Focused fluid flow: the mechanisms, geological impacts, and insights from analogue and numerical modeling

Research on naturally occurring fluid migration within geological systems under higher pressures and temperatures has shown the ability for gas to be released with different physical mechanisms leading to formation of various geological structures (e.g., gas chimneys, volcanic diatremes, hydrothermal vents, salt diapirism). More recently, it has been suggested that such systems could result from fluid injection into the subsurface (e.g., CO₂ sequestration, H₂ storage as a battery). Focused fluid flow is often transient and self-organizing, occurring within a local area in reaction to changing temperatures and pressures (e.g., overpressure). Once started, a feedback loop between fluid pressure, media deformation, and permeability can occur resulting in a continuous fluid seepage. Geological evidence across a broad swathe of systems highlights that during and after the vertical fluid flow has occurred, there is a significant, localized impact on the geological fabric of the subsurface.   

Focused fluid flow is a phenomenon that could manifest within ductile deformation settings at lower fluid pressures than those associated with brittle stress behaviour (i.e., fractures), but higher fluid pressures than what is seen when solely diffusion occurs. As such, the impact on the surrounding media also varies. With higher pressures, fracturing (i.e. failure) of the media occurs. Heterogenous layering roughly parallel with the planetary surface will work to impede vertical propagation resulting in a series of oblique, but often interconnected pathways. With lower pressures, diffusion occurs, potentially resulting in geochemical and minor physical alterations to media with higher transmissibility. However, diffusion lacks the necessary pressure to bypass sections of lower transmissibility. At these intermediate pressures, porosity waves can develop, facilitating the movement of fluids through localized, transient increases in porosity within the medium. These waves enable fluid transport even in regions of relatively low permeability by temporarily enhancing the pore space without causing structural failure. The result is a dynamic interplay between pressure gradients, fluid viscosity, and the mechanical properties of the surrounding media. This interplay leads to significant changes in fluid distribution, mineralization patterns, and the potential for localized geochemical reactions.

Here we utilize analogue and numerical modelling of focused fluid flow in order to better understand the geological and injection engineering principles that could lead to the narrow range of conditions under which porosity waves could breach sections of lower transmissibility without fracturing it. Our modelling identifies key parameters of these systems that could help us better understand both the natural and the geo-engineered, including (1) proximity to surface, (2) strength of the host rock, (3) mechanical anisotropy, and (4) injection rates and amounts. For the analogue modelling we use a Hele-Shaw cell wherein we inject a lower density fluid into a viscoelastic hydrogel. By varying the injection rate we are able to identify the narrow range within which porosity waves occur. For numerical modelling we use a finite difference pseudo-transient methods to simulate coupled fluid flow and mechanical deformation in heterogeneous media. The numerical model is calibrated using results from the Hele-Shaw cell experiments, ensuring consistency between analogue and numerical observations.