scholarly journals On the reliability of analytical models to predict solute transport in a fracture network

2013 ◽  
Vol 10 (12) ◽  
pp. 14905-14948
Author(s):  
C. Cherubini ◽  
C. I. Giasi ◽  
N. Pastore
Keyword(s):  
Solute Transport ◽  
Transport Model ◽  
Fractured Rock ◽  
Early Time ◽  
Taylor Dispersion ◽  
Breakthrough Curves ◽  
Nonlinear Flow ◽  
Analytical Models ◽  
Late Time ◽  
Tracer Transport

Abstract. In hydrogeology, the application of reliable tracer transport model approaches is a key issue to derive the hydrodynamic properties of aquifers. Laboratory and field-scale tracer dispersion breakthrough curves (BTC) in fractured media are notorious for exhibiting early time arrivals and late-time tailing that are not captured by the classical advection–dispersion equation (ADE). These "non-Fickian" features are proved to be better explained by a mobile–immobile (MIM) approach. In this conceptualization the fractured rock system is schematized as a continuous medium in which the liquid phase is separated into flowing and stagnant regions. The present study compares the performances and reliabilities of classical Mobile–Immobile Model (MIM) and the Explicit Network Model (ENM) that takes expressly into account the network geometry for describing tracer transport behavior in a fractured sample at bench scale. Though ENM shows better fitting results than MIM, the latter remains still valid as it proves to describe the observed curves quite well. The results show that the presence of nonlinear flow plays an important role in the behaviour of solute transport. Firstly the distribution of solute according to different pathways is not constant but it is related to the flow rate. Secondly nonlinear flow influences advection, in that it leads to a delay in solute transport respect to the linear flow assumption. Whereas nonlinear flow does not show to be related with dispersion. However the interpretation with the ENM model shows a weak transitional regime from geometrical dispersion to Taylor dispersion for high flow rates. The experimental results show that in the study case the geometrical dispersion dominates the Taylor dispersion. Incorporating the description of the flowpaths in the analytical modeling has proved to better fit the curves and to give a more robust interpretation of the solute transport.

scholarly journals On the reliability of analytical models to predict solute transport in a fracture network

2014 ◽  
Vol 18 (6) ◽  
pp. 2359-2374 ◽  
Author(s):  
C. Cherubini ◽  
C. I. Giasi ◽  
N. Pastore
Keyword(s):  
Solute Transport ◽  
Transport Model ◽  
Fractured Rock ◽  
Early Time ◽  
Taylor Dispersion ◽  
Breakthrough Curves ◽  
Nonlinear Flow ◽  
Analytical Models ◽  
Late Time ◽  
Tracer Transport

Abstract. In hydrogeology, the application of reliable tracer transport model approaches is a key issue to derive the hydrodynamic properties of aquifers. Laboratory- and field-scale tracer dispersion breakthrough curves (BTC) in fractured media are notorious for exhibiting early time arrivals and late time tailing that are not captured by the classical advection–dispersion equation (ADE). These "non-Fickian" features are proven to be better explained by a mobile–immobile (MIM) approach. In this conceptualization the fractured rock system is schematized as a continuous medium in which the liquid phase is separated into flowing and stagnant regions. The present study compares the performances and reliabilities of the classical MIM and the explicit network model (ENM), taking expressly into account the network geometry for describing tracer transport behavior in a fractured sample at bench scale. Though ENM shows better fitting results than MIM, the latter remains still valid as it proves to describe the observed curves quite well. The results show that the presence of nonlinear flow plays an important role in the behavior of solute transport. First, the distribution of solute according to different pathways is not constant, but it is related to the flow rate. Second, nonlinear flow influences advection in that it leads to a delay in solute transport respect to the linear flow assumption. However, nonlinear flow is not shown to be related with dispersion. The experimental results show that in the study case the geometrical dispersion dominates the Taylor dispersion. However, the interpretation with the ENM shows a weak transitional regime from geometrical dispersion to Taylor dispersion for high flow rates. Incorporating the description of the flow paths in the analytical modeling has proven to better fit the curves and to give a more robust interpretation of the solute transport.

Migration of Solutes in Unsaturated, Fractured Rock at Yucca Mountain: Measurements, Mechanisms, and Models

1995 ◽  
Vol 412 ◽  
Author(s):  
A. V. Wolfsberg ◽  
B. A. Robinson ◽  
J. T. Fabryka-Martin
Keyword(s):  
Solute Transport ◽  
Transport Model ◽  
Alternative Hypothesis ◽  
Fractured Rock ◽  
Yucca Mountain ◽  
Nuclear Waste Repository ◽  
Transport Studies ◽  
And Performance ◽  
High Level ◽  
A Site

AbstractCharacterization and performance assessment (PA) studies for the potential high-level nuclear waste repository at Yucca Mountain require an understanding of migration mechanisms and pathways of radioactive solutes. Measurements of 36C1 in samples extracted from boreholes at the site are being used in conjunction with recent infiltration estimates to calibrate a site-scale flow and solute transport model. This exercise using the flow and solute transport model, FEHM, involves testing different model formulations and two different hypotheses to explain the occurrence of elevated 36Cl in the Calico Hills unit (CHn) which indicates younger water than in the overlying Topopah Spring unit (TSw). One hypothesis suggests fast vertical transport from the surface via fractures in the TSw to the CHn. An alternative hypothesis is that the elevated 36C1 concentrations reflect rapid horizontal flow in the CHn or at the interface between the CHn and the TSw with the source being vertical percolation under spatially isolated regions of high infiltration or at outcrops of those units. Arguments in favor of and against the hypotheses are described in conjunction with the site-scale transport studies.

Solute transport in dual conduit structure: experiment and modelling

2020 ◽  
Author(s):  
Chaoqi Wang ◽  
Xiaoguang Wang ◽  
Samer Majdalani ◽  
Vincent Guinot ◽  
Hervé Jourde
Keyword(s):  
Solute Transport ◽  
Peak Concentration ◽  
Dispersion Model ◽  
Tracer Tests ◽  
Breakthrough Curves ◽  
Physical Models ◽  
Length Ratio ◽  
Tracer Transport ◽  
Transport Parameters ◽  
Total Length

<p>An important phenomenon often encountered when interpreting tracer tests in karst aquifers is the occurrence of dual-peaked breakthrough curves (BTCs). The dual-peaked BTCs are usually attributed to tracer transport through a conduit system consisting of a dual-conduit structure: an auxiliary conduit that deviates from the main conduit at the upstream and converges back at the downstream. In order to understand how the geometric configuration of the dual-conduit structure influences the BTCs, laboratory experiments utilizing plastic tubes were conducted. The physical models were constructed by varying: 1) the total length of the conduits, while fixing the length ratio; 2) length ratio between the two conduits, while fixing the length of the main conduit; and 3) conduits connection angle. The tracer experiments are then fitted by a Multi-Region Advection Dispersion model and a Transfer Function model to derive effective transport parameters. This allows us to quantitatively compare the experimental results, and thus to analyse the conduit geometry effects on solute transport and to compare the performance of the two models.</p><p>Results show that the dual-conduit structure causes the double peaks of BTCs. Keeping the length ratio of the two conduits and increasing their total length leads to a larger separation of the two peaks of the BTCs. Keeping the length of main conduit while increasing the length of the secondary conduit causes similar effects. As (θ<sub>1</sub>-θ<sub>2</sub>) increases, the first peak concentration value decreases, the second peak concentration value increases.</p><p><strong>Keywords</strong>: karst, lab experiment, dual-peaked BTCs, modelling</p>

Determination the Levels of Thief Zones Based on Machine Learning

2021 ◽  
Author(s):  
Chunhua Lu ◽  
Hanqiao Jiang ◽  
Chengcheng You ◽  
Fulong Wang ◽  
Fei Xu ◽  
...  
Keyword(s):  
Transport Model ◽  
Dropout Rate ◽  
Activation Function ◽  
Breakthrough Curves ◽  
Classification Model ◽  
Automatic Identification ◽  
Tracer Transport ◽  
Practical Applications ◽  
Tracer Testing ◽  
Model Training

Abstract The technique of interwell tracer testing is considered as one of the most effective method to identify the thief zone (TZ) in reservoirs. However, in heavy oil reservoirs, tracer breakthrough curves are mostly parabolic and unimodal, thus resulting in slight differences between curves. It is inefficient and inaccurate to identify different types of curves with traditional methods applied to characterize the levels of TZs. In this paper, convolutional neural network (CNN) is applied to construct a classification model for the automatic identification of the levers of TZs. According to the TZs criteria specified on the field, the analytical tracer transport model was applied to generate 3000 curves as the sample, which can meet the requirements of model training accuracy. In the meantime, One-hot encoding, Xavier initialization, Adam optimizer, and mini-batch normalization were used to construct the model, and the key parameters are optimized to improve the performance of the model. The results show that the appropriate activation function is ReLU and the optimal dropout rate is 0.5. Moreover, the construction of CNN with discrete data points (DDP-CNN) as input contributed to a further improvement of classification accuracy of tracer curves. The accuracy of DDP-CNN in training set is 0.96, which is 14% and 23% higher than random forest (RF) and k-means, respectively. In practical applications, DDP-CNN proves capable to correctly classify 88 of the 100 curves.

scholarly journals Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

2013 ◽  
Vol 10 (1) ◽  
pp. 221-254 ◽  
Author(s):  
C. Cherubini ◽  
C. I. Giasi ◽  
N. Pastore
Keyword(s):  
Solute Transport ◽  
Rock Sample ◽  
Breakthrough Curves ◽  
Fractured Media ◽  
Hydraulic Loss ◽  
Assessment Procedure ◽  
Tracer Transport ◽  
Flow And Transport ◽  
Experimental Apparatus ◽  
Study Laboratory

Abstract. Accurate predictions of solute propagation in fractured rocks are of particular importance when assessing exposure pathways through which contaminants reach receptors during a risk assessment procedure, as well as when dealing with cleanup and monitoring strategies. The difficulty in modeling fractured media leads to the application of simplified analytical solutions that fail to reproduce flow and transport patterns in such complex geological formations. A way for understanding and quantifying the migration of contaminants in groundwater systems is that of analyzing tracer transport. Experimental data obtained under controlled conditions such as in a laboratory allow to increase the understanding of the fundamental physics of fluid flow and solute transport in fractures. In this study laboratory hydraulic and tracer tests have been carried out on an artificially created fractured rock sample. The tests regard the analysis of the hydraulic loss and the measurement of breakthrough curves for saline tracer pulse inside a rock sample of parallelepiped (0.60 × 0.40 × 0.8 m) shape. The effect of the experimental apparatus on flow and transport tests has been estimated. In particular the convolution theory has been applied in order to remove the effect of acquisition apparatus on tracer experiment. The experimental results have shown evidence of a non-Darcy relationship between flow rate and hydraulic loss that is best described by Forchheimer's law. The observed experimental breakthrough curves of solute transport have been modeled by the classical one-dimensional analytical solution for advection–dispersion equation (ADE) and the single rate mobile–immobile model (MIM). The former model does not fit properly the first arrival and the tail while the latter provides a very decent fit.

scholarly journals Multiscale Roughness Influence on Conservative Solute Transport in Self-affine Fractures

2018 ◽  
Author(s):  
Zhi Dou ◽  
Brent Sleep ◽  
Hongbin Zhan ◽  
Zhifang Zhou ◽  
Jinguo Wang
Keyword(s):  
Numerical Simulations ◽  
Solute Transport ◽  
Large Scale ◽  
Breakthrough Curves ◽  
Nonlinear Flow ◽  
Small Scale ◽  
Single Fracture ◽  
Advection Dispersion ◽  
Flow Features ◽  
Conservative Solute Transport

Abstract. In this article, the influence of multiscale roughness on transport of a conservative solute through a self-affine fracture was investigated. The fracture roughness was decomposed into two different scales (i.e., a small-scale stationary secondary roughness superimposed on a large-scale non-stationary primary roughness) by a wavelet analysis technique. The fluid flow in the single fracture was characterized by Forchheimer's law and exhibited nonlinear flow features such as eddies and tortuous streamlines. The results indicated that the small-scale secondary roughness was primarily responsible for the nonlinear flow features. Numerical simulations of asymptotic conservative solute transport showed non-Fickian transport characteristics (i.e., early arrivals and long tails) in breakthrough curves (BTCs) and in residence time distributions (RTDs) with and without consideration of the secondary roughness. Analysis of multiscale BTCs and RTDs showed that the small-scale secondary roughness played a significant role in enhancing the non-Fickian transport characteristics. Removing small-scale secondary roughness delayed the arrival time and shortened the tail. The peak concentrations in BTCs decreased as the secondary roughness was removed, implying that the secondary roughness could also enhance the solute dilution. Fitting the one-dimensional (1D) Fickian advection-dispersion equation (ADE) to the numerical BTCs resulted in considerable errors that decreased with the small-scale secondary roughness being removed. The 1D mobile-immobile model (MIM) provided a better fit to the numerical BTCs and inclusion of the small-scale secondary roughness in numerical simulations resulted in a decreasing MIM mobile domain fraction and an increasing mass exchange rate between immobile and mobile domains.

Parameter Determination for Chloride and Tritium Transport in Undisturbed Lysimeters during Steady Flow

1992 ◽  
Vol 23 (2) ◽  
pp. 89-104 ◽  
Author(s):  
Ole H. Jacobsen ◽  
Feike J. Leij ◽  
Martinus Th. van Genuchten
Keyword(s):  
Experimental Data ◽  
Unsaturated Flow ◽  
Transport Model ◽  
Time Moment ◽  
Breakthrough Curves ◽  
Coarse Sand ◽  
Retardation Factor ◽  
Parameter Determination ◽  
Model Parameters ◽  
Water Fraction

Breakthrough curves of Cl and 3H2O were obtained during steady unsaturated flow in five lysimeters containing an undisturbed coarse sand (Orthic Haplohumod). The experimental data were analyzed in terms of the classical two-parameter convection-dispersion equation and a four-parameter two-region type physical nonequilibrium solute transport model. Model parameters were obtained by both curve fitting and time moment analysis. The four-parameter model provided a much better fit to the data for three soil columns, but performed only slightly better for the two remaining columns. The retardation factor for Cl was about 10 % less than for 3H2O, indicating some anion exclusion. For the four-parameter model the average immobile water fraction was 0.14 and the Peclet numbers of the mobile region varied between 50 and 200. Time moments analysis proved to be a useful tool for quantifying the break through curve (BTC) although the moments were found to be sensitive to experimental scattering in the measured data at larger times. Also, fitted parameters described the experimental data better than moment generated parameter values.

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