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(of oil deposits), the injection of water into oil beds in order to maintain and restore formation pressure and the formation energy balance.
Waterflooding provides high oil production rates and a relatively high degree of underground petroleum recovery, since exploitation proceeds at the most efficient water-pressure operating conditions of the bed (the oil contained in rock pores or fissures is replaced by water). Most oil-bearing regions have sources of water suitable for bed injection after simple treatment. The effectiveness of waterflooding (including its economic effectiveness) has contributed to the wide-spread adoption of this method for petroleum production in the USSR (approximately three-fourths of the oil produced in the late 1960’s). Waterflooding allows a considerable decrease in the number of oil wells and a sharp increase in their output (daily production), substantially lowering the costs for each ton of oil produced. A waterflooding system usually consists of water intakes, vessels, cleaning installations, pumping stations, water circulating systems, and pressure injection wells. Water is injected into the oil beds through a system of injection boreholes usually drilled especially for this purpose. There are different types of waterflooding, depending on the location of the injection wells in relation to the oil deposit and on the relative positioning of the injection and working (producing) wells. In transcontour waterflooding, all the injection wells are located in water-only zones of the bed beyond the limits of the oil deposit; in intracontour waterflooding, the injection wells are located in the area of the oil deposit and the water is injected into the oil-saturated part of the bed; and in section (square) waterflooding, the oil and injection wells are located according to a special network and alternate with each other in a prescribed pattern.
Well operation with transcontour waterflooding is by its very nature similar to the natural water-pressure operating conditions of the formation with active marginal (transcontour) waters. Transcontour waterflooding only intensifies this process, approaching the supply zone of the bed close to the deposit. This intensification is of decisive importance for many oil deposits, since only in this case can the deposit be worked in the necessary periods under the most effective conditions for the displacement of the petroleum by the water. A type of waterflooding known as adcontour waterflooding, in which the injection wells are located on the oil pool outline, is sometimes considered to be a separate method (it is used at deposits where the permeability of the formation at or beyond the oil pool outline is deteriorating substantially). A typical example of transcontour waterflooding is the operation of the Bavleny oil field in the Tatar ASSR, where use of this process resulted in a fourfold reduction in the number of oil wells and a long period of stable oil production.
With intracontour waterflooding, water is injected directly into the oil deposit, usually into injection wells that are located in rows, thus causing the deposit to be “cut up” by the water into separate, smaller deposits that can be worked independently. The number of operating wells located in the high pressure zone of the formation (in the vicinity of the injection wells) increases, resulting in a sharp rise in oil production rates and a decrease in deposit exploitation periods. A classic example of intracontour waterflooding is the operation of the Romashkino oil deposit, a Devonian-age pool in the Tatar ASSR. The splitting up of this huge deposit with a series of injection wells (begun in 1954) allowed the recovery period of the basic oil reserves to be reduced by several times. Longitudinal or lateral intracontour waterflooding, depending on the direction of the “cutting” rows in relation to the structure, is used for smaller deposits.
Section waterflooding is the most intensive method; in this method the phenomenon of interference of wells having the same function is reduced to a minimum and well yields are maximized, other conditions being equal. Section waterflooding is usually used either from the start of exploitation at deposits with very low formation permeability, where other types of waterflooding are not as efficient, or as a so-called secondary oil recovery method after the exploitation of a deposit without pressure maintenance.
Combinations of the different types of waterflooding methods described above are used at many oil deposits. During the exploitation process, it is often necessary to modify the waterflooding system for further intensification of oil recovery.
REFERENCESSpravochnik po dobyche nefti, vol. 1. Edited by I. M. Murav’ev. Moscow, 1958.
Proektirovanie razrabotki neftianykh mestorozhdenii. Moscow, 1962.
IU. P. BORISOV