Microbial contamination and extensive growth of bacteria in the drilling mud impact drilling operation and reservoir integrity. To reduce the damaging effect of microbial growth, drilling mud is generally treated with biocides for microbial control and preventing loss of rheological properties from biodegradation. Make-up water and drilling mud samples from reserve tank and mixing tank were collected from three drilling rigs in Saudi Aramco oilfields. The baseline number of general aerobic bacteria (GAB) and sulfate-reducing bacteria (SRB) in make-up water and drilling mud samples were determined using the Most Probable Number (MPN) method. Microbial growth in the drilling mud and microbial control using glutaraldehyde-based biocide were evaluated under the conditions simulating short-term (<24 hours) and long-term (up to 7 days) downtime of drilling operations. In short operational downtime, the current practice in Saudi Aramco with no biocide addition resulted in high number of GAB (105-107/ml) in the mud samples. In long operational downtime, addition of 1500 ppm of biocide (1/3 of normal dosage) at Day 3 was effective in controlling GAB and SRB growth in drilling mud. Biocide performance was significantly affected by sulfide content in the mud; higher dosage and more frequent treatment may be required to control the microbial growth in the drilling mud. The study indicated that biocide treatment is essential for microbial control in drilling operation, especially in the period of operational downtime. Finally, drilling mud operation and field best practice for microbial control were recommended to be implemented in all Saudi Aramco drilling rigs.

Deep drilling often requires the use of circulating fluids to facilitate the drilling process, carrying cuttings and rock fragments to the surface. In this practice, microbial contamination and extensive growth of bacteria in the drilling fluids impact drilling operation and reservoir integrity. An onsite and rapid microbial assessment technique is required to provide an early warning of microbial activities in drilling fluids, and manage microbial risks through proper countermeasures. Bioluminescence method is a rapid testing technique for quantification of living cells based on the measurement of adenosine-5'-triphosphate (ATP) found in the sample. ATP test has been widely used in food and pharmaceutical industries; however, its application in the oil and gas industry has been hindered by many interference substances present in typical oil industry samples. In this study, we have validated an ATP-based bioluminescence test for its linearity and repeatability in various types of samples from drilling rig operations (make-up water, drilling mud, and cement mix fluid). The results from ATP test showed an excellent linear relationship with the microbial numbers in the samples determined by plate count method. The study proved ATP bioluminescence technique as a reliable method for onsite determination of microbial contamination in drilling operations, obtaining results in minutes, rather than days or weeks with traditional methods. The action levels based on ATP measurements were recommended for drilling engineers to assess microbial risks in drilling fluids. The implementation of this onsite microbial monitoring technique in the field will facilitate real-time detection and immediate mitigation countermeasures for microbial control in drilling fluids, reducing risks such as loss of rheological properties of drilling fluids from biodegradation, H2S production during drilling downtime and deep in the formation, biomass plugging in the formation, and microbial corrosion. This is the first study of the use of ATP-based test to assess microbial contamination in drilling rig operations.

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