Simulation of sour‐oxic‐nitrite chemical environment in oil and gas facilities

Abstract

The nature of the chemical environment in oil and gas fluids such as produced water (PW) and soured oil or low-oxygen environments plays a vital role in microbiologically influenced corrosion (MIC). H 2 S and/or other forms of sulphur species in soured oils and PW are key factors in the corrosion and growth of microorganisms. To mitigate reservoir souring and subsequent corrosion, nitrate is injected to displace sulphate-reducing bacteria with nitrate reducers.However, nitrates and the associated nitrogen species (eg, nitrite) impact the chemistry and microbial activity, and hence the corrosion potential in the system. This study investigates the PW chemical environment in light of sulphide and nitrite chemistry, and provides information towards understanding the chemical transformations and microbial relationships. The sulphide-nitrite environment was studied as a function of temperature, pressure, nitrite level, oxygen-using equilibrium, and kinetic model approaches. Equilibrium simulation predicted the formation of FeS, FeO(OH), and Fe 2 O 3 as the key corrosion products, the amount of which varied depending on the chemistry and operating conditions. In experiments where nitrite was very low or absent, S 0 was favoured over SO 2 − 4 as the inlet H 2 S concentration increased and FeS dominated with an increase in temperature. In the presence of nitrite, Fe 2 O 3 was formed instead of FeO(OH) at temperatures above 50 C. The trend of the kinetic simulation of the sulphide-oxygen reaction in seawater was in good agreement with the wet-lab experiment in PW. The models can serve as tools to better understand and describe the chemical environment in PW systems.

Assessing Microbial Corrosion Risk on Offshore Crude Oil Production Topsides under Conditions of Nitrate and Nitrite Treatment for Souring

“…In the first 24 days, there was a lack of efficacy in sulfate reduction inhibition as sulfide was detected at close to the same level added (~2 mM, Figure 3 , right panel). This initial observation was likely due to the chemical reactivity of nitrite and sulfide, where nitrite is converted to ammonium abiotically [ 45 ] (Equation (1)). Once the stoichiometric amount of nitrite was chemically reduced to ammonium (~0.5 mM nitrite, based on 4 moles HS − : 1 mole NO 2 − , Equation (1)), approximately 4.5 mM nitrite should remain.…”