Advantages and disadvantages of MEOR
The main advantages of MEOR over conventional enhanced oil recovery (EOR) methods are: much lower energy consumption, lower cost, and low environmental toxicity. Lower costs come from the fact that after the introduction of microorganisms into the reservoir their growth increases exponentially, therefore it is possible to obtain a large number of active substances from a small volume of initial organic material. After the initial injection of microorganisms, it is only necessary to supply nutrients into the reservoir rock to enable the development of the existing bacterial population that will produce the required chemical agents.
More in details, the advantages are:
– Injected microbes and nutrients are cheap; easy to handle in the field and independent of oil prices.
– Economically attractive for mature oil fields before abandonment .
– Existing facilities require slight modifications.
– Easy application.
– Less expensive set up.
– Low energy input requirement for microbes to produce MEOR agents.
– More eﬃcient than other EOR methods when applied to carbonate oil reservoirs.
Factors aﬀecting MEOR
The important factors aﬀecting MEOR are the temperature, pressure, pH, salinity, and pore size. At high temperature, the microbes do not function properly, which means no reproduction and no metabolite production. The maximum growth rate was observed at temperatures below 80 0C. However, it is known that some microorganisms can grow at temperatures up to 120 0C ( ). Pore size and geometry may aﬀect bacterial motion. Penetration of bacteria through the reservoir is regarded possible with minimum pore diameters of at least 2 2  and ideally be in the range of 6–10 2 . If this is not the case, microbes may not be transported to the target zone. Even if we assume that the pore spaces are relatively large, losses of injectivity may occur due to well-bore plugging by bacteria.
A reservoir with high pressure allows gases to mix with fluids for which the reservoir may have strong acidic components [22, 23]. At high pressures the DNA double helix becomes denser, and therefore both gene expression and protein synthesis are aﬀected also increasing pressure increases gas solubility, and this may aﬀect the redox potential of gases participating as electron acceptors and donors, such as hydrogen or CO2 .
Nielsen  reports that acidity determines the microbial surface charge, which also aﬀects the transport of microbes. A low pH value impairs the ability of microbes to reproduce. Embedded cell proteins (enzymatic activity) play a fundamental role in trans-porting chemicals across the cell membrane. Their function is highly dependent on their ionization state, which in turn is highly dependent on pH. So far, an understanding of the interaction between pH and the microbial communities of the environment remains unknown, and research is still under development. Moreover, its physiology influences the transport of bacteria. For example, bacteria with a hydrophobic surface tend to stick together and transport in a stream, while hydrophilic bacteria often are suspended and transported alone with  fluid.
The thermodynamically favorable oxidative potential is crucial for the survival of microbes. For bacterial growth, electron donors and acceptors must be present, where they are oxidized and reduced in biochemical processes, respectively. In aerobic respiration, oxygen in the form of O2 is a terminal electron acceptor, where they receive a large amount of energy used in the processes of growth and maintenance. When there is no oxygen, only anaerobic processes occur. Especially for the oil reservoir, the redox potential is very low, and electron acceptors such as ferric ion, nitrate and sulfate are used as the background. The water phase contains sulfate and carbonate in various concentrations, which suggests that the main metabolic processes that occur naturally are sulfate reduction, methanogenesis, acetogenesis and fermentation [22, 20].
Eﬀect of capillary trapping in flow models
Capillary trapping occurs on the pore scale, whilst the flow equations used in reservoir engineering are macroscopic. How this fine eﬀect can appears in coarse models ? The principle consists in the following.
The flow of two phases in porous media is governed by the well-known Darcy’s law, which says that the flow velocity is proportional to the pressure gradient: Vi = Kki(S) rPi; i = w; o (1.1).
where Vi is the Darcy velocity; K is the intrinsic permeability of the medium; is the dynamic viscosity; P is the pressure; S is the water saturation (the volume fraction of water in the total fluid). The dimensionless functions kw(S) and ko(S) are called the relative permeabilities (RP) and have the form shonw in Fig. 1.4.
General properties of micro-biochemical systems (MBS)
1. Bacterial population does not decay. The decay of population is related to the death of individuals, which means an irreversible loss of ability to grow and multiply. The death necessary leads to the decomposition of the cell or its rotting. In contrast to multicell animals, the death of a single cell is considered as practically impossible event. In fact, a cell is dead when it has been eat by other maicroorganisms or fungi. A nutrient deficiency causes the inactivity and hibernation of bacteria, but not their death. This fact was used even in the first models of bacterial population growth . We will consider the decay (the death) of bacteria as negligible eﬀect.
2. The process is isothermal, since all the energy produced by the respiratory reaction is consumed by bacteria to their vital functions. For the displacement of a phase by another one, the pressure may be also considered as almost constant value.
3. The phases are in chemical equilibrium: i.e. the concentrations of all chemical components in water and oil (not bacteria) are controlled by the laws of phase equilibria. According to the Gibbs rule of phases, the thermodynamic degree of freedom of a two-phase equilibrium system with four dissolved chemical components and fixed temperature and pressure is equal to 2. Therefore, only two concentrations are independent. We select them as cn and cm. All other concentrations depend on them.
4. The mobility (the relative permeability and viscosity) of the phases depends on the phase saturation and on the phase composition, i.e. on cn and cm.
5. Water and oil are dilute solutions: water consists essentially of H2O, while the oil phase consists essentially of the heavy component.
6. We consider bacteria as a colloidal suspension (or « emulsion ») in water, so that the size of a single bacterium is much lower than the size of a pore. The averaged flow of such a system of particles has the same velocity as that of transporting water, so that bacteria behave as a chemical component of water, but not a separated phase.
7. The respiratory component is always present in suﬃcient amounts, so it is not necessary to take it into account. We also assume that the chemical transformations during the respiration do not produce any substances important for oil recovery. Thus, the respiration process may be entirely ignored.
8. At the first step of study, the gravity, the capillary pressure, and the diﬀusion of all components and bacteria can be neglected.
Table of contents :
1 Literature review on MEOR and bacterial dynamics
1.1 Microbial Enhanced Oil Recovery: MEOR
1.1.1 EOR objectives and forms
1.1.2 Thermal, Chemical and Miscible EOR
1.1.3 MEOR technology
1.1.4 Advantages and disadvantages of MEOR
1.1.5 Factors affecting MEOR
1.1.6 Previous studies on MEOR
1.2 Capillary trapping of oil and surfactants
1.2.1 Mechanism of capillary trapping
1.2.2 Mechanism of the action of surfactants
1.2.3 Effect of capillary trapping in flow models
1.3 Structure of Microbes and their Functions
1.3.1 Structure of a cell
1.3.2 Bacteria movement
1.3.3 Metabolism of bacteria
1.3.4 Products of metabolism: metabolites
1.4 Kinetics of population evolution
1.4.1 Population growth
1.4.2 Mathematical models of population kinetics
1.4.3 Population decay
2 Mathematical model of MEOR
2.1 General mathematical model of MEOR
2.1.1 Structure of the fluids
2.1.2 General properties of micro-biochemical systems (MBS)
2.1.3 Mass balance equations
2.1.4 Description of the impact of biosurfactant
2.1.5 Kinetic functions
2.2 Reduction to the model of kinematic waves
2.2.1 Ideal mixing and volume concentrations
2.2.2 Kinematic wave model
2.2.3 Conditions at the shocks
2.2.4 Derivation of the conditions at the shocks
3 Asymptotic solution at weak bioreactivity
3.1 Method of Solution
3.1.1 Concept of weak bioreactivity
3.1.2 Asymptotic expansion
3.1.3 Expected structure of the solution
3.2 Semi-analytical solution to the MEOR problem
3.2.1 Behaviour of concentrations
3.2.2 Stage 1: before the impact of surfactant
3.2.3 Stage 2: formation of an oil bank
3.2.4 Stage 2: moment of the appearance of the biochemical shock (t1) .
3.2.5 Moment of appearance of the third shock (t2)
3.2.6 Velocity of the third shock
3.2.7 Stage 3: collision of two mechanical shocks
3.2.8 Behaviour at large times
4 Numerical analysis of MEOR scenarios
4.1 Numerical qualitative analysis of the process
4.1.1 Description of the numerical code COMSOL Multiphysics
4.1.2 Input parameters
4.1.3 Comparison with net waterflooding
4.1.4 Impact of viscosity ratio
4.1.5 Numerical results for low injection concentration
4.1.6 Numerical results for high injection concentration
4.1.7 Impact of the form of the population kinetics
4.1.8 Impact of the characteristic kinetic rate
4.2 Simulation of a case study
4.2.1 Description of the Kazakhstan oil fields
4.2.2 On the implementation of MEOR in Kazakhstan’s oil fields
4.2.3 Data for Uzen oil field
4.2.4 Calculation of the field case for Uzen oil field
A Analytical solution to the Buckley-Leverett equation
B Numerical code Matlab: population growth for various kinetic functions