It has been discovered that condensed brines from wet gas produced up the annulus of Bakken wells can contain up to 30,000 mg/L bicarbonate. When commingling with high calcium-containing produced brine on surface, the incompatibility has led to surface line plugging with calcium carbonate. Previous theories evaluated the possibility of corrosion contributing to the elevated bicarbonate concentrations. Subsequent work discovered that distillation of produced water was able to isolate distillate containing high concentrations of bicarbonate. This paper summarizes the laboratory work conducted to understand the ability of bicarbonate to transfer from distilled produced brine to the collected condensed water as well as field work to confirm suitable mitigative strategies. Produced brine from different production basins was heated to 250 °F in an oil bath under an inert atmosphere using a distillation apparatus. Alkalinity and pH of the starting produced brine and collected distillate were measured in the lab using phenolphthalein and methyl purple indicators, and a pH probe, respectively. Alkalinity concentrations were also measured via non-dispersive infrared (NDIR) analysis to eliminate interference from other titratable species. Field work consisted of selecting 30 trial wells for batch applications consisting of diluted corrosion inhibitors in addition to only produced water. Wells were tracked for extension of mean time between failure to evaluate program effectiveness. The return on investment was calculated in terms of avoided deferred production. Surprisingly, it was discovered that simple distillation of produced brine could result in transfer of alkalinity to the collected distillate. NDIR analyses confirmed that minimal alkalinity remained in some samples while the bulk of alkalinity (as bicarbonate) was found in the collected distillate. This discovery has significant implications for the ultimate prevention of buildup of bicarbonate in the condensed brine on surface as well as strategies to mitigate the ensuing brine incompatibility. Produced brines from other production basins were also distilled to see if alkalinity transfer could be observed, or if this phenomenon appears unique to Bakken produced brines. It was found that this alkalinity transfer can be observed in distillation experiments using produced brines from other basins. The discovery of this alkalinity transfer has implication for the oilfield where condensed brine collects. While currently observed in the Bakken, it is of interest whether other basins could see a similar concentration of alkalinity in condensed brines, resulting in instances of calcium carbonate deposition of varying degree. The ability to identify alkalinity transfer into condensed brines may help identify root causes of incompatibility and subsequent suitable strategies for mitigation in other regions. This phenomenon of measured alkalinity transfer also represents a unique scenario as distilled water is assumed to contain minimal dissolved ions.

The Bakken formation is well known for producing brine very high in total dissolved solids (TDS). Halite, calcium carbonate, and barium sulfate scales all can pose substantial production challenges. Trademarks of Bakken produced brine include elevated concentrations of sodium (>90,000 mg/L), chloride (>200,000 mg/L), and calcium (>30,000 mg/L), contrasted against low concentration of bicarbonate (50-500 mg/L). In the past 3 years, operators have experienced unexpected instances of severe calcium carbonate scale on surface where produced fluids from the production tubing commingled with the gas produced up the casing. Initially treated as one-off scale deposits despite the application of scale inhibitor, acid remediation jobs or surface line replacement were typical solutions. As time has passed, this issue has become more and more prevalent across the Bakken. Investigation of this surface issue discovered a most unexpected culprit: a low TDS, high alkalinity brine (up to 92,000 mg/L alkalinity measured to date) produced up the casing with the gas. When mixing with the high calcium brine typically produced in the Bakken, the resulting incompatibility posed remarkable scale control challenges. The uniqueness of this challenge required thorough analytical work to confirm the species and concentrations of the dissolved ions in the brine produced with the gas. Scale control products were tested to evaluate their abilities and limitations regarding adequate control of this massive incompatibility. The theory that corrosion contributed to this situation has been supported by a unique modelling approach. Once corrosion was identified as the likely source of the high alkalinity brine, corrosion programs were instituted to help address the surface scaling. This paper highlights the evaluations conducted to fully grasp the severity of the incompatibility, the theories put forth to date, work conducted to try to replicate the phenomena in the lab and in models, and chemical programs used in the field to address corrosion and scale. While not known to exist in other oilfield basins, conventional or unconventional, this discovery may have implications for the broader industry if similar situations occur. The possible explanations for why this may be happening may have implications for scale control, asset integrity, and potentially even the methods by which wells are produced.