How Air Entrapment in Hydrophobic Particle-Water-Air Mixtures Changes Post-Wildfire Mudflow Composition

This paper shows critical new insights into how air entrapment affects the properties of rain-induced post-wildfire mudflows as a mixture of air bubbles, water, and hydrophobic sand. The idea of mudflows' internal structure containing trapped air bubbles is novel. Such mixtures can flow down slopes at incredible speeds, quickly blasting obstacles on the way and carrying large stone boulders and objects. The surficial soil particles turn hydrophobic due to the deposition of combusted organic matter during wildfires. Afterward, raindrops, splash, and erosion form devastating mudflows. We propose a new paradigm in which a significant amount of air remains entrapped in post-wildfire mudflow via hydrophobic particle-air attraction. Specific findings quantify the amount of air trapped within sand-water volumetric concentrations, the effect of intermixing energy, gravity, and sand particle size on outcome mudflow internal structure. As a result, little agglomerates of sand particles covering air bubbles characterize the mudflow mixture's internal structure.Comment: 19 pages, 14 figures, 3 table

Assessment of Maintenance Strategies for Bio-stabilization of Mudslides on Wildfire-affected Slopes

UC-ITS-2021-48Wildfires in California have increased due to climate change, poor forests maintenance, and human factors. Post-wildfire mudflows frequently occur during rain events on burn scars due to loss of vegetation, change of surface morphology, and soil surface hydrophobicity. Spreading Xanthan gum biopolymer on slopes after wildfires may mitigate the risk of extensive erosion of hydrophobic soil layers during rain. Experiments test identified rain intensities from 15 mm/hr to 50 mm/hr and seven natural rain events on a separate set of experiments on fine, medium, and coarse sand slopes 10\ub0 to 25\ub0. Different approaches to Xanthan gum application are considered. Surficial erosion occurs due to rain and is extreme in untreated slopes in all three sand types. Sprinkling surfaces with pure Xanthan gum leads to erosion as well. However, when xanthan gum is mixed with sand in small quantities and wetted, gel-like connections form between sand particles, which prevents further erosion when allowed to harden. Experiments focused on cured Xanthan gum and sand mix covers yielded a better understanding of coupled conditions necessary for successful erosion mitigation and the advantages and limitations of the proposed approach. Furthermore, practical guidelines for improving burned scars are recommended

New Experimental Equipment Recreating Geo-Reservoir Conditions in Large, Fractured, Porous Samples to Investigate Coupled Thermal, Hydraulic and Polyaxial Stress Processes

Abstract Use of the subsurface for energy resources (enhanced geothermal systems, conventional and unconventional hydrocarbons), or for storage of waste (CO2, radioactive), requires the prediction of how fluids and the fractured porous rock mass interact. The GREAT cell (Geo-Reservoir Experimental Analogue Technology) is designed to recreate subsurface conditions in the laboratory to a depth of 3.5 km on 200 mm diameter rock samples containing fracture networks, thereby enabling these predictions to be validated. The cell represents an important new development in experimental technology, uniquely creating a truly polyaxial rotatable stress field, facilitating fluid flow through samples, and employing state of the art fibre optic strain sensing, capable of thousands of detailed measurements per hour. The cell’s mechanical and hydraulic operation is demonstrated by applying multiple continuous orientations of principal stress to a homogeneous benchmark sample, and to a fractured sample with a dipole borehole fluid fracture flow experiment, with backpressure. Sample strain for multiple stress orientations is compared to numerical simulations validating the operation of the cell. Fracture permeability as a function of the direction and magnitude of the stress field is presented. Such experiments were not possible to date using current state of the art geotechnical equipment

Particle-fluid flow and transport within rough fractures

Proppant injection is an important part of a hydraulic fracturing programs in which fluid-particle slurry is injected into rock fractures. Injected particles are lodged between fracture surfaces during wall close-in thereby propping open the fracture, improving connectivity and production. This paper investigates behaviour of proppant particles within artificially generated rock fractures, providing insight into transport behavioural differences caused by realistic surface roughness. Better understanding of proppant behaviour within more realistic rough fracture conditions provides greater understanding of proppant transport as compared to past works where smooth walled fracture configurations were utilized. A clearer understanding is important in providing more accurate evaluation of realistic proppant flow and distribution and improving injection design. In this study a roughened surface, analogues to actual rock fracture surface, is artificially generated based on a rock surface’s fractal dimension and asperity height standard deviation. Computational representation of the rock surfaces and flow domain is generated. Resolved Discrete Element Method coupled with computational fluid dynamics (DEM-CFD) is implemented in this study to evaluate proppant particle transport behaviour within the fractures. This work highlights importance of considering fracture surface roughness in evaluating proppant flow and transport and more generally the impact of rough boundary conditions of particle-fluid systems

Evaluation and remediation of post-wildfire slope stability

Catastrophic mudflows and landslides triggered by rainfall can occur suddenly and move with high speed, damaging electrical and civil infrastructure and threatening human and wildlife. Due to the climate change and extreme weather increase, it is likely that wildfires and consequently mudflows will increase in frequency in the future. The risk of mudflows and landslides increases in post-wildfire areas mainly because of water repellent soil which forms on slopes. Water repellency, or hydrophobicity, can occur due to the burning of the accumulated organic matter in soil. Hydrophobicity repels water and prevents infiltration of water into the soil, which results in soil erosion, mudflows and landslides. In this study, a series of experimental laboratory tests are conducted on regular, hydrophobic and Xanthan gum-treated hydrophobic sand. Xanthan gum, which is an environmentally friendly biopolymer, can be substituted for chemical material used for soil improvement and decrease the CO2 emissions and enhance environmental slope protection. Xanthan gum can enhance the inter-particle cohesion and can hold a large amount of water and consequently help the recovery of the vegetation. Contact angle, direct shear and rain simulation tests are conducted on samples. Results show that in Xanthan gum treated slope the rate of erosion and the risk of mudflow decreases. It is also observed that the Xanthan gum treated sample can retain more water and consequently decreases the rate of wind erosion and helps the dust-control in burned areas

Technical review on coaxial deep borehole heat exchanger

Abstract: This review paper summarizes recent developments regarding geothermal exploitation using coaxial deep borehole heat exchangers (DBHE). Specifically, this study focuses on field tests, analytical and semi-analytical approaches, and numerical simulations. First, field tests and applications of coaxial DBHE are summarized and future work for the field tests is suggested. Then, the ongoing analytical and numerical modeling approaches on coaxial DBHE are evaluated regarding the capability and incapability of describing physical behaviors. Lastly, key factors for the design of coaxial DBHE are summarized and discussed based on collected results. Regarding field tests, future work should focus more on (1) long-term performance; (2) effect of groundwater flow within formation and fractures; (3) technology for larger diameter boreholes; (4) new and cheap materials for insulated inner pipe; (5) treatment of fluid, pipe wall, and different working fluid; (6) economic analysis of coaxial DBHE-based geothermal power plant. As for the analytical methods and numerical simulations, it is important to consider the dependence of fluid and formation properties on pressure and temperature. Besides, verification and calibration of empirical models for working fluids other than water such as CO2 should be performed based on laboratory and field tests. Different borehole properties and pump parameters should be optimized to obtain the maximum thermal power of a coaxial DBHE, and an insulated inner pipe is recommended by many researchers. An intermittent working pattern of the DBHE could be more realistic when modeling a DBHE. To further improve the performance of coaxial DBHE, continuous research to enhance heat transfer and working fluid performance is still important