Extraction of organic compounds from representative shales and the effect on porosity

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 to 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 to 1.35µm)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

The effects of solvent extraction on nanoporosity of marine-continental coal and mudstone

“…Previous studies generally found PSD curves of mudstone rise after solvent extraction, and the samples in their studies are dominantly lacustrine shale with moderate or relatively high TOC contents, and have high extract production [30,33,81]. Decreases of mesopore volumes or PSD curves of a few marine shales after solvent extraction have been reported [27,32,38]. While the PSD decreasing becomes more common for our samples.…”

“…M-1, M-3 and M-4 are in this case. Solvent extraction inducing swelling of montmorillonite and illite/smectite has been observed [38,82]. The decrease of PSD of M-1, which is dominated by kaolinite (93.1 wt.…”

“…Methoxy groups expand the interlayer space of kaolinite (the basal spacing of kaolinite and methanol-treated kaolinite are 0.72 and 0.86 nm, respectively) [83]. Hydrogen bonding solvents (such as methanol) swell clays more than polar aprotic and non-polar solvents [38], which means methanol causes stronger swelling of clay than original hydrocarbons. However, the positive effect of realizing pore spaces dominates over the negative effect of absorbing solvent if TOC is large enough, such as M2, with a TOC value of 8.43%.…”

“…However, the negative effect of kerogen swelling is not as remarkable as the positive effect of emptying pores, since the pores of the coals and M-2 (samples with high TOC contents) increased after solvent extraction. Additionally, solvent extraction may also dissolve and extract porous resins or asphaltenes [38]. Adsorbate (Ar or CO2) can dissolve in bitumen, which results in an overestimate of pore volumes of original samples [24,27].…”

“…However, chemical and physical interactions between solvent and kerogen or between solvent and minerals may alter the pore structure and cause inaccurate "real porosity" (porosity free of hydrocarbons and soluble bitumen). For examples, 1) Solvent extraction causes swelling of clay and kerogen; 2) solvent absorbs in organic matter and/or adsorbs on clay surfaces; 3) solvent extraction may completely or partially dissolve porous resins or asphaltenes [27,32,38].…”