Gelled acid

Acids were an early exception to the no water rule. It was recognized that aqueous solutions of acids would inhibit swelling of clays and shales as well as dissolve any acid-soluble minerals contained in a formation. By 1933 commercial well stimulation with hydrochloric acid was of great interest. A whole separate methodology and treatment chemistry has since evolved around acidizing and fracture acidizing(54). Water emulsions, mainly emulsified acids, and gelled acids thickened with polymeric additives were applied early in the history of well treatment. 

E.I. duPont deNemours and Company Gelled Acidic Explosive Compositions... 

One of the most successful applications of phosphoric acid has been in gelled electrolytes that are made by adding fumed silica to sulfuric acid [73,74]. Larger phosphate concentrations appear to be tolerated in the gelled acid without adversely affecting cell performance. One reason may be that the plates in these batteries are formed by the dry-charge process and the phosphate is added to the battery with the electrolyte. Addition of silica may also affect the equilibria of phosphate dissociation and/or lead phosphate formation. Further study of these effects may lead to a better understanding of how to control phosphate activities to enhance battery performance. 

Breza et al (1972) Gelled acidic explosive compositions. US Patent 3,661,659, 1972... 

The employment of foamed acid and acid-external emulsions (oil as the dispersed phase and gelled acid as the continuous phase) are other methods used to control the loss of acid solutions. The disadvantage of using oils is that a large concentration of oil is required to increase the viscosity of the emulsion formulation, which reduces the acid concentration and, therefore, the amount of acid available for fracture etching. Foaming the acid also reduces the amount of acid available for etching since less acid is present per unit volume injected. 

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. 

Unbroken gel plugging may occur in carbonate acidizing procedures in which gelled acid is used. Monitoring and analysis of posttreatment flowback fluid samples may indicate unbroken gel, which will restrict flow of formation fluids. Gel breaking should be defined and controlled through pretreatment laboratory testing by the service company, at expected downhole temperature and residence time conditions. 

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. 

The system that provides deepest penetration is an oil-external emulsified add. Add-oil emulsions can be oil eoctemal or acid external. Oil-external emulsions have higher dissolving capacity per unit volume and are generally more effective. A common emulsion mixture is 70% acid-30% oil. A limitation is that emulsified add can be difficult to pump at suffident rate in deeper wells because of high friction pressure during injection. Gelled acids provide the most friction reduction. The temperature limit of an emulsified acid is about 300°F, because of the associated well depth. 

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). 

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. 

---