During the course of normal perforating operations, special attention is paid to minimizing debris in order to prevent perforation tunnel plugging or other forms of formation damage. However, operational constraints, especially in high-temperature offshore wells, may result in freshly perforated zones being left in contact with brines or formation fluid for a long time. Exposure to these fluids can cause zinc-containing scales to form inside perforation tunnels or zinc particles to become embedded in the formation, which in turn may cause damage to the full-flow potential of the perforations. Since the damage potential increases with brine density and pressure, the risk is greater in high-temperature/high pressure wells.
The damages described above can be prevented or eliminated by using a perforating acid. However, at high temperatures acid corrosion rates must be taken into account when selecting an effective acid system. Debris can be removed with strong acid or prevented with an inhibitor, but as formation temperatures increase, metal corrosion and acid sensitivity of the formation become highly problematic. At these higher temperatures, organic acids are often used, but many of them do not have the acid strength or the capability to dissolve zinc or zinc salts.
This paper describes a highly biodegradable organic acid system that has high zinc dissolution activity but low corrosion rates to steels and chrome alloys. The corrosivity of these acids is low enough that it can be spotted above the perforation guns for long periods of time at high temperature to aid in the immediate prevention and/or clean-up of formation damage products. Corrosion tests have shown that at 350°F, the corrosion rate caused by the biodegradable organic acid is 0.001 lbs/ft2 on 22-Cr for 16 hours, a value that is 90% lower than that of acetic acid under the same conditions. Acid reaction tests have confirmed that this new acid system can effectively prevent formation damage induced by perforation at temperatures up to 400°F. These tests and their effects on well production will be described in detail in the paper.