Megan Dustin, Adam D. Jew, Anna L. Harrison, Claresta Joe-Wong, Dana L. Thomas, Katharine Maher, Gordon E. Brown Jr., John R. Bargar

Stanford University

The use of hydraulic fracturing of tight shales to produce oil and natural gas has grown significantly in recent years, yet it remains relatively inefficient, recovering only an estimated 5% and 25% of the oil and gas present, respectively. The need to improve efficiency and diminish environmental impact requires further research into fundamental geochemical reactions occurring in shales. In particular, reactions between kerogen and fracture fluid components are poorly understood. Kerogen is the precursor of the hydrocarbons of interest and contains metals in addition to organic material; it is also electron rich and therefore susceptible to oxidation and release of a variety of elements. Although some mineral phases in the shales are expected to undergo dissolution-precipitation reactions, kerogen is generally considered to be relatively unreactive. Here we have investigated reactions between isolated kerogen and a hydraulic fracturing fluid typical of that used in the Marcellus shale.  These experiments show that kerogen, as well as redox-sensitive minerals within shales, react with fracture fluid.

Firstly, kerogen exhibited more extensive release of certain metals (e.g. Al, Ba, Cu, among others) than was observed for bulk shale under the same experimental conditions. Fourier transform infrared spectroscopy results also indicate a significant change in organic content after reaction, suggesting preferential loss of certain functional groups. These findings suggest that kerogen may be far more reactive to fracture fluids than previously thought.

The addition of metals to solution suggests that kerogen may significantly impact the compositions of produced waters, which have previously been attributed solely to mineral reactions. Additionally, changes in aliphatic and aromatic fractions of kerogen will likely have effects on kerogen’s wettability, affecting transport of both hydrocarbons and fracturing fluids.

Overall, this work emphasizes the need for further characterization of kerogen and its reactions with complex hydraulic fracturing fluids.