Shale Kerogen: Hydraulic Fracturing Fluid Interactions and Contaminant Release

Abstract

The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for 2 weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen isolates were in fact altered, although by apparently different mechanisms. In particular, hydrophobic functional groups decreased in the Marcellus kerogen, which suggests the wettability of shale organic matter may be susceptible to alteration during hydraulic fracturing operations. About 1% of organic carbon in the more immature and Type I Green River kerogen isolate was solubilized when it was exposed to fracturing fluid, and the released organic compounds significantly impacted Fe oxidation. On the basis of the alteration observed in both kerogen isolates, it should not be assumed that kerogenic pores are chemically inert over the time frame of hydraulic fracturing operations. Shifts in functional group composition and loss of hydrophobicity have the potential to degrade transport and storage parameters such as wettability, which could alter hydrocarbon and fracturing fluid transport through shale. Additionally, reaction of Green River and Marcellus kerogen isolates with low pH solutions (full fracturing fluid, which contains hydrochloric acid, or pH 2 water) mobilized potential trace metal(loid) contaminants, primarily S, Fe, Co, Ni, Zn, and Pb. The source of trace metal(loid)s varied between the two kerogen isolates, with metals in the Marcellus shale largely sourced from pyrite impurities, whereas metals in the Green River shale were sourced from a combination of accessory minerals and kerogen.

Geological characteristics of shale in the Silurian (Sichuan) basin and chemical reaction of its reservoir under the action of fracturing fluid

“…At present, in shale gas exploitation in China and abroad, the flowback rate of fracturing fluid is relatively low (Yan et al 2015), and about 10-30% fracturing fluid returns to the ground (Tikhomirova et al 2011). In addition to affecting the shale gas exploitation effect, the remaining fracturing fluid may also contain a large number of salts (Cl, Br) which will promote the dissolution of carbonate minerals (Joewong et al 2015) and metal pollutant (Countess et al 2014) and heavy metal pollutant (Dustin et al 2018) which will change the permeability of rock (Harrison et al 2017). Fletcher (2012) pointed out that the overflow of fracturing fluid brought large pollution risk to the underground water resource of Pennsylvania.…”

Applied techniques for residual oil recovery from source rocks: A review of current challenges and possible developments

“…ratio 9:1), hexane extraction. Then, sequentially eluted with hexane (8 mL), hexane/DCM (12 mL; 1:1 v:v), and CH 2 Cl 2 /CH 3 OH (12 mL; 1:1 v:v) Soxhlet extraction and elution Gao et al [135] 3:1 toluene to methanol; 1:1:1 hydrofluoric acid to hydrochloric acid to deionized water Soxhlet solvent extraction and stirring Dustin et al [136] Water, 1 M NH 2 OHÁHCl at pH 4.5, 30% H 2 O 2 , 68% HNO 3 , 37% HCl…”

Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing

“…The extent and rate of mobilization of elements is influenced by the mineralogy and geochemistry of the shale and the chemistry of the hydraulic fracturing fluids (Wang et al, 2016). Moreover, the rate of pyrite oxidation by dissolved O2 is enhanced by bitumen in shale, especially at low pH (Dustin et al, 2018). Mobilized trace elements such as As, Cu, Co, Ni, Zn, and Pb from shale hydraulic fracturing activities have been identified to originate from the oxidative dissolution of pyrite (Dustin et al, 2018;Lerat et al, 2018;Xie et al, 2020).…”

“…Moreover, the rate of pyrite oxidation by dissolved O2 is enhanced by bitumen in shale, especially at low pH (Dustin et al, 2018). Mobilized trace elements such as As, Cu, Co, Ni, Zn, and Pb from shale hydraulic fracturing activities have been identified to originate from the oxidative dissolution of pyrite (Dustin et al, 2018;Lerat et al, 2018;Xie et al, 2020). Furthermore, pyrite dissolution can enhance the degradation of, for example, dazomet, a commonly-used biocide, resulting in the formation of formaldehyde and methyl isothiocyanate compounds which have potential human health and environmental impacts (Consolazio et al, 2019).…”