Modelling focused fluid flow in the subsurface: its controlling factors and the formation of seismic chimney 

Abstract

Seismic chimneys, prevalent along continental margins, intricately contribute to Earth's degassing processes, facilitating fluid migration from the deep Earth to the surface. This phenomenon carries significant implications for subsurface storage utilization. Among the principal mechanisms driving seismic chimney formation is the fluid flow instability within porous subsurface rocks. While numerical studies of geodynamical two-phase flow have successfully replicated vertical fluid flow structures, many of these models overlook elastic compaction, advection of solid and poro-space, and geological heterogeneity, thereby limiting their applicability in subsurface scenarios. In addressing these limitations, this study incorporates a viscoelastic rheology into the geodynamical two-phase flow model to explore the controlling factors influencing the formation of focused fluid flow, accounting for various rock properties, including geological heterogeneity. Initial investigations into the impact of elastic compaction by varying Deborah numbers reveal a limited influence on fluid flow compared to viscous compaction. Through a comparative analysis of models with and without advection, we observe the potential importance of solid advection in fluid migration, particularly under conditions of high background porosity (≥0.1) and relatively low permeability. This effect is accentuated when the solid matrix undergoes significant deformation. Channel widths range from 2-3 compaction lengths to a maximum of 5-10 compaction lengths, primarily contingent on the viscosity ratio between shear and bulk viscosity and the fluid supply. Further simulations involving fluid flow encountering a horizontal block with rock heterogeneity and geological heterogeneity demonstrate the potential for fluid penetration through the structure or deflection to the side, contingent upon rock properties and block size. Finally, we apply our model to a specific seismic chimney at Loyal Field in Scotland, UK, successfully reproducing the observed upward-bending structure in the seismic image with a consistent width-height ratio. This comprehensive investigation sheds light on the complex interplay of various factors influencing focused fluid flow, including geological heterogeneity, thereby contributing valuable insights to the understanding of seismic chimney formation in real-world geological settings.