New discovery boosts the use of microorganisms in oil recovery
Oilfields usually represent extreme environments, where physicochemical conditions appear at first sight to be generally unsuitable for living organisms to develop. However, these environments, usually poor in nitrates and oxygen, harbor a rich diverse community of microorganisms. The most widely represented and best-known types are sulphate-reducing, methanogenic and fermentative bacteria.
Nitrate-reducing bacteria, on the other hand, have received little research attention regarding their biology and role. Nevertheless some of these bacteria are known also to have the ability to oxidize sulphates. These components, which can result from metabolic activity of sulphate-reducing bacteria, prove dangerous for the environment and corrosive for drilling equipment.
Nitrate injection is practiced in some regions of the world in order to restrict the emission of sulphates produced during processes of exploitation of oil deposits. This input of nitrates stimulates nitrate-reducing bacteria, initially present in low quantities in the waters associated with oil reservoirs, to proliferate. They thus induce at once inhibition of the development of sulphate-reducing bacteria and oxidation of sulphides that such microorganisms produce.
The question remains of determining whether or not these nitrate inputs into the petroleum reservoirs environment can favor the growth of populations of nitrate-reduction microorganism different from those that oxidize the sulphides, in this way modifying the microbial ecology of oil wells.
Now scientists from the Institut de Recherche pour le Developpement (IRD) in Paris are investigating in the laboratory the metabolism of novel nitrate-reducing bacteria, especially those able to oxidize organic acids. These acids are often present in the waters of oil reservoirs. The IRD team surveyed oilfields in Australia with the help of partners from Griffith University in Metropolitan University of Mexico City and the Mexican Petroleum Institute.
The group has succeeded in isolating and identifying two novel nitrate-reducing bacteria, Petrobacter succinatimandens and Garciella nitratireductens , which can be distinguished by their metabolic activities. The bacterium Petrobacter succinatimandens, extracted from an oil well located in Queensland in the East of Australia, was shown to be capable of oxidizing the organic acids. It has an aerobic metabolism, which means that it develops in the presence of oxygen. Accidental introduction of oxygen by means of an input of water from outside the oil deposit (rainwater infiltrations, common practice3 of water injection while oil is being extracted) could explain the presence of this bacterium and its survival in an anaerobic environment. However, Garcialla nitratireductens, isolated from several oil wells in the Gulf of Mexico , has an anaerobic metabolism, like most microorganisms that live in these kinds of habitats.
This research work brings fundamental new information about oil reservoir ecosystems and the microorganisms that colonize them. In particular they offer the oil industry the means to gauge more accurately the biodiversity of nitrate-reducing microorganisms in the reservoirs and the impact of their metabolism on the biogeochemical cycles of matter within these environments.
Other research has been embarked upon in order to identify bacteria potentially useful for industry, characteristic of oil reservoir environments, which might be usefully deployed in aided recuperation of oil deposits by microorganism-based processes.
Scientists at the New Delhi-based Energy and Resources Institute (TERI) and their colleagues at the Ahmedabed-based Institute of Reservoir Studies have also been facing the challenge of microbial enhanced oil recovery. The scientists took a cylindrical piece of porous rock typical of that found deep in oil wells in the country and carried out a simulation experiment. The rock sample was first soaked in oil until completely saturated. Noting the amount of oil absorbed, the scientists heated the rock to 90 degrees Celsius before placing it in a cylindrical steel jacket fitted with valves on either side.
Hot water was then forced through one valve in the steel jacket. This released approximately 40 per cent of the absorbed oil via the other valve. Next, anaerobic bacteria and their nutrients were pumped under high pressure into the rock. Immediately afterwards, both valves in the steel jackets were closed and allowed to remain that way for 10-15 days.
Afterwards, the valves were opened and hot water passed through the rock once again. The remaining oil was released. As a control, the same treatment was given to a near identical piece of rock which had not had microbes injected into it. This sample did not yield any oil when hot water was passed through it for a second time. Encouraged by this experimental success, the scientists applied the technique on a large scale to 11 oil wells in Gujarat .
Approximately 120m 3 of bulk-produced microbes and nutrients were pumped into the wells at extremely high pressure. The valves of the pump were closed immediately after injection of anaerobic microbes to ensure that no air entered the well. Then the bacteria were allowed to proliferate in the reservoirs for 15-20 days. During this time, the bacteria multiplied and started producing polymers, gases such as carbon dioxide and methane, and organic acids which acted on the pores in the rocks containing oil, pushing it out. This mixture of oil and water accumulated in the reservoir and, after the prescribed period, was extracted using sucker rod pumps. The recovered mixture was then sent to the group gathering station and from there to the refineries.
The success rate in these wells was very encouraging, with production in some increasing three-fold.
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