Significant progress in hydrology, especially in subsurface flow and solute transport, has been made over the last 35 years because of sustained interest in underground nuclear waste repositories. ...

/pdf/hydrologic-issues-associated-with-nuclear-waste-repositories-1bgviebqs7.pdf

Hydrologic issues associated with nuclear waste repositories

A physically based numerical approach is presented for modeling multiphase flow and transport processes in fractured rock. In particular, a general framework model is discussed for dealing with fracture-matrix interactions, which is applicable to both continuum and discrete fracture conceptualization. The numerical modeling approach is based on a general multiple-continuum concept, suitable for modeling any types of fractured reservoirs, including double-, triple-, and other multiplecontinuum conceptualmodels. In addition, a new, physically correct numerical scheme is discussed to calculatemultiphase flow between fractures and thematrix, using continuity of capillary pressure at the fracture-matrix interface. The proposed general modeling methodology is verified in special cases using analytical solutions and laboratory experimental data, and demonstrated for its application inmodeling flow through fractured vuggy reservoirs. AMS subject classifications: 76S05

http://petroleum.mines.edu/research/emg/EMG_papers/2009-A,%20Wu%20and%20Qin,%20CiCp.pdf

A Generalized Numerical Approach for Modeling Multiphase Flow and Transport in Fractured Porous Media

Tight gas reservoirs are characterized by single-phase (gas) or two-phase (gas and liquid) flow in extremely low-permeability, highly heterogeneous porous/fractured, and stress-sensitive rock. Gas flow in such tight formations is further complicated by other co-existing processes, such as Klinkenberg effect, non-Newtonian or non-Darcy flow behavior, due to strong interaction between fluid molecules and solid materials within tiny pores, or microand macrofractures. Because of the low permeability in tight rock, the traditional double-porosity model may not be applicable for handling fracture-matrix interaction of gas flow in these reservoirs. In this work, we present a generalized mathematical model for simulating multiphase flow of gas in tight, porous/fractured reservoirs using a more general, multi-continuum modeling approach. The model incorporates the following processes: (1) Klinkenberg effect, (2) non-Newtonian behavior (i.e., threshold pressure gradient for flow to occur); (3) non-Darcy flow with inertial effects; and (4) rock deformation due to changes in the stress field. We propose to explicitly separate effects of rock mechanical deformation and molecular interaction between fluids and rock materials. The former effect is included using the intrinsic permeability and porosity relations, while the latter is accounted for by an apparent viscosity for non-Newtonian, non-Darcy’s behavior, or a modified permeability for Klinkenberg effect The proposed mathematical model has been implemented into a multiphase, multidimensional reservoir simulator. In the numerical model, specifically, a control-volume, integral finite-difference method is used for spatial discretization with an unstructured grid, and a first-order finite-difference scheme is adapted for temporal discretization of governing two-phase flow equations in tight gas reservoirs. The resulting discrete nonlinear equations are solved fully implicitly by Newton iteration. The numerical scheme has been verified against analytical solutions with Klinkenberg effect, non-Newtonian or non-Darcy flow, and flow in deformable fractured rock in our previous studies. The model’s application to actual tight gas reservoirs is an on-going research project. Introduction Since the 1960s, significant progress has been made in modeling of flow and transport processes in fractured rock. Driven by the increasing need to develop petroleum, geothermal, and other natural underground energy resources and to study the problem of subsurface contamination, researchers have developed several numerical modeling approaches and techniques (Barenblatt et al. 1960; Warren and Root, 1963; Kazemi, 1969; Pruess and Narasimhan, 1985). Numerical modeling approaches, developed in the past few decades, rely in general on continuum approaches and involve developing conceptual models, incorporating the geometrical information of a given fracture-matrix system, setting up mass and energy conservation equations for fracture-matrix domains, and then solving discrete nonlinear algebraic equations. The key issue for simulating flow in fractured rock, however, is how to handle fracture-matrix interaction under different flow conditions. To model fracture-matrix interaction in fractured porous media, investigators have developed and applied many different conceptual models and modeling approaches for dealing with fracture-matrix interaction, including: (1) an explicit discretefracture and matrix model (e.g., Snow, 1965), (2) the dual-continuum method, including doubleand multiporosity, dualpermeability, or the more general ―multiple interacting continua‖ (MINC) method (Barenblatt et al. 1960; Warren and Root, 1963; Kazemi, 1969; Pruess and Narasimhan, 1985; Wu and Pruess, 1988), and (3) the effective-continuum method (ECM) (Wu, 2000). The explicit discrete-fracture approach is, in principle, a more rigorous model. However, the application of this method to field studies is still limited because of the computational intensity involved as well as the lack of detailed knowledge of

A Multi-Continuum Model for Gas Production in Tight Fractured Reservoirs

We estimate the travel time of percolating water through a deep vadose zone at the regional scale using a transfer function model and a physical based conceptual flow model (Hydrus-1D), thereby exploiting the time series of precipitation, actual evapotranspiration and groundwater piezometry and generic vadose zone data. With the transfer function model we observe a high variability of estimated travel time varying from 0.9 to 3.1 years, corresponding to estimated vertical water flux velocities varying from 6.6 to 28.0 m/year. These results were compared with the travel time estimated from the physical based conceptual model. With the flow model, estimated travel time varies between 4.7 and 15.5 years, corresponding to water flux velocities varying between 1.7 and 4.1 m/year. The estimated travel time calculated with the flow model were therefore about five times larger than those estimated with the transfer function model. This could be explained by the fact that the transfer function model considers heterogeneous recharge from the vadose zone as well as from the vicinity of the piezometer through the so called “pushing effect”. In addition, the flow model requires various hydrogeological and hydrodynamic parameters which were estimated using generic parametrisation approaches, that are largely affected by uncertainty and may not reflect the local conditions. In contrast, the transfer function model only exploits available measurable time series and has the advantage of being site-specific.

Estimating travel time of recharge water through a deep vadose zone using a transfer function model

Transport modeling in performance assessments for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

This paper presents a large-scale modeling study characterizing fluid flow and tracer transport in the unsaturated zone of Yucca Mountain, Nevada, the proposed underground repository site for storing high-level radioactive waste. The modeling study is conducted using a three-dimensional numerical model, which incorporates a wide variety of field data and takes into account the coupled processes of flow and transport in Yucca Mountain's highly heterogeneous, unsaturated, fractured porous rock. The modeling approach is based on a dual-continuum formulation. Using different conceptual models of unsaturated flow, various scenarios of current and future climate conditions and their effects on the unsaturated zone are evaluated to aid in the assessment of the repository's system performance. These models are calibrated against field-measured data. Model-predicted flow and transport processes under current and future climates are discussed.

/pdf/a-mountain-scale-model-for-characterizing-unsaturated-flow-4waejld7nl.pdf

A Mountain-Scale Model for Characterizing Unsaturated Flow and Transport in Fractured Tuffs of Yucca Mountain

Characterizing percolation patterns in unsaturated fractured rock has posed a greater challenge to modeling investigations than comparable saturated zone studies, because of the heterogeneous nature of unsaturated media and the great number of variables impacting unsaturated flow. This paper presents an integrated modeling methodology for quantitatively characterizing percolation patterns in the unsaturated zone of Yucca Mountain, Nevada, a proposed underground repository site for storing high-level radioactive waste. The modeling approach integrates a wide variety of moisture, pneumatic, thermal, and isotopic geochemical field data into a comprehensive three-dimensional numerical model for modeling analyses. It takes into account the coupled processes of fluid and heat flow and chemical isotopic transport in Yucca Mountain's highly heterogeneous, unsaturated fractured tuffs. Modeling results are examined against different types of field-measured data and then used to evaluate different hydrogeological conceptualizations and their results of flow patterns in the unsaturated zone. In particular, this model provides a much clearer understanding of percolation patterns and flow behavior through the unsaturated zone, both crucial issues in assessing repository performance. The integrated approach for quantifying Yucca Mountain's flow system is demonstrated to provide a practical modeling tool for characterizing flow and transport processes in complex subsurface systems.

/pdf/analyzing-unsaturated-flow-patterns-in-fractured-rock-using-35wjuvthqh.pdf

Analyzing unsaturated flow patterns in fractured rock using an integrated modeling approach

This paper presents a field modeling study characterizing fluid flow and tracer transport in the unsaturated zone of Yucca Mountain, Nevada, a proposed underground repository for storing high-level radioactive waste. The 500 to 700 meter thick unsaturated zone of Yucca Mountain consists of highly heterogeneous layers of anisotropic, fractured ash flow and air fall tuffs. Characterization of fluid flow and heat transfer through such a system has been a challenge due to the heterogeneities prevalent on various scales. Quantitative evaluation of water, gas, and heat flow by means of numerical simulation is essential for design and performance assessment of the repository. A three-dimensional numerical flow and transport model will be discussed. The model has been calibrated against field-measured data and takes into account the coupled processes of unsaturated flow and tracer transport in the highly heterogeneous, unsaturated fractured porous rock. The modeling approach of the model is based on a dual-continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. As application examples, effects of current and future climates on the unsaturated zone processes are evaluated to aid in the assessment of the proposed repository's system performance.

/pdf/modeling-unsaturated-flow-and-transport-processes-in-27cwxeyxbz.pdf

Modeling Unsaturated Flow and Transport Processes in Fractured Tuffs of Yucca Mountain

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A Mountain-Scale Model for Characterizing Unsaturated Flow and Transport in Fractured Tuffs of Yucca Mountain

Analyzing unsaturated flow patterns in fractured rock using an integrated modeling approach

Modeling Unsaturated Flow and Transport Processes in Fractured Tuffs of Yucca Mountain