Pore-Scale Simulation of Mass Transfer Across Immiscible Interfaces

Advisor

Hamdi Tchelepi

Abstract

Interphase mass transfer in porous media involving multiple fluid phases is a fundamental process that appears in a large number of situations of applied science and engineering including the injection and sequestration of CO2 into the sub-surfaces, the aquifer contamination by non-aqueous phase liquids (NAPL) and the primary migration of bitumen in petroleum reservoirs. In those situations, two immiscible phases share the pore-space, oil and water, gas and liquid, etc. One component is miscible in both and able to cross the interface, a tracer, a polluting chemical component or a gas dissolving into the liquid phase for example. Quantifying this mass transfer allows not only to predict the mass loss of one phase towards the other but also to understand and model its possible effects on the flow, such as a change in wettability, physical properties of the fluids, dissolution of a gas.

However, mass exchange has proven difficult to predict. Indeed, it is highly dependent on the physical properties of both fluids and solid, but also on the interfacial area between the phases. It is, therefore, essential to be able to predict the two-phase flow, depending on wettability, solid topology, fluid injection, and previous conditions. In particular, in the processes mentioned earlier such that CO$_2$ sequestration the flow in the subsurface does not necessarily obey classical laws at the reservoir scale but gravity, viscous, and capillary instability leading to fingering has been observed. That is why in this work we go back to the pore-scale to understands the underlying phenomena affecting the mass transfer and the flow.

A solver was implemented to simulate two-phase flow at the pore scale, with a miscible component crossing the interface, with or without phase change. It was based on an existing computational fluid dynamic (CFD) software: OpenFoam. The two immiscible phases are modeled under the Volume-of-fluid (VOF) formulation. The concentration of the miscible component is treated consistently with the VOF approach, and extended to handle contact line fluid/fluid/solid. The phase change is also implemented.

The species transport solver gives results very consistent with analytical solutions, and allows calculation of mass transfer coefficient in complex porous media. The phase change solver only gives preliminary results but shows good behavior in the case of the dissolution of a droplet in a surrounding fluid.