Fresh river water is used to flush production lines to control halite deposits in unconventional carbon steel wells near, North Dakota, USA. Raw river water is stored in 400 (bbl) fiberglass tanks and is treated with oxygen scavenger before injection. Quarterly testing showed high levels of bacteria. The remote location, cold seasonal temperatures, tank design, USA EPA regulations and restricted access limit treatment options to biocide application in solid form added to tank tops. To cover all reasonable product candidates, range of both liquid and solid biocides were tested on site to determine an effective formula and dose. Selection was based on multiple factors including, effect on bacteria, compatibility with water, worker safety, product label restrictions and cost. Based on the results, calcium hypochlorite was selected for tank treatments. Hypochlorite is corrosive, however testing showed acceptable corrosion rates (<3mp/y) against carbon steel at target treatment rates for extended periods of time. Tanks were dosed not to exceed 3 ppm free available chlorine residual. Bacteria monitoring showed that bacteria levels in the tank were reduced by as much as 5 log to non-detectable levels post quarterly treatments with calcium hypochlorite.
This study describes an effort to find a method to control bacteria in 130 remote freshwater fiber glass storge tanks with an effective low-cost, convenient treatment method. Freshwater, in this application, is being used to control halite scale formed in the production from unconventional oil wells in the Williston, North Dakota, USA area. The water is sourced from local freshwater rivers and trucked to location and stored in 400 barrel (bbl) freshwater tanks. The water stored in the tanks is injected continuously, and the tanks are refilled on a variable schedule. This results in the tank volumes being replaced every 3-5 days. Bacteria population in the water storage tanks have increased over time and range from tens of thousands (4 log) to hundreds of millions of (9 log) living bacteria per milliliter. Bacteria growth in the tanks is a concern for the production operator. Bacteria population reduction and control is conducted to reduce the occurrence of microbial induced corrosion and valve failure from biofilm or water injection line clogging.
