Foamed acid fracturing is gaining importance in maximizing flowback recovery and is particularly applicable when reservoir energy is not sufficient to effectively flow back the well. Laboratory studies and field implementations during the 1980s showed application of foamed acid in addressing three fundamental issues of acid fracturing such as reactivity control, fluid loss control, and conductivity generation but it was evaluated at low temperature and in shallow wells. Recently, foamed acid has been successfully utilized to energize reservoirs to enhance flowback recovery and restore production after treatment. This paper summarizes literature reports from the last 30 years, showing improved use of foamed acid for acid fracturing.

Foamed acid offers additional benefits such as retardation, deeper conductivity generation, reduced water consumption, and improved acid diversion. Foamed acid laboratory studies from literature such as foam stability, rheology, reaction kinetics, fluid loss, diversion characteristics, and dynamic acid etching is reviewed. Comparison of CO2 and N2 foamed acid is documented in this paper to define fluid selection criteria for a typical foamed acid treatment. Foamed acid rheology at different quality is also summarized in this paper from previous studies.

The dissolution of carbonate rock is controlled by reactivity, which is greatly reduced after foaming for the same acid strength and temperature. Foamed acid having 50-60 quality could retard 60-70% acid reactivity. Excessive fluid loss is one of the challenges in acid fracturing. Conventionally, fluid loss is mitigated by using synthetic polymers to viscosify acid that controls leakoff by depositing a low permeability filter cake on the face of formation leading formation damage concerns. Foamed acid does not build filter cake and showed excellent leakoff control. Proper reactivity and fluid loss control regulates conductivity generation in acid fracturing. Conductivity generation depends on kinetic parameters — such as acid type/strength, temperature, reaction time, and flow regime. These parameters affect the amount of rock removed during the acidizing process. Case histories from different regions where recent application of foamed acid is documented show placement strategies and lessons learned. A horizontal well in one case study was treated with N2 foamed acid to achieve a 2.5 fold increase in production. N2 and CO2 foam was used as foam diverter in acid fracturing, and the productivity index increased to 38% with the use of N2 foam while in other fields the productivity index increased to 3.25 fold with the use of CO2 foam.

Laboratory studies available in the literature are not adequate to design foamed acid treatment for high-temperature, high-pressure wells. This paper summarizes published literature showing improved use of foamed acid for acid fracturing.

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