Evaluation of formation water chemistry and scale prediction: Bakken Shale

•Novel method to calculate/restore in-situ pH for clastic formations.•Explores high salinity brine in unconventional oil formation of interest (Bakken Shale).•Uses Bakken mineralogy to constrain calculations of in-situ chemistry.•Uses USGS produced water database as source of produced water analyses.

Determination of in situ formation water chemistry is an essential component of reservoir management. This paper details the use of thermodynamic computer models to calculate reservoir pH and restore produced water analyses for prediction of scale formation. Bakken produced water samples were restored to formation conditions and calculations of scale formation performed. In situ pH is controlled by feldspar-clay equilibria. Calcite scale is readily formed due to changes in pH during pressure drop from in situ to surface conditions. The formation of anhydrite and halite scale, which has been observed, was predicted only for the most saline samples. In addition, the formation of anhydrite and/or halite may be related to the localized conditions of increased salinity as water is partitioned into the gas phase during production.