Effects of salinity on the leaching of ionic species from the Duvernay Formation, a Canadian hydraulic fracturing play

  • Author / Creator
    Snihur, Katherine N.
  • Hydraulic fracturing combined with horizontal drilling has revolutionized the oil and gas industry in North America. The majority of Canada’s hydraulic fracturing operations are in Alberta and British Columbia, with the Montney and Duvernay formations ranked highest in five hydraulically fractured formations in the Western Canadian Sedimentary Basin (WCSB). During fracturing, water, and chemical additives, are mixed to make hydraulic fracturing fluid (HFF), which is then injected into an oil and/or gas rich shale formation at high pressure to create fracture networks. The resulting fractures increase the formation permeability, allowing hydrocarbons to flow freely into the well bore. After fracturing, a portion of the injected HFF returns to the surface along with hydrocarbons and formation water. This wastewater, composed of HFF and formation water, is commonly referred to as flowback and produced water (FPW). FPW contains organic compounds from the HFF chemical additives, along with organic compounds from the target formation (e.g. polycyclic aromatic hydrocarbons (PAHs)), and potentially toxic heavy metals (PTHM) (e.g. As, Ba, and Sr). FPW often has high total dissolved solids (TDS) upwards of 200,000 ppm in many Formations including the Duvernay Formation. Recently, FPW from nearby wells, called recycled produced water (RPW), has been used to supplement fresh source water as part of the HFF to reduce the load hydraulic fracturing has on nearby freshwater sources, typically to a TDS of approximately 30,000 ppm. It is from the chemistry of FPW that much of our understanding of the subsurface interactions between HFF and the target geologic formation is derived. While the chemical analysis of FPW does provide information to understand water-rock interactions in the subsurface, the mechanisms by which various ions enter into FPW remain a topic of active study.
    In this thesis, I simulated the geochemical processes occurring during hydraulic fracturing using benchtop reactors with a water:rock ratio of 18:1 at formation temperature of the Duvernay in the Eastern Shale basin (95 C) and Western Shale basin (140 C), to better understand reaction mechanisms in real time, such as the partitioning of metals between the solid and aqueous phases. I conducted these experiments in two different vessels to accommodate temperatures above and below 100 C without violent boiling. Further, I aimed to assess if stirred batch reactors can be used to understand downhole water-rock interactions that occur during hydraulic fracturing and be used to predict the inorganic chemistry of FPW and be applied to commercial scale problems. I tested my approach by conducting simple experiments to determine how the higher initial salt content of RPW can affect the leaching of PTHM. My results indicated that elemental concentration data from reactor experiments can be used to predict the assemblage of solid phases that could precipitate downhole, such as quartz, barite, and celestite, through saturation indices modeling of these FPW minerals, with SI within ± 0.5, 0.3, and 2.0 of FPW, respectively. Results of experiments simulating the use of RPW to make up HFF, which had a higher initial salinity of 30,000 ppm, showed that the higher ionic strength results in a salting-in effect that increased the concentrations of many ions in solution such as As, Ba, and Sr by as much as 937%, 1874%, and 284%, respectively. My findings are corroborated by the geochemical modeling of FPW samples collected from the Duvernay Formation, using both freshwater and RPW source waters, that reveal that higher concentrations of many elements, including PTHM, occur in FPW from wells that used RPW for fracturing. My work both illustrated the potential risks of using RPW in hydraulic fracturing operations on the environment in the event of a spill during transport or at disposal sites and provided a robust benchtop approach to predict the leaching of elements from the host rock during hydraulic fracturing.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.