DISPLACEMENT OF OIL FROM A FORMATION BY AQUEOUS SOLUTIONS OF SURFACTIVE SUBSTANCES

ABSTRACT

This article discusses the displacement of oil from a reservoir with aqueous solutions of surfactants. The system is used for reservoir-type oil deposits with natural water pressure or active elastic-water pressure regime. It involves drilling a deposit with production wells, placing them mainly in the purely oil part of the deposit in closed (“circular”) rows, parallel to the internal contour of the oil deposit. This includes oil fields with effective reduced anti-wear regimes, deposits with water-pressure and active elastic-water-pressure regimes. Waterflooding cannot be developed when formation permeability is low. As a result, modern regimes are developing in some fields.

AННOТAЦИЯ

В данной статье рассматривается вытеснение нефти из пласта водными растворами поверхностно-активных веществ. Система применяется для залежей нефти пластового типа с естественным водонапорным или активным упруго-водонапорным режимом. Он предполагает бурение месторождения добывающими скважинами, располагая их преимущественно в чисто нефтяной части залежи замкнутыми («кольцевыми») рядами, параллельно внутреннему контуру нефтяной залежи. Сюда входят месторождения нефти с эффективными пониженными режимами против изнашивания, залежи с водонапорным и активным упруго-водонапорным режимами. Заводнение невозможно разрабатывать при низкой проницаемости пластов. В результате на некоторых месторождениях развиваются современные режимы.

Keywords: Development system, impact methods, waterflooding, oil displacement methods, water-pressure mode, elastic-water-pressure mode, oil-bearing contour.

Ключeвыe слoвa: Систeмa рaзрaбoтки, мeтoды вoздeйствия, зaвoднeния, мeтoды вытeснeния нeфти, вoдoнaпoрный рeжим,  упругoвoдoнaпoрный рeжим, кoнтур нeфтeнoснoсти.

Introduction. Currently, when using natural types of energy, oil fields with effective natural regimes are being developed, for which artificial influence is not required, as well as fields with special geological conditions under which stimulation methods cannot bring the necessary results or cannot be developed [1].

Purpose of the work. When oil is displaced from the formation by a substance miscible with it, the problem of complete elimination of the interface between the oil and the displacing substance is radically solved, capillary forces “disappear”, oil dissolves in this substance, as a result of which it can be completely removed from the formation area, covered by the process of repression. But is it possible, with conventional flooding, to somehow reduce the surface tension at the oil-water interface in a porous medium, improve the wettability of the surfaces of rock grains with water so that the films are better washed from the rocks and, under the influence of the water flow, move to the producing wells? to us?

It turns out that such a possibility exists in principle. If you add a surfactant (surfactant) to the water injected into the reservoir, you can significantly reduce the surface tension at the oil-water contact and make the surface of the reservoir rock grains more wetted by water, i.e. increase its hydrophilicity.

The efficiency of oil displacement from formations by surfactant solutions depends on the degree of oil dispersion in the flooded area of ​​the formation, the structure of the pore space, the proportion of oil remaining in the form of films on rock grains, of the total residual oil, the nature of the physical and chemical interaction of the surfactant and rocks. collectors, etc. Finding the optimal conditions for the use of any particular surfactant or selecting the most effective surfactant for given reservoir conditions is a difficult matter [2].

Results and discussion. All physicochemical methods for developing oil fields, including oil displacement with aqueous surfactant solutions, polymer and micellar-polymer flooding, are accompanied by the phenomenon of sorption of surfactant additives to water on rock grains. This has a decisive influence on the process of extracting oil from reservoirs and the economics of physical and chemical methods for developing oil fields. Therefore, we will consider it in detail from the quantitative side, first of all, using the example of displacement of oil from a straight formation with an aqueous solution of a surfactant.

The equations for the filtration of oil and water in a formation when oil is displaced from it by an aqueous solution of a surfactant remain essentially the same as when oil is displaced from a formation by ordinary water.

If we use the model of non-piston displacement, then the continuity equations for filtered liquids and the generalized filtration law oil and water remain the same as when oil is displaced from the reservoir by ordinary water. However, the relative permeabilities change somewhat during the displacement of oil from the reservoir by an aqueous surfactant solution. Figure 1 shows the curves of relative permeabilities kB(s) and kH(s), constructed from data on oil displacement with ordinary water (solid lines) and an aqueous solution of surfactants (dashed lines). As can be seen from this figure, when using aqueous solutions of surfactants, the relative permeability curve for oil moves to the right compared to the permeability curve when oil is displaced by ordinary water [3].

Since the amount of residual oil in the formation decreases when oil is displaced by an aqueous surfactant solution, the corresponding value s*i > s*.

Figure 1. Relative permeability curves when oil is displaced by ordinary water and an aqueous surfactant solution:

Relative permeability: 1 – K_n for oil when it is displaced by ordinary water; 2 – Shi for oil when it is displaced by an aqueous solution of a surfactant: 3 – K_v for ordinary water; 4 – K_Bi for an aqueous surfactant solution

However, in order to construct a mathematical model of the process of displacement of oil by an aqueous surfactant solution, it is necessary, in addition to the equations of filtration of oil and water, to use the equation of surfactant transfer in the formation, taking into account its sorption in a porous medium.

Let us consider the change in the size of characteristic areas of a straight formation when oil is displaced from it by an aqueous solution of a surfactant (Figure 2). In region 1, water saturation is equal to s1, in region 2 – s2, in region 3 – s3, and in region 4 s = scв.

Figure 2. Scheme of oil displacement from a straight formation with an aqueous surfactant solution:

1 – region 1 (from х = 0 to х = х_cor); 2 – region 2 (х_сор < х < х*); 3 – region 3 (х* < х < х_в); 4 – region 4 (х_в < х < 1);

Increase in oil saturation in area 2 compared to area 1, i.e. The formation of an oil swell is associated with the movement of additionally displaced oil from area 1 to area 2.

When oils of different viscosities are displaced from reservoirs by ordinary water, the current and final oil recovery decreases with an increase in the ratio of oil and water viscosities. Polyacrylamide (PAA) is most often used as a dolimer injected into oil reservoirs. The molecular structure of  PAA is such that the molecules of this substance can be schematically represented in the form of long chains consisting of carbon, hydrogen and nitrogen atoms. Moving in a porous medium, polymer molecules in an aqueous solution seem to “cling” to the grains of this medium, creating additional filtration resistance and being sorbed on the grains of the rock surface [4].

Figure 3. Dependence of the filtration rates of water and dilatant liquid on grad p

Filtration of an aqueous solution of polymers occurs in such a way that with an increase in the pressure gradient, the speed of its movement increases more slowly compared to the speed of water according to Darcy's law. A liquid whose filtration rate nonlinearly depends on the pressure gradient, and, moreover, with each increment of the pressure gradient it increases by an ever smaller amount, is called dilatant.

Figure 3 shows the dependence of the filtration rate on the pressure gradient for ordinary water (curve 1) and for an aqueous polymer solution (curve 2). The formula for the law of filtration of an aqueous solution of PAA can be represented as

,                       n                         (1.1)

where μ_вп is the viscosity of the aqueous polymer solution.

However, taking into account the resistance factor R, this formula takes the form

                                            (1.2)

This representation of the law of filtration of an aqueous polymer solution arose in connection with the following circumstance. If you measure the viscosity of an aqueous solution of PAA on a viscometer, it will be μ_vp. If you pump an aqueous solution of  PAA through a porous medium, then the pressure drop in such a medium increases more significantly than what follows from Darcy’s law. Therefore, the resistance factor R is taken into account. From (1.2) it follows that

                                       (1.3)

As already mentioned, filtration of an aqueous solution of PAA is accompanied by its sorption by a porous medium. In this case, the sorption curve, if the concentration of PAA in water is significant, does not correspond to the Henry isotherm, and at low polymer concentrations, such an isotherm can be used with a certain approximation [5].

Polyacrylamide is produced in the form of a gel, solid granules or powder. Usually the following concentration of  PAA in water is used: for gel 1-5%, for solid polymer (in the form of granules or powder) 0.08-0.4%. Due to the high sorption of PAA, its concentration is brought to a value at which the viscosity of an aqueous solution of this full measure would be μ_vp = 5-6 μv (μv is the viscosity of ordinary water). In this case, the resistance factor R varies within 5-10.

It is believed that it is advisable to use an aqueous solution of PAA to displace oil from formations when its viscosity cn is equal to (10-30) - 10-3 Pa s.

As a result of the sorption of PAA by a porous medium in the process of oil displacement, a sorption front is formed, as in the case of oil displacement by aqueous solutions of surfactants. Ahead of the sorption front of polyacrylamide in the formation, water moves, almost free of it. The picture of the displacement of oil from the reservoir by an aqueous solution of PAA is similar to the picture of its displacement by a surfactant, shown in Figure 2, although the mechanisms of displacement in these two processes are completely different.

An aqueous solution of PAA can also be used to regulate the displacement process, taking advantage of this. To do this, PAA solution is pumped into highly permeable layers. This reduces the speed of movement along water, increase the injection pressure, and increase the rate of displacement of oil by water from layers with lower permeability.

Among the physicochemical methods for developing oil fields, the method of complex influence on an oil reservoir by injecting a mixture of surfactants into it is also known. Alcohols, petroleum solvents, water and aqueous solutions of  PAA. This method is called the micellar-polymer flooding method. According to this method, when using a relatively small amount of hydrocarbon (oil solvent, alcohol, sulfonates or other surfactants) at the oil-complex solution contact, they create an area of ​​complete mixing of oil with such a solution or on it sharply (up to 10-6 N/m ) reduce surface tension. As you move away from the direct contact of the oil - complex solution towards water injection wells, the proportion of water in the solution should increase until it turns into clean water [6].

When a certain ratio of water, surfactants, hydrocarbons and alcohol is reached in the solution, physico-chemically related groups of molecules - micelles - are formed. This solution is called micellar.

Conclusion. However, the effective viscosity of the micellar solution turns out to be greater than the viscosity of the initial substances that make it up. If near the injection line this solution turns into water, then it turns out that the latter, as a less viscous liquid, should displace a more viscous liquid - a micellar solution. In this case, the solution displacement coefficient will decrease. Therefore, an aqueous solution of the polymer is used to move the rim of the micellar solution through the formation. This effect on the formation is called micellar-polymer flooding.

Various compositions of micellar solutions are known. For example, solutions of the following composition are used (in%): 1) sulfonates – 6; surfactant OP-4 – 1.2; isopropyl alcohol – 1.2; kerosene – 51.6; water – 40; 2) sulfonate – 8: surfactant – 2; petroleum or composition of certain liquid hydrocarbons – 30; water – 60.

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