Retarded acid systems

Retarded acid systems can increase acid penetration depth by slowing or blocking acid reaction. They also can reduce the rate of acid leak-off to the matrix surrounding wormhole channels during their creation, providing deeper penetration and extension of flow channels formed. 

The formation of smaller, more branched wormholes, which is promoted by retarding add, can be benefidal, at least to a certain degree. As explained in chapter 10, excessive branching is not desirable, because it will reduce live acid penetration and flow-path conductivity. Retarding acid to the extent appropriate will result in branching, but not at the expense of the formation of dominant, deeply penetrating wormholes. 

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Quite often, acid will form predominantly single wormholes from limited numbers of perforations, without significant branching. That is the case with strong acids, such as HCl. Weaker acids, such as carbox)dic acids (e.g., acetic add), and retarded acid systems tend to create more branching of wormholes, which is desirable but only to a certain extent. Retarded acid systems include viscosified acids (e.g., polymer- or surfactant-gelled acid, emulsified acid, and foamed acid) or chemically retarded (surfactant-retarded) acid. The nature of wormholes created depends on injection rate, temperature, and formation reaction characteristics as well. 

Retarded acid systems can extend the length and number of wormholes. Such systems include slightly gelled acid, chemically modified acid, surfactant-retarded acid, emulsified acid, and foamed acid. However, the time it takes for acid to spend is still short in most cases. Usually, only the formation near the wellbore can be treated effectively. Thus, effective uniform matrix treatment beyond several feet from the wellbore is exceptional. 

As in fracture acidizing, the retarded acid system that provides the deepest live acid penetration is an oil-external emulsified acid. The high friction pressures experienced with such systems are especially limiting in matrix stimulation, however. 

Interacts with GABA and excitatory amino acid systems also may block opiate receptors Retards the reuptake of serotonin... 

Chemisorption studies with probe molecules could provide valuable information on metal dispersion and metal-promoter interactions and as shown in the present work, cou d help in identification of the cative phase/alloy composition. Both ensemble and ligand effects could simultaneously be operative, as has been demonstrated in Pt-Sn system. While the ensemble effect promotes activity, ligand effect could improve selectivity and stability. The major role of alkali promoters is to control acidity and retard acid catalysed side reactions and thus improve selectivity and stability. 

Solvents used for paint removal are able to dissolve or considerably swell physically drying binders (e.g., vinyl chloride copolymers, cellulose nitrate, polyacrylates) and chemically cross-linked coatings (e.g., oil-based paints, dried alkyd resins, cross-linked polyester-melamine resins, cross-linked epoxy and isocyanate coatings) [14.237]. A combination of dichloromethane with low-boiling ketones or esters is particularly suitable. Small amounts of high-boiling solvents with a low volatility (e.g., tetrahydronaphthalene, solvent naphtha, methyl benzyl alcohol, or benzyl alcohol) are added to these mixtures to retard evaporation and increase the solvency. Modern paint removers do not contain chlorinated hydrocarbons, they are formulated on the basis of high boilers (e.g., dimethylformamide, dimethyl sulfoxide, propylene carbonate, and yV-methylpyrrolidone) in combination with alcohols and aromatics, or consist of aqueous, frequently alkaline or acidic systems. 

Acids commonly used in sandstone formations include hydrofluoric-based acids. Full strength mud acid (12wt% HCl + 3wt% HF) has been used to stimulate sandstone reservoirs for several decades [J]. Beeause of the fast reaction of hydrofluoric acid with clay minerals, various retarded mud acids were introduced. In one system, aluminum ehloride was added to mud acid systems to ensure deep acid penetration [4. In a seeond system, boric acid was used to achieve the same goal [5]. Reeently, LuUo and Rae [6] introduced a new acid system, consisting of phosphonic acid and hydrofluoric acid. For high temperature applications, mixtures of organic acids (formic or acetic) and hydrofluoric acids are used [7]. 

Novel acidizing systems come and go. These serve to differentiate service company providers and thus do not necessarily enhance acidizing response. However, certain retarded HF systems can be uniquely benefidal in various appbcations. Such appfications are removal of deep damage, treatment of highly add-sensitive sands (or sands with unusual mineralogies), and treatment of high-temperature formations. Certain retarded (or buffered) systems are less corrosive and easier to mix on-site, as well, and may be considered with those features in mind. 

Viscous acid fracturing uses viscous acid systems such as gelled, emulsified, and foamed acid, or chemically retarded adds, to both create the fracture and differentially etch the fracture face. Treatments with viscous acid are applicable in heterogeneous carbonates such as dolomites or impure limestones. 

Add retardation can be accomplished by adding unique, usually oilwetting surfactants to acid. These surfactants coat pore surfaces, thereby temporarily preventing or slowing the rate of acid attack on the pore walls. These systems are desirable for the sake of their simplicity. Surfactant-retarded acid is useful in high-temperature appUcations as well. Both HCl and organic acids can be retarded with surfactant. 

Surfactant-gelled acid systems (also known as viscoelastic acid systems) represent a more recent development. Such systems have found success in carbonate matrix acidizing applications in particular. Certain special surfactant formulations can be added to acid that is above a certain concentration (e.g., >15% HCl) at which the surfactant does not impart appreciable viscosity. However, as the acid is injected and reacts in the formation, the surfactant generates viscosity (as a function of dissolved chloride ion and pH), thereby retarding reaction and providing, potentially, in situ diversion. As acid spends further, viscosity breaks back to reduced level (in the ideal case). 

Remember that the need for deep penetration is often overrated. Most conventional acidizing treatments with some degree of retardation or fluid-loss control (viscosity), using ball sealers for diversion, can effectively bjqjass formation damage within several feet of the wellbore. In any case, simpler acid systems are preferable. 

Main acid stage (seccnd) Retarded HF system (service company specified) 75... 

Main acid stage HCI-HF (variable strength) or service company-specified retarded HF system —... 

Uses O/w emulsifier, opacifier, and thickener for cosmetic and phann. o/w emulsions, esp. low pH fonnulatlons, topical skin preps., lotions, creams, powds., conditioners, rinses, foundations, cleansers, moisturizers, makeup, sunscreens Features Decreases water evap. and retards freezing reduces greasiness stable In acidic systems... 

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