2.3. The main sources of reservoir energy.

Energy is not taken out of nothing. It is released due to the initial state of the system changing; in this case we are speaking about the reservoir. During the process of wells’ operation the pressure on the bottom is decreasing, but when the injection wells are working pressure on the bottom is increasing. From the formulae of the previous paragraph it is obvious that the energies depend on pressure. Reservoir energy is manifested in the process of pressure decreasing, depression creation. Only the pressure change and presence of depression cause the fluids and gases filtration, their moving forward to the bottomholes of producing wells [3,4,5].

Depending on the geological structure of the deposits, pool-reservoir properties, the sources of reservoir energy can be:

1) Energy of the produced water pressure.

2) Energy of free gas expansion (gas cap).

3) Energy of dissolved gas in oil expansion.

4) the energy of the elastic deformation of fluid and reservoir.

5) Energy of oil pressure.

The above-mentioned types of energy can come together.

Reservoir energy depends on pressure, fluid and reservoir elasticity, fluids volume, the amount of the gas dissolved in oil.

Reservoir energy is spent to overcome the resistance forces such as: capillary and gravitationalforces, forces of viscosity.

2.4. Operation modes of oil deposits.

The operation mode of reservoir is called a manifestation of dominant type of reservoir energy in the development process.

According to the dominant type of reservoir energy there are determined some reservoir operation modes.

Elastic drive is characterized by the excess of reservoir pressure above the saturation pressure, hydrocarbons i.e. oil is in single-phase state. The filtration of oil happens due to the elastic strain energy and reservoir pressure. The pressure from the external reservoir boundary to the bottom-hole pressure changes according to the logarithmic law. There is a depression cone [5] around the bottom-hole. See figure 2.2.

Fig. 2.2. Depression cone. p_c – bottom-hole pressure,

p_пл - external reservoir boundary pressure, R- external reservoir boundary radius

Depression is defined as Δp=p_пл-p_с. Pressure is constant on the external reservoir boundary for the stationary filtration, and it is maintained by edge stratum water or by generated injection wells’ pressure. More info about stationary oil filtration is set out in Annex 1.

If the deposit is controlled by lithology, so it is closed-elastic mode. In this case, the pressure on the external reservoir boundary falls, depression is reduced, wells’ production is decreased.

2.5. Elastic-water drive

Elastic-water drive is made when there are active edge stratum waters. Oil displacement happens due to the elastic energy of stratum waters if the zone is well permeable and enough water saturated.

Since the beginning of depression cone spread outside the inner oil pool outline and its invasion to the edge water zone, water is embedded to the oil zone, the process of displacement of oil by water to the bottom-holes of the producing wells happens and collaborative filtration of oil and water starts, the output begins to water out, although the water injection to the stratum has not happened yet.

If the quantity of selected liquid (oil+water) is equal to the quantity of the supplied liquid to the pure oil zone of the stratum water, this mode (drive) is called hard water drive. Under natural conditions such drive is practically not found, but its allocation promotes reliable and successful design process of oil extraction. Violation of the balance between liquid extraction and its input into the formation considers the presence of other energy forms.

The quantity of the suppled water to the reservoir from the edge zone can be defined by the Pilatovskiy formula [2,13].