A relatively simple apparatus was used to measure the rate of reaction of dissolved oxygen with hydrogen sulfide and with added chemical scavengers in oilfield waters. Results show that natural waters may catalyze or retard the reaction of sulfur dioxide or sodium sulfite with dissolved oxygen.

Introduction

Dissolved oxygen usually increases the corrosion rate of steel in natural waters. The effect is particularly severe in brines because the presence of dissolved oxygen causes pitting. Therefore, various methods are employed to remove oxygen from oilfield waters, both fresh and brine, to protect enormous investments in steel pipe and process equipment. Both mechanical and chemical methods are used to remove dissolved oxygen from water. Common mechanical methods have been described in detail by Speller and by Weeter. Probably the most widely used mechanical method in the oil field is to strip dissolved oxygen from water by countercurrent contact with a gas. This process requires a source of oxygen-free gas, usually methane. The most common way of removing oxygen from oilfield waters is to add sulfur dioxide or sodium sulfite because very little capital investment is required. These compounds react with dissolved oxygen as follows:

(2)

Thus, four parts by weight of sulfur dioxide or eight parts of sodium sulfite are required to react with one part of dissolved oxygen. These two reactions have been studied extensively over the years, but the experiments generally were performed in pure water with controlled additions of various chemicals. Tittoff and Bigelow in 1903 showed that the reaction of sulfite ion with dissolved oxygen was very sensitive to positive and negative catalysts; e.g., cupric ion and stannic ion, respectively. This work was repeated later by Fuller and Crist, who concluded that a catalyst was required for the reaction of sulfite with dissolved oxygen and the reaction rate was independent of oxygen concentration. Pye demonstrated that the reaction was catalyzed very effectively Pye demonstrated that the reaction was catalyzed very effectively by cobalt ion (Co++), and to a lesser degree by copper (Cu++), manganese (Mn++), and iron (Fe++). More recently, Winkelmann used a polarographic method, rather than the previously used wet analytical methods, to study the reaction between sulfite and dissolved oxygen. He found that the reaction rate decreased as the pH was lowered, although pH 7.0 was the lowest he investigated, These results show that the reaction of sulfite ions with dissolved oxygen is extremely sensitive to pH and to positive and negative catalysts. The treatment of pure water is straightforward: addition of a slight excess of SO2 or Na2SO3 and 0.005 ppm COSO4 effects a rapid removal of dissolved oxygen. However, natural ground waters and sea water contain a number of influential dissolved species" that may speed up, slow down, or even prevent the reaction between sulfite and dissolved oxygen. Thus it is impossible to predict the kinetics of the sulfite-oxygen reaction in natural waters. In a previous paper we measured rates of reaction of sulfides, sulfur dioxide, sodium sulfite, and hydrazine with dissolved oxygen in prepared waters using a polarographic technique.

JPT

P. 443

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