In-situ Pore Physics
- Characterize the pore network structure for selected oil sand/oil shale resources using Computed Tomography (CT)
- Perform Lattice Boltzmann simulations of flow through pore network structures to predict transport properties such as permeability
- Conduct CT analysis of pore network structures during pyrolysis reactions over a range of temperatures using drill cores (1.8 cm in diameter and 5 cm in length) from the Mahogany zone of Green River oil shale samples and oil sands samples from the U.S. and Canada
Department of Energy, National Energy Technology Laboratory
In a carbon-constrained world, transportation fuel production from oil shale and oil sands resources will require an understanding of processes that occur over a wide range of length and time scales from the structure of kerogen and how it binds to an inorganic matrix to the fluid flow resulting from in situ processing of an oil shale interval that covers hundreds of acres. In this regard, parameters which are important for the analysis of in situ oil shale pyrolysis include:
- Kerogen conversion to oil, gas and coke
- Nature of the pore space before and after pyrolysis
- Porous media characteristics after pyrolysis
- Permeabilities and relative permeabilities.
This project addresses the challenging characterization problems presented by items 2 to 4. Project researchers will characterize and digitize the pore space of the oil shale samples before and after pyrolysis using the multi-scale, non-invasive, non-destructive 3D imaging technique known as x-ray micro/nano CT (XMT/XNT) and specialized software. With these tools, the 3D network of the pores, kerogen/mineral phases, crack network and flow channels of oil shale samples. Figure 1 shows the 3D volume rendered images from the reconstructed multi-scale XMT data for a Mahogany oil shale core sample before pyrolysis. Lamellar structures (kerogen-rich and silicates-rich) are observed. The middle column shows the distribution of the kerogen phase. At a 60 nm voxel resolution, individual grains can be identified. Figure 2 shows the same set of 3D images for a Mahogany oil shale core sample after pyrolysis. Crack networks, developed during pyrolysis, are evident and well defined within two distinct regions. Inside region A (silicates-rich lamellar structure), cracks and voids as small as 100 nm are observed. Inside region B (kerogen-rich lamellar structure from high resolution XMT or HRXMT images), larger, anisotropic cracks and voids have developed.
Figure 1 (left): Volume rendered images of Mahogany oil shale drill core sample MD-10 from reconstructions of multi-scale x-ray CT data. Gray scale indicates variations in density and atomic number of material. Middle column shows kerogen phase distribution (in purple and brown colors for XMT, HRXMT and XNT, respectively).
Figure 2 (right): Volume rendered images of Mahogany oil shale drill core sample after pyrolysis (400°C, N2 flow) from reconstructions of multi-scale x-ray CT data.
Once the digital representation of the pore space is established, the Lattice Boltzmann method (LBM) is used to calculate flow properties such as absolute and relative permeabilities. For region A, the estimated permeability from LB simulation of oil shale after pyrolysis was 0.00363 µm2 or 0.363 mD (millidarcy). Because the absolute permeability is highly anisotropic, the estimated permeability in region B is 3.87x10-8 cm2 or 3.87 darcy, four orders of magnitude higher than in region A. Anisotropic features of oil shale permeability are being quantified and may be the first 3D imaging of pyrolysed oil shale by HRXMT and nano-CT.
In addition, oil shale core samples after pyrolysis at three reaction temperatures (300°C, 350°C, and 400°C) and heating rates of 1, 10 and 100°C/min have been imaged using HRXMT to establish the pore structure of the core after reaction (~5 micron voxel resolution). The porosity variation with drill core sample height as measured from the CT data clearly correlates with position of the kerogen layers.
Future research includes the analysis of fresh oil shale core and its comparison with the initial oil shale samples, the determination of directional (anisotropic) permeability of the new oil shale samples after pyrolysis reactions at different temperatures and loading conditions using XMT analysis and LB simulation, and calibration for phase identification with results from QEM/SCAN.