Definitely not a circle: on the variability of the zone of influence in porous media flows

Yana Nec

When fluid motion is induced through a porous medium, the distance travelled depends on the energy exerted and overall resistance to flow. Ostensibly intuitive, this concept of reach has proven elusive not only in delineation, but even mathematical definition. In wells surrounded by a homogeneous medium it is a circle and thus characterised by a single number referred to as the radius of influence. The simple definition thereof as the locus of zero radial velocity yields an infinite or severely overestimated result, and tempts custom modifications to reconcile the estimates with reality. In environmental applications such as aquifer sparging wells, hydraulic and petroleum wells, natural gas and landfill gas wells, the medium is often highly heterogeneous. We show that the zone of collection or dispersal changes shape dramatically based on the interrelation of resistance to flow posed by subdomains of distinct permeabilities. In contrast to the isotropic case, the mere presence of heterogeneity suggests a natural and unambiguous definition of this zone of interest as a set of separatrices connecting stagnation points in the flow field, and results in realistic estimates. The variability of its shape is remarkable and relates to the structural diversity of the parameter space underpinning the flow field. The findings help explain the longstanding problem of a reliable delimitation of these wells’ reach in practice.

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