Corrosion, Control & Repair of Wells
Recent surveys show that approximately 40% of the world’s wells suffer from losses of production rates because of the effects of corrosion and the development of leaks and fractures in well tubing and/or casing. This is an especially prevalent problem in older, or remediation, wells. EnSys Yocum multiphase/single phase flow performance models are employed to determine the locations of the corrosion leaks and tubing/casing fractures. There are several methods that are applied to eliminate the production losses in these wells by repairing and restoring the wells to original operating conditions.
In some cases, rapid corrosion can completely breakdown a well. One example that EnSys Yocum encountered involved a high pressure/high temperature (HP/HT) condensate well in the Middle East. The well casing and tubing lasted only 1.5 years due to the effects of rapid corrosion.
Reservoir and well attributes:
- Pressure: 15,000 psig
- Temperature: 270 degrees F (at the perforations)
- Well Depth: 10,200 feet with a perforated upper section of the reservoir.
- Casing Diameter: 7 inches (schedule 80)
- Perforations: for ~50 feet
- Reservoir bound by edge water and partial bottom water aquifers
- 4% hydrogen sulfide and carbon dioxide
A complete well workover was conducted to sustain production for a longer period of time in this highly corrosive reservoir. The new well included:
- Production up the 7 inch casing into a 4.5 inch tubing
- Installation of an orifice choke at the bottom of the tubing
- Initially at 3/4ths orifice based on multiphase flow rate calculations
- Installation of inner tubing with 1.5 inches diameter inside of the 4.5 inch tubing
- Corrosion inhibitor pumped down the 1.5 inch tubing and injected via a sparger to evenly distribute the corrosion inhibitor liquid evenly on the 7 inch casing.
- Installation of a second 1.5 inch tubing that transmitted electric power to an actuator in order to control the settings of the orifice choke from the surface over the range of 1/8 inch to 2 inches (this provided control of the multiphase flow rate and acceptable pressure limits for the wellhead and downstream flow line/surface facilities.
Results: The corrosion inhibitor coating on the casing and tubing minimized corrosion which resulted in the wells lasting more than 10 years in addition to controlling tubing head pressures and the multiphase flow rates in an optimum range. However, the electric cable connection to the orifice actuator did not last long in these wells. Thus it was not possible to change the orifice setting.