Field Monitoring of Bacterial Growth in Oil and Gas Systems

This standard describes field test methods for estimating bacterial populations commonly found in oil and gas systems. Although these techniques have been successful in the oil field, they are not the only methods that are used. Regardless of the method chosen, all techniques should be applied in a consistent manner. It should be recognized that transportation of samples from the field before analysis may significantly change the viability of the bacteria and therefore, whenever practical, analysis should be initiated in the field. It is not the intent of this standard to exclude additional techniques that can be proved useful. However, caution should be exercised with any technique that is at variance from those outlined here.

The presence of bacteria in a system does not necessarily indicate that they are causing a problem. In addition, bacterial populations causing problems in one situation, or system, may be harmless in another. Bacterial population determinations are one more diagnostic tool useful in assessing oilfield problems.

A glossary of terms used in this standard is provided in Appendix A (Mandatory).

This standard deals only with oilfield bacteria generally recognized as harmful in oilfield systems and does not consider other organisms that may be found in oilfield fluids, such as phytoplankton (algae), protozoa, or marine organisms such as zooplankton (copepods).

Emphasis is given to sampling methods that are suitable for use in oilfield conditions because effective sampling is essential to any successful analysis.

Not all bacteria and archaea species can be cultured, and newer molecular microbiology methods are provided in Appendix B (Nonmandatory) to help in identifying microorganisms that cannot be cultured and assessing their roles in the oilfield. Media formulations for enumerating some oilfield bacteria commonly recognized as harmful are given in Appendix C (Nonmandatory).

This standard describes dose-response (constant concentration versus time-kill) testing for evaluating biocides used in oilfield applications. Minimum inhibitory concentration testing versus biostat inhibitor concentration needs to be addressed. Minimum inhibitory concentration refers to the amount of inhibitor required to create the desired result. In some cases, the desired result is simply to retard the activity and/or growth of the bacteria in the system. In this case, a biostat may be used with more efficiency than a biocide. In cases in which the desired result is for there to be as few viable bacteria available as possible, a biocide may be necessary. There may be cases in which a biostat and a biocide are the same chemicals applied at different dosages. The minimum inhibitory concentration should be determined by testing using the methods outlined in this and other standards. The minimum inhibitory concentration varies with the required result, type of inhibitor, and required dosage to accomplish the objective.

Methods for evaluating surface attached (sessile) bacteria are addressed in Section 3. The importance of these bacteria in oilfield problems is usually not adequately considered. Attached bacterial populations are often the most important component of a system’s microbial ecology.¹

Methods for the rapid enumeration of bacterial populations’ activity are addressed in Appendix B. The user must determine the applicability of these methods to the site/system. Similarly, there are a number of commercially available “test kits” for detecting various types of microorganisms that are not discussed in this standard; however, the user could use this standard to evaluate the suitability of these test kits for any particular situation.

Additional information on the corrosion problems associated with bacterial growth in oilfield systems is given in NACE Standard TM0106 and NACE Standard TM0212.^2,3