The presence of microorganisms is recognized as a major concern in the oil and gas industry. In addition to existing in petroleum reservoirs, microbes often can be introduced into oilfield systems at the initial stages of drilling and well completion; during shut-ins; and during secondary and tertiary oil recovery.
Microbiological activity in oilfield systems can create the formation of biofilms and provide an environment for anaerobic sulfate-reducing bacteria (SRB), which produce hydrogen sulfide (H2S) and can reduce the quality of oil and gas produced and increase equipment corrosion risks.
The process by which corrosion is initiated and/or accelerated by the activities of microorganisms is commonly known as microbiologically influenced corrosion (MIC) and detecting this activity from oilfield samples is an important initial step in evaluating a system’s risk of MIC.
Detection methods for MIC focus on the application and implementation of biological techniques that make use of the bacteria’s features. In the oil and gas industry, detecting microbiological activity is still primarily based on cultivation techniques.
Culture-based MIC Testing
Culture-based methods involving bacteria culturing in specific artificial growth media have long been established as the standard technique for bacterial identification in the oil and gas industry. The most commonly used culture-based method is the serial extinction dilution technique, with the test consisting of a serial dilution step followed by an incubation step using a growth medium selected for the bacteria of interest.
For example, if the bacteria of interest are SRB, vials containing a specifically formulated SRB growth medium will be used in the test. If possible, the serial dilution part of the test should be carried out within a few hours of obtaining the subject samples (typically done on site). Following the incubation period, the SRB population density in the original sample is determined to the nearest order of magnitude by the number of vials in each dilution series that turn black because of bacterial sulfide production.
According to current standards, the prescribed incubation period for SRB is a minimum of 28 days; however, a very good indication of the SRB count usually can be obtained after 10 to 14 days of incubation. To improve the accuracy of such methods, threefold or fivefold replicate serial dilution enumeration is typically carried out to allow a mean probable number (MPN) of bacteria to be determined from standard MPN tables.
A common criticism of culture-based methods is their inherent bias regarding strain isolation and growth, meaning that only a small proportion of the microorganisms in a sample are cultivable and, therefore, analyzed. Consequently, these techniques may not be adequate for detecting all the microorganisms potentially involved in the corrosion processes.
Molecular Microbiological Methods for Detecting MIC
The limitations associated with the culture-based methods have led to the development of other culture-independent methodologies, which are gaining increased acceptance in the oil and gas industry. There are several different types of culture-independent methods, such as the measurement of adenosine triphosphate levels and specific enzyme activity, and the powerful molecular microbiological methods (MMM).
MMM involve the extraction of deoxyribonucleic acid (DNA) directly from the microorganisms in a sample. The polymerase chain reaction (PCR) is then used to amplify copies of a particular gene in the sample to such an extent that the DNA fragments can be used to identify the bacteria. A progression of the traditional PCR method is the real-time quantitative PCR (qPCR) technique, which can provide more accurate and quantitative reproductive enumeration data regarding microbial communities, in addition to bacterial characterization.
Microbiological monitoring based on advanced molecular microbiological analysis has been applied successfully to offshore production pipelines. These techniques have been demonstrated to be a faster, more accurate means of microbiological monitoring; and they accommodate the rapid identification and quantification of all microorganisms present in a sample, including the clear majority that cannot be cultured.
Is It Time to Change?
The evolution of microbiological monitoring techniques from culture-dependent methods to more state-of-the-art and advanced culture-independent molecular techniques has undoubtedly yielded improvements in detecting and monitoring microbiological activity in oilfield systems. For oilfield systems where implementation of a new microbiological monitoring program is required, MMM are the obvious choice as these methods can better estimate the total amount of bacteria in a specific environment
Before deciding whether a change to culture-independent monitoring methods is beneficial for existing microbiological monitoring programs, it is important to have a comprehensive understanding of the system under threat from MIC. For example, the use of viable culture techniques would be sufficient for microbiological monitoring in systems where the control of microbial population and biofilm growth has been proven to be successfully achieved through a biocide treatment. Typically, biocides are nonselective, affect a broad spectrum of bacteria, and are applied to control all possible microbiological activity. Therefore, even monitoring only the culturable bacteria will allow the time point and trend measure data required to optimize a treatment in the field.
When a biocide treatment proves to be unsuccessful, a more detailed analysis of the system using MMM can provide the additional information required to both characterize and quantify which specific bacteria present in the system are responsible for MIC (both directly and indirectly). The application of MMM, therefore, allows for improved optimization of remedial actions (e.g., adjusting biocide treatments to target the specific bacteria involved in corrosion processes) and MIC monitoring procedures.
The widespread application of molecular techniques throughout the industry could improve understanding of MIC and the specific microbes involved in the varied operational environments where bacteria can be present and cause problems. MMM is potentially the new standard for improving knowledge and understanding of how to effectively manage microbiological activity in oilfield systems.
Find microbiologically influenced corrosion resources from AMPP here.
Source: Originally appeared on materialsperformance.com, authored by Steven Loftus, senior corrosion engineer and oil and gas industry consultant with ROSEN UK; detailed article references found at Materials Performance.