Due to possibility of sub hydrostatic pore pressure, a nitrified water system was selected in a 2 well underbalanced drilling (UBD) campaign in West Malaysia for reservoir characterization and productivity. The first well was drilled with nitrified seawater in the first UBD run in the 8.5? basement section, but it encountered severe foaming and corrosion issues. Extensive laboratory studies were undertaken for proper diagnosis of these issues, resulting in successful mitigation measures for the second UBD run in the same section. More improvements were made based on rigorous laboratory testing for the successful drilling of the second well.

In UBD operations where nitrified drill water was selected as the drilling fluid, the presence of 3 - 5% oxygen in the nitrogen generated on site posed a serious problem with oxygen corrosion. In an operation where temperature exceeds 300 degrees F and pressure above 5000 psi, the continuous presence of dissolved oxygen (average of 800 – 1200 scfm) would accelerate the corrosion process, resulting in BHA failures if not properly mitigated. This paper shares laboratory and field experience where three different treatment approaches were taken to address corrosion and foaming problems in nitrified drill water in a high temperature UBD environment.

The first approach involved the use of filming amine and oxygen scavenger (ammonium bisulfite), with seawater as a base fluid. The specifications called for reduction of dissolved oxygen to below 0.05 mg/l and to lower the corrosion rate to less than 50 MPY. The corrosion rates recorded were excessively higher than this specification. This approach also has a higher tendency to promote foaming as well as corrosion.

The second approach involved the use of filming amine and a phosphate ester based corrosion inhibitor coupled with an oxygen scavenger (ammonium bisulfite). Laboratory and field data, however, indicated that the corrosion rates recorded were unable to fully achieve these objectives. Additionally, the use of filming amine was found to cause foaming, which interrupted the UBD operation.

The third approach followed a concept from industrial cooling water treatment where it was readily acknowledged that it was impossible to remove all dissolved oxygen. A combination of cathodic (sodium molybdate, Na2MoO4) and anodic (HEDP phosphonate, hydroxyethyledene diphosphonate) corrosion inhibitors were used instead.

All of the above approaches underwent laboratory testing, using a HTHP laboratory stirred autoclave to test their effectiveness against corrosion of carbon steel, in simulated well temperature and pressure conditions. High-pressure nitrogen with the representative oxygen ratio was used to simulate the membrane nitrogen injection.

In both laboratory and field use, the approach of using sodium molybdate and neutralized HEDP phosphonate to control corrosion of nitrified drill water during the UBD operation was proven to be more successful and was thus adopted as a standard operating procedure for future UBD applications.

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