Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Thoughts on MEOR from the DTI Workshop

Lewis R Brown, PhD, Professor of Microbiology, Department of Biological Sciences, Mississippi State University gives his thoughts on MEOR, following the recent DTI Workshop. (lrbsr@ra.msstate.edu)

There are several aspects of the discussions on MEOR that I think should be emphasised. To wit:

It needs to be understood that only a limited amount of growth is necessary to alter the path of injection water in the formation. Take, for example, an orifice of 20-micron diameter with one bacterium attached to the wall. An average bacterium would be about 1.0 micron by 0.5 micron or cover an area of 0.5 square microns. Since bacteria reproduce by binary fission i.e. one cell divides into two and each of these divides into two, etc., in nine generations there would be 512 cells occupying 81% of the opening. Obviously, the length of time required for a bacterium to divide varies from species to species and with the environmental conditions under which they are growing. While some species will divide every 20 minutes during logarithmic growth under ideal conditions, the doubling time for the bacteria in the reservoir is probably hours, if not days. Thus, even with a doubling time measured in days, a significant shift in injection water flow would occur in a very short period of time. Therefore, large amounts of bacterial growth are not required to achieve a positive effect from MEOR.

It also should be understood that growth is not required for MEOR to be effective. For example, once activated, the bacteria will continue to metabolise even if they are not actively reproducing. This is especially important when the bacteria are producing polymer that is helping restrict fluid flow in the reservoir or producing acid that will react with cementing material, such as carbonate, to open up new channels. Since the substrate form from which the desired by-products are produced is the oil per se, the other nutrient requirements are drastically reduced since they are only required for repair rather than reproduction.

It does not make sense to depend on growth of non-oil-degrading bacteria to bring about the desired changes in the reservoir. The idea of growing bacteria in an oil reservoir on a carbon source other than oil increases the cost immensely and poses the hazard of plugging the well with excessive growth. My experience has shown that the indigenous oil-degrading bacteria are capable of producing most, if not all of the materials helpful in MEOR (e.g. acids, gases, emulsifiers, polymers and solvents). Therefore there is no need to foster the growth on non-oil-degrading bacteria, only to direct the by-product formation by the indigenous bacteria. This can be accomplished by modifying the nutrients.

It is not necessary to have a constant supply of nutrients available all of the time. When a bacterium depletes its supply of a required nutrient, it will take up much more of the nutrient than it actually needs when it becomes available. This phenomenon is known as biosorption. Therefore, the bacteria can be metabolising continually for quite a while without an ever-present supply of a given nutrient in the injection water.

Alteration of the composition of the oil is not a problem. When one considers the tiny amount of oil that would be modified, it is virtually impossible to detect alien compounds because of the massive dilution effect. It is, however, possible to stop the production of hydrogen sulphide using nitrate and this, of course, increases the value of the oil.

Other than bacteria that get into the reservoir from injection water or are introduced in other ways, most if not all indigenous bacteria will be in the form of ultramicrobacteria (UMB). These survival forms of bacteria usually cannot be activated with normal strength microbial nutrients and dilute nutrient solutions are required. Therefore, processes involving too much nutrient will not activate these indigenous bacteria and thus they will not perform their desired mission.

I would certainly agree with other speakers that starting the introduction of nutrients in the life of the oil field would be far better than waiting until production has nearly played out.

I would also agree with many of the other participants about not injecting micro-organisms into the reservoir for MEOR. It is not inconceivable that they could be pumped very far into the reservoir except in the larger channels. In water purification plants, the rapid sand filter (4-feet of unconsolidated sand) removes 98-99% of the bacteria from the incoming water. Admittedly, floc helps to trap bacteria, but experience has shown that very few bacteria can be pumped through even rather porous sandstone. Furthermore, water travels very slowly in the reservoir. For example, in our work in the North Blowhorn Creek Oil Field water travelled at the rate of 1-6 feet per day, which means that it would take a considerable amount of bacteria any distance from the injector. Anyway, bacteria are known to be present in oil reservoirs. We have analysed cores from 13 wells and found viable bacteria in all of them, even from cores taken from nearly 15,000 feet.

Finally, I want to thank the DTI once again for inviting me to the workshop. Not only was it an enjoyable experience, but also an opportunity to learn from the results of others.

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