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.

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