3.8. Circle water flooding.

When the circle water flooding is used the balance of reservoir energy maintenance and buildup are carried out by means of water injection directly into oil-bearing part of the reservoir.

Currently, it is the most intensive and efficient way of influence on productive formations. According the nature of mutual arrangement of oil-producing and injection wells there are several varieties of circle water flooding:

-cutting of oil deposit by the rows of injection wells on separate areas; splitting into separate blocks of independent development;

- grid water flooding;

- spot and selective water flooding;

- center-to-edge water flooding;

- barrier water flooding;

3.8.1. Direct line drive systems. Their varieties – block systems.

The systems of circle water flooding with cutting of the deposit in some separate areas are used on large fields with wide, in project, pure oil and oil-water zones. Extensive water-oil zones are cut off from the main part of the deposit and developed independently. When the deposits are of medium-size there is used block system of flooding.

The lines of producing and injection wells are arranged in the transverse direction. The number of producing wells’ lines between the lines of injection wells defines the lane of the system. There are applied one-three-five-line drive development systems fig.3.8.The number of producing wells must be odd. The central row in three - or five-line drive system is called astringent. If the central row is absent, the oil reserves in this area of the formation will remain unmined.

The grid density for these water flooding systems is almost the same as in edge water flooding systems (S_c 25-30 ha/well).

Splitting the deposits in areas and the blocks is used in the development of Pravdinskaya, Mamontovskoye, Samotlor, Zapadno-Surgutskoye deposits with 3-5 lines of producing wells. The improvement of block systems is block-square system with periodic change of the water flow direction.

Fig. 3.8. L_{п –} the width of block and row; 2_н and 2_{в –} the distance between the producing and injection wells in the row; l₀₁, l_12,l₂₃ – the distance between the rows of the producing wells.

Line drive systems are more preferable during the development when the formation thickness is heterogeneous.

3.8.2. Grid water flooding systems.

Grid water flooding is characterized by dispersed water injection into the reservoir throughout the area of its oil content. Grid water flooding systems are determined by the number of wells of each deposit element with one injection or producing well located in the center of it. If injection well is located in the centre of the development element, such system is called reverse. If production well is located in the center of the development element, such system is called direct. In practice the reverse systems are used.

The most used are 5-7-9-spot systems

Fig.3.9. Five-spot system.

An element of five-spot system is a square in the angles of which there are situated the producing wells and in the center there is an injection well.

Fig. 3.10. Seven-spot system.

An element of seven-spot system is a hexagon with the producing wells in the angles and injection well in the center.

Fig. 3.11. Nine-spot system.

Nine-spot system is a square with the producing wells in the angles and in the medium and injection well in the center.

When the five-spot system is watered out it is transformed into nine-spot system by means of additional drilling.

Fig. 3.12. The transformation of five-spot system into nine-spot.

1- producing wells; 2 – unrecovered oil; 3- extra-drilled wells; 4- watered zone; 5 – injection well

Grid system is used when the formations are highly heterogeneous throughout the area.