The Influence of Grain Shape and Size on the Relationship Between Porosity and Permeability in Sandstone

An accurate and reliable description of the relationship between porosity and permeability in geological materials is valuable in understanding subsurface fluid movement. This is of great importance for studies of reservoir characterisation, useful for energy exploitation, carbon capture, use and storage (CCUS) and groundwater contamination and remediation. Whilst the relationship between pore characteristics and porosity and permeability are well examined, there is scope for further investigation into the influence of grain characteristics on porosity and permeability due to the inherent relationship between grains and related pores. In this work we use digital image analysis (DIA) of reconstructed 3D X-ray micro computed tomographic (μCT) images to measure porosity, permeability and segment individual grains enabling the measurement of grain shape (sphericity) and size (Feret diameter). We compare two marker-based watershed workflows to grain boundary segmentation before applying the most reliable one to our images. We found there to be a positive relationship between grain sphericity and porosity according to ϕ=1.22ϕs-0.42 whereas no such relationship exists with grain size. We applied our grain shape and size measurements to calculate a Kozeny-Carman (K-C) porosity-permeability fit which was found to be unsatisfactory, possibly due to significant deviation from the K-C assumption that grains are spherical. Therefore, we show that a simpler fit of the form K= 10^5.54 ϕ^3.7, excluding any influence of grain characteristics, is most suitable for the studied materials and that grain shape and size is not influential on the porosity-permeability relationship in a K-C paradigm.

The Influence of Grain Shape and Size on the Relationship Between Porosity and Permeability in Sandstone

An accurate and reliable description of the relationship between porosity and permeability in geological materials is valuable in understanding subsurface fluid movement. This is of great importance for studies of reservoir characterisation, useful for energy exploitation, carbon capture, use and storage (CCUS) and groundwater contamination and remediation. Whilst the relationship between pore characteristics and porosity and permeability are well examined, there is scope for further investigation into the influence of grain characteristics on porosity and permeability due to the inherent relationship between grains and related pores. In this work we use digital image analysis (DIA) of reconstructed 3D X-ray micro computed tomographic (μCT) images to measure porosity, permeability and segment individual grains enabling the measurement of grain shape (sphericity) and size (Feret diameter). We compare two marker-based watershed workflows to grain boundary segmentation before applying the most reliable one to our images. We found there to be a positive relationship between grain sphericity and porosity according to ϕ=1.22ϕs-0.42 whereas no such relationship exists with grain size. We applied our grain shape and size measurements to calculate a Kozeny-Carman (K-C) porosity-permeability fit which was found to be unsatisfactory, possibly due to significant deviation from the K-C assumption that grains are spherical. Therefore, we show that a simpler fit of the form K= 10^5.54 ϕ^3.7, excluding any influence of grain characteristics, is most suitable for the studied materials and that grain shape and size is not influential on the porosity-permeability relationship in a K-C paradigm.

Submarine canyon systems focusing sub-surface fluid in the Canterbury Basin, South Island, New Zealand

This work uses a high-quality 3D seismic volume from offshore Canterbury Basin, New Zealand, to investigate how submarine canyon systems can focus sub-surface fluid. The seismic volume was structurally conditioned to improve the contrast in seismic reflections, preserving their lateral continuity. It reveals multiple pockmarks, eroded gullies and intra-slope lobe complexes occurring in association with the Waitaki Submarine Canyon. Pockmarks are densely clustered on the northern bank of the canyon and occur at a water depth of 500–900 m. In parallel, near-seafloor strata contain channel-fill deposits, channel lobes, meandering channel belts and overbank sediments deposited downslope of the submarine canyon. We propose that subsurface fluid migrates from relatively deep Cretaceous strata through shallow channel-fill deposits and lobes to latter seep out through the canyon and associated gullies. The new, reprocessed Fluid Cube meta-attribute confirms that fluids have seeped out through the eroded walls of the Waitaki Canyon, with such a seepage generating seafloor depressions in its northern bank. Our findings stress the importance of shallow reservoirs (channel-fill deposits and lobes) as potential repositories for fluid, hydrocarbons, or geothermal energy on continental margins across the world.

Subsurface fluid migration associated with feeding systems of mud volcanoes in Northwestern Caucasus, results of passive seismic studies

<p>The results of passive seismic studies of subsurface fluid transport systems associated with mud volcanic phenomena in Northwestern Caucasus and the Taman mud-volcanic province are presented. Comparative analysis of results of geophysical cross-sections featuring the deep subsurface structures of several mud volcanoes obtained by means of passive microseismic sounding approach with respect to previous studies has demonstrated advantages of the ambient noise seismic prospecting. It has been shown that subvertical pathways of hydrocarbon migration and so feeding systems of mud volcanoes represent nearly-ideal case of local geological heterogeneities affecting the amplitudes of low-frequency microseismic noise. The analysis of the results was performed with respect to available geological as well as geomorphological data. At the same tine, results of past active seismic experiments with controlled vibroseismic sources were reanalyzed and followed by mathematical modeling of processes of hydrodynamic outflow under various mechanisms of mud volcanic eruptions. For several mud volcanoes there were outlined three-dimensional subvertical feeding structures in sedimentary layers and deeper in the crust, responsible for fluid migration and eruptive activity. Specific features of volcanic products (gas components and mineral inclusions in breccia) were analyzed with respect to the new geophysical data obtained.</p>

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

Occurrence of earthquakes related with geothermal reservoirs highlights the possibility that subsurface fluid injection may reactivate critically stressed faults and trigger seismicity. We report on laboratory experiments conducted at T = 100-250 °C, σc = 110 MPa and Pf = 42-63 MPa and show that two prevalent minerals, epidote and chlorite, impact frictional properties and fault stability under conditions relevant to typical geothermal reservoirs. Numerous geothermal reservoirs worldwide exhibit metamorphic epidotization and chloritization. Shear experiments on simulated epidote-rich fault gouges indicate potentially unstable frictional behavior - more pronounced at elevated temperatures and pore pressures. Increased proportions of chlorite in fault gouges stabilize the faults, which indicates that gouge composition exerts significant control on fault stability. Our results imply a high potential for induced seismicity on faults containing epidote found in many geothermal reservoirs.