RU2343274C1 - Method of evaluation of space distribution of oil saturated regions in watered beds - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: invention refers to development and operation of oil deposits under water drive mode, particularly to methods of increasing oil withdrawal from bed. Method includes charting of deposit maps by means of isochrones of flooding. According to invention by means of hydrodynamic investigations of water flooded wells there is obtained a dependence of production flooding on yield of extracted fluid. Oil saturated regions in water flooded, non-uniform by permeability beds are evaluated by the character of change of the above said dependence and by the time of existence of positive effect of decreased production water flooding. Monotonous decrease of production flooding at increase of liquid yield and its constancy at the set mode of operation testify of presence of patches of regions with residual oil saturation washed up with water during operation. Monotonous increase of production water flooding at increase of liquid yield testify of presence of regions stretched for tens and hundreds of meters and distributed uniformly along the whole bed including regions with primary oil saturation not participating in filtration. Also variations of flooding values in time testify of non-uniform in stretching distribution of oil-saturated regions. Abrupt decrease of flooding of production at increase of liquid yield, achieving maximum water flooding and following abrupt increase of flooding tell of presence of space stretched regions with intermediary oil saturation - they are transitive regions of hydrodynamic capillary displacement.

EFFECT: increased oil withdrawal from bed.

1 tbl, 1 dwg

Description

The invention relates to the field of development and operation of oil fields under water pressure conditions, in particular to methods for increasing oil recovery.

In the oil industry, a method for developing fields is known, which consists in setting the analytical dependences of all the main technological and economic indicators of the main parameters of oil reservoirs, fluids filtered through them, the development systems used and the dynamics of technical measures [1]. The economic and mathematical model is built on the basis of taking into account the reserves of the field, the parameters of the physical and geological properties of the rocks and the necessary set of measures carried out both at the beginning and at the subsequent stages of the formation operation.

The disadvantages of the known method [1] include the fact that the applied methods based on analytical calculations use the parameters of the reservoir-well-pump system with very large spatial errors. The lack of real analytical relationships between the main parameters of production wells does not allow an objective assessment of the effectiveness of measures taken to increase oil recovery, reduce water cut and reduce non-recoverable reserves.

Of the known technical solutions closest to the claimed method, which is also the base, is the isochron method of flooding deposits [2, p. 37-61]. The method of irrigation isochron is to build maps of the advancement of the internal contour of oil along the strike of the reservoir area using isochron - lines for the simultaneous appearance of water in the production of producing wells. The isochrones of flooding the area of the deposit, depending on the possible detail, are carried out after 0.5-1.5 years of operation. The balance method for the accumulated oil production and its initial reserves is determined at the dates of the fixed watering front by the current oil recovery and waterflood coverage rates for the reservoir as a whole. Based on the dynamics of these indicators for the previous development period, a forecast is made for the subsequent development period.

The known method [2] has the following main disadvantages. The waterflooding of productive formations by the method of isochron flooding is estimated approximately due to the limited number of determinations of the current position of the oil-water contact along strike and the complex shape of this contact in the volume of the reservoir when water is introduced into the heterogeneous formation [2, p. 47]. This method is suitable only for reservoir-type deposits when the frontal advancement of water occurs. For deposits of massive type, with vertical introduction of water, the development of the method is required [2, p. 61].

The aim of the proposed method is to increase oil recovery.

This goal is achieved by the fact that with the help of hydrodynamic studies of wells:

- the spatial distribution of oil-saturated areas in water-flooded formations is determined;

- the reasons leading to the formation of oil-saturated areas in water-flooded formations are clarified;

- measures for stimulation of the reservoir are proposed that increase oil recovery.

In the exploitation of oil fields developed under pressure, water usually does not completely fill the pore space of the reservoir, which was originally occupied by oil. This leads to a joint filtration of the water that has invaded the formation and the oil gradually washed out in it. Features of filtration of immiscible fluids depend on geological heterogeneities of the formation (zoning, layering, discontinuity, etc.) and technological indicators of well operation. To identify the relationship of geological and technological indicators on the nature of waterflooding, the hydrodynamic method of researching wells at stationary operating conditions or the method of steady-state selection was used [3, pp. 96-116]. During the research, the operation modes of the production pump were changed without stopping the downhole equipment using a variable frequency drive (VFD). The use of VFD allowed to change the productivity of the production pump 1.4 times downward and 1.6 times upward from the nominal capacity of the production pump [4]. The use of VFD allows the measurement of parameters over a wide range of changes in the flow rate of the produced fluid.

Field research was carried out in 2000-2003 at the existing fields of NK “Lukoil” (Kogalym, Western Siberia), operated more than 20 years. In the experiments, 28 wells were involved, the water cut of which exceeded 80%. By changing the frequency of the supply current, the operating modes of the production pump were scanned in the range of stable operation of the reservoir-well-pump system. At the same time, in each mode, the parameters were recorded: flow rate of the produced fluid Q _w ; its water content In; electric power W supplied to the production pump; dynamic level N _d ; overpressure at the wellhead R _y . When switching from one operating mode of the production pump to another, the parameters were measured no earlier than 3-5 hours, with the obligatory establishment of the well flow rate and a stable dynamic level. The establishment of a stable dynamic level testified to the balance of fluid flow from the reservoir to the well and its selection using a production pump. Stable and strictly fixed power supplied to the production pump, as well as the possibility of changing it using the VFD without stopping the process, revealed the dependence of the measured parameters of production wells on the production rate of the produced fluid.

Well production was measured using an automated group metering device (AGZU) of the "Sputnik AM-40" type with a relative measurement error of 4% [5]. The water cut of the produced fluid was determined using the device described in [6], and in the laboratory by the chemical-analytical method using samples taken at the wellhead. The relative measurement error of water cut by the indicated methods did not exceed 8%. The electric power supplied to the production pumps was measured with a relative error not exceeding 2%. The dynamic level was measured using the SUDOS-Avtomat sonar with a relative measurement error not exceeding 1%. When analyzing the experimental data, the initial information was the dependence of the measured parameters on the frequency or electric power supplied to the production pump. Examples of constructing these dependencies in graphical form are given in [7, 8], in which the optimal operational parameters of the production pump were determined from these dependencies.

Table 1 presents the operational parameters of 11 of the 28 wells tested. A smaller number of wells was taken for reasons of visual presentation of the experimental material in graphical form. Designations adopted in table 1: B - water content of the extracted products; Q _W - flow rate; Q _n - oil flow rate. From the point of view of increasing oil recovery, the results of hydrodynamic studies are conveniently interpreted graphically in the form of the dependence of the water cut on the production rate of the produced fluid B = f (Q _W ). This dependence has quantitative information characterizing the change in the proportion of water in the product depending on the flow rate of the produced fluid. To build this dependence, it is enough to know the change in two parameters: the liquid flow rate Q _w and the corresponding water cut value B. The figure shows the dependences B = f (Q _w ) for 11 wells, the operational parameters of which are contained in table 1.

The experimental data presented in the drawing clearly demonstrate the complex dependence of the water cut on the flow rate of the liquid. For analysis, all the studied wells, the data of which are presented in the drawing, can be divided into three groups.

The first group includes wells 2422, 4154, 8141. These wells are characterized by a monotonic decrease in water cut with an increase in the production rate of the produced fluid.

The second group includes wells 2244, 5528, 2367, for which an increase in the production rate of the produced fluid leads to a monotonic increase in water cut.

The third group includes wells 2248 and 1790, for which with an increase in production a sharp decrease in water cut is observed, reaching a minimum value of water cut, after which there is a sharp increase in water cut. Wells 1641 and 2246 should also be assigned to the third group of wells. For these two wells, when expanding the range of changes in the fluid flow rate, a minimum value of water cut should be observed, but this requires the replacement of the production pump.

Only one well 2227 of 11 wells has features characterizing the wells of the first and second groups. With an increase in fluid flow rate, at first a monotonic increase in water cut is observed, and then a monotonic decrease in water cut.

Such a complex expression of the dependence of B = f (Q _w) connected with the processes occurring in the pore space of the formation and resulting in a change in permeability. The main forces that prevent the joint filtration of immiscible liquids in the pore space are: surface (capillary) forces, viscous drag forces (hydrodynamic) and gravitational forces that act together [9, p. 9]. Phase permeabilities depend on the saturation of the formation with the displacing phase (water). In real conditions, in the reservoir at a late stage of field development, areas distinguished by the degree of reservoir depletion are distinguished:

- with residual oil saturation, i.e. areas washed with water during operation;

- with initial oil saturation, these are areas that did not participate in the filtration;

- transitional regions of hydrodynamic capillary displacement with intermediate oil saturation.

The formation of these areas during waterflooding is associated with macroinhomogeneity (layering) and heterogeneity of the internal structure of the pore medium (microinhomogeneity).

For the first group of wells, a monotonic decrease in the water cut of the product with an increase in the flow rate is due to the residual oil saturation of the drained areas of the formation. Drained areas of the reservoir contain pillars of small sizes (from tens of centimeters to several meters) with intermediate saturation, formed under the action of capillary forces in an inhomogeneous pore medium. With increasing depression, the role of hydrodynamic forces increases, which leads to a gradual leaching of oil from the pillars.

For the second group of wells, a monotonic increase in water cut of the product with an increase in production rate is due to the fact that the drained areas of the formation are characterized by layered heterogeneity and include interlayers with initial oil saturation. When oil is displaced from layer-by-layer heterogeneous formations, water occupies more permeable layers, and the phase distribution is determined by viscous friction forces, i.e. hydrodynamic forces [9, p. 9]. Filling highly permeable interlayers, water begins to be absorbed into low permeable interlayers, displacing oil from them. The smaller the depression, the greater the distance that water penetrates into low-permeable interlayers and the more oil displaces them into highly permeable interlayers. Therefore, with an increase in depression, the role of surface forces decreases, and the role of hydrodynamic forces increases, which leads to an increase in water cut in production.

The change in water cut in well production 2227 (see the drawing) indicates a different nature of the spatial distribution of the residual oil saturation of the drained areas of the formation. The depending portion = f (Q _w) with monotonically increasing water content indicates a layered formation heterogeneity drained areas. The plot of dependence B = f (Q _g ) with a monotonously decreasing water cut indicates the presence of pillars of small size with intermediate saturation.

The nature of the change in water cut in the wells of the third group indicates the presence of spatially extended areas with initial oil saturation in the drained area of the formation. For example, a well has a spatially formed watering cone blocking the formation zones with initial oil saturation. Another example is gas blocking of oil-saturated areas of the formation during intensive fluid withdrawal, when the pressure in the bottom-hole zone of the formation becomes lower than the saturation pressure. For the plot of dependence B = f (Q _g ) with decreasing water cut, a predominance of surface forces over hydrodynamic is characteristic. At low flow rates, the pressure in the drained area of the formation is high, which creates favorable conditions for the introduction of water into oil-saturated areas and the displacement of oil from them into water-saturated areas. To move oil displaced from oil-saturated areas to production wells, certain values of depression are necessary. This explains the decrease in water cut with increasing fluid flow rate. The increasing portion of the dependence B = f (Q _g ) is characterized by a decrease in the role of surface forces and an increase in the role of hydrodynamic forces. The minimum water cut value for the wells of the third group corresponds to the equality of the influence of surface and hydrodynamic forces.

Well studies to determine the dependence of water cut on the production rate of the produced fluid (see drawing) do not allow us to fully evaluate the volumes of oil-saturated areas. When switching wells to the regime with a minimum value of water cut, the expected effect may be short-lived. The lifetime of a positive effect depends on the spatial dimensions of the oil-saturated areas.

The location and amount of residual oil in the reservoirs depends on the predominant wettability of the rock with water or oil. When immiscible liquids (water and oil) move in a porous medium, each of the phases moves along its own system of pore channels, maintaining continuity [9, pp. 8-10]. In general, the phase distribution is determined by surface forces, viscous resistance forces, and gravity. At a certain flow rate of the displacing water, capillary equilibrium is established, and oil recovery tends to a certain limit. Each of the phases (water and oil) flows in the pore space of the reservoir occupied by it under the influence of “its own” pressure [10, p.118-127]:

u ₁ = - (k · f ₁ / µ ₁ ) · gradP ₁ , u ₂ = - (k · f ₂ / µ ₂ ) · gradP ₂ ,

where index 1 is for water, index 2 is for oil, k is absolute permeability, f ₁ and f ₂ are relative phase permeabilities, μ ₁ and μ ₂ are phase viscosities. The pressure difference in the phases (P ₁ -P ₂ ) is equal to the capillary pressure P _with :

,

,

where α is the interfacial tension; m is the porosity; J (S) is the Leverett function characterizing the dependence of capillary pressure on water saturation. Thus, the phase distribution in a porous medium is determined by two main parameters: capillary forces and saturation of the pore space with water.

Changing the operating mode of the production pump leads to a change in pressure and the propagation of pressure disturbances in the porous medium. At the interface between the non-flooded low-permeability and water-flooded high-permeability areas of the formation, a pressure gradient arises normal to the contact surface, as well as capillary forces aimed at equalizing oil and water saturation in adjacent areas of the formation. This leads to the flow of liquids. Water wetting the rock is absorbed through the pores of a small cross section in the reservoir area, saturated with oil, and displaces the oil through the pores of a large cross section. In [10, p.125], estimation formulas are given for determining the characteristic time of unsteady pressure redistribution t ₁ = L ² / æ and the characteristic displacement time t ₂ = L / u, where æ is the piezoelectric conductivity coefficient, L is the characteristic size of the oil-saturated region, u - average filtration rate. Typically, the filtration rate in the pore medium is 10 ⁻³ cm / s and the piezoelectric conductivity coefficient is 10 ⁴ cm ² / s. Therefore, the characteristic times of pressure redistribution t ₁ and oil displacement from oil-saturated regions t ₂ depend on the size of oil-saturated regions L. For example, if the length of the oil-saturated region of the reservoir is 500 m, then the characteristic time of unsteady pressure redistribution will be

t ₁ = L ² / æ = (5 · 10 ⁴ cm) ² / (10 ⁴ cm ² / s) = 2.5 · 10 ⁵ s≈3 days,

and the typical displacement time is

t ₂ = L / u = (5 · 10 ⁴ cm) / (10 ^-3 cm / s) = 5 · 10 ⁷ s = 19.3 months ~ 1.6 years.

The characteristic displacement time determines the duration of the well operating mode with lower water cut values.

To determine the duration of the existence of a positive effect using VFD, a well operating mode is established with a minimum water cut value and parameters are monitored: fluid flow rate, water cut, dynamic level. Moreover, the fluid flow rate using VFD is kept constant. If over time the water cut of the product does not change, then the well is operated in this mode.

For the wells of the first group, the constant water cut of the product at the established well operation mode means that the drained area of the reservoir contains small pillars with intermediate saturation, distributed uniformly throughout the volume of the drained area of the formation. For wells of the second group, the invariability of water cut in the established mode indicates the presence in the drainage area of the formation of extended along the strike streaks (tens and hundreds of meters) with initial oil saturation, distributed uniformly throughout the volume of the drained area of the formation. If, over time, fluctuations in the water cut value are observed, then this indicates an uneven distribution of pillars and interlayers in the drained area of the formation along the strike. By the duration of the change in the water cut of the product, one can judge the spatial distribution of the pillars and interlayers in the drained area of the formation relative to the producing well. If, over time, the effect of reducing water cut disappears, then non-stationary oil recovery technologies should be used [11].

For wells of the third group, the output of wells using VFD to the regime with a minimum water cut value corresponds to a decrease in depression and an increase in pressure in the drained area of the reservoir. In this case, equality of influence of surface and hydrodynamic forces is achieved. The surface (capillary) forces are directed normal to the surface separating the oil-saturated and water-saturated regions, and contribute to the introduction of water through the pores of a small section and the displacement of oil through the pores of a large section. Hydrodynamic forces proportional to the depression are directed in a horizontal plane along the radius to the producing wells. The simultaneous action of these forces leads not only to the displacement of oil from oil-saturated areas to lower water-saturated areas, but also in the horizontal direction to production wells. This leads to the destruction of the cone of flooding, blocking the formation zone with initial oil saturation. The increase in pressure in the bottomhole formation zone leads to the destruction of gas blocking of oil-saturated areas of the formation. If during the withdrawal of wells of the third group to the minimum water cut value, the effect of reducing the water cut of the product is short-lived, then it is necessary to reduce the fluid flow rate to the minimum possible using VFD. An increase in pressure in the bottom-hole zone of the formation leads to a decrease in the influence of hydrodynamic forces and an increase in the influence of surface forces, which contributes to a more effective destruction of the cone of flooding or gas blocking. Then the fluid flow rate is brought to the regime with a minimum value of water cut of the product and keep it constant during well operation. For wells in which the residual oil reserves occupy spatially extended areas of the reservoir with the initial oil saturation, the typical displacement time can be years.

It is a hydrodynamic study of waterlogged wells, which consists in establishing the dependence of water cut on production rate, determining the nature of the change in water cut on production rate and the time of existence of the positive effect of reducing water cut in a producing well, which allows to establish the spatial distribution of oil-saturated areas in water-filled formations, is the essence of this invention .

Thus, the claimed method meets the criteria of the invention of "novelty." When studying other technical solutions in this technical field, the features that distinguish the claimed invention from the prototype were not identified, and therefore they provide the claimed technical solution with the criterion of "significant differences".

Using the proposed method for determining the spatial distribution of oil-saturated areas in water-flooded formations allows, in comparison with existing ones: to establish the spatial distribution of oil-saturated zones in drained areas of the formation; to find out the reasons leading to the formation of oil-saturated areas in heterogeneous permeability flooded formations; to choose measures on the impact on the reservoir, increasing oil recovery. It should be noted that well research, bringing wells to a predetermined mode and maintaining a given mode of well operation is carried out using a variable frequency drive without stopping the process.

Information sources

1. Lysenko V.D. Optimization of oil field development. - M .: Nedra, 1991 .-- 296 p.

2. Surguchev M.L. Methods of monitoring and regulating the process of developing oil fields. - M .: Nedra, 1968 .-- 301 p.

3. Mishchenko I.T. Downhole oil production. - M .: Oil and gas, 2003 .-- 816 p.

4. Maksimov V.P., Semenchenko P.G., Khanzhin V.G. Adjustable drive control of submersible pumps. - M.: Overview of VNIIOENG. Ser. "Machines and oil equipment", 1981. - 32 p.

5. Belov V.G., Soloviev V.Ya. Modernization of the Sputnik AM-40 AGZU and measurement methods. // Oil industry, 2000, No. 10, p.118-121.

6. Belov V.G., Ivanov V.A., Soloviev V.Ya. Measurement of water cut in oil well products. // Oil industry, 2003, No. 4, p.111-113.

7. Patent RU No. 2240422. Bull. Number 32. 11/20/2004. A way to optimize the process of extracting oil from the reservoir. / Alexandrov G.F., Soloviev V.Ya., Nazarov A.E. and etc.

8. Belov V.G., Ivanov V.A., Musaev H.C., Soloviev V.Ya. Determination of the optimal operational parameters of the oil reservoir-well-pump system. // Oil industry, 2004, No. 7, pp. 100-102.

9. Kuznetsov G.S., Leontiev E.I., Rezvanov R.A. Geophysical methods of control and development of oil and gas fields. - M .: Nedra, 1991 .-- 223 p.

10. Barenblatt G.I., Entov V.M., Ryzhik V.M. The movement of liquids and gases in reservoirs. - M .: Nedra, 1984. - 211 p.

11. Vladimirov I.V. Non-stationary oil production technologies. - M.: VNIIOENG OJSC, 2004. - 216 p.

Table 1 No. Well Pump type AT, % Q _W , m ³ / day Q _n , m ³ / day one 2422 ESP-80 92 90 7 2 2227 ESP-80 89 101 eleven 3 8141 ESP-50 one hundred 85 0 four 4154 ESP-80 99 104 one 5 1790 ESP-50 87 61 eleven 6 1641 ESP-30 one hundred 36 0 7 2246 ESP-125 89 122 13 8 5528 TD-1200 85 168 25 9 2367 TD-1750 90 221 22 10 2248 ESP-80 90 116 eleven eleven 2244 ESP-125 83 97 16

Claims (1)

A method for determining the spatial distribution of oil-saturated areas in water-filled formations, including constructing reservoir maps using isochron irrigation, characterized in that, using hydrodynamic studies of waterlogged wells, the water cut is dependent on the production rate of the produced fluid, the nature of the change of which and the lifetime of the positive decrease effect water cuts characterize oil-saturated areas in permeable non-uniform waterfloods five layers, namely: a monotonic decrease with increasing water-cut production fluid flow rate and its invariability of the operation mode set indicates the presence of areas of small dimensions with a residual oil saturation, washed with water during operation; a monotonous increase in water cut of the product with an increase in fluid flow rate indicates the presence of tens and hundreds of meters extended along strike, and areas distributed uniformly throughout the reservoir, including areas with initial oil saturation, not taking part in the filtration, while fluctuations in water cut values over time indicate uneven along the strike distribution of oil-saturated areas; a sharp decrease in the water cut of the product with an increase in the liquid flow rate, the achievement of the minimum water cut and the subsequent sharp increase in the water cut indicate the presence of spatially extended regions with intermediate oil saturation - transitional regions of hydrodynamic capillary displacement.

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