GROWTH OF EXOPOLYSACCHARIDE (EPS) BIOFILM IN RESERVOIR SANDSTONES AND MICROBIAL ENHANCED OIL RECOVERY (MEOR)

Porosity-blocking microbial enhanced oil recovery (MEOR) is valuable primarily as an adjunct to waterflooding operations. Microbial permeability profile modification involves adding nitrogen- and phosphorus-containing fertilizer to the injection water of a conventional waterflood operation. These nutrients stimulate growth of in situmicrobes, not injected microbes, diverting water flow from more porous zones to unswept zones, increasing waterflood sweep efficiency. A series of experiments were performed to investigate the change in shape, size, and distribution of microbes and biofilms during MEOR. Live sandstone cores were fed and samples examined by SEM over the course of four weeks. Analyzed samples were preserved using 10% glutaraldehyde fixation or ethanol dehydration and hexamethyldisilazane. Bacteria were scarce and biofilm absent in unfed control samples.

After feeding, <200 nm spherical and ovoid forms, that we interpret as ultramicrobacteria, were present in the rocks. Nascent exopolysaccharide (EPS) occurred as an exceedingly thin coating on bacteria and mineral surfaces. The electron beam easily damaged this elementary biofilm making it appear darkened as if burnt. As time progressed the ultramicrobacteria became less common and full-sized (~1 mm) bacteria with similar shape became more common. Concurrently the EPS biofilm thickened and occurred as ropy or globular masses (which resemble in size and shape purported nannobacteria) and as thin sheets. Subsequently the slime layer continued to thicken and become more widespread in occurrence. Large (~10 mm) balls of slime were also observed, as were weblike strands of EPS, however, these later features may have been dehydration artifacts.

After four weeks, flow tests showed that the permeability of the cores had been completely degraded. The morphology of the EPS slime at this stage was a thick, continuous layer that coated and bridged grains and occluded pore throats. This morphology was possibly produced by the dehydration and deflation of a thick, pore-filling mass of slime. The large volume of slime versus the relatively small number of bacteria observed in these experiments indicates that it is the presence of exopolysaccharide biofilm, not pore blockage by bacterial bodies, that is the dominant plugging mechanism in the MEOR procedure.