Bacteria and Oil Production

A funded project that my students and I are working on is an investigation of bacteria in subsurface hydrocarbon reservoirs. Over 50% of original oil in place in the United States is still in the ground and cannot be recovered economically with today's technology. Most of this is in economically vulnerable wells classified as marginal. Without economic enhanced oil recovery (EOR) technologies this resource is at risk of being abandoned forever. Waterflooding is by far the most widely used EOR technology, producing over half of U.S. oil. It is estimated that 50-75% of all fields have been or will be waterflooded.

Microbial permeability profile modification (MPPM) involves adding nitrogen- and phosphorus-containing microbial nutrients to the injection water of a conventional waterflood operation. The nutrients stimulate growth of in situ microbes, not injected microbes, diverting water flow from more porous zones to unswept zones, increasing waterflood sweep efficiency. It is a reservoir process, not just treatment of individual wells. Since the nutrients are commonly used plant fertilizers and only microbes already present in the reservoir are involved, it is a very environmentally friendly process. Compared to other EOR technologies it is a relatively low cost method.

MPPM has been successfully applied at Hughes Eastern Corp.'s North Blowhorn Creek Unit (NBCU) in Lamar County, Ala. Production is from Mississippian-age Carter sandstone at a depth of about 2,300 ft. The process has extended the economic life of the field 5-11 years past normal waterflooding and generated ~$350,000 in taxes and royalties with potential for $1.3-1.9 MM. The incremental cost of the MPPM was $1.32 per barrel. MSU and MPPM has received a prestigious Hart's Oil and Gas Award, and has been recognized by the Secretary of Energy. Our research deals with understanding the relationships between the bacteria and the rocks in order to help us better understand the MPPM process.

Scanning electron microscopic preservation techniques for bacteria indigenous to the NBCU have been tested. Five techniques were tested, including air drying, 10% glutaraldehyde fixation, standard ethanol dehydration with hexamethyldisilazane, ethanol dehydration with critical point drying, and ethanol/acetone dehydration with critical point drying. Ethanol dehydration and critical point drying preserved the bacteria but greatly changed the morphology of the associated polysaccharide capsule (Fig. 1). Air-drying and glutaraldehyde fixation preserved the polysaccharide biofilm, but bacteria were distorted or collapsed. Our conclusion is that an accurate investigation requires two samples, one preserved by glutaraldehyde fixation for characterization of the biofilm, and one by ethanol dehydration for examination of the bacterial bodies themselves.

Several experiments have been performed on NBCU bacteria and rocks. In the first experiment, sandstone samples were inoculated with indigenous NBCU bacteria from a laboratory culture and incubated for two weeks. SEM examination of the samples showed a polysaccharide slime layer forming an irregular but continuous sheet that draped across sand grains and stretched across pore throats and crevices. Bacteria were uncommon and randomly distributed throughout the samples. In places, two different morphologies of polysaccharide slime were present: a beaded, globular layer overlain by a smooth, sheetlike layer (Fig. 2).

Fig. 1. Web-like morphology of polysaccharide slime produced by dehydration preservation.
Fig. 2. Grain-coating morphology of polysaccharide slime preserved by simple air-drying.

In the second experiment small pieces of live NBCU core were feed with nitrogen- and phosphorus-rich nutrients on the same schedule as the NBCU cores that have been flow tested. After two weeks, SEM analysis showed that the sandstone core pieces were so completely covered with biofilm that the entire mineral surface was obscured (Fig. 3). Fewer bacteria were observed than in cultured-bacteria experiments. The speed with which the bacterial capsule grew and thickly and completely covered the samples was a surprise. Future experiments with shorter feeding times are in progress. Other experiments comparing the rate of growth in samples with dissimilar mineral compositions are also underway.

It has been long debated whether the efficacy of MEOR is due to pore blockage by bacterial bodies or by polysaccharide capsule. These experiments suggest that is the polysaccharide slime layer that is almost entirely responsible for the plugging of sandstone pores and that this is accomplished not by completely filling the pore spaces but by stretching across pore throats in a weblike morphology (Figs. 4, 5, and 6). Our research on MEOR will soon be shifting gears as we begin an investigation of bacteria in a carbonate reservoir where acid produced by the bugs will actually increase the porosity and permeability of the rock. This research is a collaborative effort with Dr. Lewis Brown in the Dept. of Biological Sciences. Much of our work is accomplished in the Electron Microprobe Center at MSU.

Fig. 3. Biofilm completely covering bacterial bodies and mineral grains.
Fig. 4. Polysaccharide capsule (biofilm) on and between kaolinite flakes.

Fig. 5. Polysaccharide slime meniscus partially occluding porosity.
Fig. 6. Sandstone porosity partially filled with polysaccharide slime.