ARTICLE | doi:10.20944/preprints202205.0019.v1
Subject: Mathematics & Computer Science, Applied Mathematics Keywords: seepage velocity; heavy metals; reservoirs pollution
Online: 5 May 2022 (03:19:54 CEST)
Increasing rate of seepage velocity from several formation characteristics, such as permeability and porosity, in water aquifer environment greatly prompt pollution of water reservoirs within a short period of time. Considerably, migration rate of dissolved heavy metals from Solid Waste Dumpsites (SWD), such as municipal dumpsites and landfills, through heterogeneous aquifer environment, and finally into nearby water reservoirs are mainly influenced by variation of seepage velocity within the soil and water environment. This presents a dynamic system for water pollution that was studied using a formulated mathematical model to describe the transport process of dissolved heavy metals, mainly characterized by seepage velocities, within the water aquiferous environment. Permeability, porosity, fluid pressure and concentration of heavy metals in aquiferous environment were used as principal parameters that influence seepage velocity of the metals, in dissolved state, through the structural formation of water aquifers. The derived mathematical equations that constitute the model of this study were generated through Darcy’s law and the equation of continuity. The model was validated on structural river aquifer sediments, and it was solved using graphical method through matlab open-source software. The initial and boundary conditions were obtained by discretizing the geological setting of flow region so as to transform the gradient of the head, into the time domain.
ARTICLE | doi:10.20944/preprints202009.0606.v1
Subject: Earth Sciences, Atmospheric Science Keywords: fracture; theoretical model; seepage pressure; rock deformation
Online: 25 September 2020 (11:36:54 CEST)
The formation of rock fractures in nature has a certain relationship with water seepage. In order to analyze the shape of rock fracture, we propose a rock deformation theoretical model in three-dimensional space considering with the condition of seepage water pressure, and establish the mass conservation equation, seepage equation, surrounding rock displacement equation, so as to deduce the rock deformation control equation under seepage pressure action. The numerical analysis of the nonlinear quadratic partial differential equation obtains the fracture deformation distribution in the example, and verifies with the calculated fracture morphology. And we further use this principle to analyze the fracture morphology in nature and prove the rationality of the theory.
ARTICLE | doi:10.20944/preprints202205.0106.v1
Subject: Earth Sciences, Geology Keywords: seepage characteristics; single fracture; roughness; numerical simulation; Fluent
Online: 9 May 2022 (06:13:55 CEST)
A single fracture is the basic unit of fracture medium, and the roughness of fracture wall surface is an important factor influencing hydraulic characteristics of the ﬂow in bedrock fracture. However, effects of the shape and density of roughness elements (various bulges/pits on rough fracture wall surfaces) on water ﬂow in a single rough fracture have not been thoroughly discovered. Thus the water ﬂow in single fracture with different shapes and densities of roughness elements was simulated by using Fluent software in this study. The results show that in wider fractures the flow rate mainly depends on fracture aperture, while in narrow and close fracture medium the surface roughness of fracture wall is the main factor of head loss of seepage; there is a negative power exponential relation between the hydraulic gradient index and the average fracture aperture, i.e. with the increase of fracture aperture, the relative roughness of fracture and the influence weight of hydraulic gradient both decrease; and in symmetrical-uncoupled fractures there is a super-cubic relation between the discharge per unit width and average aperture. Above results would help to deepen the understanding of rough fracture seepage.
ARTICLE | doi:10.20944/preprints201810.0619.v1
Subject: Earth Sciences, Geology Keywords: BSR; gas hydrate; methane; seepage; active margin; Chile Triple Junction
Online: 26 October 2018 (06:18:51 CEST)
Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a zone of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here we present an analysis of the spatial distribution, concentration, estimate of gas phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism and forearc sediments offshore Taitao (45.5° - 47° S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values <10% of the total rock volume, and extremely high geothermal gradients (<190 °Ckm-1). Gas hydrates are located in shallow sediments (90-280 meters below the seafloor). The large amount of hydrate and free gas estimated (7.21x1011 m3 and 4.1x1010 m3, respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous geothermal conditions, set the stage for potential massive releases of methane to the ocean mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological and ecological research.
ARTICLE | doi:10.20944/preprints202112.0043.v1
Subject: Engineering, Civil Engineering Keywords: hydro-damage; red-bed mudstone; hydrodynamic; X-ray image; seepage–diffusion
Online: 3 December 2021 (09:46:59 CET)
Nonlinear catastrophes caused by geological fluids are a fundamental issue in rock mechanics and the geoengineering hazard field. For the consideration of hydrodynamic force on red-bed mudstone softening damage, X-ray visualization test on the fissure flow in mudstone block failure under hydrodynamic force was performed in this study based on block scale and the physical phenomena of fissure seepage and nonlinear diffusion were further explored. A new method for evaluating the hydro-damage degrees of rocks using an X-ray image analysis was proposed, and the quantitative relation of diffusion coefficients of hydro-damage and seepage was established. The research results revealed that the hydrodynamic force promoted the fluid-filled fissure behavior in mudstone specimen failure. Also, the seepage and diffusion phenomena of fluid in rocks during failures were indicated using X-ray imaging. A dual mechanical behavior was presented in the nonlinear seepage and abnormal diffusion of a red mudstone geological body under hydrodynamic conditions. The damaged degree of mudstone was aggravated by the effect of hydrodynamic force, and the initial seepage–diffusion coefficient with respect to lower hydro-damage was larger than the final seepage–diffusion coefficient with respect to higher hydro-damage of rocks with a decreasing nonlinear trend.
ARTICLE | doi:10.20944/preprints201805.0384.v1
Subject: Engineering, Civil Engineering Keywords: fracture grouting; cement-silicate grout; geophysical prospecting; seepage; Yellow River Embankment
Online: 28 May 2018 (05:45:00 CEST)
Fracture grouting has been a widely used mitigation measure against seepage in the Yellow River Embankment. However, there is currently a lack of systematic investigation for evaluating the anti-seepage effectiveness of fracture grouting employed in this longest river embankment in China. Therefore, in this work, laboratory and in-situ experiments are carried out for investigating the reinforcement effect of fracture grouting in the Jinan Section of the Yellow River Embankment. In particular, firstly, the laboratory tests concentrate on studying the optimum strength improvement for cement-silicate grout by varying the content of backfilled fly ash and bentonite as admixtures. Flexural strength and Scanning Electron Microscope photographs are investigated for assessing the strength and compactness improvement. Subsequently, based on the obtained optimum admixtures content, in-situ grouting tests are carried out in the Jinan Section of the Yellow River Embankment to evaluate the anti-seepage effectiveness of fracture grouting, where geophysical prospecting and pit prospecting methods are employed. Laboratory results show that, compared with pure cement-silicate grouts, the gelation time of the improved slurry is longer and gelation time increases as fly ash content increases. The optimum mixing proportion of the compound cement-silicate grout is 70% cement, 25% fly ash and 5% bentonite, and the best volume ratio is 2 for the investigated cases. Geophysical prospecting using the Ground Penetrating Radar and High Density Resistivity methods can reflect the anti-seepage effectiveness of fracture grouting on site. It shows that the grouting material mainly flows along the axial direction of the embankment. The treatment that is used to generate directional fracture is proved to be effective. The injection hole interval distance is suggested to be 1.2 m, where the lapping effect of the grouting veins is relatively significant. For the investigated cases, the average thickness of the grouting veins is approximately 6.0 cm and the corresponding permeability coefficient is averagely 1.6 × 10−6 cm/s, which meets the anti-seepage criterion in practice.
ARTICLE | doi:10.20944/preprints202007.0571.v1
Subject: Keywords: native hydrogen; deep-seated source of natural hydrogen; H2 seepage; radiolysis; serpentinization; draining faults
Online: 24 July 2020 (05:23:26 CEST)
Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by i) a relatively high geothermal gradient, ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, iii) an archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing an important amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The very high fluxes of H2 escaping out of these structures which have been recently documented are, however, in disagreement with the newly developed H2 production model in the Precambrian continental crust. They either question the validity of these models or the measurement methodology.
ARTICLE | doi:10.20944/preprints202009.0527.v1
Subject: Engineering, Civil Engineering Keywords: local-saturated zone of subgrade; fine particles migration; two-phase seepage characteristics; deformation characteristics; volume fraction of fine particles
Online: 23 September 2020 (03:34:20 CEST)
The fluid seepage in local-saturated zone of subgrade promotes the migration of fine particles in the filler, resulting in the change of pore structure and morphology of the filler and the deformation of solid skeleton, which affects the fluid seepage characteristics. Repeatedly, the muddy interlayer, mud pumping and other diseases are finally formed. Based on the theory of two-phase seepage, the theory of porous media seepage, and the principle of effective stress in porous media, a two-phase fluid-solid coupling mathematical model in local-saturated zone of subgrade considering the effect of fine particles migration is established. The mathematical model is numerically calculated with the software COMSOL Multiphysics○R, the two-phase seepage characteristics and the deformation characteristics of the solid skeleton in local-saturated zone of the subgrade are studied. The research results show that due to the continuous erosion and migration of fine particles in local-saturated zone of the subgrade, the volume fraction of fine particles first increases then decreases and finally becomes stable with the increase of time. And the volume fraction of fine particles for the upper part of the subgrade is larger than that for the lower part of the subgrade. The porosity, the velocity of fluid, the velocity of fine particles, and the permeability show a trend of increasing first and then stabilizing with time; the pore water pressure has no significant changes with time. The vertical displacement increase first and then decrease slightly with the increase of time, and finally tend to be stable. For a filler with a larger initial volume fraction of fine particles, the maximum value of the volume fraction of fine particles caused by fluid seepage is larger, and the time required to reach the maximum value is shorter. It can be concluded that in actual engineering, the volume fraction of fine particles in the subgrade filler should be minimized on the premise that the filler gradation meets the requirements of the specification.