The invention relates to methods for creating and managing an underground storage facility for natural gas in water-bearing geological structures and, in particular, to physicochemical methods for controlling the movement of gas-water contact (GWC) in these structures during gas extraction from an underground storage facility.

The prior art describes the practice of creating and placing underground gas storage facility (UGSF) in commercial operation in water-bearing reservoir beds, comprising two stages:

- creation of an artificial gas deposit in a porous medium and annual test cycling of increasing injection and gas extraction;

- cyclic operation of UGSF immediately after the design volume of gas stored in the reservoir bed is reached.

Herewith, the design volume of the gas stored in UGSF is always equal to the sum of the active (extracted) and inactive (passive) gas volumes. The function of the inactive gas volume is to create UGSF in the reservoir bed at the end of the gas extraction from its gas-saturated zone with a certain pressure, which ensures the necessary gas extraction rate from the storage facility, to restrict the encroachment of production wells, to reduce the gas compression ratio at the gas-compressor station during the gas transfer to the consumption area and to comply with the requirements of the conservation of mineral resources.

The prior art describes engineering solutions for the creation of UGSF in water-bearing structures, which provide for the formation of the compact highly gas-saturated volume in the reservoir and ensure the achievement of stable volume ratios of active and inactive gas (author's certificate SU 190272, pubd. 16.12.1966, application US 3330352, pubd. 11.07.1967 and US 3393738, pubd. 23.07.1968).

The said solutions are based on the use of physicochemical methods for intensifying the stratal water displacement by natural gas in the reservoir bed during the gas injection in UGSF at each operating cycle throughout the entire production well stock. Foams created by different technologies from aqueous solutions of foaming surfactants, which are used in the form of rims between natural gas injected into the bed and displaced water, are used as a means for intensifying the displacement of water by natural gas. Due to the physicochemical phenomena at the phase interface in the porous medium and the anomalous nonequilibrium rheological characteristics of the foam, the ratio of water displacement by gas significantly increases (as compared with the usual displacement method). As a result, favorable conditions allow for the formation of the compact, highly gas-saturated volume of UGSF in the porous medium, ensuring balance of active and inactive volumes of the stored gas, due to the restriction of uncontrolled gas flow in the layered inhomogeneous porous medium during the injection and the prevention of progressive water intrusion into the storage during the gas extraction.

Considering the cyclical pattern of the UGSF operation, the above- mentioned methods for intensifying water displacement by gas should be used in each cycle when gas is injected into UGSF throughout the entire production well stock, the number of which in some domestic UGSF exceeds 300 units. Thus, the use of these methods is associated with significant material and operating costs, costs for the purchase of the necessary chemicals and execution of processing procedures to implement the methods of intensification.

The prior art describes methods for creating UGSF when chemical production gas, synthetic gas or any other non-hydrocarbon gases are injected into the storage facility in order to increase the technical-and-economic efficiency by reducing the cost of the inactive gas (author's certificate SU 398803, pubd. 27.09.1973), or when inactive natural gas is replaced with carbon dioxide or nitrogen until the complete extraction of natural inactive gas before the removal of UGSF (patent RU 2508445, pubd. 27.02.2014).

The prior art describes methods for creating UGSF that provide for the replacement of inactive natural gas with any other less expensive non-hydrocarbon gases that are similar to methane in their physicochemical properties, for example, nitrogen, carbon dioxide, exhaust gases of compressors, turbochargers, etc. [Levykin E.V. To the use of exhaust gas engine compressors as inactive volume filler when creating underground gas storage facilities. Ref. inform. Ser. "Transport and gas storage". - M., VNIIEgazprom, Vol. 8, 1976. - P. 29-32 .; Karvatsky A.G. C0 ₂ is an effective substitute for inactive gas of UGSF. - Gas industry, 1985, N° 7, p. 30-31], patent RU 2532278, pubd. 10.11.2014.

According to the above-mentioned methods, in the first stage of the UGSF construction, non-hydrocarbon gases are injected into the reservoir bed in the volume corresponding to the design volume of inactive gas produced by the UGSF, and then natural gas is injected until the design volumes of inactive and active gas are reached and then transferred to the second stage of the UGSF operation associated with cyclical gas extraction and injection.

The disadvantage of the use of non-hydrocarbon gases as inactive gas that are similar to methane in their physicochemical properties includes complications resulting from the diffusion mixing of hydrocarbon gas and non-hydrocarbon gases, resulting in the decrease of calorific capacity of this mixture and the increase of corrosiveness of acid components in the natural gas transferred to consumers.

Also, the prior art describes a method for creating an underground gas storage facility in geological structures filled with non-hydrocarbon gas by injection of natural gas up to the admissible limit value of rock pressure (patent RU 2458838, pubd. 20.08.2012). The thickness of the transition zone (trap) of the gas mixture of methane and carbon dioxide is up to 73 m with a thickness of the productive section of 100 m.

The major disadvantage of this method is the mixing with direct contact of stored natural gas and carbon dioxide, which creates technological and technical complications. In addition, the above-mentioned dimensions of the "trap" for water-bearing beds do not exist.

Among the prior art engineering solutions similar to that proposed in terms of technical substance and achieved results is a method for creating an underground gas storage facility in water-bearing bed of inhomogeneous and lithological structure, based on the isolation of the lower zone of the reservoir bed and providing injection and extraction of gas from the wells of the UGSF. Therewith, during gas injection, when the gas-saturated volume of UGSF is formed, the lower part of the reservoir bed is isolated by cementing, opened and the gas is withdrawn from the upper part of the reservoir bed (patent RU 2085457, priority 11.01.1995).

However, this method has the following disadvantages:

- cementing puts the abandoned section out of operation, resulting in the need to drill the formed cement plug in the following cycle of operation;

- in the process of cementing, the well is isolated in the certain section of the bottomhole formation zone, and the pressure decrease in the gas-bearing zone during gas extraction leads to the stratal water rise, flowing round this zone, which reduces the net gas thickness of UGSF bed.

The most similar method to the proposed one in technical substance and achieved results is a method (patent RU 2588500, pubd. 27.06.2016) to create an underground gas storage facility in water-bearing geological structure by drilling wells in the upper zone of the water-bearing structure, through which natural gas is injected until the gas-water interface reaches the hypsometric marks corresponding to the design volume of the storage, then the gas is sequentially injected through the drilled wells into the section of the gas-water contact of water solution of foaming surfactants, and then non-hydrocarbon gas similar to natural gas in its physical and chemical properties is injected in the section of the water-bearing structure, located below the gas-water contact, and the volumes of water solution of foaming surfactant and non-hydrocarbon gas are selected on the basis of the ratio of 1 : 1 ÷ 6, which provides for the formation of a stable bed insulating screen from the foam, acquired as a result of mechanical mixing of water solution of foaming surfactant and non-hydrocarbon gas when they are co-filtered in a porous medium, in the process of cyclical selection and injection of natural gas, the foam screen is created with a small permeability and thickness determined from the condition of screening and filtration of bottom water through it with intensive gas extraction from the storage during 90-120 days.

This method has apparent advantages over the method according to patent RU 2085457 from technological point of view, however, it has the following technical and economic disadvantages:

- the creation of an areal solid screen by injecting water solution of surfactant and gas through several wells drilled into the section of gas-water contact requires significant financial investments;

- it is very difficult to create an areal screen through vertical wells, drilled in the section of GWC, because due to the layered heterogeneity of the water-bearing bed, the surfactant solution is filtered into the zone of higher permeability, which, as a rule, does not coincide with the section of gas and water contact and does not allow controlling the screen thickness that provides an extension of dry mode (cost advantage);

- to create an areal screen, it is necessary to drill special wells in a certain order using 3, 4, 5, 7-point schemes according to a single-stage scheme or with a central relief well, the products of which (highly mineralized bed water) should be disposed.

The present invention is aimed at the technical result, which involves improving the efficiency of natural gas storage by increasing the active volume of gas and extending the mode of dry cyclic operation of UGSF at the increased rates of gas extraction.

The technical result is ensured by the fact that according to the method for creating and operate an underground gas storage facility in water-bearing geological structure including drilling of wells in the upper zone of the waterbearing structure, through which natural gas is injected until the gas-water interface reaches the hypsometric marks corresponding to the design volume of the storage, formation of low-permeable screen from the dispersed system that consists of water solution of foaming surfactants, natural or non-hydrocarbon gas similar to natural gas in its physical and chemical properties in the section of the gas-water contact, and the volumes of water solution of foaming surfactant and the above- mentioned gas are selected on the basis of the ratio of 1 : 1 ÷ 6, which provides for the formation of a stable bed insulating screen from the foam, acquired as a result of mechanical mixing of water solution of foaming surfactant and non-hydrocarbon gas when they are co-filtered in a porous medium, and the foam screen is created with a small permeability and thickness determined from the condition of screening and filtration of bottom water through it with intensive gas extraction from the storage during 90-120 days, the screen is created through a multihole well drilled in the central part of the underground gas storage facility up to the level of the design gas and water contact with at least two side horizontal legs in the section of the design gas and water contact.

Depending on the configuration of the isolated zone, the side horizontal legs of the multihole well have inclined directions and/or form angles from 45 to 180 degrees.

The method of creating low-permeable areal screen in a porous medium during the operation of an underground gas storage facility is illustrated by the figures, where:

fig. 1 - presents the calculated and experimental dependence of the front gas saturation when using the foam solution of surfactant OP- 10;

fig. 2 - shows the intermediate and final forms of the screen, created through

3 wells according to the single-step technology. 1,2,3 are the numbers of wells through which the areal screen is created. The screen is created for 35 days. The dotted circle represents the circuit of the "lithological window" with a radius of 139 m, which should be blanked off;

fig. 3 - shows the intermediate and final forms of the screen, created through

4 wells according to the single-step technology. 1,2,3 and 4 are the numbers of wells through which the areal screen is created. The screen is created for 37 days. The dotted circle represents the circuit of the "lithological window" with a radius of 139 m, which should be blanked off.

fig. 4 - shows the intermediate and final forms of the screen, created through 4 wells according to the two-step technology. 1,2,3 and 4 are the numbers of wells through which the areal screen is created. The screen is created for 37 days, the volume of stratal water pumped out from the central well is 6.66 thous. m ³ . The dotted circle represents the circuit of the "lithological window" with a radius of 139 m, which should be blanked off.

fig. 5 - shows the intermediate and final forms of the screen, created through 4-hole well. The dotted circle represents the circuit of the "lithological window" with a radius of 139 m, which should be blanked off. The screen is created for 28 days.

The proposed method is performed in the following way.

In the water-bearing geological structure, the estimated number of production wells in the upper zone of the water-bearing structure and one multihole well in the central part are drilled up to the level of the design GWC, through which 2 and more side horizontal legs are implemented at the level of the design GWC. Natural gas is injected through production wells until the GWC reaches the hypsometric marks corresponding to the design volume of the storage, and then the gas is sequentially injected to the central well with horizontal legs in the gas-water contact of water solution of foaming surfactants, further natural or non hydrocarbon gas is injected, that is similar to natural gas in its physicochemical properties, and the volumes of the water solution of foaming surfactants and gas are selected in such ratios, which form the estimated value of a stable low- permeable areal screen during mechanical mixing and collaborative filtration in reservoir bed conditions. Volumes of water solution of foaming surfactants and natural or non-hydrocarbon gas are in the ratio 1 : 1 ÷ 6.

The theoretical and estimated background for the creation of low-permeable screens are empirical dependencies of relative phase permeabilities (Karimov M.F. Operation of underground gas storage facilities, M, Nedra, 1981, p. 104), which have a correlation coefficient in the range of 0,8 < R ² < 0,99 for surfactants and their concentrations given in Table 1, when using the equations:

k ^* = 1; 0 < s < 0,l;

where s is the gas saturation of the porous medium, dimensionless value;

c is the concentration of foaming surfactant, % wt;

k 1 is the relative permeability of the porous medium to liquid, dimensionless value; k _g ^* is the relative permeability of the porous medium to gas, dimensionless value.

These dependencies are used in further calculations on a computer.

As foaming surfactants it is possible to use various surfactants, examples of which are listed in the below-mentioned table 1. It is more preferable to use a solution of a synergistic surfactant composition (a foam solution) consisting of a main foam non-ionic surfactant and an auxiliary anionic surfactant in stratal water. For example, a composition consisting of the main foam non-ionic surfactant in the form of nonylphenol ethoxylate of the grade OP-7 or OP- 10, or sodium salts of carboxymethylated ethoxylated isophenols Sinterol AFM-12 and auxiliary anionic surfactant in the form of spent sulphite liquor (KSSL or SSL), has a synergistic effect due to better adsorption of KSSL or SSL on the rock surface (Hydrodynamics and filtering of single-phase and multi-phase flows, Proceedings of the MIU and GP n.a. Gubkin I.M., M., Nedra, 1972, p. 76). Herewith, the losses of the main surfactant decrease to 60% wt. Preferably, the specified surfactants (OP- 10: KSSL or SSL) in mass ratios from 0,6: 1 to 1 :1 are used in the synergistic composition. When preparing a solution, it is important to use stratal water from the horizon where the screen should be created. It ensures maximum retention of strength and structure of the reservoir bed. The concentration of the synergistic composition in the stratal water is 0.8 % -1.0 % wt.

To ensure a stable screen thickness, the amount of injected natural or nonhydrocarbon gas into each well for foam formation under reservoir conditions is preferably from 1 to 6 volumes of the used foam surfactant solution. Determination of surfactant concentration in the solution of foam surfactants to create an effective screen is implemented considering the chemical composition of the stratal water, sorption properties of the porous medium and the type of surfactant. Recommended surfactants to create screens, depending on the salinity of the stratal water are presented in table 1.

The experimental values of front gas saturation and values of front gas saturation when substituting solutions of surfactant with gas in a porous medium calculated using formulas (1) and (2) with surfactant OP- 10, are presented in figure 1, where the following designations are used: M = 1% is substitution of surfactant solutions with gas in the stratal sodium bicarbonate water with mineralization of 1 % wt; M = 15 % is substitution of surfactant solutions with gas in the stratal calcium chloride water with mineralization of 15 % wt.

According to the above-mentioned data, the foam formation in a porous medium of non-equilibrium dispersed systems increases gas saturation at the displacement front to 0.7-0.8. Herewith, the relative permeability for water decreases as well. Thus, non-equilibrium dispersed systems can be effectively used both for screening the gas volume from the cross flow beyond a certain isohypse and for screening water intrusion into the gas-saturated volume of the UGSF.

The lateral dimensions of the insulating low-permeable screen are determined by the following methods.

The isohypse, within which the design volume of UGSF is provided, is determined by geological studies. The area bounded by this isohypse is determined by a structural map using a computer method or is approximated by a polygon or circle, oval, ellipse, or divided into separate sections, the areas of which are also approximated by a part of circle, oval, ellipse, polygon, or a combination of such figures, the total area of which is the required area of the design gas-water contact.

The volume of the required screen is calculated by multiplying the determined area and the estimated screen thickness. The volume of the low- permeable screen calculated in this way should consist of one part of the foam surfactant solution and 1-6 parts of gas under reservoir bed conditions. The period of creating the screen is determined by the preparatory works, the injection of solution and the injection of gas.

The key parameter of the screen that determines the effectiveness of its operation is the thickness of the screen. The screen thickness is determined based on the fact that a particle of the bottom water should be filtered through the screen within the extraction cycle during the time 9, which is technologically justified by the condition of reliable isolation of the bottom water intrusion into the gas-bearing section during cyclical operation of UGSF. Depending on the geological and technological features of UGSF, the time can amount to 90 - 120 days.

The thickness of the screen (the required vertical cross dimension l _w , for reliable protection of the gas volume from the the bottom water intrusion is determined from the equation: where:

l _w is the screen thickness, m;

9 is the required time of stratal water screening, s;

P _j and P ₂ are pressure values at the borders of the screen, MPa;

K = k ^* · k is relative permeability coefficient to water, m ² ;

k is absolute permeability of the reservoir bed in the isolated zone, m ; m _M> is water viscosity under the reservoir bed conditions, mPa _* s;

m is porosity coefficient of the reservoir bed in the isolated zone.

According to the formula, the thickness of the screen depends on the parameters of the reservoir bed, i.e. permeability k and porosity m.

The screen thickness is determined taking up the required screening time of the intrusive stratal water.

An example of the implementation of the proposed method is presented below. There is UGSF in water-bearing structure with a total stored gas volume of 3 billion m and with an active volume of 1.5 billion m . In the bottom part of the storage there is a circular lithological window with a radius of 139 m, which needs to be blanked off with a horizontal low-permeable screen. The screening period lasts 90-120 days. The thickness (cross vertical dimension) of the screen, the surfactant composition, the volume of the solution and the mass of surfactant necessary to create the screen are determined. The period of gas extraction before the water intrusion with and without the screen is determined by computer simulation.

Initial data:

The bed depth is H=1000 m;

Chlorine-calcium stratal water according to Sulin with total mineralization M=150 g/i;

Reservoir pressure varies in the range of 8-10 MPa, i.e. maximum screen load is 2 MPa;

The thickness of the gas-bearing part of the reservoir bed is h = 20 m;

Average absolute permeability is k=0.65* 10 m ;

Porosity is m =0,20;

Gas viscosity is 0.014 mPa*s;

Stratal water viscosity is 1.8 mPa*s.

The method is performed in the following order. 1 . According to Table 1, the main foam surfactant is chosen, for example, OP-10SNCC, a solution with a preferred concentration of 0.967 % is prepared and the synergistic component of the surfactant is added, i.e. 0.3% KSSL or SSL.

2. The front saturation s is determined by the curves in fig.1 , depending on the determined concentration (0,967%) s=0,7.

3. According to the formulas (1) and (2) the relative permeability to gas and liquid is determined at s = 0,7: k _g ^' = 0, 0001; k] = 0, 003 , consequently ^=0,0001 · 0,65 · 10 - ¹² m , _/ =0,003 · 0,65 · 10 ^‘12 m = 0,00195 · 10 ^'12 m ² .

4. The design thickness (vertical cross dimension) of the screen l _w is determined.

The minimum cross vertical dimension of the screen is determined by the condition of passage of the bottom water particles (for the period of intensive screening, for example, 90 days) when gas is taken from UGSF. The value l _w is determined from the equation (3):

90 · 86400 · (10 · 10 ⁶ - 8 · 10 ⁶ ) · 0, 003 · 0, 65 · 1 O ¹²

9,2 m 1,8 · 10 ^-3 - 0,2 where P _j and P2 are pressure values at the borders of the screen, MPa;

k _w is relative permeability coefficient to water, m ;

m is porosity coefficient;

m _w is stratal water viscosity under the reservoir bed conditions, mPa*s.

5. The area of UGSF is calculated by one of the above-mentioned methods.

6. The required volume of the surfactant solution to blank off the lithological window is determined based on the creation of a screen with the thickness of 9.2 m in the lithological window:

Area of a circle S= pt ² =3,14· 139 ² =59832 m ² .

The volume of the circular cylinder saturated with foam,

F=7U-r ² 7 _w =3,14- 138 ² -9, 2=550, 46· 10 ³ m ³ .

The volume of foam in the pore volume of the circular cylinder 10, M0 ³ m ³ . The amount of solution of the foam composition in the foam, consisting of one part of the solution and 4 parts of gas under reservoir bed conditions: 22, 326 0 ^J m ^s .

Volume of natural or non-hydrocarbon gas in the screen under the conditions of the reservoir bed:

The mass of OP- 10 in the solution according to table 1 m 0,2 9.7 kg

M, OP- 10 = n - r ·/„— C ₀ = 3,14 - 139 -9, 2 = 2l5,9 - l0 ³ Kg.

m

The weight of the synergetic component of SSL or KSSL at the rate of 0.3%: 2 3 Kg

66, 98 10 ³ Kg.

m

7. The parameters of UGSF are calculated, when“lithological window” is blanked off, by computer simulation in various ways:

- on the single-stage technology through one well, existing or drilled in the central part of the isolated zone;

- on the single-stage technology through three wells, existing or drilled in the isolated zone at the comers of the equilateral triangle, the steps of creating the screen are shown in Fig.2a. 2b.;

- on the single-stage technology through four wells, one of which is central, when all the wells are injected with a synergistic composition and gas solution; visualized process of creating a screen using this technology is shown in fig.3a and 3b;

- on the two-stage technology through four wells, one of which is central, when the stratal water is pumped out of the central well before the appearance of the markers of neighboring injection wells in it, indicating the completion of the first stage and the beginning of the second stage, the visualized results are shown in fig.4a and 4b ;

- through a central multihole well with cross-edged completion, the length of horizontal parts is a equal to the radius of the isolated circular“lithological window”;

this visualized process of a screen creation is shown in fig. 5a and 5b.

The formation of a water insulating screen in the section of GWC provides for the limitation of the bottom water intrusion into the storage during cyclical operation of the UGSF. It ensures the gas-dynamic stability of the natural gas volume to be stored in water-bearing structure and, as a result, leads to reduction in expenses for forming and maintaining the optimal volume of inactive gas required for the operation of the UGSF in design modes. Considering the significant area of GWC at the UGSF and the required volume of gas to create a screen by foam formation in a porous medium that remains in the reservoir, it is proposed to use low-value non-hydrocarbon gases similar to natural gas in their physical and chemical properties (nitrogen, carbon dioxide, exhaust gases from gas compressors, turbochargers, etc.) as a gaseous agent. If there is no such an opportunity, it is possible to use natural gas, which slightly increases the cost of the project.

In order to determine the effectiveness of the proposed method for the operation of UGSF, the problem of stratal water intrusion to production wells during the gas extraction from UGSF has been considered.

Unsteady liquid and gas filtration, described by second-order differential equations in partial derivatives of parabolic type, due to the difficult consideration of the geological structure of the object, does not allow to obtain accurate analytical solutions suitable for engineering calculations. In this regard, the problem has been considered computationally using the MatLab interactive package. Various screen installation options have been simulated with blanking off coefficient h = ⁵ screen _as ^ imension _{ratj0 Q} f effective screen area to the

S ht.wmdow

“lithological window” (Table 2).

The simulation has shown that installation of a screen with a permeability of 0.001 is equivalent to installation of a leak-proof screen. However, the installation of a leak-proof screen is problematic under the field conditions of real storage facilities. In this regard, the use of the screen with a permeability of 0.01 has been studied, which creates a triple reliability margin.

The results of the interactive simulation of the GWC movement using the MatLab package are listed in Table 3.

According to the comparison of the results of the interactive simulation of the screen installation in various ways on the parameters of UGSF operation, it is preferrable to create an areal screen through a multihole well, because using this method

- the increased accuracy of screen installation can be achieved, which is the basis to obtain the expected effect;

- the shortest time to create a screen compared to other methods;

- minimum consumption of reagents to create a screen;

- relative cheapness compared to other ways to create a screen;

- the maximum effect during the gas extraction, due to the maximum accurate blanking off the "lithological window".

Thus, the described method of creating and operating an underground gas storage facility in water-bearing geological structure makes it possible to limit the uncontrolled water intrusion of UGSF during its cyclic operation and significantly increase the period of dry gas extraction and, therefore, the active volume of UGSF. Method for creating and operating an underground gas storage facility in a water-bearing geologic structure

Table 1

Table 2

Method for creating and operating an underground gas storage facility in a water-bearing geologic structure

Table 3