Reservoir Souring: Analysis of Surveys and Experience in Sour Waterfloods
Authors: Khatib Z.I. , Salanitro J.R 
Published: December 2007

Mechanisms of reservoir souring are reviewed in general. A survey was conducted on several seawater and source-water floods to determine the factors that could be responsible for reservoir souring. Data from Shell's offshore Gulf of Mexico field (Cognac Platform) were analyzed more closely, and the results are presented as a case history. All seawater floods examined in the survey were found to be soured to varying degrees. The main factors responsible for the souring of seawater floods appear to be the sulfate ion concentration, the organic acid content, and the salinity of the produced water. The role of sulfate-reducing bacteria in souring of several Shell waterfloods is discussed.

Introduction

Many offshore fields require pressure maintenance in order to recover oil and gas reserves. Often there are no source-water sands available, and seawater is the only available injection water. Formation souring with the injection of relatively low salinity and high sulfate content brines, such as seawater, has been observed at some time during the producing life of the field. The number of sour wells within a field is variable; some wells are noticeably sour (up to 100 ppm of H2S in the produced gas), while others remain free of H2S. This souring is generally attributed to sulfate-reducing bacteria (SRB) activity. It is difficult to keep any injection system sterile as well as maintain bacteria-free operations during well drilling and completion.

The main goals of this investigation wereTo initiate an industry survey of seawater floods in the Gulf of Mexico, Alaska, and California,To identify factors that could cause formation souring,To develop a methodology to predict the likelihood and magnitude of H2S generation, andTo outline the field and research studies necessary to understand reservoir souring mechanisms.

Incentives to understand reservoir souring include (1) predictions that would impact the facilities and downhole tubing design and material selection and, perhaps, the oil value of future deepwater developments, and (2) knowledge on how to control, chemically or biologically, the extent of sour gas production in sweet reservoirs for reducing potential equipment and pipe corrosion failures, formation plugging, and environmental and human health hazards. Goals 1,2, and 4 were achieved. However, due to the limited data, the development of a method to predict the magnitude of H2S generation was not possible.

This paper presents the current understanding of reservoir souring mechanisms and an analysis of survey data that were compiled on seawater floods and selected source-water floods. Data from SOI's Cognac flood were reviewed more closely, and the results are presented as a case history. A general review of field experience on the role of SRB in souring Shell waterfloods is also presented.

MECHANISMS OF RESERVOIR SOURING

The following section is a summary of the current proposed mechanisms of reservoir souring. The microbial and abiotic geochemical mechanisms are discussed.Microbial sulfate ion (SO4=) or sulfur (S) reduction. It is well known that the SRB are a physiologically diverse and ubiquitous group of anaerobic bacteria capable of reducing so: to H2S when grown on several "oxygen-containing" substrates such as short-chain volatile acids (formic, acetic, propionic, and butyric), lactic acid, phenols, and benzoates. Volatile fatty acids are present in many oilfield produced waters, and they may be a predominant factor in the growth of sulfate-reducers in oil reservoirs and the sour gas formation during waterflooding for enhanced recovery.

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