Chapter 3 Numerical simulation of sedimentary basin-scale hydrochemical processes

Abstract

Abstract Sedimentary basins represent large-scale porous media, are important hosts to a significant portion of the world's economic energy and mineral resources. Processes occurring in sedimentary basins include groundwater flow, heat transport, and reactive mass transport. Quantitative models of flow and transport in these settings can provide insight into the processes that control the evolution of sedimentary basins by enabling the examination of processes that may occur too slowly to be observed in the field or laboratory. In many cases, such models may be the only available tool for studying processes occurring over geological time and space scales. In addition, it is important to consider the behavior of the processes occurring in sedimentary basins simultaneously, since they are generally coupled. Groundwater flow is controlled by the boundary conditions and the distribution of hydraulic conductivity; as a result, flow velocities vary spatially and temporally. This circulation is capable of transporting thermal energy and dissolved mass. In general, flow rates will be sufficiently small that the water will reach approximate equilibrium with each lithology along the flow path at the ambient temperature and pressure. These successive equilibria produce changes in the chemical composition of the fluid, resulting in reactions with the medium (i.e., precipitation, dissolution), which in turn modify the porosity and permeability. This modification may be insignificant at a human time scale, but very significant at the geological time scale. The hydrogeological flow field then is a coupled hydrological-thermal-geochemical system, requiring solution to three sets of coupled partial differential equations. This paper reviews developments over the past several years in numerical simulation of these coupled processes. The governing conservation equations are presented, and solution procedures discussed; the finite element equations are developed for the case where local chemical equilibrium is assumed. Application of coupled models to a variety of geological problems is discussed, such as the propagation of mineral reaction fronts in one spatial dimension. These studies have noted the importance of hydrodynamic dispersion and its control on the spatial distribution of reaction rates and products. Relatively few two-dimensional simulations are available in the literature, but these few are reviewed, including the formation of uranium ore deposits, mixing-zone reactions in carbonate aquifers, and sandstone diagenesis. These studies note the importance of transport-controlled reaction-front propagation, fluid mixing, and gradient reactions, which all occur to varying degrees in a heterogeneous sedimentary basin. Future developments will require greater computer capability, and are likely to focus on application to well-documented field problems and greater inclusion of natural geological heterogeneity, but results presented to date show promise of enabling quantitative study of coupled hydrological, geochemical, and thermal processes in evolving sedimentary basins.