Dissolution phenomena in porous media play an important role in the transport of sequestered CO2. Petrophysical properties can be dramatically altered by resulting pore structural changes, including the formation of high conductivity channels, known as “wormholes”. As it is difficult to forecast the emergence of wormholes based on experimental information, it is of vital importance to be able to accurately predict these processes from pore-scale simulations. We use a hybrid pore-scale dissolution model coupling a lattice Boltzmann (LB) method for fluid flow to a finite-volume method for chemical transport . A comprehensive chemical model is solved in the fluid and at mineral boundaries, which are able to dissolve over time. Additionally, the Coulombic coupling in multi-component ion diffusion is captured, as are chemical activities computed using the mixed-solvent electrolyte (MSE) model . Calculations are enabled on representative-scale pore-space images, obtained from high resolution micro-CT scanning, using a parallel GPU implementation.
As a first step towards simulating complex reaction behaviour of saturated CO2-brine systems, we use HCl acid in carbonate rocks. This chemistry does have applications such as matrix acidizing operations in low permeability formations, however here it provides simpler chemical basis for validating the detailed reactive transport model. This is because the surface reaction rate constant is fast compared to transport rates, ensuring that dissolution does not occur inside the microporous grains. This is likely to happen with the slower H2CO3 – CaCO3 reaction pathway.
First, we validate the model against a simple experimental system consisting of a small channel drilled through a calcite single crystal, with HCl flowed through it. Morphological changes were imaged over time using micro-CT imaging and directly compared with simulated dissolution profiles. We find excellent agreement with the experimental dissolution rate and use the simulation to reveal the complex interplay between chemical transport, surface reaction rates and equilibrium reactions in the fluid which control the overall dissolution rate and resulting morphology.
Then, we apply the model to a representative image of Ketton carbonate, obtained using micro-CT imaging. HCl is again injected at a high flow rate which leads to wormhole-type dissolution. This is compared to a corresponding micro-CT experiment, and good agreement in the overall dissolution rate and permeability are found. We show how the particular flow path, which takes over as the dominant wormhole during the wormhole competition phase, depends strongly on the inlet configuration. A detailed understanding of the chemical and transport mechanisms is again enabled by the simulation.
- Gray, F., J. Cen, and E.S. Boek, Simulation of dissolution in porous media in three dimensions with lattice Boltzmann, finite-volume, and surface-rescaling methods. Physical Review E, 2016. 94(4): p. 043320.
- Springer, R.D., P. Wang, and A. Anderko, Modeling the Properties of H2S/CO2/Salt/Water Systems in Wide Ranges of Temperature and Pressure. Society of Petroleum Engineers, 2015.
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