Acid Fracturing of Gas Wells by Use of an Acid Precursor in the Form of Solid Beads: Lessons Learned From First Field Application

Summary Acid-fracturing treatments are used commonly to enhance the productivity of carbonate formations with low-permeability zones. Various forms of hydrochloric acid (HCl) are used to create deep etched fractures. However, regular HCl reacts very fast with limestone and high-temperature dolomite formations and, unless retarded, will produce a fracture with low conductivity. In addition, concentrated HCl-based acids are very corrosive to well tubulars, especially at high temperatures. To address problems associated with concentrated acids, various retarded acids were introduced. Organic acids were used also in some cases. These organic acid systems were used successfully to acid fracture several wells in a deep gas reservoir in Saudi Arabia. Field data, however, indicated that there is a need to create deeper and more-conductive fractures. To achieve this goal, it was decided to conduct a field trial with a newly developed acid system. The new acid system is an ester of an organic acid in the form of solid beads. The ester reacts with water (hydrolyzes) at bottom-hole temperature and produces lactic acid, which reacts with carbonate minerals and etches the surface of the fracture. The system was examined thoroughly in the laboratory and showed promising results. The treatment was conducted in the field without encountering operational problems. After successful placement of the solid beads in the fracture, the well was shut in for 24 hours to give ample time for the ester to hydrolyze and for the generated acid to react with the formation rock. The well was allowed to flow, and samples of the fluids produced were collected to understand chemical reactions that occurred during the treatment. The treatment has resulted in a slight increase in gas production, and no significant improvement was noted over a 9-month period. Consequently, the well was matrix acidized with 28 wt% HCl and responded positively to the treatment. This paper will discuss major reactions that occurred during these treatments and how they impacted well response. Lessons learned and recommendations to improve the results of this new acid system will be given.