Published on Nov 30, 2023
It is well known from several core scale experiments that microbial activity inside a core may lead to enhanced oil production. In this work we argue that the only realistic microbial mechanism that contributes to oil production is that of the biofilm type, simply because of the low concentration of microbes inside the porous media. Microbial activity can lead to formation of a biofilm on the rock surface and the oil water interface. By modelling the microbes as immiscible drops we show that they can change the wetting properties of the rock. The model used is a Lattice Boltzmann algorithm for solving the multiphase Navier- Stokes equations.
Experiments with two strains of microbes from oil fields have been performed. The experiments are focused on studying the ability of microbes to attach to interfaces and surfaces and thereby change the wetting properties of oil, brine and rock. The first type is a microbial capillary tube experiment where microbes grown inside capillary tubes may change the interfacial or wetting properties of the tubes. A change in interfacial tension or wetting characteristic can be observed as a change in height of the oil water interface. The second type is a sessile drop experiment, where the contact angle of an oil drop has been observed over time, while subjected to microbial activity.
Microbial enhanced oil recovery (MEOR) is motivated by the fact that numerous core scale experiments have shown an increased oil production due to microbial activity. In some cases the increased oil production has been extremely high while in some cases very low (see Bryant and Lockhart (2000)). The experimental evidences are convincing that something is going on inside the core which increases the oil production. The interpretation of core scale experiments is complicated due to one simple reason that when oil is released, one never really knows what kind of mechanism is responsible for the increased oil production. Even if the microbes or product produced by the microbes was responsible for the extra oil produced, one does not know precisely what they did.
The core acts as a black box. In order to do a field trial or pilot one needs to understand in detail what the microbes are doing. As an example core scale experiments are often performed on water wet cores. If the mechanism for extra oil production is wettability change towards more oil wet behaviour, then one needs to take into account that the reservoir is probably not water wet, but mixed wet.
Biofilms can grow on the surface of the porous rock, which may lead to a change of surface properties and/or a decrease in permeability (Gandler et al. (2006)). Permeability reduction can not explain increased oil production from water wet cores. The properties of the biofilm will be different from the rock properties. The change in surface properties inside the porous rock can thus lead to a change in the wetting properties. If the microbes locally change the wettability close to a trapped oil cluster, this oil cluster can be mobilized when the receeding contact angle is reduced sufficiently. In addition microbes attached to the oil water interface will not detach easily (we will return to this point). Microbes would then be transported with the oil cluster to a new location and may induce new oil mobilization.
Chemical methods (chemical flooding) are claimed to have significant potential based on successful laboratory testing, but the results in field trials have not been encouraging. Furthermore, these methods are not yet profitable. In these processes, chemicals such as surfactants, alkaline solutions, and polymers are added to the displacing water in order to change the physicochemical properties of the water and the contacted oil making the displacement process more effective. In surfactant flooding, by reducing the interfacial tension between the oil and the displacing water and also the interfacial tension between the oil and the rock interfaces, residual oil can be displaced and recovered. Moreover, in caustic flooding, the reaction of the alkaline compounds with the organic acids in the oil forms insitu natural surfactants that lower the oil-water interfacial tension. In addition to surfactant and alkaline flooding, polymers are used to increase the viscosity of the displacing water to improve the oil swept efficiency.
The underlying principle behind miscible displacement processes is to reduce the interfacial tension between the displacing and displaced fluids to near zero that leads to the total miscibility of the solvent (gas) and the oil, forming a single homogeneous moving phase. The displacing fluid (injected solvent or gas) could be carbon dioxide, nitrogen, exhaust gases, hydrocarbon solvents, or even certain alcohols.
Another tertiary method of oil recovery is microbial enhanced oil recovery, commonly known as MEOR, which nowadays is becoming an important and a rapidly developed tertiary production technology, which uses microorganisms or their metabolites to enhance the recovery of residual oil In this method, nutrients and suitable bacteria, which can grow under the anaerobic reservoir conditions, are injected into the reservoir. The microbial metabolic products that include biosurfactants, biopolymers, acids, solvents, gases, and also enzymes modify the properties of the oil and the interactions between oil, water, and the porous media, which increase the mobility of the oil and consequently the recovery of oil especially from depleted and marginal reservoirs; thus extending the producing life of the wells (Lazar et al., 2007; Belyaev et al. 2004; Van et al. 2003).
In MEOR process, different kinds of nutrients are injected to the reservoirs. In some processes, a fermentable carbohydrate including molasses is utilized as nutrient (Bass & Lappin-Scott, 1997). Some other reservoirs require inorganic nutrients as substrates for cellular growth or as alternative electron acceptors instead of oxygen. In another method, water containing a source of vitamins, phosphates, and electron acceptors such as nitrate, is injected into the reservoir, so that anaerobic bacteria can grow by using oil as the main carbon source (Sen, 2008).
The microorganisms used in MEOR methods are mostly anaerobic extremophiles, including halophiles, barophiles, and thermophiles for their better adaptation to the oil reservoir conditions (Brown, 1992; Khire & Khan, 1994; Bryant & Lindsey, 1996; Tango & Islam, 2002). These bacteria are usually hydrocarbon-utilizing, non-pathogenic, and are naturally occurring in petroleum reservoirs (Almeida et al. 2004). In the past, the microbes selected for use, had to have a maximum growth rate at temperatures below 80ºC, however it is known that some microorganisms can actually grow at temperatures up to 121ºC (Kashefi & Lovley, 2003). Bacillus strains grown on glucose mineral salts medium are one of the most utilized bacteria in MEOR technologies, specifically when oil viscosity reduction is not the primary aim of the operation.
The most outstanding advantages of MEOR over other EOR technologies are listed below (Lazar, 2007):
1. The injected bacteria and nutrient are inexpensive and easy to obtain and handle in the field.
2. MEOR processes are economically attractive for marginally producing oil fields and are suitable alternatives before the abandonment of marginal wells.
3. Microbial cell factories need little input of energy to produce the MEOR agents.
4. Compared to other EOR technologies, less modification of the existing field characteristics are required to implement the recovery process by MEOR technologies, which are more cost-effective to install and more easily applied.
5. Since the injected fluids are not petrochemicals, their costs are not dependent on the global crude oil price.
6. MEOR processes are particularly suited for carbonate oil reservoirs where some EOR technologies cannot be applied efficiently.
7. The effects of bacterial activity within the reservoir are improved by their growth with time, while in EOR technologies the effects of the additives tend to decrease with time and distance from the injection well.
8. MEOR products are all biodegradable and will not be accumulated in the environment, therefore are environmentally compatible.
9. As the substances used in chemical EOR methods are petrochemicals obtained from petroleum feedstock after downstream processing, MEOR methods in comparison with conventional chemical EOR methods, in which finished commercial products are utilized for the recovery of raw materials, are more economically attractive.
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