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Purpose: The objective of this paper is to establish a solution strategy for obtaining dual solutions, namely trivial (conventional) and nontrivial (unconventional) solutions, of coupled pore-fluid flow and chemical dissolution problems in heterogeneous porous media. Design/methodology/approach: Through applying a perturbation to the pore-fluid velocity, original governing partial differential equations of a coupled pore-fluid flow and chemical dissolution problem in heterogeneous porous media are transformed into perturbed ones, which are then solved by using the semi-analytical finite element method. Through switching off and on the applied perturbation terms in the resulting perturbed governing partial differential equations, both the trivial and nontrivial solutions can be obtained for the original governing partial differential equations of the coupled pore-fluid flow and chemical dissolution problem in fluid-saturated heterogeneous porous media. Findings: When a coupled pore-fluid flow and chemical dissolution system is in a stable state, the trivial and nontrivial solutions of the system are identical. However, if a coupled pore-fluid flow and chemical dissolution system is in an unstable state, then the trivial and nontrivial solutions of the system are totally different. This recognition can be equally used to judge whether a coupled pore-fluid flow and chemical dissolution system involving heterogeneous porous media is in a stable state or in an unstable state. The proposed solution strategy can produce dual solutions for simulating coupled pore-fluid flow and chemical dissolution problems in fluid-saturated heterogeneous porous media. Originality/value: A solution strategy is proposed to obtain the nontrivial solution, which is often overlooked in the computational simulation of coupled pore-fluid flow and chemical dissolution problems in fluid-saturated heterogeneous porous media. The proposed solution strategy provides a useful way for understanding the underlying dynamic mechanisms of the chemical damage effect associated with the stability of structures that are built on soil foundations. [ABSTRACT FROM AUTHOR]

This paper deals with how to implement perturbations in the computational simulations of chemical dissolution‐front instability (CDFI) problems in fluid‐saturated porous media. On the basis of theoretical analysis, it is found that the application of a perturbation to the chemical dissolution front is equivalent to the application of an alternative perturbation to the dimensionless pore‐fluid normal velocity (relative to the planar chemical dissolution front) in the chemical dissolution zone, where the chemical dissolution front is located. This avoids the difficulty to find the spatial coordinates of the chemical dissolution front locations in the computational simulations of CDFI problems. Based on this new finding, a novel algorithm for implementing perturbations in the computational simulations of CDFI problems is proposed. The key point of the proposed algorithm is that the perturbed pore‐fluid normal velocity (relative to the planar chemical dissolution front) is used to directly replace the original pore‐fluid normal velocity (relative to the planar chemical dissolution front) in the related mathematical governing equations (MGEs), so that the proposed algorithm works for the porosity‐velocity‐concentration scheme when it is used to solve CDFI problems in fluid‐saturated porous media. In addition, the related theoretical analysis in this study has answered the previous unanswered question why the application of a perturbation to porosity works in the porosity‐pressure‐concentration scheme but does not work in the porosity‐velocity‐concentration scheme for solving the same CDFI problems in fluid‐saturated porous media. Through solving two illustrative examples with two different distributions of initial porosity, in which one is homogeneous and another is heterogeneous in the chemical dissolution system, the validity and usefulness of the proposed algorithm for implementing perturbations in the computational simulations of CDFI problems have been demonstrated. [ABSTRACT FROM AUTHOR]

Pore-fluid flow associated with seepage instabilities can play an important role in controlling large mineralisation patterns within the upper crust of the Earth. To demonstrate this process, two kinds of seepage instability problems in fluid-saturated porous rocks are considered in this paper. The first kind of seepage instability problem is caused by the temperature-induced buoyancy of pore fluid, so that it can be called the buoyancy-driven seepage instability problem, while the second kind of seepage instability problem is caused by chemical dissolution reactions that are commonly encountered in the upper crust of the Earth, so that it can be called the chemical-dissolution-driven seepage instability problem. After the mathematical governing equations of and computational methods for these two kinds of seepage instability problems are introduced, two numerical examples are used to elucidate how and why these two kinds of seepage instabilities can provide favorable places for the formation of large mineralisation patterns within the upper crust of the Earth. The related computational simulation results have demonstrated that: (1) the convective pore-fluid flow caused by the buoyancy-driven seepage instability not only can dissolve minerals at the lower part of the upper crust, but also can transport the dissolved minerals from the lower part to the upper part of the upper crust, resulting in large mineralisation patterns near the surface of the Earth's upper crust. (2) The chemical-dissolution-driven seepage instability in fluid-saturated porous rock can provide some favorable places, such as finger-like channels created by porosity enhancement in the porous rock, for the formation of large mineralisation patterns within the upper crust of the Earth. [ABSTRACT FROM AUTHOR]

Temporal-spatial distributions of geochemical data in geoinformatics are very important for mineral exploration in the upper crust of the Earth and for the treatment of geoenvironmental issues such as CO 2 sequestration in geological formations. To understand ore-forming processes associated with mineral exploration, it is necessary to acquire geochemical information regarding orebody formation and mineralization in hydrothermal systems within the upper crust of the Earth in the past. On the contrary, to generate a safe design for CO 2 sequestration in a geological formation, it is necessary to predict the composition of geochemical environment after CO 2 sequestration and test such predictions through acquiring geochemical information in the future, so that the safety and feasibility of the design can be adequately assessed. The computational simulation approach is used, in this paper, to facilitate and guide these acquisition procedures. To demonstrate the feasibility and usefulness of the computational simulation approach in this aspect, a simplified ore-forming system associated with the Australian Broken Hill Pb and Zn mine, which belongs to a natural system, and a fluid-rock reaction system involving CO 2 sequestration in a geological formation, which belongs to a man-made system, are considered to model geochemical information. The related computational simulation results demonstrate that the computational simulation method is not only applicable for simulating the ore-forming processes in hydrothermal systems within the upper crust of the Earth, but also useful for acquiring both the ancient geochemical information associated with ore-forming systems and the future geochemical information associated with geoenvironmental systems. [ABSTRACT FROM AUTHOR]

Numerical modeling has now become an indispensable tool for investigating the fundamental mechanisms of toxic nonaqueous phase liquid (NAPL) removal from contaminated groundwater systems. Because the domain of a contaminated groundwater system may involve irregular shapes in geometry, it is necessary to use general quadrilateral elements, in which two neighbor sides are no longer perpendicular to each other. This can cause numerical errors on the computational simulation results due to mesh discretization effect. After the dimensionless governing equations of NAPL dissolution problems are briefly described, the propagation theory of the mesh discretization error associated with a NAPL dissolution system is first presented for a rectangular domain and then extended to a trapezoidal domain. This leads to the establishment of the finger-amplitude growing theory that is associated with both the corner effect that takes place just at the entrance of the flow in a trapezoidal domain and the mesh discretization effect that occurs in the whole NAPL dissolution system of the trapezoidal domain. This theory can be used to make the approximate error estimation of the corresponding computational simulation results. The related theoretical analysis and numerical results have demonstrated the following: (1) both the corner effect and the mesh discretization effect can be quantitatively viewed as a kind of small perturbation, which can grow in unstable NAPL dissolution systems, so that they can have some considerable effects on the computational results of such systems; (2) the proposed finger-amplitude growing theory associated with the corner effect at the entrance of a trapezoidal domain is useful for correctly explaining why the finger at either the top or bottom boundary grows much faster than that within the interior of the trapezoidal domain; (3) the proposed finger-amplitude growing theory associated with the mesh discretization error in the NAPL dissolution system of a trapezoidal domain can be used for quantitatively assessing the correctness of computational simulations of NAPL dissolution front instability problems in trapezoidal domains, so that we can ensure that the computational simulation results are controlled by the physics of the NAPL dissolution system, rather than by the numerical artifacts. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper primarily deals with the computational aspects of chemical dissolution-front instability problems in two-dimensional fluid-saturated porous media under non-isothermal conditions. After the dimensionless governing partial differential equations of the non-isothermal chemical dissolution-front instability problem are briefly described, the formulation of a computational procedure, which contains a combination of using the finite difference and finite element method, is derived for simulating the morphological evolution of chemical dissolution fronts in the non-isothermal chemical dissolution system within two-dimensional fluid-saturated porous media. To ensure the correctness and accuracy of the numerical solutions, the proposed computational procedure is verified through comparing the numerical solutions with the analytical solutions for a benchmark problem. As an application example, the verified computational procedure is then used to simulate the morphological evolution of chemical dissolution fronts in the supercritical non-isothermal chemical dissolution system. The related numerical results have demonstrated the following: (1) the proposed computational procedure can produce accurate numerical solutions for the planar chemical dissolution-front propagation problem in the non-isothermal chemical dissolution system consisting of a fluid-saturated porous medium; (2) the Zhao number has a significant effect not only on the dimensionless propagation speed of the chemical dissolution front but also on the distribution patterns of the dimensionless temperature, dimensionless pore-fluid pressure, and dimensionless chemical-species concentration in a non-isothermal chemical dissolution system; (3) once the finger penetrates the whole computational domain, the dimensionless pore-fluid pressure decreases drastically in the non-isothermal chemical dissolution system. [ABSTRACT FROM AUTHOR]

Abstract: The main purpose of this paper is to investigate the effects of different domain shapes in general and trapezoidal domain shape in particular on the morphological evolution of nonaqueous phase liquid (NAPL) dissolution fronts in two-dimensional fluid-saturated porous media. After the governing equations of NAPL dissolution problems are briefly described, the numerical procedure consisting of a combination of the finite element and finite difference methods is used to solve these equations. The related numerical simulation results have demonstrated that: (1) domain shapes have a significant effect on both the propagating speed and the morphological evolution pattern of a NAPL dissolution front in the fluid-saturated porous medium; (2) an increase in the divergent angle of a trapezoidal domain can lead to a decrease in the propagating speed of the NAPL dissolution front; (3) the morphological evolution pattern of the NAPL dissolution front in a rectangular domain is remarkably different from that in a trapezoidal domain of a large divergent angle; (4) for a rectangular domain, the simplified dispersion model, which is commonly used in the theoretical analysis and numerical simulation, is valid for solving NAPL dissolution instability problems in fluid-saturated porous media; and (5) compared with diverging flow (when the trapezoidal domain is inclined outward), converging flow (when the trapezoidal domain is inclined inward) can enhance the growth of NAPL fingers, indicating that pump-and-treat systems by extracting contaminated groundwater might enhance NAPL dissolution fingering and lead to less uniform dissolution fronts. [Copyright &y& Elsevier]

Abstract: This paper presents a general discussion on some fundamental issues associated with the current research status in the field of computational hydrodynamics of mineralization. Based on the scientific characteristics of mineral forming systems within the upper crust of the Earth, the fundamental issues under consideration include: (1) simulation of multi-process aspects of a mineral forming system; (2) simulation of multi-scale aspects of a mineral forming system; (3) simulation of fluid convection associated with a mineral forming system; (4) simulation of fluid focusing and mixing associated with a mineral forming system; and (5) simulation of reactive mass transport associated with a mineral forming system. After the current status of each fundamental issue is briefly summarized, the future development in the field of computational hydrodynamics of mineralization is discussed on the basis of further improving the treatments of the fundamental issues considered in this investigation. [Copyright &y& Elsevier]

Abstract: Mass and energy should be conservative in nature and ore forming systems within the upper crust of the earth are no exception. Thus, the mass conservation law is valid not only for a closed system, but for an open system as well. In the latter case, exchange between the system and its surroundings must be considered. Based on the mass conservation law, this paper considers a number of different roles of geofluid flow in ore forming systems. Due to the concurrence of these roles, interactions between fluid flow, heat transfer, mass transport and chemical reactions need to be considered in a comprehensive manner. Through theoretical analysis and computational simulations of several typical generic models for ore forming systems, it can be demonstrated that for an ore forming system in chemical equilibrium, the aqueous species of which are produced by the chemical dissolution reactions, the approximate form (i.e. the rock alteration index and the improved rock alteration index) of the mineralization rate can be used to predict mineralization patterns in the ore forming system. However, for a chemically non-equilibrated ore forming system, the detailed interaction between solute advection, solute diffusion/dispersion and chemical kinetics needs to be considered to determine potential mineralization patterns in the ore forming system. [Copyright &y& Elsevier]

• To study mathematical transform on analyzing chemical-dissolution front instability. • Two different mathematical transforms were considered in the study. • Mathematical transforms have no effect on the dispersion equation of the system. • Mathematical transforms affect evolution patterns in the system of a finite domain. Reactive mass transport, in which chemical dissolution front may become unstable, is a common phenomenon in the field of groundwater hydrology. The use of mathematical transforms can convert many scientific and engineering problems from the conventional time–space domain into a generalized time–space domain, so that analytical solutions, which are impossible to be directly obtained in the conventional time–space domain, can be derived in the generalized time–space domain. The theoretical analysis and computational simulation of dissolution-timescale chemical dissolution-front instability within fluid-saturated porous media is no exception. To investigate how different mathematical transforms can affect the theoretical analyses and computational simulation of chemical dissolution-front instability within fluid-saturated porous media, two different approaches are considered to select mathematical transforms in this study. In the first approach, the mathematical transform mainly consists of a much larger timescale than the dissolution timescale and a length-scale that is independent of the mineral dissolution ratio (MDR), while in the second approach, the mathematical transform mainly consists of the dissolution timescale and the MDR-dependent length-scale. The related theoretical and computational results have demonstrated that: (1) the use of two different mathematical transforms has no effect on the analytical solution to the dispersion equation of a chemical dissolution-front instability problem in the original physical time–space domain. (2) The use of different mathematical transforms may have significant effects on computationally simulating the evolution patterns of unstable chemical dissolution-front in finite space domains, which are filled with fluid-saturated porous media. (3) If the mathematical transform is appropriately selected, then the dissolution-timescale chemical dissolution-front instability problem in fluid-saturated porous media is mathematically solvable. [ABSTRACT FROM AUTHOR]