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

Abstract Acid fracturing treatments are commonly used to enhance the productivity of carbonate formations with low permeability zones. Various forms of HCl acid are used to create deep etched fractures. However, HCl reacts very fast with the carbonate rocks 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 also used in some cases. These acid systems were successfully used 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 temperature and produces an organic acid. The organic acid then reacts with carbonate minerals and etches the surface of the fracture. The system was thoroughly examined in the lab and showed promising results. The treatment was conducted in the field without encountering operational problems. After successful placement of the solids 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 as a result, the well was matrix acidized using 28 wt% HCl and responded very well to the matrix treatment. This paper will discuss major reactions that occurred during this treatment, and how they impacted well response. Lessons learned and recommendations to improve the results of this new acid system will be discussed in detail.