Real-Time Field Monitoring To Optimize Microbe Control
Microbial growth in oil and gas systems can cause numerous problems that result in production downtime, lost revenue, and safety concerns. Unfortunately, microbial populations are ubiquitous in many oil and gas production zones and minimizing their impact is challenging. The most common effects of microbial growth are corrosion, hydrogen sulfide (H2S) production, and biofouling.
The ability to minimize the negative impact of microbes is confounded by the current field monitoring technology, which is unable to detect many of the organisms. These monitoring technologies are culture-based, meaning that organisms must be grown in culture to be detected and quantified. It is well documented in many systems that less than 1% of the total organisms present are culturable. In addition, the culture-based methods commonly take days or weeks to produce a result. Thus, operators must wait a considerable time before taking control measures, and the impact of microbial populations can increase during the long period of undetection.
New Development in Monitoring Method
Two essential parameters for any new field monitoring technology are the ability to obtain a result in real time and the ability to detect all organisms, not just the ones that can be grown in culture. The industry has evaluated several of these technologies over the years, including a first-generation adenosine triphosphate (ATP) assay in which the level of ATP is used to measure the number of actively growing microbes in a sample. ATP is the molecule used by cells to drive any process that requires energy. This includes metabolism, protein translation, DNA repair, and cell division. Although there are differences in the absolute quantity of ATP in different species or organisms, it is assumed that actively growing bacterial cells hold relatively similar amounts of ATP, and a calculation has been determined to quantify microbial cell numbers based on a measurement of ATP.
The major challenge of using this technology in oilfield systems is that the enzymatic assay used to quantify ATP is very sensitive to the quality of the fluids tested. This means that many oilfield fluids such as oil, emulsions, produced water with significant solids, high-salinity fluids, and fluids contaminated with certain chemicals would not yield meaningful results. As an example, the firstgeneration ATP test was not generally accepted by oilfield operators and has been little used. However, a second-generation ATP test that addresses the concerns has been developed recently. Specifically, additional steps were developed to enable accurate testing of fluids that contain oil, high salinity, residual chemicals, and solids. This technology also meets the other necessary criteria—real-time results and detection of all microbes, not only those that grow in culture.
The first key step in determining the utility of the second-generation test was to evaluate its efficacy in an oil/water emulsion and compare it with results from a water sample. Both samples were inoculated with microbes that are known to grow in conventional culture media. This would allow for a direct comparison of the ATP and culture-based methods in a setting in which the results should be nearly identical. Microbial enumeration was performed in each sample through the use of the second-generation ATP test as well as conventional plating on lysogeny broth (LB) agar plates (Fig. 1). Samples were tested in a 5-fold dilution series over a 3 log range.
The results demonstrated an excellent correlation of microbial numbers between the two methods, confirming that data generated by the second-generation ATP test is accurate and that oil/water emulsions do not lead to abnormal results. The calculated correlation was greater than 0.9 using linear regression analyses, confirming the accuracy of the ATP measurements.
There are multiple areas in oil and gas systems in which this novel technology is applicable, such as monitoring at the wellhead and in production separation equipment, monitoring of produced water systems, monitoring of water injection and disposal systems, microbial enumeration during drilling and fracture applications, and evaluation of stagnant fluids. To date, Nalco has used this technology successfully in each of these areas. An example of its use in a production system follows.
An onshore oil and gas producer in the northwestern United States was experiencing high levels of H2S contamination in its produced fluids. The fluids produced at each well were routed to a central facility where oil, gas, and water separation was performed. The increasing H2S levels were prevalent throughout the central facility, with recorded measurements of 1,400 ppm to 1,700 ppm in the run tank and 1,500 ppm to 2,400 ppm in the incoming tank.
At these levels, the H2S was a severe threat to safe production, and frequent treatment was required to bring it under control. The levels were assumed to have resulted from microbial growth, specifically sulfate-reducing prokaryotes. Initially, batch treatment of the bulk fluid with biocide was performed to reduce the bacterial levels. The H2S measurements dropped to a range of 200 ppm to 400 ppm but within two days returned to their original levels, as this proved not to treat the source of the microbial growth. Furthermore, treatment of the entire fluid stream was not cost-effective.
To determine the source of the H2S in the system rapidly and accurately, the second-generation ATP test was incorporated as a field monitoring tool. The purpose of the testing was to obtain quick feedback on the exact location from which the increased microbial activity was originating and to provide the fastest method of identifying the organisms causing the H2S production. Samples were initially tested on the two incoming fluid lines (north and south) to the processing facility. The lines are each fed by 50 to 100 wells and merge in the central facility. It was quickly determined from microbial activity that only the north line contained elevated microbial levels. Based on these results, the individual producing wells feeding the north line were tested for microbial activity.
There were nine wells where measurement in each case exceeded 1 million microbes per mL of fluid, suggesting that these wells may have been responsible for the H2S production. All nine wells were immediately batch treated with biocide through the wellhead, with the dose proceeding into the flowline and the treater. As opposed to the initial method, which treated the total fluid volume without targeting specific locations, this option directly addressed the source of the microbes and required much less chemical use to control the microbial growth.
During the next two weeks, batch biocide treatment at the nine problematic wells resulted in a 92% decrease in the total number of bacteria entering the north line (Table 1). More importantly, immediately after treatment of the individual wells, the H2S level at the central facility was reduced to 200 ppm—meeting the customer’s immediate goal for operating at lower H2S levels.
The key value of the second-generation ATP quantification technology was the rapid detection of the microbes in the fluid stream. The entire field was surveyed, with corrective action taken at the needed locations in fewer than two days. Had enumeration been performed by means of conventional culture-based methods, the customer would have had to wait for days or weeks to obtain results and initiate treatment.