Towards modelling physical and chemical effects during wettability alteration in carbonates at pore and continuum scales

Understanding what controls the enhanced oil recovery during waterflooding of carbonate rocks is essential as the majority of the world’s remaining hydrocarbon reserves are contained in carbonate rocks. To further this understanding, in this thesis we develop a pore-scale simulator that allows us to look at the fundamental physics of fluid flow and reactive solute transport within the porous media. The simulator is based on the combined finite element – finite volume method, it incorporates efficient discretization schemes and can hence be applied to porous domains with hundreds of pores. Our simulator includes the rule-based method of accounting for the presence of the second immiscibly trapped fluid phase. Provided that we know what chemical conditions initiate enhanced oil recovery, our simulator allows us to analyse whether these conditions occur, where they occur and how they are influenced by the flow of the aqueous phase at the pore scale. To establish the nature of chemical interactions between the injected brines and the carbonate rocks, we analyze the available experimental data on the single-phase coreflooding of carbonate rocks. We then build a continuum scale simulation that incorporates various chemical reactions, such as ions adsorption and mineral dissolution and precipitation. We match the output of the continuum scale model with the experimental data to identify what chemical interactions the ions dissolved in seawater are involved in

Efficient flow and transport simulations in reconstructed 3D pore geometries

Upscaling pore-scale processes into macroscopic quantities such as hydrodynamic dispersion is still not a straightforward matter for porous media with complex pore space geometries. Recently it has become possible to obtain very realistic 3D geometries for the pore system of real rocks using either numerical reconstruction or micro-CT measurements. In this work, we present a finite element-finite volume simulation method for modeling single-phase fluid flow and solute transport in experimentally obtained 3D pore geometries. Algebraic multigrid techniques and parallelization allow us to solve the Stokes and advection-diffusion equations on large meshes with several millions of elements. We apply this method in a proof-of-concept study of a digitized Fontainebleau sandstone sample. We use the calculated velocity to simulate pore-scale solute transport and diffusion. From this, we are able to calculate the a priori emergent macroscopic hydrodynamic dispersion coefficient o f the porous medium for a given molecular diffusion Dm of the solute species. By performing this calculation at a range of flow rates, we can correctly predict all of the observed flow regimes from diffusion dominated to convection dominated

Pore-scale modeling of chemically induced effects on two-phase flow

We present a finite element-finite volume simulation method for modelling fluid flow and solute transport accompanied by chemical reactions in experimentally obtained 3D pore geometries. We solve the stationary Stokes equation on the computational domain with the FE method using the same set of nodes and the same order of basis functions for both velocity and pressure. The resulting linear system is solved by employing the algebraic multigrid library SAMG. To simulate large 3D samples we partition them into subdomains and treat each separately on a different computing node. This approach allows us to use meshes with millions of elements as input geometries without facing limitations in computer resources. We apply this method in a proof-of-concept study of a digitized Fontainebleau sandstone sample. We use the calculated velocity profile with the finite volume procedure to simulate pore-scale solute transport and diffusion. This allows us to demonstrate the correct emer ging behaviour of sample s hydrodynamic dispersivity. Finally, we model the transport of an adsorbing solute and the surface coverage dynamics is demonstrated. This information can be used to estimate the local change of a sample wettability state and the ensuing changes of the two-phase flow characteristics