23-01-2013, 04:02 PM
Enhanced Oil Recovery (EOR)
Oil Recovery
Oil Recovery Factor: also called overall hydrocarbon displacement efficiency, the volume of hydrocarbon displaced divided by the volume of hydrocarbon in place at the start of the process measured at the same conditions of pressure and temperature.
Microbial Enhanced Oil Recovery
Microbial enhanced oil recovery refers to the use of microorganisms to retrieve additional oil from existing wells, thereby enhancing the petroleum production of an oil reservoir. In this technique, microorganisms are introduced into oil wells to produce harmless by-products, such as slippery natural substances or gases, all of which help propel oil out of the well. Because these processes help to mobilize the oil and facilitate oil flow, they allow a greater amount to be recovered from the well2.
Mechanisms of MEOR
The use of microorganisms and their metabolic products to enhance oil production involves the injection of selected microorganisms into the reservoir and the subsequent stimulation and transportation of their in-situ growth products in order that their presence will aid in further reduction of residual oil left in the reservoir after secondary recovery is exhausted. The MEOR is unlikely to replace conventional EOR methods, because MEOR itself has certain constraints. This unique process seems superior in many respects, however, because self-duplicating units, namely the bacteria cells, are injected into the reservoir and by their in-situ multiplication they magnify their beneficial effects3. Some of the mechanisms proposed by which these microbial agents could stimulate oil release are shown in Table 1.
History of Microbial Enhanced Oil Recovery
In 1895,Miyoshi first reported the growth of a mould culture on n-alkanes5. In 1926, Bastin did the first extensive microbiological study describing the widespread presence of SRB in oil-producing wells6. At the same year, Beckman suggested that microorganisms could be used to release oil from porous media7. Later in 1946, as the most important founder of MEOR, ZoBell patented a process for the secondary recovery of petroleum, using anaerobic, hydrocarbon-utilizing, sulfate-reducing bacteria such as Desulfovibrio species in situ8. The first field test was carried out in the Lisbon field, Union County, AR in 19549. Kuznetsov et al. found that bacteria discovered in some oil reservoirs in the Soviet Union produced 2 gm of CO2 per day per ton of rock, in 196310.
From 1970s to late 1990s, MEOR research was boosted by the petroleum crisis and later became a scientific substantiated EOR method. Many international meetings were periodically organized on the MEOR topic and proceedings volumes with the advances in the knowledge and practice of MEOR have been published. Several books on MEOR were also published4.
From late 1990s, modern biological methods began to be applied on the MEOR research, such as Molecular Ecological Technique of Microbes, Protoplast Fusant Technology, and Recombination DNA Technology11,1
Current Status of MEOR
The research of MEOR has been done worldwide, and most of oil produce countries have applied this technology into oil fields for pilot tests. Recently this technology has been widely used in oilfields of China, such as Daqing, Shengli, Jilin, Dagang, Liaohe, Henan, Changqing, Xinjiang, and Qinghai.
Classification of MEOR
Mainly, MEOR is classified as surface MEOR and underground MEOR based on the place where microorganisms work. For surface MEOR, biosurfactand (Rhamnolipid13), biopolymer (xanthan gum14), and enzyme15 are produced in the surface facilities. These biological products are injected into the target place in the reservoirs as chemical EOR methods. While, for underground MEOR, microorganisms, nutrients and/or other addictives are injected into the reservoir and let them sustain, grow, metabolize, and ferment underground.
Based on the source of microorganisms, underground MEOR is categorized into in-situ MEOR and indigenous MEOR. While according to procedures of processes, underground MEOR is sorted as:
• Cyclic Microbial Recovery (Huff and Puff, Single Well Stimulation)
• Wax Removal and Paraffin Inhibition (Wellbore Cleanup)
• Microbial Flooding Recovery
• Selective Plugging Recovery
• Acidizing/Fracturing
Cyclic Microbial Recovery
A solution of microorganisms and nutrients is introduced into an oil reservoir during injection. The injector is then shut in for an incubation period allowing the microorganisms to produce carbon dioxide gas and surfactants that help to mobilize the oil. The well is then opened and oil and products resulting from the treatment are produce. This process may be repeated. Fig.1 illustrates this technology.