradation can exacerbate the potential of creating easy pathways for toxic gases to the surface. Several advanced analytical techniques including X-Ray Diffraction (XRD) and Inductively Coupled Plasma (ICP) were used to evaluate the condition of the cement before and after the fluid exposure in our experiments. The results indicated that, unlike what has been reported in the literature, acid can significantly degrade a major portion of the exposed Class-G cement within less than two hours if the exposed cement surface area to acid is considered. The exposure to hydrogen sulfide and carbon dioxide did impact the cement degradation, but not significantly. The degradation was found to be a function of acid concentration, fluid additives, and environment conditions. To determine whether cement catalyzes the corrosion process, the average corrosion rates were also calculated. The results were in agreement with field observations and beneficial to optimize the cement stability and to minimize the related operational problems.

Abstract Synthetic polymers have long been used as fluid-loss additives (FLAs) in hydraulic cement slurries for cementing subterranean zones. Typically used synthetic polymeric materials include N,N-dimethylacrylamide and sulfonated acrylamide monomers, which are preferred for high temperature applications. However, as these materials are less environmentally acceptable, the search for more environmentally acceptable chemical increases. Although preferred materials are biopolymers, they sometimes impart high slurry viscosity at ambient temperature, and cause thinning and settling of the cement slurries at elevated temperatures. Thus, there is a need for an additive composition that is environmentally acceptable and performs better over a wide temperature range. This paper presents a novel idea of using a combination of two biopolymers, a hydrophobically modified (HM) biopolymer and a hydroxypropyl derivative of cyclodextrin (DCY) for controlling fluid loss, while imparting better cement slurry properties, stability, and viscosity. HM biopolymer as an FLA by itself causes excessive surface slurry viscosification at ambient temperature. However, when DCY was added into the cement slurry, optimum slurry viscosities were observed at ambient temperature, thus avoiding pumping issues. Laboratory experiments showed that the fluid-loss control was excellent, but the viscosity of cement slurry was high at ambient temperature when the HM biopolymer was used as a FLA by itself. The addition of DCY reduced the surface viscosity without compromising fluid-loss control and without affecting other properties. Studies conducted at elevated temperatures revealed that the HM biopolymer was left intact to maintain the cement slurry viscosity at that temperature, thereby helping prevent settling issues. Also, no significant effect on compressive strength development and thickening time of the cement slurry was observed with the addition of DCY molecules. The results of laboratory experimentation at ambient and elevated temperatures support the formation of in-situ inclusion complex formation of DCY molecules with the HM biopolymer.

Abstract Hydrophobically modified hydrophilic polymers (HMHPs) have numerous applications in the petroleum industry, among which stand out relative permeability modifiers (RPMs) for acid diversion, drilling, fracturing, water control, water-injection profile modification, or improved oil recovery (IOR) operations. The efficacy of the polymeric RPM is attributed to a bimodal mechanism of action, which involves the association of the hydrophobic appendages in addition to adsorption of the amphoteric material to the formation surface. As a result, one challenge associated with the use of HMHPs is the pressure increase often observed when the material enters the formation. A second limitation is the inability to reverse the effect of the treatment fluid, which can be significant when the HMHP is overdisplaced, resulting in a reduction of flowback fluid, or when operational requirements call for placement of a deactivated HMHP that can be selectively activated at a desired time and location. Because of the self-diverting nature of the HMHP discussed in this paper, field deployment has been limited to near-wellbore (NWB) applications. Within this context, a linear surfactant additive [sodium dodecyl sulfate (SDS)] has been an effective mitigation strategy against such self-diverting characteristic; however, this also inhibits other desirable HMHP properties. Restoration of the native RPM properties was achieved using a surfactant control agent that allows on-demand RPM reactivation. Recently, it was realized that the sodium dodecyl sulfate has a much higher affinity for hydroxypropyl-β-cyclodextrin (HPCD) than HMHP. This paper discusses developments that allow for improved injectivity of HMHPs when used as RPMs for water-control operations. In addition, this study attempts to advance the existing understanding of the performance and associative mechanism of action of RPMs. Currently, there is a lack of consensus about the role hydrophobic modifications play in HMHP performance. Using host-guest attraction, the nature of the hydrophobic moieties is revealed, which compliments the current body of literature related to the use of HMHP in the petroleum industry. To this end, adsorption, fluid loss, and coreflow results prove the increased affinity of the surfactant for HPCD serve to release HMHP from the HMHP/linear-surfactant complex, reactivating the HMHP to its original associative mechanism-based RPM-performance profile.

Summary Water-based-mud (WBM) formulations inclusive of nanosilicas offer the possibility for improved shale inhibition with reduced environmental impact vs. conventional shale inhibitors. These additives may be effective in maintaining well stability and in preventing equipment problems, such as bit balling, which may be experienced in the absence of good shale inhibition. A series of different nanosilicas has been tested as shale inhibitors in WBMs. Differences in shale inhibition are observed depending on the kind of nanosilica that is used. The nanosilicas described in this article are tested in fresh water and seawater to determine their applicability in both onshore and offshore scenarios. In seawater, nanosilica muds appear to be more powerful inhibitors than silicate/potassium chloride (KCl) muds. The data presented in this report indicate that a nanosilica mud achieves less shale erosion in seawater than a conventional silicate/KCl mud. This increased performance of nanosilica is accompanied with greater ease in handling the lower-pH fluid (which can range from pH 8.5 to 10.0) compared with silicate muds that often exceed pH 12. In addition to being more-potent inhibitors and more safe to handle, nanosilicas offer the possibility of lower environmental cost. High concentrations of KCl are not necessarily required when nanosilicas are included in a mud design for shale inhibition.

Abstract Achieving quality cement performance in deep water wells with large amounts of salt present requires fundamental understanding of the salt effects on the interactions of cement additives with cement on the slurry performance. Many problems encountered later in the life of the well can be traced to slurry performance issues during placement and the initial setting process. Cement slurry designs for an oilwell can range from simple to highly complex depending on the geology, lithology, placement logistics, wellbore conditions and long-term performance. Logically designed cement slurry formulations and dependable additive behavior are critical to meeting cement performance requirements. Formulations can include mineral admixtures added in substantial quantities and additives in smaller (<2% by weight of cement) quantities. The performance of the additives in cement slurries depends strongly on competitive adsorption on cement and mineral surfaces. Adsorption interactions are directly influenced by downhole temperature, the nature of the formation, and the mix water composition. These aspects underline the need for understanding additive interactions with cement under wellbore conditions. Of particular relevance to deep water cementing is the performance of additives in sea water containing monovalent and divalent salts with chloride and various other anions present in amounts close to 4% by weight. The problem of additive performance becomes even more of a concern when cement is placed against salt formations. High ionic strength of sea water or of a sea/fresh water slurry placed against a salt formation, can compress the electrical double layer of cement clinker particles, alter chain conformations of ionically charged or hydrophobically modified polymeric additives, or change the solubility of additives. Additives may perform extremely well in fresh water slurries, but may perform poorly in slurries with high salt content. Additionally, the salt itself can affect cement performance. For example, sodium chloride is a set accelerator in small quantities (≤13% by weight of cement) while it functions as a retarder in large quantities. The objective of this presentation is to gain molecular level understanding of relationships among additives based on chemical structures, adsorption onto cement surfaces, and the mix water ionic strength. In this study, typical additives for dispersion and retardation are contacted with Portland cement in deionized water, synthetic sea water, water containing 2% to saturation level of NaCl or divalent salts in a formation brine. The reactions were analyzed by isothermal calorimetry, UV/Visible spectroscopy and rheology. The interpretation and implications of laboratory results are presented.

Strength retrogression of set Portland cement at elevated temperatures, for example at temperatures greater than about 230°F, has been known for many years, and is believed to be due to the formation of a variety of crystalline phases at the expense of amorphous calcium silicate hydrate phase. Ground quartz silica is typically added to prevent the strength retrogression. Calcium hydroxide that is generated from cement hydration reacts with silica in pozzolanic reactions to generate amorphous calcium silicate hydrate. It appears from the literature search that there is no consensus as to how much silica is needed to effectively prevent strength retrogression. The amount of silica proposed as sufficient varies from the generally accepted value of 35% by weight of cement to more than 60%. In offshore situations, the problem is more complicated due to the high content of salts present in sea water. The chloride content of seawater has the potential to form crystalline phases, for example Friedel’s salt. The effects of seawater on the required amounts of silica flour for prevention of strength retrogression have not been explored. The objective of this study is to identify the optimized amounts of silica required for a given application temperature in both fresh water and sea water, and search for correlations between macroscopic strength related-properties and molecular level structural features. It is hoped that the information and knowledge will be useful in designing cement slurries for any offshore hot zones. As part of this study, cement formulations containing different amounts of silica were tested in both fresh and sea water in the 250-400°F range using Ultrasonic Cement Analyzer (UCA). The strength development/retrogression patterns were observed. The cured samples were analyzed by X-ray diffraction studies. In these studies, cement to water ratio is maintained constant to ensure similar hydration rates, and the densities were allowed to vary based on the amount of silica added. The results clearly indicate that the amount of silica needed to prevent strength retrogression depends on temperature. Highly intriguing strength development patterns, dependent on temperature and the amounts of silica, were observed prior to stabilization at ultimate strengths. Implications of such patterns on well construction strategies are discussed, and molecular level clues from X-ray studies for such patterns are pursued.

Abstract Conformance gels based on nanosilicas are used as environmentally acceptable materials for plugging and sealing water- or gas-producing zones and controlling water production. Because of the lower pH(compared to sodium silicate solutions) of these solutions, they are environmentally acceptable in the North Sea area. The gelling agents required to activate the materials to produce gels are typically common salts, such as sodium chloride and potassium chloride. The reduced environmental impact of nanosilica has led to its commercial success in the North Sea. At temperatures greater than 80°F, the gel time of nanosilica can be controlled to desirable values by adjusting concentrations of the gelation agents. The nanosilicas that are currently used in the oilfield, however, are not suitable for low-temperature applications (e.g., below 80°F) because of extremely long gel times. This paper describes a new nanosilica system that demonstrates non-spherical-shaped nanosilica particle assemblies, such as rod-like and string-of-pearl geometries that display superior gelation activity in the presence of salts, exemplified by sodium chloride, compared with their spherical counterparts. These materials can gel in a reasonable time frame in the presence of an activator at temperatures as low as 50°F. A trend of increasing reactivity in nanosilicas based on their geometries is presented in this paper. In particular, nanosilicas with a high aspect ratio render stable gels in a shorter period of time than their spherical counterparts. It is proposed that the aspect ratio could enable formation of a networked superstructure of silica at lower concentrations than required for a networked structure of nanosilica spheres of similar diameter.

Abstract Enzyme breakers have been widely used in water-based fracturing fluids for more than three decades. Enzyme breakers have several significant advantages compared to traditional oxidizer chemical breakers. First, enzymes specifically break long-chain polymers without causing undesirable damage to the wellbore, formation, or fracturing equipment. Second, because of the catalytic nature of the enzyme, they are not consumed, thereby requiring minimal amounts. Third, enzymes are non-toxic compared to oxidant breakers. However, enzymes operate under a narrow pH range and often become inactive at high pH values. In addition, at elevated temperatures, their activity decreases or completely diminishes as a result of denaturing. This paper discusses a new method to extend the breaker activity of a mannanase enzyme to higher pH and temperatures by the addition of lignin-based additives. Results show that the addition of lignosulfonates stabilizes the enzyme at an elevated temperature and pH, thereby extending enzymes breaker performance beyond what was previously achievable. Another aspect of fracturing fluid breakers that is difficult to control is break time. Generally, varying the amount of breaker provides customizable break times. For example, if a faster break time is required, then a larger amount of breaker is added to the fluid. However, enzymes are highly expensive, and the use of greater amounts to achieve a faster break adds significant cost. On the other hand, if lesser amounts of enzyme are used, extended break times are achievable, but the polymer breakdown becomes incomplete, which could lead to formation or propped fracture permeability damage. In this study, it was possible to modulate the break times of guar-based fluids by varying the amount of lignosulfonate and keeping the enzyme loading constant. The higher loading of the less-expensive lignosulfonate aids in faster breaking of the fluid viscosity. Consequently, this approach is more economical as it requires less enzyme loading. Additionally, the polymer breakdown is complete despite the low amounts of the enzyme used. The effects of different structural types of sulfonated lignins on enzyme activity are also discussed. To the best of the authors' knowledge, there are no mannanase enzyme stabilizer additives available in the oilfield industry. Introduction of these new additives will aid in improving higher temperature and pH tolerance of mannanase enzyme breakers.

Abstract Horizontal wellbores can present unique problems during water and gas shutoff operations. The primary challenges include the extended length of many horizontal wells, which could be outside the reach of coiled tubing (CT), and the fact that many are completed with open holes, gravel packs, or slotted liners. With gravel pack and slotted liner completions, it can be extremely difficult, if not impossible, to obtain complete zonal isolation. Even when the offending water or gas producing zones are clearly demarcated from the producing zones, chemical remediation remains arduous. Mechanical plugs can be set inside gravel pack screens or slotted liners and a chemical sealant can be pumped into this isolated internal section. However, it is difficult to limit fluid flow in the outer annular space. Because most chemical sealants are designed to have relatively low viscosity, they tend to slump to the low side of the wellbore, either inside or outside of this isolated section. Consequently, the water and/or gas zones are not uniformly exposed to the treatment, which could result in an incomplete seal. Because more horizontal wells are being drilled, water/gas shutoff systems are necessary for these types of completions. An ideal sealant system would exhibit flow properties facilitating pumping and placement while, immediately upon placement in the desired location, resist slumping by behaving as a thick paste. This paper presents three systems designed to address the challenges associated with treatment of zones within horizontal wellbores. The objective was to develop sealant systems that would not slump upon placement and subsequently undergo polymerization, crosslinking, or setting and provide lasting seals to prevent water/gas flow. The fluids discussed include polymeric and monomeric gels and a Sorel cement based system, which additionally contains additives to provide slump resistant properties. The chemistries that impart the slump resistant properties are discussed. In addition, laboratory data is presented to include slump measurements, gelation times, and flow resistance measurements.