Numerical modeling of gas hydrate recycling in complex media:
Implications for gas migration through strongly anisotropic layers
Abstract
Burial driven recycling is an important process in the natural gas
hydrate (GH) systems worldwide, characterized by complex multiphysics
interactions like gas migration through an evolving gas hydrate
stability zone (GHSZ), competing gas-water-hydrate (i.e.
fluid-fluid-solid) phase transitions, locally appearing and disappearing
phases, and evolving sediment properties (e.g., permeability, reaction
surface area, and capillary entry pressure). Such a recycling process is
typically studied in homogeneous or layered sediments. However, there is
mounting evidence that structural heterogeneity and anisotropy linked to
normal and inclined fault systems or anomalous sediment layers have a
strong impact on the GH dynamics. Here, we consider the impacts of such
a structurally complex media on the recycling process. To capture the
properties of the anomalous layers accurately, we introduce a fully mass
conservative, high-order, discontinuous Galerkin (DG) finite element
based numerical scheme. Moreover, to handle the rapidly switching
thermodynamic phase states robustly, we cast the problem of phase
transitions as a set of variational inequalities, and combine our DG
discretization scheme with a semismooth Newton solver. Here, we present
our new simulator, and demonstrate using synthetic geological scenarios,
a) how the presence of an anomalous high-permeability layer, like a
fracture or brecciated sediment, can alter the recycling process through
flow-localization, and more importantly, b) how an incorrect or
incomplete approximation of the properties of such a layer can lead to
large errors in the overall prediction of the recycling process.