Drilling Fluids Microbial Impact and Mitigation

Microbial contamination and extensive growth of bacteria in water-based drilling fluids impact the overall drilling operation cost and performance and may affect reservoir integrity. To prevent microbial growth, drilling fluids should be treated with effective biocides for microbial control to prevent polymers biodegradation and loss of the mud rheological properties Make-up water and drilling mud samples from reserve fields and mud mixing tanks were collected from active drilling rigs. Baseline numbers of general aerobic bacteria (GAB) and sulfate-reducing bacteria (SRB) in the make-up water and the drilling mud being used 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 short-term (< 24 hours) conditions and long-term conditions, up to 7 days downtime of drilling operations. In short operational downtime, without biocide addition to the stagnate mud system, resulted in high number of GAB (105-107/ml) in the mud. In long operational downtime, addition of 1500 ppm of biocide (1/3 of normal dosage) at Day 3 was found to be effective in controlling GAB and SRB growth in drilling mud. Biocide performance was significantly affected by the mud sulfide content; 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, especially in the period of drilling operational downtime. Based on this evaluation study, Drilling mud treatment programs and field best practice have been developed and implemented in by all Saudi Aramco active drilling rigs.