Overview of H2S Souring Cases in Argentina Reservoirs: Origin and Mitigation Scenarios

Abstract

Abstract H2S is an element toxic to life that can be associated to natural gas, oil and production water. It is very dangerous to operational staff and causes corrosion -cracking and pitting of steels-, especially when it is associated to water. The petroleum fields can early show H2S associated to the original fluids or it can appear later in mature stages. The generation mechanisms of H2S have been classified as biotic (biological sources) or abiotic (geological or geochemical sources). The first one is related to the development of Sulphate Reducing Bacteria (SRB) in reservoirs. The best-known reason is the injection of SRB with seawater in combination with nutrients of formation waters. Abiotic mechanisms involve only chemical reactions between organic, inorganic phases and water. Temperature and pressure are critical parameters: themochemical sulphate reduction (TSR), hydrolysis of metallic sulfhurs, cracking of organic compounds, cracking of kerogen and volcanogenic sources are examples of abiotic mechanisms. According to this classification, most of the H2S souring cases in Argentina can be, on some level, related to the development of SRB, but alternative sources have been studied to explain the new reports of H2S on fields with high contents of original H2S. This paper classified the fields in Argentina with H2S in three categories;-The oilfields that show progressive H2S souring after a secondary recovery project. SRB were introduced with the injection of foreign waters and three examples are shown: Chihuido Lomitas, Barrancas and Las Heras-Cerro Grande (Cases A, B, C).-A second group of oilfields report low concentrations of H2S still in primary production without previous history of H2S. SRB could be indigenous or introduced with drillings fluids. El Alba and Grimbeek fields are presented in this paper as Case D.-A third group of fields show medium and high values of original H2S (>2000 ppmv) since the firsts development activities. A possible contribution of abiotic H2S sources is now considered. Some fields in the Neuquen area could show a mixture of mechanisms (biotic and abiotic). The characteristics of these fields are summarized in Case E. All reservoirs require H2S simulation to design surface pipelines and environment management. However, according to the origin of the H2S, a variety of mitigation and control technologies have already been implemented in these fields. This paper summarizes the three groups of fields in Argentina that produce H2S, altogether with their water-chemistry, evidences of H2S origin, associated problems and control practices. Introduction H2S is a toxic gas, heavier than air and flammable. It burns with a blue flame producing sulfhur dioxide (SO2), which is itself a toxic gas that can have harmful effects on heath and environment. Concentrations of 1 ppm (0,0001%) of H2S are detectable by the human nose (rotten eggs odour), 10 ppm is considered to be the short time exposure limit (maximum of 4 exposures a day of less than 15 minutes each), 100 ppm suppresses the sense of smell and concentrations above 500 ppm cause respiratory problems and unconsciousness. H2S is also a corrosive compound. It has been estimated that H2S alone is responsible for about 20% of the metal corrosion losses in the petroleum industry (Elshashawi H., and Hashem M., 2005). Many reservoirs world-wide even show variable H2S concentrations associated to original fluids, around 70% of water-flooded reservoirs worldwide have turned sour, and it appears to be a systematic increase in the sulphur content of crude oils over the past 10–20 years (Elshashawi H., and Hashem M., 2005).