Assessment of Oilfield Souring Mechanisms by Mass Spectrometry Analysis of the Stable Sulfur Isotopes Ratio

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractReservoir souring is defined as the increase mass of gaseous Hydrogen Sulfide (H 2 S) per unit mass of produced fluids. Unless the onset of H 2 S takes place either during the appraisal of the well or in the very early stages of oil and/or gas production, it is not quite straightforward to identify the origin of souring. Under reservoir conditions, H 2 S is extremely reactive thus being able to react with native H 2 Ssequestering agents and be converted to metallic sulfides, elemental sulfur or organic sulfur compounds. This phenomenon can account for the delay of the onset of souring in the producer wells.It is a common practice in offshore facilities to prepare the drilling, completion and work-over fluids using seawater which is rich in sulfate and contains sulfate-reducing-bacteria (SRB). Should these fluids be lost to formation there is potential of (anthropogenic) reservoir souring promoted by these bacteria. In most situations, these fluids have always to bear the blame of reservoir souring (and many other problems). But it is not always correct to blame these fluids, because huge amounts of treated seawater are also injected in the formation (water flooding) for secondary recovery. Besides, H 2 S may be derived from non-anthropogenic sources. Consequently, a given souring problem may be derived from different sources and the existence of multiple possibilities render the souring identification a kind of difficult sometimes. But, this problem can be solved with the aid of instrumental analytical tools.The sulfur isotopic ratio ( 34 S/ 32 S) determination by Mass Spectrometry (MS) has been used to identify the H 2 S genesis, a key to properly address this controversial issue. This paper focuses on the use of this analytical tool to determine the sulfur isotope ratio and, therefore, to identify the source of souring. This paper also presents case histories on the use of MS to address souring problems that took place after workover jobs done in Campos Basin. Some aspects of reservoir souring mechanisms and SRB behavior are discussed as well.

Isotopic insights into microbial sulfur cycling in oil reservoirs

“…To date, field applications of isotopes to the souring problem have largely been restricted to using sulfur isotopes to differentiate between H 2 S formed from highly fractionating microbial sulfate reduction and H 2 S from abiotic thermochemical sulfate reduction (Aplin and Coleman, 1995 ; Poli et al, 2002 ; Cavallaro et al, 2005 ; Martins and Marques, 2006 ). This is based on the observation that thermochemical sulfate reduction often shows an apparent zero fractionation between reservoir minerals and H 2 S, thought to result from when the sulfate reduction itself is kinetically faster than the release of sulfate to solution (Machel et al, 1995 ).…”