Abstract

This paper reports the results of several field tests of the ability of bacteria, indigenous to oil reservoirs, to reduce and eliminate hydrogen sulfide (H2S) from the production stream. The effort was a logical progression from laboratory studies reported previously, to field testing under controlled conditions. The field testing ranged from production tanks to individual well treatments to large groups of wells. The treatment consists of providing small quantities of essential nutrients to denitrifying bacteria, which utilize the volatile fatty acids present in many oil reservoirs as a carbon source. These denitrifying bacteria in the reservoir compete in the microecosystem for the volatile fatty acids also required by the sulfate reducing bacteria. This process is called Biocompetitive Exclusion. The field tests show the ability of the biocompetitive exclusion process to be a viable field treatment for the reduction and elimination of iron sulfide and hydrogen sulfide problems in producing oil wells. Different treatment practices, quantities, frequencies, and candidate selection were examined as part of this testing program. An attempt was made to quantify production increases resulting from the treatment. Recommendations are made to increase the effectiveness of the application of this process in a field environment. Treatment of wells with the Biocompetitive Exclusion Process was both a technical and an economic success.

Introduction

The classical interpretation of microbial enhanced oil recovery (MEOR) is to introduce a microorganism along with a food source to effect a positive change in the recovery mechanism of an oil reservoir. A common treatment is to clean up paraffin problems in producing wells. More sophisticated processes take advantage of the microbe's ability to produce surfactants, acids, alcohols, and polymers to improve oil recovery. In the biocompetitive exclusion process, bacteria already existing in the reservoir are stimulated to effect favorable changes in the production stream.

Sperl and Sperl have shown that nutrients can be introduced into reservoir systems to stimulate indigenous microorganisms. This commonly occurs when reservoirs are flooded with water containing significant sulfates. This sulfate influx stimulates the indigenous sulfate reducing bacteria (SRB) population which metabolize the sulfate into hydrogen sulfide gas. The hydrogen sulfide then reacts with metallic compounds such as iron to form iron sulfide, apparent in many producing systems as a black scale soluble in hydrochloric acid. Iron sulfide scale often plugs flow paths in the reservoir, perforations, pump intakes, and tubulars, causing restricted production. In a similar manner, the introduction of nitrate salts as a nutrient to the denitrifying bacteria (DNB) population causes them to flourish and metabolize the sulfides out of the system, producing byproducts commonly used as agents for improved oil recovery.

Hitzman and Sperl have recently discovered that volatile fatty acids (VFA), such as acetate, butyrate, formate, lactate and propionate, play a key role in the microecology of petroleum reservoirs. This is an important step in understanding reservoir microecology and effecting positive change using the biological system.

Volatile fatty acids have been found in many petroleum reservoirs. They act as a carbon source for microbial action. They are generally metabolized by sulfate reducing bacteria, such as Desulfovibrio desulfitricans, generating H2S gas as a by-product.

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