Abstract

Reservoir souring refers to the generation of hydrogen sulphide (H2S) in originally sweet reservoirs that have been subjected to production operations such as (sea)water flooding. The most plausible cause of reservoir souring is the growth of plausible cause of reservoir souring is the growth of sulphate-reducing bacteria (SRB) in the zone where seawater mixes with formation water. In the mixing zone the components that support the SRB's life - oxidant and nutrients - are present.

Inorganic reactions are not considered important in the generation of H2S. They are important, however, in the scavenging of H2S, since many iron-containing minerals are capable of reacting with H2S, forming pyrite or pyrrhotite. For this reason, the types and quantities of iron-containing minerals within the reservoir have been studied using petrographic and isotopic techniques. petrographic and isotopic techniques. As a first step towards quantifying the effects of H2S production by SRB, a 1D analytical transport model has been developed. It describes the production of H2S in the mixing zone and the compound's transport through a reservoir. The partitioning of H2S between the fluid phases and the possibility of scavenging by iron-containing minerals have also been included in the model.

The model has served to calibrate a numerical simulator, which can be used for modelling souring under more realistic settings. It is demonstrated how the distribution of nutrients, sulphates and H2S actually observed in a producing field can be used to assess the existence of souring chemical reactions as well as the mechanisms that give rise to them.

Introduction

Many North Sea oil fields that originally had low indigenous levels of hydrogen sulphide (H2S) have started to produce increasing amounts of H2S after being flooded with seawater to the point of breakthrough. As seawater injection operations expand in scale, reservoir souring is of increasing concern. Therefore, a model of the generation and transport of H2S in an oil reservoir has been developed that can be applied to forecast H2S levels in the produced gas, oil and water phases.

The set-up of such a model requires knowledge about the mechanisms causing the reservoir souring. A number of mechanisms have been proposed. proposed. P. 369

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