ABSTRACT:

Aeolian deposition is significant to the landscape formation on Earth and even other planetary bodies. Particle size controls the transport distance of particles. However, the understanding of particle-size effects on the structure and mechanical behavior of aeolian accumulations remains limited. We investigate the effect of particle size on the structure and collapse of aeolian sediments by one-dimensional compression and collapse tests following the initial air-fall deposition of particles of various sizes from sand to fine silt. Results reveal that the relevance between interparticle forces and gravity controls the formation of two distinct sediment structures as the particle size varies. A loose structure with a packing density close to the simple cubic packing forms for sands and is not collapsible. An open structure with higher porosity forms as particle size becomes smaller. The structure can stack vertically. The stacking height, porosity, and collapsibility increase as particle size decreases. A dimensionless granular bond number Bo captures the relevance between van der Waals forces and gravity. A threshold particle size corresponding to Bo = 10 is identified, below which open and collapsible structures form. The structure forms due to distinctive accumulation behavior. Particle interaction forces create porous aggregates, stabilize the aggregate as it contacts the deposit, and retain open pores. The effect of fine particles on properties of deposited mixtures is also analyzed. Results are relevant to understanding the formation of collapsible aeolian deposits on Earth and may contribute to the estimation of the physical properties of aeolian sediments on other planetary bodies.

ABSTRACT:

Resistivity index curves describe the relationship between electrical resistivity and water saturation of porous media and are critical in formation evaluation and geophysical subsurface process monitoring. Archie’s second equation enforces a linear relationship between resistivity index and water saturation in log-log plots which has been widely used for the assessment of in situ hydrocarbon saturation. However, resistivity index curves that deviate from Archie’s equation are ubiquitous in reservoirs, especially complex carbonates exhibiting bimodal pore-size distributions, where the effects of pore-scale controlling factors on rock resistivity remain unclear. We implement pore-network models built under controlled conditions of pore shapes, bimodal pore-size distributions, pore connectivity, micropore fractions, and anisotropy to investigate the effects of pore shapes and pore-space heterogeneity on resistivity index curves. Results indicate that percolating wetting films associated with pore shapes decrease the resistivity index at low values of water saturation and cause a decrease in saturation exponent. In cases of bimodal pore-size distributions, micropore fractions control the connectivity between macropores, thus affecting water drainage and resistivity index curves. At low fractions of micropores, the resistivity index curve is governed by the connected macropore system at high water saturation and is then determined by micropores, thereby resulting in non-Archie behavior. Pore-size distributions and spatial anisotropy also affect the resistivity index curves. We summarize the observed pore-space heterogeneity effects on resistivity index curves and compare model predictions with numerical results; both the geometric mean model and effective medium theory provide acceptable estimates of the electrical properties of bimodal porous media.