No abstract

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.

This paper highlights efforts taken in answering the following question: "Can horizontal unconventional shale wells be successfully squeezed for scale control?". The Bakken shale formations in North Dakota, Montana and Alberta have presented unique operational challenges during the unconventional play boom. Despite the ability to control scale formation with conventional scale inhibitors under Bakken conditions, scale formation (primarily calcium carbonate) can still remain an operational challenge due to well design and sub-optimal scale inhibitor deployment. Due to limited experience in the industry in scale squeezing fractured long reach horizontal wells, scale squeezes have not been frequently applied in the Bakken. As a result of sub-optimal scale control despite application of suitable scale inhibitors, an in-depth evaluation of scale squeeze chemistries, application methods and scale squeeze modeling has been ongoing in the Bakken. These successful applications are being studied to improve current scale squeeze modeling approaches for horizontal, fracked wells in addition to understanding the factors that impact Bakken scale squeezes. The lessons learned in modeling, application and monitoring of the scale squeezes will be discussed in this paper. Squeeze9 and Place-iT™ field history matching indicate the primary impact to squeeze life is the amount of scale inhibitor (both concentration and volume) used while overflush volumes have less of an impact. This varies from traditional scale squeezes that combine scale inhibitor and overflush volumes to achieve the desired scale squeeze lifetime. Due to the unique brine chemistry of the Bakken, squeeze monitoring has relied less upon traditional ion tracking and almost exclusively upon more advanced environmental scanning electron microscopy (ESEM) of suspended solids within the produced brine samples. Examples of successful Bakken squeezes lasting more than 1 year will be highlighted. The successful applications of scale squeezes in the Bakken are bringing a new method of efficient, cost effective, long term scale control to unconventional plays. The lessons learned in the Bakken, and the resulting advancement of unconventional scale squeeze models and theories, have implications for the global industry as unconventional plays across the world are identified, explored and produced.