Direct characterisation of transport and mixing in unsaturated porous media

Multiphase flow and hydrodynamic dispersion in porous media are the key processes in many subsurface and engineering applications, such as vadose zone contamination, soil remediation, and hydrocarbon recovery. To secure sustainable access to clean water and energy, which are identified as the global challenge areas, we urge to enhance our understanding of fundamental processes and improve our predictive capability.
Hydrodynamic dispersion in porous media is controlled by the spatially and temporally variable velocity field, described by the advection-dispersion equation for saturated porous media. However, in the case of an unsaturated porous medium (where two or more immiscible fluids are present), the Fickian advection-dispersion equation is not anymore valid. In unsaturated porous media, not only the velocity field varies with the change of saturation topology, but also the spatial variation of velocity can change by orders of magnitude, such that even in homogeneous media some regions become hydrodynamically stagnant. Transport time scales in the stagnant and flowing regions can be different by orders of magnitude, which makes the concentration profiles very skewed. This feature is referred to as the non-Fickian behaviour that varies dramatically with the stagnant saturation and flow dynamics. The existing non-Fickian theories (e.g. the MIM theory) have been used to inversely model the externally-measured concentration profiles. The satisfactory "inverse modelling" results are generally regarded as an indication of the validity of these theories, although this perception has been challenged in the recent 2D micromodel experiments.
In this project we articulate the major misconceptions and gaps in our understanding and we hypothesize that the concept of stagnant saturation has been incorrectly employed in the existing theories. The stagnant saturation is a two-phase flow variable, which depends on fluids topology. It should be regarded as the key "variable" to couple two-phase flow and the non-Fickian transport modelling in a hydrodynamically-consistent way, which is absent in the literature.
To address the gaps and misconceptions in understanding, we have envisaged a novel experiments and modelling in two work packages (WP).
In WP1, we will deliver the first direct visualisation of dispersion and mixing of a tracer in oil-wet and water-wet unsaturated porous media using the fast elapsed-time (4D) X-ray micro-tomography. All experiments will be computationally designed using the in-house developed pore-scale models and the experimental setup will be tested in Henry-Moseley X-ray Imaging Facilities. The 4D experiments are envisaged to take place in the Diamond Light Source (DLS), facilitated by the Diamond Manchester Collaboration.
In WP2, the 4D X-ray images will be analysed to extract the concentration field, saturation field at different flow conditions. Transport properties including stagnant saturation, dispersion coefficient, and mass transfer rate will be quantified from the obtained images. Finally, the validity of the existing established models will be tested and a new theoretical framework will be developed.
The project will benefit from the collaborations with world-renowned scientists, Prof. M. Celia (MC) from Princeton University, Prof. H. Steeb (HS) from Stuttgart University, and Prof. Brian Berkowitz (BB) from Weizmann Institute of Science and the partial support of two PhD students of the PI's team. MC will bring his key knowledge in multiphase flow and transport in porous media and their applications in large-scale problems of environmental engineering. BB has a renowned track record in mathematical analysis of non-Fickian transport in heterogeneous porous media. HS is an expert in microCT imaging of complex porous media processes and supports the WP1 by offering experimental equipment. Also, the DLS beamline scientist, Dr Nghia Vo, will support the project in data acquisition and reconstruction.

Planned Impact
This project has direct impacts on fundamental science of "fluid dynamics in porous media" and the findings will open a new way to interpret and model dispersion and mixing in unsaturated porous media that can found in many engineering and natural applications, such as contaminant transport in vadose zone, in-situ soil remediation, and hydrocarbon recovery. These applications are parts grand challenge areas to secure sustainable access to clean water and energy.
Dispersion ubiquitously occurs in systems where the velocity field is spatially variable. Under two-phase conditions, the variability in the velocity field - even in homogeneous porous media- is saturation and flow dynamics dependent. This leads to ambiguous transport and mixing processes which are not fully resolved. We aim to enhance the fundamental knowledge by filling the gaps and identifying the misconceptions in the existing theories of dispersion and mixing in unsaturated porous media. This research will create a paradigm shift in the existing "school of thought" for modelling, by establishing the concept of coupling the two-phase flow and transport in a hydrodynamically-consistent way.
The outcome of this research can be fully integrated in the next generation of "predictive models" which have significant impacts on any discipline of engineering, where flow and transport in porous media are the key processes. Noticeable examples, to which the research outcomes will directly contribute, are:
- Quantitative assessment of contaminant transport in unsaturated zone.
- Predictive modelling of the removal of dense non-aqueous phase liquids from the contaminated sites.
Moreover, the UK has strategic energy plans for future water and enhanced oil recovery as shown in the recent £180m governmental investment in Oil and Gas Technology Centre. Our precursor research on this topic has already received significant attention from BP and Shell who invited the PI to present the new research results. Thus, it is highly plausible that on a longer timescale, these industries will be interested in going down this road to further explore deployability of the ideas for the environmental-friendly cost-effective low salinity water flooding technology. The project outcomes will seriously impact the predictive modelling and risk analysis models used for this technology. Moreover UK has a strong expertise on contaminated land revival and remediation as it is one the first countries to suffer from environmental issues due to industrialisation (the UK has over 400,000 hectares of contaminated land according to the UK Government). Therefore the project serves the legacy of clean-up by an innovative and revolutionary understanding supported by sound experimental data. Thus, the economy of aquifer remediation and soil clean-up projects will benefit from the enhanced modelling accuracy and predictive capacity that this project brings forth.
As reflected in the supporting letters, the following organisations have clearly expressed their interests in getting involved in the project.

Netherlands Environmental Assessment Agency (PBL in Dutch): PBL has stated that they are keen to explore the possibility to incorporate the findings of this project in their on-going Integrated Model to Assess the Global Environment (IMAGE) project. One of the key components in the IMAGE project is the modelling and assessment of contamination of vadose zone and groundwater, which is the focus of this project.

British Geological Survey: As highlighted in the BGS supporting letter, the outcomes of this research will directly impact the on-going research on "nitrate contamination and transport in unsaturated zone" in the group of Prof. D. Gooddy.
Finally, we envisage that the industrial knowledge transfer will be favoured via the Knowledge Transfer Networks of the Innovate UK Network, which bring together academic and industrial partners, in which VJN is already a member of the Energy group.