Micromechanical Modeling of the Soil Water Retention Curve using a Coupled Discrete Element-Lattice Boltzmann Method

The Soil Water Retention Curve (SWRC) is a key constitutive relationship describing the behavior of variably saturated soils. The objective of this research is to assess the performance of a hydro-mechanical model, developed by coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM), for micromechanical simulation of the SWRC. The DEM-LBM model is used to examine the effects of wave propagation on fluid-solid interaction. A multi-phase LBM is then employed within a static particle array generated by the DEM to examine the effects of initial fluid density distribution. The SWRCs are generated by recording the liquid pore pressure and the degree of saturation within a porous medium subjected to imbibition for two cases: randomized fluid density simulation (non-unified wetting front) and droplet simulation (unified wetting front). The coupled DEM-multiphase LBM model is shown to be a promising tool to characterize capillary regime in partially saturated porous media

Porous media drying and two-phase flow studies using micromodels

In this thesis, we report an investigation of porous media drying and steady-state two-phase flow behaviour at the pore scale using micromodels based on thin section images of real rocks. Fluid distributions (and the deposition of solid salt in the case of drying) were imaged in real-time using optical microscopy. Computer simulations of the two-phase flow was initially compared to micromodel experiments and then used to predict behaviour in geometries not available in the lab. We performed evaporation experiments on a 2.5D etched-silicon/glass micromodel based on a thin section image of a sucrosic dolomite carbonate rock at different wetting conditions. NaCl solutions from 0 wt% (deionized water) to 36 wt% (saturated brine) were evaporated by passing dry air through a channel in front of the micromodel matrix. For deionized water in a water-wet model, we observed the three classical periods of evaporation: the constant rate period (CRP) in which liquid remains connected to the matrix surface, the falling rate period (FRP) and the receding front period (RFP), in which the capillary connection is broken and water transport becomes dominated by vapour diffusion. The length of the deionized water CRP was much shorter for a uniformly oil-wet model, but mixed wettability made little difference to the drying process. For brine systems in water-wet and mixed-wet micromodels, the evaporation rate became linear with the square root of time after a short CRP. Although this appears similar to the RFP for water, salt continued to be deposited at the external surface of the matrix during this period indicating that a capillary connection was maintained. The reduction of evaporation rate appears to be due to the deposited salt acting as a partial barrier to hydraulic connectivity, perhaps allowing dry patches to grow on the evaporating surface. The mechanism causing the square root time behaviour is therefore unlike the case of deionized water where capillary disconnection from the fracture channel is followed by a diffusion controlled process. In completely oil-wet micromodels capillary disconnection prevented salt deposition in the fracture. The resulting permeability impairment was also measured, for the water-wet model, we observed two regions of a linear downward trend in the matrix and fracture permeability measurements. A similar trend was observed for the mixed-wet systems. However, for the oil-wet systems, fracture permeability only changes slightly even for 360g/L brine, a result of the absence of salt deposits in the fracture caused by the early rupture of the liquid wetting films needed to aid hydraulic connectivity. Overall, matrix permeability for all wetting conditions decreased with increasing brine concentration and was almost total for the 360g/L brine. Furthermore, drying with air was compared with drying with CO2 gas, with the latter having important applications in CO2 sequestration processes. We observed that using CO2 rather than air as carrier gas makes the brine phase somewhat more wetting especially in the deionized water case, with the result that hydraulic connectivity was maintained for longer in the CO2 case compared to dry-out with air. Steady-state two-phase flow experiments were also conducted to study the effect of viscosity ratio, flow rate and capillary number on flow regimes and displacement processes using a 2.5D etched-silicon/glass micromodel based on a thin section image of a Berea sandstone rock. Of particular interest here was a new type of pore-scale behaviour, termed dynamic connectivity, previously identified in steady-state two-phase flow experiments in real rocks at the transition to ganglia flow by X-ray tomography. Micromodels have the potential to resolve the dynamics of these displacement processes due to the high speed resolution of optical techniques. Depending on the mean-size, prevalence, and connectivity of the non-wetting phase, four flow regimes were identified: connected pathway flow (CPF), big ganglia flow (BGF), big-small ganglia flow (BSGF) and small ganglia flow (SGF). These flow regimes move from CPF to SGF as the capillary-viscous balance of the system is altered by increasing the total flow rate of the system. The boundaries of the flow regimes are indistinct, however the domain of the BGF increases (and/or SGF decreases) with a decrease in the viscosity ratio of the system. That is the BGF regime persisted to higher capillary number for the water/squalane system than the water/decane system because it is harder for big blobs to split into smaller blobs at low viscosity ratio. However, dynamic connectivity was not observed in these micromodel experiments even after replicating the experiments with the same fluid pair (Nitrogen/Deionized water) used in the real porous media experiment. Therefore, we speculate that the constant depth of the micromodel used in this study does not provide a suitable geometry for dynamic connectivity to develop. One potential reason for this is the compressed range of capillary pressures due to the single etch depth. Hence, a multi-depth non-repeat micromodel was designed based on a single confocal image of a Bentheimer sandstone. Prototypes of small sections of the multi-depth model were produced by 3D printing but it was not possible to fabricate a functioning model due to time constraints. Simulation was therefore used to explore the multiphase flow behaviour of the new geometry. Initially a Lattice Boltzmann code (developed in another project) was applied to simulate flow in a small region of the single depth geometry and compared to the experimental results as a validation step. The LB model was then used to predict flow behaviour in the multi-depth geometry, however only connected pathway and ganglia flow regimes were seen unambiguously. It is therefore likely that the lack of 3D connectivity rather than capillary pressure limitations prevent the appearance of dynamic connectivity.Open Acces

Review on pore-network modeling studies of gas-condensate flow: Pore structure, mechanisms, and implementations

Gas-condensate flow is a critical process in the near-well region where the well production efficiency is strongly affected by the production of condensate dropout. Pore-scale simulations have provided an understanding of the underlying processes such as snap-off and the effect of the interplay between viscous and capillary forces on gas-condensate flow and its induced blockage within the pore spaces. Among various modeling approaches used to explore these phenomena, pore-network modeling, due to its computational efficiency and the ability to simulate relatively large sample sizes, has appealed to researchers. This article presents a review of the development of pore-network models to simulate gas-condensate flow, particularly in the near wellbore regions. This contribution reviews pore-scale mechanisms that should be included in simulating the gas-condensate flow, together with the involved processes and the peculiarities pertinent to such modeling efforts. After a brief review of different pore scale studies and their differences, advantages, and disadvantages, the review focuses on pore-network modeling, and the application of pore-network modeling in gas-condensate flow in the recent studies. The employed methodologies, highlights, and limitations of each pore network study are examined and critically discussed. The review addresses pore-space evolution, flow mechanisms, and the involved flow and transport parameters. The formulations of capillary entry pressure in different pore geometries, the corresponding conductance terms, snap-off criteria, and conditions for the creation of condensate bridging in different pore structures are presented. Additionally, three major approaches used in pore-network modeling of gas condensation, namely quasi-static, dynamic methods and dynamic compositional pore-network modeling, are presented and their main governing equations are provided using various tables. Finally, the significance of gas-condensate flow modeling including its modeling challenges together with the main similarities and differences among pore-network studies are provided

Measurement and Modeling of Reduced-Gravity Fluid Distribution and Transport in Unsaturated Porous Plant-Growth Media

The effect of reduced gravity on the balanced management of liquid, gaseous and ionic fluxes in unsaturated porous media remains a central challenge for plant-based bio-regenerative life support systems needed for long-duration space missions. This research investigated how shifting capillary and gravitational forces alter the sample-scale transport and distribution of fluids in mm-sized porous ceramic aggregates. Measurements in variably saturated media conducted on the International Space Station in microgravity ($sim1cdot10^{-3} g_{earth}$) and measurements during parabolic flight in variable gravity encompassing microgravity, terrestrial gravity and hypergravity ($sim1.8 g_{earth}$) were supported by numerical modeling based on fundamental, earth-derived soil-physical relationships. Measurements of water fluxes in rigid saturated media suggested Darcian flow unaffected by gravity. Observations of hydraulic potential and sample water content were used to estimate the primary draining and wetting water-retention characteristic (WRC). Terrestrial parameterizations of the WRC were largely unaffected by reduced gravity. However, because the WRC is hysteretic, heterogenous water-content distributions resulted within the confines of the primary draining and wetting characteristics. Ensuing distributions were fundamentally different from terrestrial observations and were stable in the absence of a significant gravity gradient. We showed that these distributions, though unexpected, could be predicted using the Richards equation. One consequence of altered water distribution could be the reduction in, and increased tortuosity of, continuous gas-filled pathways for diffusive transport compared to terrestrial estimates. Measurements of oxygen diffusion in microgravity suggested reduced diffusivities during draining. These observations, particularly for the smaller particle-sized media, were suggestive of the delayed formation of critical air-filled pathways at lower water contents. This dissertation further uses a case history of a stratified root-zone developed based on water-retention characteristics of different particle-sized media. The root-zone design provided a more uniform water-content distribution at terrestrial gravity suggested to provide more optimal conditions for root growth. Additionally, the design and testing of a novel integrated sensor for measurements of water content based on the dissipation of heat and estimation of nutrient status based on electrical resistivity are discussed. These results should provide insights into microgravity fluid distribution and transport contributing to the design and implementation of controllable plant-growth systems for use in microgravity and future planetary habitats