Polymer-based fluids are widely used in geothermal drilling operations due to high effectiveness and decreased costs associated with them. Most of these polymers have various specimens of bacteria used during their fermentation process. This bacterial activity needs active monitoring. Increased bacterial activity, if not controlled, not only decreases fluids performance during drilling, but also increases the risk for rig personnel by H2S formation. Moreover, uncontrolled bacteria existence results in plugged pores and reduced permeability of the formation. These, in the end, result in higher chemical treatment costs and increased drilling and well-completion times. In this paper, we investigated two major types of biocides (amine- and triazine-based) and their effectiveness in tackling different classes of bacteria in polymer-based fluids. Extensive laboratory experiments were conducted for the analyses. Sterile polyethylene bottles with thioglycolate broth were used for waste drilling fluid sampling to keep all aerobic and nonaerobic bacteria alive while transportation from the well-site to the laboratory. The concentration of bacteria in the fluid samples were determined by dilution methods. Spread plate technique was applied with the mineral salt medium. The inoculated plates were inverted onto their lids. A 9.0 cm Whatman filter paper was placed in each plate and saturated with sterile waste drilling-fluid as the carbon source. All of these plates were incubated at 35°C (±2°C) for 7 days. The microorganisms growing on the agar plates were purified and isolated. Semi-automated Analytical Profile Index (API) test and fully-automated Phoenix systems were used in defining the bacteria classification. Amine- and triazine-based biocides were tested to investigate the effectiveness of them against all different classes of bacteria by the quantitative dilution method. 16 different types of bacteria were observed in the polymer-based drilling fluid samples. 11 of these were identified as gram-negative while the other 5 was identified as gram-positive bacteria. In the oxygen demand, it was observed that the aerobic bacteria existed in the samples taken from shallow zones. They were replaced with facultative anaerobe and anaerobe bacteria at deeper zones. It was also observed that amine-based biocide, for more than 50% of the strains, was much more effective in tackling the bacteria formation on gram-negative strains than Triazine-based biocide. Amine- and triazine-based biocides were both equally effective on gram-positive strains. This paper summarizes the experimental results of the most common microorganisms in polymer-based drilling fluids and the effectiveness of amine- and triazine-based biocides to control the bacteria activity. Making a better selection of chemicals increases the effectiveness of biocide, therefore decreasing overall drilling fluid maintenance and well completion costs.

Press the Back button in your browser, or search again.

Copyright 2019, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.