Surfactant-gelled acid systems

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). 

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. 

Currently, the commercial systems available that will result in the deepest penetration of acid are emulsified acid and, perhaps, surfactant-gelled acid. Viscous acids may contain a fluid-loss additive, such as an oil-soluble resin or polymer, to reduce leak-off. Particulate diverters are not effective in fracture acidizing, but in matrix treatments, they can make a difference. 

Hild to the skin. Compatible with anionic, cationic and nonionic surfactants. Excellent viscosity builder and gelling agent. Hard-water tolerance permits equally good foaming in hard and soft water. Stable in high-electrolyte solutions and will help solubilize other surfactants into these systems. Stable in acidic and alkaline conditions, functioning as cationic in acid media and as anionic in alkaline. Lime soap dispersant. 

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. 

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