Optimizing the Drilling Completion Brine Design in the Presence of Martensitic Stainless Steel Equipment

Abstract

Completion fluids, typically chloride or bromide brines, based on density requirements are used to control the well during some operations and remain either in the tubing until well is put on production or in the annulus above the packer for the duration of well life. Under normal conditions, the well casing is a closed system where the brine is protected from ingress of H2S/CO2 and oxygen. However, brines may be exposed to oxygen ingress from the surface through a leak at the wellhead, and /or to H2S / CO2 ingress through a potential leak through the packer, their dissolution in the brine, affecting significantly the corrosion resistance of the steel. In spite of its proven efficiency with martensitic stainless steels, sodium bromide based completion brines are quite expensive. To explore possible less expensive alternatives, without compromising corrosion resistance of the tubing, ADNOC Onshore conducted a comprehensive testing program to identify suitable, less expensive alternative brine systems with the same or improved corrosion behavior in well conditions. In the study, the general and pitting corrosion, and the Sulphide Stress Cracking (SSC) resistance of 13Cr and S13Cr samples in NaCl, NaBr and CaCl2 brines were assessed. Samples were tested for a period of 30 days in three brine systems, under inert conditions, under 1.6psi (6.5psi) H2S / 165psi CO2, at 120°C and under oxygen ingress conditions at 49°C, in an autoclave. Pitting and general corrosion were assessed using weight loss coupons, whereas the susceptibility to SSC was tested using C-ring specimens in accordance with NACE TM0177 - Method C, at stress levels of 0,2% of the material proof stresses. Relative pitting susceptibility of the steels under oxygen contamination of the different brine systems was also assessed by electrochemical polarisation tests, at 49°C. The most significant results obtained is that none of the steels presented SSC under all conditions and brine systems. For both alloys, in all test conditions, the general corrosion rates decreased in the order CaCl2 > NaBr > NaCl brines, the exposure to H2S/CO2 presenting 2 to 5 times higher corrosion rates as compared to the inert gas conditions, with the 13Cr alloy presenting higher rates in all conditions, as expected. Pitting was inexistent / negligible in all testing conditions for S13Cr. In sour environment and in oxygen ingress conditions, 13Cr showed relevant pitting in all brines. Under oxygen contamination, deeper and broader pits were observed in the NaCl as compared to the CaCl2 brine, while no pitting was found on NaBr brine specimens. Electrochemical polarisation tests showed that the pitting onset and the repassivation potentials were shifting towards the cathodic direction in the order NaCl, NaBr and CaCl2. The conclusions of the study is that chloride brine systems are a technically viable option for application with S13Cr, without introducing additional corrosion or HSE risks, leading to cost saving of $81MM over five years whereas for 13Cr, the use of bromide based brines cannot be avoided.