Hydrochloric acid (HCl), is the most typical acid used in acidizing treatments, due to its low price, availability and fast reaction rate. In lower temperature, and often shallow wells, the fast reaction of HCl does not often cause any problems. However, in deeper carbonate reservoirs, which naturally have a higher temperature, this can prove to be problematic.
HCl reaction kinetics are complex, and the reaction which occurs downhole can be broken down into three stages:
- Transfer of the acid from the main stream of treatment acid to the acid reaction surface
- The reaction that occurs at the surface
- Transfer of the spent acid from the reaction surface back to the main stream of treatment acid.
In extreme cases, the reaction can be limited either by the reaction rate of the acid, in this case, minimal acid spending occurs before transferring back to the main stream, or by transfer rate, where the acid completely spends before transferring back to the main stream. In the former case, acidizing treatments will have minimal effect on the permeability improvement around the wellbore and therefore not increase production as desired. In the latter case, the acid spends too close to the wellbore, causing face dissolution and well integrity issues. Both cases, though extreme, are ineffective acidizing treatments.
Temperature has a strong influence on the reaction rate, along with injection rate, and the acid used itself. High-temperature causes the reactions to occur extremely fast (mass transfer limited), and will cause face dissolution and achieve poor reach into the reservoir, as well as cause corrosion to tubulars. Therefore, the industry has developed a number of methods which will inhibit the reaction rate. These include, but are not limited to, pH buffered packages, acid emulsifying systems, gelled acids and use of organic acids.