Many people have proposed the use of microorganisms as blocking agents (see Refs. 36-57 in Ref. 28). In most of these proposals, placement of the microorganisms is dictated by placement of the nutrients. Since the flow properties of the nutrients are no different from those of gelants, their placement characteristics are similar to those of gelants. Specifically, for a given distance of penetration into a high-permeability zone, the distance of penetration into a less-permeable zone will be no less for the nutrient (and the microorganism) than for a gelant with a water-like mobility. If a viscous nutrient is used (e.g., molasses or corn syrup), microorganism penetration into less-permeable zones increases.1,44
From one perspective, microorganisms could be viewed as particles. Because of their narrow size distribution, certain microorganisms could, in concept, provide the advantageous placement characteristics associated with monodisperse particles (discussed earlier). A suspension of microorganisms could penetrate readily into a high-permeability zone, while size restrictions prevent them from entering less-permeable zones. Bae et al.75 proposed the use of spores to act by this mechanism. They observed spores that propagate through Berea sandstone with permeabilities greater than 710 md but that do not propagate through cores with permeabilities less than 380 md. Once placed, nutrients could be provided so that the microorganisms could restrict flow (i.e., by growing or generating biomass or polymers). Thus, properly sized microorganisms conceptually could provide a placement similar to that for intermediate-sized particles suspended in a gelant.
Two important restrictions must be noted when using microorganisms in this mode. First, growth, aggregation of microorganisms, and adsorption onto pore walls must be limited during placement. Otherwise, these phenomena could greatly limit the distance of microorganism penetration into the high-permeability zones. Second, the microorganisms should be near-spherical in shape during placement. Elongated microorganisms act as particles with a significant size distribution.28 As mentioned earlier, the placement advantage for particles will be lost unless the size distribution is very narrow.