FRACTAL ANALYSIS OF THE EFFECT OF BACTERIAL MUCILAGE ON POROSITY AND PERMEABILITY IN SANDSTONE

In Microbial Permeability Profile Modification (MPPM), dormant, in situ bacteria are exposed to K and N solutions, which result in growth, production of mucilage, and reduction of flow within the most permeable strata. This bacterial growth diverts flow of injected waters from higher to lower permeability strata, thereby increasing production. Photomicrographs of the Cretaceous Stanley sandstone, Eutaw Formation, Jasper County, Mississippi, at 4,796 feet, are used to quantify behavior of the bacterial mucilage in MPPM samples.

Under SEM, particle edges appear as grayscale intensity maximums, countable as number of resolved spikes (N) per chord-length (d). Thus a single chord, imaged at eight sequential magnifications, can be used to graph eight data points (d, N). Along a single chord that crosses pores in dry sandstone, sampled grayscale maximums result in fractal distributions.

For samples of Stanley sandstone exposed to nutrient solutions and thus bearing pore-filling biofilm, sampling edge-features where the chord passes over a meniscus feature yields a different distribution of data points where the edge-feature power-law distribution tends to flatten out to a slope of n=0 (i.e., an integer) and the 10 – 20µm data conspicuously fall out.

In contrast to the dry sandstone, the samples with biofilm yielded power-law distributions with integer slopes. This shift in behavior indicates that as the pore-surfaces become coated with biofilm by-products, the stochastic behaviors of surfaces imaged shift from fractal to Euclidean, i.e., from power-law distributions with powers which are fractions to those that are integers. The conspicuous change in 10 – 20 µm data suggests that, although visible porosity remains high, significant permeability modification occurs ubiquitously within pore and throat spaces of 10 µm to 20 µm diameter. Based upon quantitative analysis of photomicrographs, bacterial mucilage does not completely fill porosity but does clog pore throats, thus significantly impacting permeability.