Representative elementary volume analysis of porous media using X-ray computed tomography

The concept of representative elementary volume (REV) is critical to understand and predict the behaviour of effective parameters of complex heterogeneous media (e.g., soils) in a multiscale manner. Porosity is commonly used to define the REV of a given sample. In this paper we investigated whether the use of a REV for porosity can be used as a REV for other parameters such as particle size distribution, local void ratio and coordination number. X-ray computed tomography was used to obtain 3D images (i.e., volumes) of natural sand systems with different particle size distributions. 3D volumes of four different systems were obtained and a REV analysis was performed for these parameters utilizing robust 3D algorithms.Findings revealed that the REVmin for porosity may not be adequate to be considered as a REV for parameters such as particle size distribution, local void ratio and coordination number. The REVmin for these parameters was observed to be larger than the REVmin for porosity. Heterogeneity of the systems was found to be an important factor to determine the REV for the parameters analyzed in this paper. The REV analysis revealed that as the uniformity coefficient increased, a larger volume was required to obtain the REVmin for the distribution of particle sizes and coordination number whereas a smaller volume was required to obtain the REVmin for local void ratio. Therefore, determination of the REV for parameters described in this paper or any microscale parameter of concern should not be derived based on REV for porosity and should be determined based on their distributions over different volumes. © 2010 Elsevier B.V

Pore networks to characterize formation damage due to fines at varied confinement and sand shape

Carbon sequestration in geological formations is in demand for many applications, especially energy production from hydrates. During gas production in a sandy hydrate reservoir, two phase flow and changes in confinement takes place. Nine fully saturated sand systems were scanned three times; before, during and after CO2 gas injection. The confinement pressure was altered, by placing a vertical spring that presses against the upper port of the sediment cylinder. 3D images were analyzed by direct visualization, followed by quantification and pore network analysis. Outcomes demonstrated that shape of sand particles affects how the unconsolidated media will impact the flow, in angular sediments with high confinement pressure, there is more friction between the grains, this results in no dislocations of sand, the fines clog the throats, and more formation damage is noted. In rounded grains with lower confinement pressure, sand grains dislocated; opening large pathways for gas flow; this resulted in lower formation damage. Measures done using pore networks, showed that because of micro-fractures, permeability of the system can increase during hydrate production. This is in contrast to the other systems, where throat sizes shrunk, decreasing the permeability; because of fines migration toward the throats and the small sand grains dislocations. EAGE 2019.This research was made possible by the National Priorities Research Program (NPRP) grant #NPRP8-594-2-244 from Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. The SMT images were collected using the X-ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), Argonne National Laboratory. The authors thank Dr. Mark Rivers of APS for help in performing the SMT scans. They also acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation, USA, Earth Sciences (EAR-1128799), and the US Department of Energy (DOE), Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source, an Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, was supported by DOE, USA under contract no. DE-AC02-06CH11357.Scopu

Grouping minimum REV of porosity and tortuosity based on descriptors of sand grains

In sandy reservoirs, selecting a representative elementary volume (REV) for the pore scale, is essential for predictive upscaling toward the reservoir scale. The porosity and tortuosity of a sand media are used for REV selection. The profile and size of sand grains, forms the voids morphology; as a result, hypothetically the grains size distribution can provide an indication to whether a volume size is representative of the media. Linking voids based characteristics such as tortuosity and porosity, to their solids counterparts like grains distribution; can help in standardizing REVs for rocks and sands. The aim of the study is to use grains size, uniformity coefficient and conformity coefficient; for categorizing the REV of porosity and tortuosity. Synchrotron X-ray micro-computed tomography of 15 unconsolidated sand system was studied. In order to determine the minimum REV of porosity and tortuosity, 20 sub-volumes for each system was generated. Micro tomography was shown to be an effective tool in measuring sand grains and voids space characteristics. REV analysis showed that a bigger size for porosity was always required compared to that of tortuosity. Categorizing sand systems based on the uniformity and conformity indices, was shown to be ineffective for the purpose of REV selection. EAGE 2019.This research was made possible by the National Priorities Research Program (NPRP) grant #NPRP8-594-2-244 from Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. The SMT images were collected using the X-ray Operations and Research Beamline Station 13-BMD at Argonne Photon Source (APS), Argonne National Laboratory. The authors thank Dr. Mark Rivers of APS for help in performing the SMT scans. They also acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation, USA, Earth Sciences (EAR-1128799), and the US Department of Energy (DOE), Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source, an Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, was supported by DOE, USA under contract no. DE-AC02-06CH11357.Scopu

Pore size distribution and soil water suction curve from micro-tomography measurements and real 3-D digital microstructure of a compacted granular media by using direct numerical simulation technique

Predictive measurement of capillary pressure - saturation relationship of a porous media is obtained based on the actual microstructure obtained from high resolution tomography data. X-ray micro-tomography provided a high contrast for silica phase, and actual geometry of sand particles and void distribution. The morphological opening (erosion + dilation) is used to get a pore size distribution using the concept of granulometry. Full-morphology approach is used to model the quasi-static wetting and non-wetting phase distribution of a primary drainage process. Predicted soil water suction curves for a compacted silica sand sample is presented along with the effects of assumed contact angle between water and silica surface