Water-based drilling mud formulations routinely include organic polymers as viscosifying and fluid loss control agents. These organic polymers, which are generally of either plant or microbiological origin (hence, "biopolymers"), can be degraded and utilized as nutrients for growth by naturally-occurring oilfield bacteria. This may occur despite the inclusion of biocide in some polymer preparations. Microbial growth in the mud can result in contamination of the well and near-wellbore zone. Fouling, corrosion and reservoir souring may then occur during subsequent operations. If growth in the mud is extensive, significant biopolymer degradation can be expected, with consequent loss of the mud's rheological properties. In these circumstances, additional polymer must be added to the contaminated drilling mud in an attempt to maintain viscosity and prevent excessive fluid loss. As additional biopolymer is added, and biodegradation continues, the cost of the drilling mud increases.
This paper addresses the requirement for control of microbiological activity in water-based drilling muds representative of those used in Saudi Arabia. A laboratory investigation of the relative efficacy of alkaline pH (lime) and biocide (glutaraldehyde) treatments in limiting microbiological activity in drilling muds is presented, and minimum treatment levels for absolute control of general aerobic bacteria (GAB) and sulfate-reducing bacteria (SRB) are provided. Application of these findings can be expected to minimize drilling mud biodegradation, and costly post-drilling operational problems such as production tubing and pipeline corrosion.