Dispersion effects in buoyancy-driven flow in porous media: local vs. distributed drainage
Creators: Sheikhi Mohammadabadi, Saeed
Publisher: name: University of Alberta Library
Year: 2024

<p>Understanding the dynamics of injecting fluids into porous media is essential for enhancing the reliability of subsurface storage/sequestration of fuels such as hydrogen or combustion byproducts such as carbon dioxide. In either case, significant progress has been made overcoming a variety of technical challenges, such as understanding capillary trapping in the context of carbon sequestration. Conversely, the dynamics of mixing, whether for miscible or immiscible  systems, remain poorly understood. This work specifically focuses on the case of miscible systems and so is especially relevant when considering the flow of hydrogen into cushion gas, i.e. the gas that remains in formation in a depleted natural gas reservoir. The overarching goal of this thesis is to clarify the dispersive mixing dynamics of a source fluid injected into a saturated porous medium characterized by cap rock that is leaky and possibly inclined. Our study employs both theoretical analysis and numerical simulations and assumes small density differences between the source and ambient fluids, consistent with the Boussinesq approximation.<br>
To address the above objective, three interrelated problems are considered. First, we consider the mixing dynamics of a gravity current fluid that propagates along cap rock layer characterized by an isolated fissure. The gravity current thereby experiences dispersion and local drainage. We present a reduced-order theoretical model that includes coupled, non-linear partial differential equations, the solution of which yields estimates for the gravity current shape and density. We<br>
validate this theoretical model using COMSOL numerical simulations that mimic laboratory experiments. Our findings demonstrate that the fissure permeability and dimension impact the degree of dispersive mixing, as do the dip angle and flow conditions upstream of the fissure.<br>
The second research component replaces the localized drainage of the above paragraph with distributed drainage, which is experienced along the length of a (thin) interbed layer. Our theoretical model and accompanying COMSOL simulations suggest that the relative intensity of dispersion is<br>
influenced by the amount of mixing experienced by the fluid that drains through the interbed layer. To this end, we consider two extreme scenarios for the mixing of this drained fluid: no mixing and perfect mixing. Comparing the theoretical model predictions with COMSOL output, it is observed that the no mixing model performs better at early times, while the perfect mixing model is more reliable at late times. In this context, the degree of dispersive mixing experienced by the gravity current depends on the effective permeability of the interbed layer and the dip angle.<br>
The third research component is to validate our reduced-order theoretical model for underground hydrogen storage in depleted gas reservoirs. Thus do we compare predictions from the theoretical model described in the previous paragraph against the output of reservoir-level simulation software<br>
such as CMG and OpenGoSim. Relative to the reduced-order models, these reservoir simulator packages take into account more complex factors e.g. thermodynamic effects including non-linear equations of state and concentration-dependent viscosities. By performing this comparison study, we find that the theoretical model is often successful in predicting both the amount of hydrogen that dispersively mixes with the surrounding ambient gas and the shape of the gravity current.<br>
The overarching contribution of this thesis is to present a straightforward hydrodynamic model that describes the evolution and dispersive mixing of miscible, leaky porous media flows reasonably well, all the while neglecting the kinds of thermodynamical details that would otherwise render the<br>
model very computationally expensive to solve.</p>