A Case Study of Reservoir Souring Despite Injection of Low-sulphate Seawater

Abstract

The use of sulphate rejection membrane (SRM) technology to generate low-sulphate seawater 
(LSSW) for water injection is becoming more commonplace, primarily to avoid severe barium sulphate 
scaling problems but it is also being increasingly applied to avoid reservoir souring.
This paper presents some operational data and results of an investigation into hydrogen sulphide 
appearance in Hess’s South Arne production facilities. The field produces from a 115°C chalk reservoir, and 
is located in the Danish sector of the North Sea.
In addition to achieving the primary objective of barium sulphate scaling control, 10 years of historical 
data from South Arne show that the injection of LSSW also significantly reduced the severity of reservoir 
souring to levels acceptable for the metallurgy used in the field’s wells and production facilities. This is 
important information for oilfields in general, and especially for deep-sea fields where high-strength steel 
risers must be used, but which are susceptible to sulphide stress corrosion cracking.
Despite using SRM technology since December 2000, South Arne production facilities saw increasing 
concentrations of H2S in late 2008, up to 15 ppm in the bulk gas and 35 ppm in gas from individual wells. 
Analyses of the produced water’s ionic composition and H2S concentrations in oil, water and gas showed 
that approximately 50% of the remaining sulphate introduced into the reservoir in the injection water was 
converted to H2S.
Stable isotope measurements of sulphide in H2S were consistent with sulphate-reducing prokaryotes 
metabolizing a high proportion of the available sulphate. Molecular microbiological methods (MMM) 
showed high concentrations of sulphate-reducing archaea (a group of thermophilic microorganisms 
producing H2S, but genetically very different to sulphate-reducing bacteria) present in the produced fluids.
Although some H2S was produced, despite use of LSSW from the very beginning of water injection, one 
study conclusion was that the residual microbiological sulphate reduction may have minimized the field’s 
barium sulphate scaling risk by lowering the sulphate concentrations in the waterflood water below those 
achievable by SRM alone.
Overall, the use of SRM to enable injection of LSSW was very successful in the control of potentially 
severe barium sulphate scaling and reservoir souring, although not entirely eliminating souring.