Abstract
As the location and accessibility of hydrocarbons is key to understanding and improving the extractability of hydrocarbons in hydraulic fracturing, this study is an attempt to understand how native organics are distributed with respect to pore size to determine the relationship between hydrocarbon chemistry and pore structure in shales. First, selected shale cores from the Eagle Ford and Marcellus formations were subjected to pyrolysis gas chromatography (GC), thermogravimetric analysis, and organic solvent extraction with the resulting effluent analyzed by GC-mass spectrometry (MS). Organics representing the oil and gas fraction (0.1–1 wt %) were observed by GC-MS. For most of the samples, the amount of native organic extracted directly related to the percentage of clay in the shale. The porosity and pore size distribution (0.95 nm–1.35 μm) in the Eagle Ford and Marcellus shales was measured before and after solvent extraction using small angle neutron scattering (SANS). An unconventional method was used to quantify the background from incoherent scattering as the Porod transformation obscures the Bragg peak from the clay minerals. The change in porosity from SANS is indicative of the extraction or breakdown of higher molecular weight bitumen with high C/H ratios (asphaltenes and resins). This is mostly likely attributed to complete dissolution or migration of asphaltenes and resins. These longer carbon chain lengths, C30–C40, were observed by pyrolysis GC, but either were too heavy to be analyzed in the extracts by GC-MS or were not effectively leached into the organic solvents. Thus, experimental limitations meant that the amount of extractable material could not be directly correlated to the changes in porosity measured by SANS. However, the observable porosity generally increased with solvent extraction. A decrease in porosity after extraction as observed in a shale with high clay content and low maturity was attributed to swelling of pores with solvent uptake or migration of resins and asphaltenes.
| Original language | English |
|---|---|
| Pages (from-to) | 646-660 |
| Number of pages | 15 |
| Journal | Journal of Natural Gas Science and Engineering |
| Volume | 35 |
| DOIs | |
| State | Published - Sep 1 2016 |
Bibliographical note
Publisher Copyright:© 2016 Elsevier B.V.
Funding
J.M. funding came from the Research Partnership to Secure Energy for America, contract NFE-11-03260. AGS and ADG research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, L.L.C., for the U.S. Department of Energy. V.H.D. and L.M.A. were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Tuition support for V.H.D. and support for initial measurements came from the Bredesen Center for Interdisciplinary Research at the University of Tennessee in Knoxville. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The authors would like to thank Michael Cheshire for many discussions about the effect of clays on the SANS data.
| Funders | Funder number |
|---|---|
| U.S. Department of Energy | |
| Office of Science | |
| Basic Energy Sciences | |
| Oak Ridge National Laboratory | |
| University of Tennessee | |
| Chemical Sciences, Geosciences, and Biosciences Division | |
| UT-Battelle |
Keywords
- Eagle Ford shale
- Marcellus shale
- Pore size distribution
- Small angle neutron scattering
- Solvent extraction from shales