One of the most important factors that determine the outcome of matrix acidizing treatment is acid placement across the target zone. Chemical and mechanical means are used to enhance diversion during matrix acid treatment in various well completions; extended reach and multi-lateral wells. In-situ gelled acid that is based on polyacrylamide-based polymers has been used in the industry for several years.1,2 Previous studies highlighted that this acid can be used for diversion in formations with almost no hydrogen sulfide and permeability greater than 50 md.3
This acid consists of a polyacrylamide-based polymer, a cross linker, a buffer and a breaker. The cross-linker reacts with the polymer at a certain pH value and forms a gel, which helps in diverting the acid in tight or damaged zones. A breaker is used to reverse the gelation reaction at pH values that depend on the system. There are various cross-linkers available including those that are based on iron, aluminum and zirconium. The objective of this study is to examine the effect of type of cross linker and acid additives on the performance of in-situ gelled acids. Coreflood tests and rheological studies were conducted in this work.
The results obtained indicated that the gelation pH depends on the type of cross-linker used. Aluminum formed a gel at much higher pH values ((pH 4.3) compared the gel formed with iron (pH 2.6). Corrosion inhibitor reduced the pH of gelation in the case of iron, whereas an opposite effect was noted with the Al-based acid. Mutual solvent (ethylene glycol monobutylether) was very effective in reducing gel residue at pH values greater than 5.
Core tests indicated the in-situ gelled acid increased the permeability of tight cores from 35 to 2,000 md. Unlike previous studies, which showed damaged due to this system, no polymer residue was noted after flushing the core with mutual solvent solutions.