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Applied Geochemistry, 55 ISSN:0883-2927 ISSN:1872-9134

In a forward chemical equilibrium problem (FCEP), the state of minimum Gibbs energy for a chemical system is sought, in which temperature, pressure, elemental amounts, and thermodynamic model parameters are prescribed. We herein present a mathematical framework for characterizing and solving inverse chemical equilibrium problems (ICEP), a class of problems for which one or more of those prescribed conditions in a FCEP are unknown in advance. In an ICEP, complementary conditions must be imposed, which are referred to here as equilibrium constraints. Examples of ICEPs include those in which a certain property is known at equilibrium (e.g., volume is specified instead of pressure; enthalpy is specified instead of temperature; pH is specified instead of the amount of element H). The equilibrium constraints may also be specified by equations that govern the relationship between several equilibrium properties (e.g., the equations relating temperature, pressure, density, energy, and velocity of the gases produced during the detonation of an explosive). Chemical Engineering Science, 252 ISSN:0009-2509

Computational Geosciences, 17 (1) ISSN:1420-0597 ISSN:1573-1499

Abstract Reactive transport simulations using our CSMP++GEM coupled code were applied to study the major controls on replacement dolomitisation and the development of dolomite geobodies in a hydrothermal setting. A series of 2D simulations show how elevated temperature and reactive surface area increase the rate of dolomitisation, and result in a dolomite replacement front that is both sharper and inclined at a higher angle from vertical. This inclination, an effect of gravity segregation, is apparent in thick homogeneous units, but in layered systems the lithological contrast determines the shape of the dolomite front. The increase in permeability resulting from porosity generation upon replacement of calcite by dolomite has a major effect on accelerating the overall progress of dolomitisation. In contrast, the changes in fluid density due to chemical reactions and the pressure dependence of thermodynamic data have a minor influence under simulated conditions. Primary dolomite forms slowly after complete replacement of host calcite, leading to porosity decrease, and is only locally important around the source of the hydrothermal fluid. For a simple layered system, our model results are in excellent agreement with those obtained using TOUGHREACT code. They do, however, show the advantage of unstructured triangular over structured rectangular meshes for resolving complex curved/inclined front shapes. Such meshes also offer benefits in simulating fault-controlled hydrothermal dolomitisation. Our simulations predict dolomite geobodies comparable in scale and morphology to natural examples documented at outcrops, and underline the importance of understanding the permeability structure within and around the fault zone.