книги / Организация закачки воды. Система поддержания пластового давления

water, flooding became a high-grade and the only means of influence on the formations during oil field development.

At present flooding is the most intensive and cost-efficient means of influence allowing to significantly reduce the number of producing wells, to increase their output, to lower the costs per 1 ton of oil. Due to this the USSR managed to produce more than 90 % of oil in the early 80-s.

Flooding types

Depending on the location of injection wells in relation to oil deposits one can differentiate between the following: edge waterflooding, marginal waterflooding and boundary flooding. Many fields combine these various types.

Edge water flooding

In this case formation pressure maintenance is fulfilled by the water pumping through injection wells located beyond oil-bearing area of productive formation in a water-occupied zone (beyond outer oil-drainage boundary). Pressure line is marked at some distance from the outer oil-drainage boundary. Usually the injection wells are positioned as near as possible to the outer oil-drainage boundary – at a distance of 0 – 200–300 m. For homogenous highly porous formations containing high-gravity oil with low viscosity, with a good flow connection between the deposit and water-bearing zone, with high reservoir permeability (0,4–0,5 mkm2 and more), on the fields of 4–5 km width, the method of edge water flooding is quite efficient providing oil recovery level which is very close to natural water drive condition. But in practice the natural system (deposit) combining these two factors is rare. During the edge water flooding, the natural process is not disturbed but intensified instead bringing the recharge area closer to the deposit.

Edge water flooding was not frequently implemented from the very beginning of the field development but in some time later when there took place some formation pressure drop. Nevertheless, water pumping into the edge water zone allowed to restore the reservoir energy store in one or two years at such a level that it finally stabilized.

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Implementing of oil formation flooding resulted first in technological obstacle connected with a low injection capacity of the injection wells. Formations that were to absorb the designed water flow during pressure drops according to Dupuis formula practically did not take in water. Wide application of methods of influencing on bottomhole zone such as hydraulic fracturing of formation and acidizing, and mainly – increased discharge pressure leaded to significant increase in injection capacity of the injection wells, thus to the solution of their development

Marginal water flooding

Marginal water flooding is applied for the formations with low permeability in marginal zone. Here the injection wells are drilled in oil-water zone of the formation between the internal and outer oil-drainage boundaries.

The advantages of marginal water flooding are obvious. Marginal deposits right up to the outer oil-drainage boundary differ in low capacities of oil-reservoir rocks that have no practical development application. Producing wells are not backfilled in the zones of low capacities (1–3 m) within large platform reservoirs.

Marginal water flooding method in comparison to other less intensive methods cannot shortly provide a maximum oil output but it enables to maintain a rather high stable production capacity for a longer period of time.

Boundary water flooding

The results of oil reservoirs edge water flooding initiated further modernization of petroleum deposits development and resulted in use of boundary water flooding, especially in large-scale deposits with formation cutting with rows of injection wells into separate areas and sections.

At boundary water flooding formation the energy balance is maintained or recovered by water injection directly into oil-saturated portion of reservoir.

In Russia the following types of boundary water flooding are used:

•Cutting petroleum deposit by rows of injection wells into separate sections;

•Barrier water flooding;

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•Cutting into separate sections of autonomous development;

•Center to edge water flooding;

•Focal water flooding;

•Area water flooding

The system of water flooding with cutting petroleum deposit into separate sections is used in large-scale platform-type deposits with large oil-water zones. These zones are cut off from the basic deposit and developed using a separate system. In middle and not large deposits are used transverse cutting with rows of injection wells into sections (block waterflooding). Extent of areas and sections is chosen taking into account the ration of viscosity and intermittence of the formations (lithologic replacement) within the distance up to 3–4 km. Inside an odd number of producing wells rows is placed (not more than 5–7).

Cutting into separate areas and sections was used on Romashkin (Tataria), Arlan (Bashkiria), Muhanov (Kuibishev Region), Osinsk (Perm Region), Pokrovsk (Orenburg Region), Uzensk (Kazakhstan), Pravdinsk, Mamontov, ZapadnoSurgutsk, Samotlorsk (Western Siberia) and other deposits.

In Sovietsk, Samotlorsk, Mamontov and other deposits starting from 60-s there were block flooding systems extensively used, so called “active” (intensive) systems with location between two injection rows of not more than 3–5 producing wells rows. At little oil viscosity (up to 3–5 mPa sec) for objects with relatively homogeneous formation structures of the flooding system sections of 3,5–4 km. can be less active. For aggravated conditions the system activity should rise and extent of sections should diminish up to 2–3 km. and less. At homogenous formations with less than 500 t/(MPа daily) productivity five-row systems proved to be effective and at 10–50 t/(MPa daily) productivity three-row systems were effective.

As a result of further investigation and proceeding from the experience in development it was determined that the most effective is cutting the developed formations with no more that five rows of injection wells in line. This is how the

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modern type of row system appeared – the block systems of oil deposits development: one-row, three-row and five-row systems.

Usage of development systems with boundary cutting made it possible to increase the development rates 2–2, 5 times in comparison with the boundary flooding and to essentially increase engineering-and-economical performance of the development. Block row systems were largely used at oil deposit development in many oil-producing regions, especially in Western Siberia.

In further work, to locate reserve wells, to intensify and regulate deposit development, there was started the usage of focal and selective water flooding schemes, in which injection and producing wells are located not in accordance with the accepted ordered development system but on separate sections of a formation.

At present this is the most intensive and efficient type of influence on productive formations. According to the type of relationship of oil-producing and injection wells there can be several types of boundary water flooding discerned.

Center to edge water flooding. At this type of flooding a row of injection wells are located in the center of the structure or near it. If deposit size exceeds the optimal one, this flooding is combined with the edge one. Center to edge water flooding is divided into: axis water flooding, peripheral water flooding and center water flooding.

Axis water flooding provides for formation pressure maintenance by means of locating injection wells along the long structure axis. It is assumed that this type of flooding can be chosen because of significant aggravation of permeability in the peripheral part of the deposit or sharply aggravated permeability in boundary part.

Axis water flooding was applied in USA in Wisson (1948) and Kelly-Snider (1954) deposits, in Russia it was applied at Novodmitriyevsk, Yakushinsk and Ust-Balyksk deposits (group А formations).

Peripheral water flooding. Peripheral row of injection wells with radius of approximately 0,4 of the deposit radius, cuts the deposit into the central and the peripheral areas. (Romashkinsk deposit).

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Center water flooding as a sort of peripheral flooding (along circle of 200–300 m in radius 4–6 injection wells are located and inside it there is one or several producing wells).

Focal water flooding at present time is applied as an additional measure to the basic water flooding system. It is applied on deposit areas which do not produce oil because of non-homogenous formation, lensing of sands and other reasons. The location of ejection and producing wells is defined in such a way, that a more complete area of oil deposit operation could be enabled. Amount of water flooding focuses is determined by the size of oil-producing area. It is also used in combination with the edge one and especially with the boundary one to deliver oil deposits from areas not covered by basic systems. It has a larger effect at later stage of development. It was introduced in oil fields of Tataria, Bashkiria, Perm and Orenburg regions, etc.

Selective water flooding is applied in deposits with sharply inhomogeneous formations. Peculiarity of this type of flooding is that initially the wells are drilled in proportional square spacing without dividing into operational and injection ones, and after investigation and some period of development the most effective injection wells are chosen out. Owing to this, having fewer wells, the most intensive water flood system is implemented and the most complete range of water flood is achieved.

Pattern water flooding is characterized by dispersal water intake into formation over all the oil-bearing area. Pattern water flooding systems, if judging by amount of well-points in each deposit element with one producing well in its centre can be four-, five-, sevenand nine-point, as well as of linear structure (fig. 1).

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Fig. 1 Four-(а), five-(b), seven-(c), nine-point (d) and linear (e, f)

pattern water flooding (with distinguished elements)

Linear system is a one-row system of block water flooding where the wells are located in staggered order. The ratio of injection and producing wells is 1:1. A rectangle with sides 2L and 2σ n = 2 σ d = 2σ. If 2L = 2σ, the linear system transforms into five-point system with the same well ratio (1:1). The five-point system is symmetric and reverse order of wells with injection well in center can also be assumed as an element (inverted five-point system). The nine-point system implies that one producing well has three injection wells (wells ratio 3:1), because four wells out of eight injection wells correspond directly to two and four neighbor elements. In inverted nine-point system (with injection well in the centre of the rectangle) the ratio of injection and producing wells is 1/3. In triangular well spacing we have fourpoint (inverted seven-point) and seven-point (inverted four-point) system with ratio of injection and producing wells correspondingly 1:2 and 2:1.

Pattern water flooding is effective at developing low-permeability formations. Its effectiveness is increased with increase of homogeneity, thickness of formation and decrease of oil viscosity and depth of deposit occurrence.

Water flooding of oil formations with its modifications at present time is the basic method of influence on oil formations for oil extraction.

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3. RISING THE FORMATION PRESSURE MAINTENANCE

EFFECTIVENESS WHEN DEVELOPING OIL DEPOSITS

Individual mediums, different solutions, compositions of substances being byproducts or waste products of high-tonnage oil extraction process, petrochemical industry or other industries, as well as some other liquids, are used for pumping into oil formation to maintain formation pressure and enhance oil-and-gas production rate.

All mediums injected into a formation can be divided into two large groups, characterized by change of phase condition at change of thermobaric conditions of transportation and injection: technologically stable mediums (ТSS) and technologically unstable mediums (ТNS).

Technologically stable mediums (ТSS) keep monophaseness at any condition, including irregular states (failure, stoppage, fill-up etc.). They are as follows: fresh, mineralized or run-off formation water, polymer solutions, surfactant species solutions and solutions of other substances in water (fig. 2).

Technologically unstable mediums (TNS) can change their phase condition in process of their moving from source to formation. Among TNS the most distributed in oil development is carbon dioxide (СО2), unstable condensate, ShFLU, ethane, propane, methanol, etc.

Under influence of external factors the viscosity of polymer solutions often decreases. They lose their basic technologic quality – to increase volumetric efficiency coefficient n cov. Destruction of solution structure under influence of external factors leads to decrease in oil-driving properties of fringes from polymer solutions.

In gas-and-oil developing system different technologic liquids are used for formation pressure maintenance, which, except for injection into formation, are also used widely for targets as follows:

1)Increase in oil-and-gas recovery;

2)Underground wells repair;

3)Well-workover operation;

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4)Bottomhole formation zone processing to intensify inflow and restrict water inflow;

5)Deletion of asphalt, pitch and paraffin deposits;

6)Destruction of mineral admixture deposits.

Fig. 2. Classification of injected mediums

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Notwithstanding the variety in fields of technological liquids usage, basically it is used for formation pressure maintenance and for Increase in oil-and-gas recovery. This field is on first place for scope of technological liquid usage. This field takes 85–95 per cent in total value of technological liquids usage in oil fields.

Effective injection, (primarily water) on new fields provides for specified dynamics of oil and gas withdrawal and what concerns the old fields – provides for retarding the production decline rate.

Applying new techniques for enhancing oil recovery

When the oil wells are developed, not all the oil deposits are extracted, but only a part of them. The end oil recovery ratio makes up to 0,4–0,5 in carbonate reservoirs and 0,4–0,8 – terrigenous reservoirs. This is achieved at maximum favourable conditions (little oil viscosity, good permeability, formation homogeneity, arrangement of water flooding system, dense wells spacing etc.).

One of the oil recovery enhancement techniques is applying new oil recovery techniques.

Experience of new oil recovery techniques introduction shows that their effectiveness depends significantly on proper choice of technique for specific oil field conditions. There are three basic factor groups:

–Geological-and-physical (oil viscosity, formation permeability, seam depth, thickness, homogeneity, oil saturation, formation pressure etc.);

–Technological (injected agent, its concentration, fringe size, density of wells spacing, development system, etc.);

–Technical (machinery and equipment supply, their quality, availability and location of raw material (agent), state of well stock, climate conditions etc.).

Proceeding from laboratory research, experimental-industrial and industrial tests, specific criteria for oil recovery techniques have been developed.

They are divided into three main groups:

–Physicochemical (water flooding with service-active substances, water flooding with polymer thickeners, acid, alkaline and other agents injection etc.);

–Techniques of mixing displacement (application of carbon dioxide (СО2), carbon gas, water-and-gas mixture, micellar solutions etc.);

–Heat techniques (hot water, steam injection, wet in-situ combustion).

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Viktor D. Grebnev

4.WATER SOURCES FOR THE FPM. WATER INTAKE COMPLEXES

Formation-pressure maintenance calls for use of large volumes of water. The solution of water supply problem boils down to finding a reliable water-abundant source (with assessment of water reserve and potential consumption), justification of quality of water, and working out the technology of its treatment. The flow rate of water to be injected is determined by the stage of oil field development (fig. 3).

Fig. 3. Dynamics of relative fluid draw-off, water injection, requirement in water (v) and oil recovery ( η) in respect to time (by M.L. Surguchyev):

1, 1’ – oil production and production rate, respectively, at dissolved-gas drive;

2, 2’, 3 – oil production, production rate, and fluid draw-off at flooding;

4 – water requirement at full return of waste water; 5 – flow rate of injection water. Hatching: vertical – benefit in oil production due to flooding;

diagonal – water saving (return)

The sources of injection water are different. At present, at various fields they use: water from open water bodies (rivers, lakes, water-storage basins, seas), underground water (infrabed and artesian), deep water (upper and lower deep waterbearing horizons), waste water.

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