Abstract
The pore structure of shales, including pore morphology, connectivity, pore volume, specific surface area (SSA), and pore size distribution (PSD), is a significant factor in controlling gas storage and transport and the migration mechanisms of hydrocarbons. However, the lack of comprehensive characterization for both accessible and inaccessible pore structure increases the difficulty of gas-in-place estimation and gas exploration. In order to investigate the nanoscale pore system, integration of high-pressure mercury intrusion porosimetry (MIP), low-pressure N2/CO2 adsorption (LNA/LCA), and small-angle neutron scattering (SANS) were employed to obtain a multi-scale quantitative characterization of the nanopore structure of organic-rich marine shale samples from the Longmaxi and Niutitang Formations in China. PSDs obtained from the combined techniques appropriately cover an overall nanopore size range of shale (0.35–15,000 nm) and overcome the limits of the individual method. Uni-, bi, and multi-modal PSDs were observed, but the sizes of a significant portion of the nanopores observed in these shales range from 0.35 to 100 nm. Pore volumes and surface areas of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) were characterized based on the best performance window of each technique: LCA for micropores; LNA and SANS for mesopores; and MIP for macropores. It was found that micropores are the major contributor to the total SSA for both the Longmaxi and Niutitang shales. With respect to pore volume, however, the contribution to the total pore volume has a trend of micropore < mesopore < macropore for the Longmaxi shale samples, but micro-/mesopore volumes are greater than macropore volumes for samples of the Niutitang shale. Strong correlations were also observed between total organic carbon (TOC) content and micropore volume and surface area, which implies that organic matter is a controlling factor in the micropore system of organic-rich shales. In addition, strong correlations between methane adsorption capacity and both micro-/mesopore volumes and SSAs indicate that micro-/mesopores are governing factors for methane storage. Furthermore, the fractions of accessible mesopore volume and surface area were quantitatively estimated by SANS and LNA. Correlation analyses suggest that the accessibility of the mesopore surface area could be an indicator for gas transport and storage in mesopores in organic matter. Thus, a shale with higher connectivity could have higher gas diffusion capability but lower gas adsorption capacity, and vice versa.
| Original language | English |
|---|---|
| Article number | 103343 |
| Journal | International Journal of Coal Geology |
| Volume | 217 |
| DOIs | |
| State | Published - Jan 2 2020 |
Funding
The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (41802183, 41972169) and the National Science and Technology Major Project (2017ZX05035004-002). Work by L.M. Anovitz was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Access to NG7-SANS was provided by the Center for High-Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. Any opinions, findings, and conclusions or recommendations expressed in this study do not necessarily reflect the views of the U.S. Department of Energy, the National Institute of Standards and Technology or the National Science Foundation. The authors declare no competing financial interest. The authors would like to acknowledge the financial support of the National Natural Science Foundation of China ( 41802183 , 41972169 ) and the National Science and Technology Major Project ( 2017ZX05035004-002 ). Work by L.M. Anovitz was supported by the U.S. Department of Energy , Office of Science , Office of Basic Energy Sciences , Chemical Sciences, Geosciences, and Biosciences Division . This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Access to NG7-SANS was provided by the Center for High-Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. Any opinions, findings, and conclusions or recommendations expressed in this study do not necessarily reflect the views of the U.S. Department of Energy, the National Institute of Standards and Technology or the National Science Foundation. The authors declare no competing financial interest.
| Funders | Funder number |
|---|---|
| Center for High-Resolution Neutron Scattering | |
| L.M. | |
| Office of Basic Energy Sciences | |
| National Science Foundation | DMR-1508249 |
| U.S. Department of Energy | |
| National Institute of Standards and Technology | |
| Office of Science | |
| Basic Energy Sciences | |
| Oak Ridge National Laboratory | |
| Chemical Sciences, Geosciences, and Biosciences Division | |
| National Natural Science Foundation of China | 41802183, 41972169 |
| National Aerospace Science Foundation of China | |
| National Major Science and Technology Projects of China | 2017ZX05035004-002 |
Keywords
- Low-pressure N and CO adsorption
- Mesopore connectivity
- Nanopore structure
- Organic-rich marine shale
- Small-angle neutron scattering
