Downhole chemical injection failure caused tubing leak

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Pitted hole found in the upper section of the 3/8” injection line.

Downhole chemical injection of Scale Inhibitor(SI) has been applied into an oil field operated by Statoil on the Norwegian Continental Shelf[1]. Unaware of at the time, the applied Scale Inhibitor has very low pH (0-1), although it was reported as <3. The scale inhibitor pitted the downhole capillary string (Inconel 825) and was sprayed onto the outer surface of the tubing and inner surface of the casing. The 8.5mm thick tubing wall was corroded through in less than 8 months[2]. Similar effect was observed on the casing.

Contents

Sequence of event

Severe corrosion attack on the 7” 3% Chrome tubing. The figure shows the corrosion attack after scale inhibitor sprayed from the pitted chemical injection line onto the production tubing.

Scale risk for this oil field was identified and continuous downhole SI injection was chosen as the mitigation. The scale inhibitor applied had originally been qualified for topside and subsea application. Recompletion of the well was followed by installation of Deep Downhole Chemical Injection (DHCI) point at 2446 m MD. The downhole injection of the topside scale inhibitor was started without further testing of the chemical.

After one year of operation leakages in the chemical injection system were observed and investigations started. Similar events occurred for several wells and some of themhad to be shut-in while the investigation was ongoing.

The production tubing was pulled and studied in detail[3]. The corrosion attack was limited to one side of the tubing, and some tubing joints were so corroded that there was actually holes through them. Approximately 8.5 mm thick 3% chrome steel had disintegrated in less than 8 months due to exposure to concentrated scale inhibitor leaked from the chemical injection line.

The 9 5/8” casing was also cut and pulled and similar effects were observed; with corrosion in the upper section of the well on one side only. The induced leak was caused by bursting the weakened section of the casing.

Gaps in chemical qualification

The actual scale inhibitor had been qualified and used in topside and subsea applications for several years, which is typically in the temperature range of 4-20 °C. The downhole temperature could be as high as 90 °C, but no additional qualification was done prior to being injected into the wells.

Tests were later performed by the operator showing that the scale inhibitor was highly corrosive for the materials in the production tubing and production casing, with corrosion rates exceeding 70 mm/year. The chemical's compatibility with Inconel 825, the chemical injection line material, was not tested either prior to application.

The investigation also revealed that the scale inhibitor as concentrated solution had reported pH of <3.0. However, the pH had not been measured. Later the measured pH showed very low value of pH 0-1. This illustrates the need for measurements and material considerations in addition to given pH values.

The hydrostatic pressure in the chemical injection line at the injection point exceeds the well pressure, which causes uncontrolled flow of chemical into the well. This could have created low pressure regime in the upper part of the chemical injection line, which can further exacerbate the corrosion risk.

Lessons learned

  • For each application, extended corrosivity testing has to be done before injection of chemicals can be implemented.
  • Test procedure for corrosivity of chemicals (e.g.acidic scale inhibitor) for continuous injection downhole should be developed to closely mimic field conditoins.
  • All chemicals should be compatible with seals, elastomers, gaskets and construction materials used in the chemical injection system under their actual application conditions.
  • The effects of partial solvent loss on the corrosivity of the chemicals (including pH change) and gunking need to be evaluated.

References

  1. Fleming, N., Ramstad, K., Eriksen, S.H., Moldrheim, E. and Johansen, T.R. 2006. Development and Implementation of a Scale Management Strategy for Oseberg Sør. Paper SPE 100371 presented at the SPE International Oilfield Scale Conference held in Aberdeen, UK, 30 May-1 June.
  2. Britt Marie Hustad, Odd Geir Svela, John Helge Olsen, Kari Ramstad and Tore Tjomsland, Statoil ASA, "Downhole Chemical Injection Lines - Why Do They Fail? Experiences, Challenges and Application of New Test Methods", SPE International Conference on Oilfield Scale, 30-31 May 2012, Aberdeen, UK
  3. Olsen, J.H. 2011. Statoil Experiences and Consequences Related to Continuous Chemical Injection. Paper SPE 146625 presented at the SPE Annual Technical Conference and Exhibition, Denver, USA, 30 October-2 November.