Reactive Transport Modelling of Dolomitisation Using the New CSMP++GEM Coupled Code: Governing Equations, Solution Method and Benchmarking Results

Abstract

Reactive transport modelling (RTM) is a powerful tool for understanding subsurface systems where fluid flow and chemical reactions occur simultaneously. RTM has been widely used to understand the formation of dolomite by replacement of calcite, which can be an important control on carbonate reservoir quality. Dolomitisation is a reactive transport process governed by slow dolomite precipitation and cannot be correctly simulated without a kinetic rate model. The new CSMP++GEM coupled RTM code uses the GEMS3K kernel for solving geochemical equilibria by the Gibbs energy minimization method with the CSMP++ framework that implements a hybrid finite element-finite volume method to solve partial differential equations. The unique feature of the new coupling is the mineral reaction kinetics, implemented via additional metastability constraints. CSMP++GEM is able to simulate single-phase flow and solute transport in porous media together with chemical reactions at different pressure, temperature and water salinity conditions. This RTM assures mass conservation which is crucial when simulating transport of solutes with low concentrations over geological time. A full feedback of mineral dissolution/precipitation on the fluid flow is provided via corresponding porosity/permeability evolution and two source terms in the pressure equation. First, the mass source term accounts for the mass of solutes released during mineral dissolution or taken from the solution by mineral precipitation. The second source term attributes to the fact that the solution density is affected by mineral dissolution/precipitation, too. This effect is included

Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code: Effects of temperature and geological heterogeneity

“…The reactive transport system controlling even a simple replacement of calcite by dolomite is so complex that an analytical solution is not possible. Numerical solution is never exact and direct comparison of RTM simulations performed with different codes is challenging (Yapparova et al, 2017).…”

“…The new CSMP++GEM reactive transport code (Yapparova et al, 2017) was applied to simulate hydrothermal dolomitisation in 2D. A subset of models also uses TOUGHREACT (Xu et al, 2004) in order to provide a comparison between the codes.…”

“…The kinetic rate of mineral precipitation is incorporated into the equilibrium calculations via the additional metastability constraints on mineral amounts (for a detailed description see Thien et al, 2014;Yapparova et al, 2017).…”

“…Porosity is updated based on the current mineral volumes, permeability is updated from the porosity using Kozeny-Carman empirical relation (Yapparova et al, 2017):…”

“…For example, Corbella et al (2014) performed a sensitivity analysis on the influence of temperature on the rate of dolomitisation, but the RTM code RETRASO used in their study did not include the permeability update from changing porosity. Both CSMP++GEM (Yapparova et al, 2017) and TOUGHREACT (Xu et al, 2004) have this feature implemented. Other features of CSMP++GEM, such as the dependence of thermodynamic data on fluid pressure, and of fluid properties on chemistry, are also lacking in many other RTMs.…”