Abstract

Major problems in oil and gas operations result from the biogenic formation of H2S in the reservoir. The presence of H2S results in increased corrosion, iron sulfide formation, higher operating costs, reduced revenue, and constitutes a serious environmental and health hazard. A new technology which removes and prevents the formation of biogenic H2S has been developed. The technology is based on the addition of low concentrations of a water soluble nutrient solution which selectively stimulates the growth of an indigenous microbial population, thereby inhibiting the detrimental Sulfate Reducing Bacteria (SRB) population which cause the generation of the H2S. This deliberate and controlled modification of the microflora and reservoir ecology has been termed "Biocompetitive Exclusion". This directed alteration of the microbial ecology is achieved by the introduction of a nutrient formulation which favorably influences the dynamics of the reservoir microflora. The treatment system does not require the addition of microbial cultures. Periodic well treatments have been shown to dramatically reduce or completely eliminate sulfide levels and establish control of the system in diverse field demonstrations supported by the DOE. Field data in a variety of reservoirs will be presented which demonstrate the effectiveness of this new approach to treating and remediating wells with high levels of H2S. The versatility and low cost of this novel technology offers the petroleum industry a practical and cost effective methodology for the control of H2S in oil and gas wells.

Introduction

The presence of H2S in reservoir fluids is a major problem for the petroleum industry and is associated with reservoir souring, iron sulfide deposition, poor sweep efficiency, and increased corrosion. Its occurrence may cause the early abandonment of many oil and gas reservoirs by increased costs, reduced revenue, and environmental concerns. In many cases reservoirs which initially did not contain sulfide have become sour as a result of operations. This progressive increase in sulfide levels is most notable in reservoirs which were flooded with sea water. It is now known that such increasing sulfide concentrations are a result of the biogenic formation of sulfide. This biogenic sulfide generation results from the metabolic activities of the anaerobic Sulfate Reducing Bacteria (SRB) which reduce sulfate to sulfide. A new technology which has been termed "Biocompetitive Exclusion" reduces microbial souring of a reservoir by the selective manipulation of the reservoir microflora and thereby turns the detrimental role of microorganisms into an advantage.

Current treatment practices for the control of SRB, prevention of the biogenic formation of sulfide, and reduction of corrosion include the use of biocidal agents and corrosion inhibitors. The application of corrosion inhibitors is for the protection of the metal from the actions of the SRB and their metabolic products, but such products are not usually designed for the elimination of the cause of the problem. Although corrosion inhibitors are used successfully to treat corrosive waters, such treatment must continue since the source and generation of H2S persists and may even increase. An alternate approach is to eliminate the SRB by the use of biocides including glutaraldehyde, formaldehyde acrolein, quaternary amines chlorine, and many proprietary formulations. Most of these biocidal agents are very toxic, require special handling, and can be an environmental hazard. The biocidal approach to preventing SRB growth can be considered as a preventive or remedial treatment. In the preventative methodology the system is treated to eliminate all contamination and biocides would be introduced at the start of drilling operations. Although virgin oil and/or gas reservoirs have been generally considered to have no microbial activity, there is increasing evidence that indigenous microorganisms may be present. Certainly contamination introduced by penetration of the reservoir during drilling and production operations can cause many changes in the reservoir due to the growth of microorganisms. In most cases, microbial actions have been detrimental, leading to increased sulfide production by SRB and resulting corrosiveness.

The preventive biocidal approach would be unsuccessful should indigenous SRB occur throughout the formation.

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