The neutron scattering length density of kerogen and coal as determined by CH3OH/CD3OH exchange

Abstract

Small angle neutron scattering (SANS) is an attractive technique for characterizing the structure of shale at length scales less than 100nm that are relevant to the storage and transport of hydrocarbons. The fine structure of hydrocarbon-bearing shale is in large part governed by the mass fraction and thermal maturity of the immobile organic phase known as kerogen. To correctly interpret and normalize SANS data requires knowing the neutron scattering length density (SLD) of the individual phases in the material; for shales this includes mineral, kerogen, and pore phases. While the SLD of many phases can be estimated with reasonable accuracy from compositional data, it is not clear that this is the case for kerogen, as its SLD is expected to vary considerably with type and thermal maturity. To characterize the importance of this variation, we measured the SLD of pure kerogen samples that were separated from the host shale by acid demineralization. By varying the kerogen type (I, II or III) and the associated thermal maturity, samples with a broad range of hydrogen to carbon atomic ratios, 0.5<H/C<1.4 were obtained. The SANS measurements consisted of using deuterated methanol exchange to vary the SLD of the pore space in contact with the kerogen, which allowed the SLD of the solid (kerogen) phase to be directly determined. The SLD of the kerogen was also calculated from the measured elemental composition and mass density. We find that the SLD of kerogen varies over a rather large range, from 1 to 4×1014m−2, increasing significantly with decreasing H/C, and that the measured and calculated values are in fairly good agreement. An implication of our results is that for shales containing immature kerogen with an SLD close to 1, the scattering between the kerogen and mineral phases may be significant, such that the shale cannot be treated as a two-phase material of solid and pore. Moreover, the surface area of extracted kerogen does not increase with thermal maturity. This result is in contrast to the contemporary literature which clearly indicates that catagenesis leaves nanopores in kerogen. Therefore we conclude that the acid demineralization procedure alters the structure of kerogen.