Contaminant mobilization by reactions between Marcellus Shale and organic additive in synthetic hydraulic fracturing fluids

Open Access
Author:
Tasker, Travis Lindsay
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 22, 2015
Committee Members:
  • William D Burgos, Thesis Advisor
Keywords:
  • hydraulic fracturing
  • Marcellus Shale
  • fracturing fluids
Abstract:
Hydraulic fracturing of unconventional fossil fuel reserves poses complex environmental problems. Well permits for extracting natural gas from Marcellus Shale in Pennsylvania, US have been steadily increasing since 2007. To extract the gas, up to 5 million gallons of fluid, composed of acid, inorganics, organics, and salt, is pumped into the shale formation at high pressure. Approximately 10-30% of this fluid returns to the surface along with inorganic, organic, and radioactive contaminants from the geologic formation and initial fluid additives. A mechanistic understanding of the relationship between injected fracture fluids and flowback contaminant origination within the shale formation will be a challenge to attain. However, such knowledge could help redesign fracturing fluids and promote methods for pollution prevention. Highly organic fracturing fluids could promote metal dissolution from shale through partitioning of organically bound shale metals or complexation reactions. In this study we reacted two synthetic fracturing fluids (SFFs), fluids with high and low organic content based on chemicals used in drilling logs posted on fracfocus.org, with Marcellus Shale outcrop from Frankstown, Pennsylvania. The Marcellus Shale samples were pulverized, sieved, and then reacted with the SFFs for up to 36 h in a high pressure (85 bar), elevated temperature (80oC), stirred-tank reaction cell. The reacted fluids were analyzed for a suite of analytes including: total organic carbon, chemical oxygen demand, organic speciation by GC, gross alpha and beta radioactivity, pH, conductivity, total dissolved solids, and metals. The outcrop used for studying fluid-shale interactions was highly weathered and resulted in dissolved metal and radioactivity concentrations significantly lower than unconventional flowback waters. Common halite salts in flowback fluids were not detected in the shale sample. Additionally, the concentrations and ratios of gross beta to gross alpha radioactivity in solutions reacted with the outcrop sample were different than flowback fluids, suggesting that radioactivity and halite evaporites are associated with water soluble fractions of the shale that were leached during weathering. The metals present were preferentially bound to specific phases in the Marcellus Shale, affecting their ability to be mobilized by various fluid chemistries. Iron, aluminum, strontium, and barium were mostly associated with oxides and organic material that were only mobilized under low pH, oxidizing conditions. Calcium and magnesium were mostly associated with surface exchangeable positions and carbonates that were easily mobilized from the shale outcrop. The chromatographic characteristics of the synthetic fracturing fluids changed after reacting at high pressure and temperature and mobilized more metals from the shale depending on their fluid chemistries. Low pH synthetic fracturing fluids mobilized the most metals from the shale. The organic content of the fluids and high pressure conditions had little effect on metal dissolution. However, the organic composition did change significantly depending on the pH, shale, and pressure conditions. Reacting synthetic hydraulic fracturing fluids at high pressure and temperature resulted in a 40-80% loss of gelling additives, friction reducers, and 1-octanol and d-limonene in the surfactant additive, indicating that the functionality and abundance of additives in solution change in down-hole fracturing environments. Longer chain surfactants, such as ethoxylated alcohols, were not affected by high pressure and temperature. A considerable portion of the organic additives were also taken up by the shale, influencing their effective fracturing concentrations and mobility in flowback fluids. Changes in pH also affected organic fate, with highest additive losses occurring at a pH of 3.5 in the presence of shale. These results suggest that characterizing transformation byproducts and organic partitioning of the additives used in hydraulic fracturing fluids could be useful in optimizing these fluids, identifying additives with high mobility in the environment, and determining potential toxic byproducts. Additionally, similar work involving high pressure and temperature experiments with Marcellus Shale samples could be important in understanding the geochemistry of shale and factors affecting metal dissolution.