Method and system for underground gas injection

Description

Domaine technique

[0001] The present invention relates to the field of gas injection into an underground formation. Gas injection into the subsurface is a widespread operation in the petroleum industry, particularly within the framework of Enhanced Oil Recovery (EOR) techniques, through the injection of natural gas or CO2 into oil reservoirs to improve oil recovery. There are currently numerous CO2 reinjection projects for EOR worldwide. Alternating water and gas (WAG) injection is also implemented for EOR.

[0002] Injecting CO2 underground is also used in CO2 sequestration projects to limit the greenhouse effect and reduce its impact on climate change. There are currently four industrial-scale CCS (Carbon Capture and Sequestration) projects operating worldwide, capable of storing 5 million tons of CO2 per year.

[0003] The gas can be injected alone with a wellhead pressure corresponding to a gas pressure gradient throughout the wellbore. This solution requires significant gas compression equipment, resulting in high energy consumption and investment costs. Technique antérieure

[0004] Technologies for the simultaneous injection of gas and liquid into the same well, based on the complete dissolution of the gas in the liquid within the well, with gas injection performed via a central injection boom, are known, for example, through the Carbfix solution (a collaborative research project), tested and approved at the Hellisheidi site in Iceland, or through US patent application 4,632,601 A. To achieve complete dissolution of the gas in the liquid, a low gas-to-liquid ratio is required. Consequently, these technologies limit the gas injection rate in the well.

[0005] Other devices enabling the simultaneous reinjection of a gas and a liquid in a two-phase manner have been proposed. Magma Energy Italia, GeothermEx, and Schlumberger Software, along with patent application JP 2012 207 605 A, propose an injection rod centered in the wellbore. The use of this small-diameter rod precludes intervention using a "smooth cable," a method used for maintenance operations (e.g., valve removal, plug installation, pressure and / or temperature measurements). Patent application JPH 09 177 507 A uses a specific device to mix the liquid and gas in the wellbore. These simultaneous two-phase injection technologies use only a single injection point, complicating the two-phase flow initiation process and / or limiting the two-phase flow to a short length of the wellbore.

[0006] US patent application 5,022,787 A proposes mixing gas and water directly at the wellhead: gas injection therefore cannot be carried out in a stabilized water column. This solution does not allow for easy mixing of these two fluids, nor does it guarantee two-phase flow in the well, as initiating flow can be difficult in the case of reservoirs with low pressure and high injectivity.

[0007] We also know of US patent applications 2017 / 145 800 A1, JP 2008 307 483 A and RU 2 512 156 C1 which relate to methods of injecting a gas and a liquid into a well and US3 194 175 A which relates to a valve.

[0008] Among these systems, some require high gas pressures at the wellhead, resulting in high installation costs.

[0009] Furthermore, a drawback of these systems concerns the potentially significant energy consumption, particularly when the system is starting up.

[0010] Furthermore, these systems lack flexibility with respect to the variability of operating conditions and the actual conditions of injections at the site.

[0011] To overcome these drawbacks, the present invention relates to a method and device for injecting a gas into an underground formation by means of a well, which comprises at least two coaxial cylinders defining a central volume and an annular volume, and a liquid / gas mixture outlet. The wellhead, located at the top, above ground level, includes a liquid inlet and a gas inlet. The coaxial cylinders serve to circulate liquid and gas, each from a separate inlet, to a common liquid / gas mixture outlet located at the bottom of the well. Several communication ports (injection points) between the coaxial cylinders, equipped or not with opening / closing means, allow the injection of one of the fluids from one of the two volumes into the other, in order to create a mixing zone. The method comprises the following steps: a) Injection of liquid from the liquid inlet and gas from the gas inlet separately, one into the central volume and the other into the annular volume; b) Mixing of liquid and gas. To achieve this, one of these two fluids flows radially through the central cylinder to join the fluid located on the other side of the central cylinder. According to the invention, the central cylinder extends substantially along the entire length of the well. Mixing occurs through injection points distributed longitudinally along the cylinder, which can be opened or closed; c) Discharge of the resulting liquid / gas mixture at the gas / liquid mixing outlet for transfer into the underground formation.

[0012] This process allows for a high degree of modularity and optimization of injection conditions. In particular, the gas injection pressure at the wellhead can be reduced, thereby decreasing the energy consumption of compression at the surface unit level. The modularity of the process relies on the ability to modify the injection altitude(s) to adapt the injection altitude(s) to the context (well conditions, injectivity, pressure, flow rates, etc.). The process according to the invention also ensures the stability of the two-phase flow formed within the well and guarantees the recompression of the mixture.

[0013] The invention also relates to a system for implementing the process, having the characteristics mentioned above. Reducing the pressure required for gas at the wellhead leads to a reduction in the cost of gas compression installations and a decrease in energy consumption. Résumé de l'invention

[0014] The invention relates to a method for injecting a gas into an underground formation by means of a well, the well comprising two coaxial cylinders defining a central volume and an annular volume. The well comprises a wellhead. The wellhead comprises a liquid inlet and a gas inlet, and the well also comprises a gas / liquid mixture outlet. The gas flows from the gas inlet to the liquid / gas mixture outlet, and the liquid flows from the liquid inlet to the liquid / gas mixture outlet. The method comprises the following steps: a) Said liquid is injected from said liquid inlet and said gas from said gas inlet separately, one into said central volume and the other into said annular volume, b) Said liquid and said gas are mixed by passing said central cylinder radially through one of these two fluids by opening or closing at least one of at least two injection points, said at least two injection points being distributed along said central cylinder, the central cylinder extending substantially over the entire length of the well and, c) A liquid / gas mixture thus obtained is brought out at said gas / liquid mixture outlet for transfer into said subsurface formation.

[0015] Advantageously, in step b), the openings or closings of said at least two injection points follow an opening / closing sequence.

[0016] Preferably, said opening / closing sequence includes the opening of an injection point when the pressure of the fluid passing through said central cylinder is greater than the pressure of the other fluid, at said injection point, and when the pressure of said liquid is greater than a predetermined pressure threshold, at said injection point.

[0017] Preferably, the predetermined pressure threshold is different for each of the injection points.

[0018] According to one embodiment of the process according to the invention: a) Said liquid is injected separately into said central volume and said gas into said annular volume, b) Said liquid and said gas are mixed by passing said gas radially through said central cylinder by said gas from said annular volume to said central volume through the opening of at least one injection point among at least two injection points distributed along said central cylinder, c) Said liquid / gas mixture thus obtained is discharged at said gas / liquid mixture outlet, said liquid / gas mixture outlet being located at the foot of said central volume.

[0019] According to another embodiment of the process according to the invention: a) The liquid is injected separately into the central volume and the gas into the annular volume, b) The liquid and the gas are mixed by passing the liquid from the central volume radially through the central cylinder to the annular volume through the opening of at least one injection point among at least two injection points distributed along the central cylinder, c) The liquid / gas mixture thus obtained is brought out at the level of the gas / liquid mixture outlet, the liquid / gas mixture outlet being located at the foot of the annular volume.

[0020] According to another embodiment of the process according to the invention: a) The liquid is injected separately into the annular volume and the gas into the central volume, b) The liquid and the gas are mixed by passing the liquid from the annular volume radially through the central cylinder through the opening of at least one injection point among at least two injection points distributed along the central cylinder, c) The liquid / gas mixture thus obtained is brought out at the level of the gas / liquid mixture outlet, the liquid / gas mixture outlet being located at the foot of the central volume.

[0021] According to another embodiment of the process according to the invention: a) The liquid is injected separately into the annular volume and the gas into the central volume, b) The liquid and the gas are mixed by passing the gas radially through the central cylinder through the opening of at least one injection point among at least two injection points distributed along the central cylinder, c) The liquid / gas mixture thus obtained is brought out at the level of the gas / liquid mixture outlet, the liquid / gas mixture outlet being located at the foot of the annular volume.

[0022] Advantageously, the liquid and gas are mixed by at least two regularly distributed injection points on the central cylinder, along the well.

[0023] Preferably, the opening / closing sequence includes successive openings of the injection points starting from the lowest injection point of the well towards the highest injection point of the well.

[0024] According to a variant of the process according to the invention, the opening / closing sequence includes successive closures of the injection points starting from the lowest injection point of the well towards the highest injection point of the well.

[0025] According to one embodiment of the process according to the invention, during the step of said mixing of said liquid and said gas, the injection flow rate of said fluid radially passing through said central cylinder is progressively increased.

[0026] According to a variant of the process according to the invention, during the step of said mixing of said liquid and said gas, the injection pressure of said fluid radially passing through said central cylinder is controlled.

[0027] Advantageously, the surface velocities of fluid movement are greater than 1m / s.

[0028] The invention also relates to a gas injection system in an underground formation for implementing the process according to one of the preceding features, comprising a well. Said well comprises two coaxial cylinders delimiting a central volume and an annular volume, and a wellhead. Said wellhead comprises a gas inlet and a liquid inlet, and said well comprises a gas / liquid mixture outlet. Said system comprises a means for circulating said gas from said gas inlet to said liquid / gas mixture outlet, and a second means for circulating said liquid from said liquid inlet to said liquid / gas mixture outlet. Furthermore, said system comprises a liquid injection means in said wellhead for injecting said liquid into one of the two volumes and a gas injection means in the wellhead for injecting said gas into the other volume.The said system also includes at least two means for the radial passage of one of the fluids through the said central cylinder, the said central cylinder extending substantially over the entire length of the said well, the said at least two means for radial passage being distributed along the said well, these means for radial passage comprising means for opening and closing, for example flaps or valves.

[0029] According to the invention, said radial passage means includes at least one valve and a non-return valve, said valve including a valve opening system when the pressure of said liquid, at the level of said valve, exceeds a predefined pressure threshold, said non-return valve including a valve opening means when the pressure of the fluid passing through said central cylinder exceeds the pressure of the other fluid, at the level of said non-return valve.

[0030] According to a variant of the system according to the invention, said system comprises a sealing means at the base of the volume where no liquid / gas mixture forms. Preferably, said sealing means is an annular element when the volume where said liquid / gas mixture forms is said central volume, or said sealing means is a cylindrical element when the volume where said liquid / gas mixture forms is said annular volume.

[0031] According to one embodiment of the system according to the invention, said system comprises a restriction means, said restriction means being positioned above said radial passage means, in the volume where said liquid / gas mixture is formed.

[0032] According to the invention, said at least two radial passage means are regularly distributed along said well.

[0033] Advantageously, said valve is calibrated by a gas charge equal to said predetermined pressure threshold.

[0034] The invention also relates to the use of the method or system according to any one of the preceding claims for a geothermal or CO2 storage application. Liste des figures

[0035] Other features and advantages of the process and system according to the invention will become apparent from the following description of non-limiting examples of implementations, with reference to the figures attached and described below. [ Fig 1 ] illustrates one embodiment of the system according to the invention. Fig 2 ] illustrates the reduction of gas injection pressure at the wellhead using the system / method according to the invention. Fig 3a ] illustrates another embodiment of the system according to the invention. Fig 3b ] illustrates an embodiment of a radial passage means for the system according to the invention. Fig 4 [ ] represents the synoptic diagram of an embodiment of the process according to the invention. ] Fig 5a [ ] represents a graph of pressure evolution as a function of depth according to one implementation of the process, according to a first step. ] Fig 5b [ ] represents a graph of pressure evolution as a function of depth according to one embodiment of the process in another step. ] Fig 5c [ ] represents a graph of pressure evolution as a function of depth according to one embodiment of the process in another step. ] Fig 5d ] represents a graph of pressure evolution as a function of depth according to one embodiment of the process, according to another step. Fig 6 ] illustrates the pressure evolutions for a process according to the prior art and for a process according to the invention. Description des modes de réalisation

[0036] The invention relates to a method for injecting gas into an underground formation using a well. The well comprises two coaxial cylinders defining a central volume and an annular volume. These two volumes are separated from each other by the wall formed by the central cylinder. The outer cylinder, with a larger diameter, can, for example, correspond to the well casing; the inner cylinder, with a smaller diameter, can, for example, correspond to the well tubing and extends substantially along the entire length of the well. The casing is the cylindrical casing installed immediately after drilling to secure the resulting well. It is generally constructed from cement. Several casings are fitted together successively to construct a well. The tubing is generally a metal tube inserted into the well. It serves to complete the well.Well completion refers to the equipment used to finish a well, enabling its safe operation (production and / or injection of fluids). The tubing is of a diameter that allows the use of a slickline, enabling maintenance work (e.g., valve removal, plug installation, pressure and / or temperature measurements) to be performed remotely from the wellhead. This is done by passing the necessary tools through the tubing. The minimum internal diameter of the tubing is approximately 45 mm to allow the slickline to pass through. The tools are then guided by the slickline using a winch, hence the term "slickline operation."

[0037] At the top of the well, the well includes a wellhead that rises above ground level. In addition, the well includes an outlet into the underground formation. The wellhead includes at least one liquid inlet and at least one gas inlet, allowing the fluids to flow from separate inlets to a common gas / liquid mixture outlet located at the bottom of the well to introduce the gas / liquid mixture into the underground formation.

[0038] In its lower section, the well lies within the underground formation. It may consist of an extension of the casing, which in this area includes multiple radial ports to allow the gas / liquid mixture to escape radially into the underground formation. Alternatively, the gas / liquid mixture may also flow vertically into the underground formation, as the casing may be left open at its lower longitudinal end, the well being classified as an open hole at its lower end.

[0039] According to the invention, the process comprises the following steps: a) The liquid is injected from at least one liquid inlet and the gas from at least one gas inlet, one into the central volume and the other into the annular volume. Thus, the injections of the two fluids are separate and distinct into two volumes. They cannot therefore mix during injection. The liquid and gas are mixed. To do this, several injection points distributed along the central cylinder are opened or closed to allow one of these two fluids to flow radially through the central cylinder. To allow injection, at least one injection point is in the open position during the operation. An injection point is defined as a level (in the sense of altitude) of injection longitudinally in the well. Thus, the injection point comprises a single injection altitude along the longitudinal axis of the well.However, it can include several radial injection positions at the same altitude. For example, the injection point can have four radial injection positions at 0°, 90°, 180°, and 270°, or slightly spaced vertically (preferably with less than 10 meters of altitude difference between the shallowest and deepest positions). The diameters of the injection points can also be modified using a smooth cable to promote liquid / gas mixing. The open / closed status of the different injection points can either change during the operation or remain constant. Thus, the proposed system is particularly modular. Specifically, the injection depth can be adjusted by opening / closing the injection points according to the conditions observed in situ. This modularity is made possible by the multiple injection points distributed along the well.The opening and closing of injection points can be performed either manually via equipment connected to the smooth cable and the insertion and removal of a packer, or remotely via a control system, for example, automatic electrical controls, or by using specific valves with a trigger threshold and possibly a check valve. Once the fluid in question has passed through the wall represented by the central cylinder, which serves as a separation between the central volume and the annular volume, it mixes with the other fluid that was initially present in that volume. The fluid passing through the central cylinder is then called Fluid1, and the fluid not passing through the central cylinder is called Fluid2. Fluid2 thus remains in the volume into which it was injected at the wellhead. Once Fluid1 has passed through the central cylinder, liquid / gas mixing is possible.Fluid 1 then changes its circulation volume, while Fluid 2 circulates from its inlet to the outlet in a single volume. The use of multiple injection points provides flexibility for operating in different situations and facilitates start-up operations. This also helps to secure the two-phase flow of the liquid / gas mixture where the gas is only slightly or not at all dissolved in the liquid. b) The resulting liquid / gas mixture is drawn from the gas / liquid mixture outlet for transfer and injection into the underground formation. The gas is then stored within the underground formation. This avoids releasing toxic gases and / or greenhouse gases, such as CO2, H2S, or natural gases, into the atmosphere.

[0040] Generally, the liquid is water or brine. Generally, the gas is a non-condensable gas, a toxic gas or a greenhouse gas, for example, CO2, H2S, natural gases or mixtures of these gases.

[0041] Preferably, the opening / closing of the injection points can follow a sequence of openings / closings. This sequence can be predefined automatically or activated manually by equipment connected to the smooth cable.

[0042] Advantageously, the opening / closing sequence may include the opening of an injection point which occurs when, on the one hand, the pressure of the fluid passing through the central cylinder is greater than the pressure of the other fluid, at the level of the injection point concerned, and on the other hand, when the pressure of the liquid is greater than a predetermined pressure threshold, at the level of the injection point concerned.

[0043] Indeed, when the liquid pressure exceeds a certain threshold, it can be ensured that the liquid column level rises above the injection point. Injecting gas (for example, water) into such a liquid column promotes liquid / gas mixing and the establishment of a two-phase flow.

[0044] Furthermore, by ensuring a Fluid1 pressure greater than the Fluid2 pressure, we ensure that Fluid2 cannot effectively change its circulation space (the circulation space being the central volume or the annular volume), as the pressure difference is not favorable.

[0045] According to one embodiment of the process according to the invention, the predetermined pressure threshold can be different for each of the injection points. Varying this parameter along the well can improve gas injection performance.

[0046] In a first embodiment of the process according to the invention: a) The liquid can be injected separately into the central volume and the gas into the annular volume. b) The liquid and gas can be mixed by passing the gas radially through the central cylinder from the annular volume to the central volume via at least one of the at least two injection points distributed along the central cylinder. The liquid remains in the central volume, with the liquid / gas mixture outlet located at the base of the central volume. c) The resulting liquid / gas mixture can be discharged from the gas / liquid mixture outlet.

[0047] This embodiment is particularly advantageous. Indeed, it allows for efficient recompression of the liquid column contained in the central volume by injecting the gas into the column, thus limiting the gas injection pressure at the wellhead.

[0048] Furthermore, injecting the gas into a stable liquid column promotes mixing and two-phase flow.

[0049] Furthermore, this solution facilitates start-up operations by limiting the gas injection pressure required at the wellhead, as the liquid column has a greater hydrostatic pressure gradient than the gas.

[0050] According to a second embodiment of the process according to the invention: a) The liquid can be injected separately into the central volume and the gas into the annular volume. b) The liquid and gas can be mixed by radially injecting the liquid from the central volume to the annular volume through the central cylinder via at least two injection points distributed along the length of the central cylinder, with the liquid / gas mixture outlet located at the base of the central volume. c) The resulting liquid / gas mixture can be discharged from the gas / liquid mixture outlet, with the liquid / gas mixture outlet located at the base of the annular volume.

[0051] According to a third embodiment of the process according to the invention: a) The liquid can be injected separately into the annular volume and the gas into the central volume. b) The liquid and gas can be mixed by injecting the liquid radially through the central cylinder from the annular volume to the central volume via at least two injection points distributed along the length of the central cylinder, with the liquid / gas mixture outlet located at the base of the central volume. c) The resulting liquid / gas mixture can be discharged from the gas / liquid mixture outlet, with the liquid / gas mixture outlet located at the base of the central volume.

[0052] According to a fourth embodiment of the process according to the invention: a) The liquid can be injected separately into the annular volume and the gas into the central volume. b) The liquid and gas can be mixed by injecting the gas radially through the central cylinder from the central volume to the annular volume via at least two injection points distributed along the central cylinder, with the liquid / gas mixture outlet located at the base of the central volume. c) The resulting liquid / gas mixture can be discharged from the gas / liquid mixture outlet, with the liquid / gas mixture outlet located at the base of the annular volume.

[0053] In this case, after passing through the central cylinder, the gas is injected into the liquid column that has formed in the annular volume. Injecting the gas at an altitude below the surface of the water column (the surface of the water column corresponds to the air / water interface of the water column) promotes mixing and ensures stable two-phase flow.

[0054] Advantageously, the liquid and gas can be mixed at at least two injection points evenly spaced along the central cylinder, along the well. By evenly spaced, we mean that the injection points are longitudinally spaced at least several meters apart, preferably at least ten meters, or even at least one hundred meters apart.

[0055] In this way, the start-up operations for gas injection into the underground formation are facilitated. Indeed, by using a first injection point in the lower part of the well, the liquid / gas mixture can begin in a relatively shallow liquid column, which facilitates start-up, particularly when the well's injectivity is high (injectivity is the well's capacity to receive and discharge fluids into the underground formation). When the liquid / gas mixture is created after the gas injection begins in the liquid column, the liquid / gas mixture has a hydrostatic pressure gradient between that of the liquid and that of the gas. This results in the liquid column rising to the surface within the affected volume, since the pressure at the injection point increases due to the column's weight reduction.

[0056] The gas can then be injected at an injection point located at a higher level in the well and gradually create a mixture over a significant portion of the well, depending on the injectivity conditions at the bottom of the well.

[0057] Furthermore, using injection points regularly distributed on the central cylinder, along the well, can allow flexibility of the installation depending on the gas, the liquid, the solubility characteristics of one in the other, the injectivity of the fluid (the injectivity of the well being its ability to introduce the mixture into the underground formation. It depends both on the fluids involved and on the porosity / permeation characteristics of the underground formation).

[0058] Preferably, the opening / closing sequence can include the successive opening of different injection points, starting from the lowest injection point in the well and progressing to the highest. This feature facilitates the initiation of gas injection into the underground formation by utilizing a reduced gas injection pressure at the wellhead, thanks to the presence of a liquid column and its favorable hydrostatic pressure gradient. As the liquid column rises due to the formation of the liquid / gas mixture, gas injection can then be performed at an injection point located higher in the well, without requiring an increase in the gas injection pressure.

[0059] Advantageously, the opening / closing sequence can also include the successive closure of injection points, starting from the lowest injection point in the well and moving towards the highest. Indeed, during the rise of the liquid column induced by the formation of the liquid / gas mixture, gas injection can then be carried out at an injection point located higher in the well, without having to increase the gas injection pressure. The lower injection point can therefore be closed, thus avoiding the need to use a higher gas pressure to inject into the column and also preventing the formed liquid / gas mixture from entering the other volume through this injection point if the pressure of the liquid / gas mixture at the altitude of the injection point in question becomes higher than that of the gas at the same altitude.

[0060] These successive closures can occur simultaneously with successive openings or with a slight delay. For example, successive closures can occur slightly after successive openings: thus, over a short period, two points can be open simultaneously. This prevents the gas injection from stopping, which could be detrimental to maintaining the two-phase flow.

[0061] According to a variant of the process according to the invention, during the liquid-gas mixing step, the gas flow rate can be progressively increased. In this way, the formation of the mixture and / or the level of the resulting mixing column can be controlled, as well as the introduction, or not, of Fluid 1 into Fluid 2 through the multiple injection points. This variant is particularly advantageous for establishing passive control of the opening / closing of the various injection points based on relative and / or absolute pressures and / or the resulting flow rates.

[0062] Alternatively, during the liquid-gas mixing stage, the gas injection pressure can be controlled. In this way, the opening / closing of the injection points can be passively controlled using relative and / or absolute pressures.

[0063] The opening and closing of injection points can also be controlled via a specific control system, for example, using pilot-operated valves. This feature allows the injection points to be opened and / or closed at the user's discretion. It can be beneficial when the mixture's behavior is not as expected or in abnormal operating situations such as variations in the injectivity of the underground formation. This provides greater operational flexibility for the installation.

[0064] Preferably, the surface velocities of the fluids are greater than 1 m / s. The surface velocity of a fluid is defined as the ratio of the flow rate of that phase alone to the flow rate of the passage cross-section where that phase circulates. It thus corresponds to the average velocity of that phase if it were the only phase flowing through the passage cross-section in question. Therefore, the flow conditions are favorable for the entrainment of the gas by the liquid phase and the establishment of a two-phase flow ensuring good recompression.

[0065] The invention also relates to a gas injection system for an underground formation, for implementing the process according to one of the preceding characteristics. This system includes, in particular, a well comprising at least two coaxial cylinders defining a central volume and an annular volume. These cylinders may, in particular, be the casing and tubing of the well, installed after drilling is complete and the well is finished. At the top, the well has a wellhead that rises above ground level. This wellhead includes, in particular, a gas inlet and a liquid inlet. At the bottom of the well, the well also includes a gas / liquid mixture outlet for introducing the gas / liquid mixture that has formed in the well into the underground formation.The system includes a gas circulation means for circulating gas from the gas inlet to the liquid / gas mixture outlet, a liquid circulation means for circulating liquid from the liquid inlet to the liquid / gas mixture outlet, a liquid injection means for injecting liquid into one of the two volumes, and a gas injection means for injecting gas into the other volume. The system includes at least two radial flow means for one of the fluids to pass through the central cylinder, distributed along the wellbore. These radial flow means allow the injection points of one of the two fluids into the other fluid. To this end, the radial flow means include opening and closing mechanisms, which may, for example, be check valves and / or open / close valves.By distributed along the shaft, we mean that the radial passage means are spaced longitudinally along the central cylinder, the spacing representing a significant area of ​​the central cylinder. For example, the longitudinal spacing between the radial passage means can be several meters, preferably at least ten meters, and even more preferably several hundred meters.

[0066] At least two radial passage methods are used to improve the flexibility of use and operation of the well on the one hand and to facilitate the start-up of the installation on the other.

[0067] According to the invention, each radial flow means comprises at least one valve and one check valve. The valve may, in particular, include a mechanism for opening when the liquid pressure at the valve exceeds a predefined pressure threshold. Thus, the valve may, for example, open only when the level of the liquid column above the valve is sufficient. Furthermore, the check valve includes a means for opening when the pressure of the fluid flowing through the central cylinder exceeds the pressure of the other fluid at the check valve. The check valve prevents, in particular, Fluid 2 from returning to the volume into which Fluid 1 is initially injected.

[0068] The non-return valve is connected to the Fluid 1 volume on one side and to an intermediate chamber, which is itself connected to the valve. The valve is connected to the Fluid 2 volume on one side and to the intermediate chamber, which is connected to the non-return valve. The intermediate chamber can contain either Fluid 1 or Fluid 2.

[0069] Preferably, the valve is calibrated by a gas charge whose pressure is equal to the predetermined pressure threshold. As a result, the valve performs passive control.

[0070] Advantageously, the system may include a sealing means at the base of the volume where no liquid / gas mixture forms. Preferably, the sealing means is an annular element when the volume where the liquid / gas mixture forms is the central volume. Preferably, the sealing means is a cylindrical element when the volume where the liquid / gas mixture forms is the annular volume.

[0071] In this way, the sealing device, such as a plug, seals the bottom of the volume where no mixing occurs. The fluid contained in this volume is then forced to pass through the central cylinder via the dedicated means for this purpose. This improves the control of liquid / gas exchange. Liquid and gas flow rates are also better controlled thanks to the presence of the sealing device. Furthermore, this sealing device prevents gas from circulating in a loop: gas not drawn into the reservoir could rise into the volume where no liquid / gas mixing should normally occur, thus circulating between the injection point and the bottom of the well.

[0072] Preferably, the system may also include a restriction means positioned above the radial flow means, within the volume where the liquid / gas mixture is formed. If the volume where the liquid / gas mixture is formed is the central volume, this restriction means may restrict the internal diameter of the central cylinder. If the volume where the liquid / gas mixture is formed is the annular volume, this restriction means may locally restrict the diameter of the outer cylinder and / or locally enlarge the external diameter of the central cylinder. This restriction means may, for example, be a valve. Thus, the flow area within the volume considered, whether it be the central volume or the annular volume, is locally reduced.The position of this restriction means at a longitudinal level of the well, located above the level of the radial passage means, prevents, once the liquid / gas mixture has been produced, the gas from being tempted to move up the column, for example if the surface velocities are too low (less than 1 m / s), in the opposite direction to that expected, the expected direction being towards the liquid / gas mixture outlet located at the bottom of the well, at the level of the zone where the mixture is introduced into the underground formation.

[0073] According to the invention, the at least two radial access points are regularly distributed along the well. Specifically, the radial access points are spaced longitudinally at substantially constant intervals, preferably at least ten meters and, even more preferably, at least one hundred meters. This feature facilitates the start-up of gas injection into the underground formation.

[0074] The system and method according to the invention make it possible to reduce the gas injection pressure at the wellhead, thereby reducing the cost of associated compression equipment and energy consumption, particularly at start-up.

[0075] There [ Fig 1 [ ] illustrates, schematically and without limitation, one embodiment of the system and method according to the invention. The system 100 consists of a well comprising at least one external cylinder 10 and an internal cylinder, also called the central cylinder, 20. These two cylinders are coaxial, with axis xx, which also corresponds to the axis of the well.

[0076] The outer cylinder 10 can be a well casing, that is, the cemented part of the well just after drilling to secure it; the inner cylinder 20 can be the well completion tubing used for gas production or injection.

[0077] In this figure, the reference T represents ground level. Thus, the well has a wellhead 60 which is the part of the well located above ground level T.

[0078] A first fluid, F1, is injected into the wellhead through a first inlet. A second fluid, F2, is also injected into the wellhead through a second inlet. One of these two fluids, F1 or F2, is a gas, and the other is a liquid. Preferably, this liquid is water or brine. The gas is preferably a toxic gas or a greenhouse gas, such as CO2, H2S, or natural gases.

[0079] Fluid F1 flows in the annular space between the outer cylinder 10 and the inner cylinder 20, while fluid F2 flows in the inner cylinder 20. According to an alternative, the reverse is possible.

[0080] At the bottom of the well, the outer cylinder 10 can be extended by a part 50 which has several radial orifices, on several longitudinal levels so that the liquid / gas mixture can enter the underground formation.

[0081] At the bottom of the well, at the lower end of the inner cylinder 20, a sealing means 40, consisting for example of a plug, also called a "packer", is placed between the outer cylinder 10 and the inner cylinder 20. Thus, the fluid F1 contained in the annular space cannot rise into the inner cylinder 20. It is blocked in the annular space delimited by the two cylinders 10 and 20 and by the sealing means 40.

[0082] Finally, the central cylinder 20 has several radial passage means 30 distributed over the central cylinder 20. These radial passage means allow one of the two fluids, for example fluid F1, to pass radially through the central cylinder 20 and thus find itself in the volume where initially only fluid F2 is contained. Thus, the mixing between F1 and F2 begins as soon as F1 enters the radial volume of F2 (on the [ Fig 1 [ ], this refers to the central volume contained within the central cylinder 20). The circulation of F2 occurs along the longitudinal axis xx of the well. Thus, at the beginning of the mixing phase, the two fluids have orthogonal directions of movement, favoring liquid / gas mixing. Then, the two fluids flow towards the outlet located at the bottom of the well. Therefore, the two-phase mixture subsequently has a substantially longitudinal flow direction.

[0083] There [ Fig 2 ] illustrates, schematically and without limitation, the impact on the reduction of gas injection pressure at the wellhead thanks to the system and method according to the invention compared to the prior art system.

[0084] There [ Fig 2 ] gives the evolution of the pressure P on the abscissa as a function of the depth D, represented on the ordinate, the direction of the ordinate going towards the bottom of the well while level 0 corresponds to the ground level.

[0085] The C1 curve represents the evolution of gas pressure as a function of depth, in the case of a direct injection according to prior art. The curve is defined, on the one hand, by the pressure existing in the underground formation and its injectivity, and on the other hand, by the hydrostatic gradient of the gas. This hydrostatic gradient corresponds to the slope of the C1 curve. For a gas, this gradient is low, and the C1 curve is close to vertical. This is why the injection pressure is high at the wellhead.

[0086] The slope of curve C2 corresponds to the hydrostatic gradient of the liquid portion. Curve C2 is much steeper than curve C1 because the hydrostatic gradient of the liquid is higher than that of the gas (due to the difference in densities). Thus, if one wished to inject a liquid into the well, the injection pressure of the liquid at the wellhead would be much lower, or even zero (vacuum zone at the wellhead), than the injection pressure of the gas required to achieve the same pressure at the bottom of the well.

[0087] Within the framework of the system and method according to the invention, the [ Fig 2 ] schematically represents an injection point in order to understand the operation of the system and the process.

[0088] Curve C2 represents the injection of liquid from the wellhead to point I. Point I will be detailed later.

[0089] Curve C1' represents the gas injection from the wellhead to an altitude at the same level as point I (same ordinate D). Curve C1' is essentially parallel to curve C1 because the hydrostatic gradient is an intrinsic characteristic of the gas, apart from density variations and pressure losses induced by the pressure reduction.

[0090] At the altitude of point I, a radial passage means is positioned on the central cylinder in order to allow one of the fluids to pass through the central cylinder in order to be mixed with the second fluid.

[0091] When the gas pressure at the altitude of point I of the injection point exceeds the liquid pressure, corresponding to the abscissa of point I, at the same altitude of point I, the process allows the gas to pass into the central cylinder to mix with the liquid, which is the case on the [ Fig 2 Alternatively, the reverse path from liquid to gas is possible.

[0092] After the gas is introduced into the liquid column, a two-phase liquid / gas mixture occurs. The hydrostatic gradient of this mixture is intermediate between that of the liquid and that of the gas. Thus, the slope of curve C3, corresponding to the liquid / gas mixture occurring at an altitude below the injection point (or below point I), is intermediate between the slope of C2 and the slope of C1 or C1', subject to hydrodynamic conditions of gas entrainment by the liquid.

[0093] It can thus be observed that, by using a two-phase mixture, the gas pressure required for wellhead injection, P2, is significantly lower than the gas pressure required at the wellhead, P1, by direct gas injection into the well. G corresponds to the reduction of this pressure thanks to the process or system according to the invention.

[0094] There [ Fig 3a ] represents another embodiment of the system and method according to the invention, in a schematic and non-limiting manner.

[0095] In this figure, the references have the same designation as those of the [ Fig 1 ] correspond to the same elements.

[0096] In this example implementation, fluid F1 is the gas and fluid F2 is the liquid.

[0097] The radial flow means here are valve / non-return valve assemblies 35, which allow the radial flow of gas through the central cylinder 20 to reach the central volume. In this valve / non-return valve assembly 35, a valve opens when the liquid pressure in the inner cylinder 20, at the valve, exceeds a predefined pressure threshold. For example, this pressure threshold corresponds to a liquid level in the liquid column that forms in the inner cylinder 20. This pressure threshold ensures that the liquid level is above the level of the valve / non-return valve assembly 35, thus ensuring gas injection into a column full of liquid. This improves the gas / liquid mixture. The valve closes when the liquid pressure at the valve falls below the predefined pressure threshold.

[0098] Once the valve of the valve / check valve assembly 35 is open, the check valve of the valve / check valve assembly 35 opens when the existing gas pressure exceeds the liquid pressure at the check valve. This ensures that gas injection into the liquid is possible, while liquid injection into the gas is hindered. When the liquid or gas / liquid mixture pressure exceeds the gas pressure at the check valve, the check valve closes.

[0099] Preferably, the non-return valve and the valve of the valve / non-return valve assembly 35 are substantially at the same level in the well.

[0100] In system 100 of the [ Fig 3a [ ], it is observed that only the second check valve assembly 35, counting from the bottom of the well, is open. The other check valve assemblies 35 are closed. Thus, gas can only pass through the second check valve assembly from the bottom of the well. Above the level of the second check valve assembly 35, there is a height in the inner cylinder 20 where the column is filled only with liquid L. Then, below this liquid L-filled section, the column is filled with a flowing liquid / gas mixture L+G. The section above the liquid section is a water-vapor mixing zone at the water vapor pressure, where the water falls into the central cylinder until it reaches the liquid recompression zone L and then the liquid / gas mixing zone L+G.

[0101] There [ Fig 3a [ ] represents three valve / non-return valve assemblies 35, corresponding to three levels longitudinally along the axis xx of the well. On the two lower levels, the valve / non-return valve assembly 35 consists of several elements positioned radially around the cross-section of the inner cylinder 20. Preferably, these elements are spaced radially at regular intervals around the inner cylinder 20. For example, two elements can be spaced 180° apart or four elements spaced 90° apart. The regular radial spacing improves the injection and mixing of the gas in the liquid to obtain the most homogeneous liquid / gas L+G mixture possible. Specific internal equipment can also be added, for example, a convergent / divergent system of the type described in patent application JP2899604, to facilitate liquid / gas or gas / liquid mixing.

[0102] There [ Fig 3b ] represents, schematically and without limitation, a schematic element of the valve / non-return valve assembly 35. This element includes a non-return valve 70. On the [ Fig 3b [ ], the non-return valve is shown in the open position. When it closes, for example when the gas fluid pressure is lower than the liquid pressure, it comes to rest against the seat of orifice 60.

[0103] The element also includes a valve 90, shown in the open position in the figure. When it closes, for example when the liquid pressure is below a predefined pressure threshold, the valve 90 comes to rest against the valve seat 80.

[0104] Valve 90 and non-return valve 70 can be substantially coaxial. The gas arriving at E first encounters the non-return valve 70, then, when this opens, the gas passes through valve 90 to exit at S and encounter the liquid.

[0105] Valve 90 is set to a predefined pressure threshold by a gas charge contained in volume 200. Valve 90 can be a check valve.

[0106] An intermediate chamber 110 can be in communication with both the non-return valve 70 and the valve 90. It can contain either Fluid 1 or Fluid 2. The non-return valve 70 is in communication, on the other side, i.e., at the inlet E, with the volume containing Fluid 1. The valve 90 is in communication with the volume of Fluid 2 at the outlet S. Immediately after outlet S, Fluid 1, arriving from inlet E, enters the volume of Fluid 2, thus mixing with it.

[0107] There [ Fig 4 ] represents a schematic and non-limiting overview of an embodiment of the process according to the invention.

[0108] In this figure, it is assumed that the well has three valve / non-return valve assemblies, spaced along the well axis. The system used for this process may correspond to that of the [ Fig 3a ].

[0109] The first step involves starting the injection of IL liquid into the inner cylinder to create a liquid column within it. Liquid injection continues throughout the process, with possible temporary interruptions.

[0110] When the liquid level in the inner cylinder is sufficient, the valve of the first valve / check valve assembly opens. This is the valve opening step #1, counting from the bottom of the well, OGL1.

[0111] Once OGL1 is established, the injection of IG1 gas can begin with a certain gas flow rate.

[0112] When the gas pressure in the annular space formed by the space between the inner and outer cylinders is greater than the liquid pressure contained in the inner cylinder, the opening of the non-return valve No. 1, starting from the bottom of the OCNR1 well, is created.

[0113] The injection of gas into the liquid (mixture) begins and requires an increase in the gas flow rate AIG1. The gas injection into the liquid changes the density of the liquid / gas mixture. Consequently, the pressure at the injection point gradually increases as the gas lightens the column of the mixture, and thus the level of the liquid column above the injection point rises.

[0114] The level of the liquid column in the inner cylinder continues and when the liquid or liquid / gas mixture pressure at the valve of the second valve / check valve assembly from the bottom of the well exceeds a pressure threshold, which may be equal to or different from the predefined pressure threshold of the first valve from the bottom of the well, the opening of the second OGL2 valve occurs.

[0115] Then when the gas pressure at the second check valve from the bottom of the well is greater than the liquid (or liquid / gas mixture) pressure, the opening of the OCNR2 check valve occurs and induces the closing of the first check valve from the bottom of the well, FCNR1.

[0116] Once again, the AIG2 gas injection is increased. The liquid / gas mixture column sees its density decrease again, and thus the level of the liquid / gas mixture column in the central cylinder rises.

[0117] When the pressure of the liquid / gas mixture or the liquid reaches the predefined pressure threshold of valve #3, i.e., the highest valve, valve #3 opens. This is step OGL3, the opening of valve #3.

[0118] When the gas pressure exceeds the pressure inside the inner cylinder, at the level of the check valve, the opening of check valve no. 3, that is to say the highest check valve in the well, OCNR3, occurs, causing the closing of check valve no. 2 FCNR2.

[0119] The injection of gas into the well then continues in the regime established by the third valve / non-return valve assembly.

[0120] THE figures 5a à 5d show the evolution of pressures P as a function of well depth D during the start-up phase of gas injection into the underground formation according to an embodiment of the process according to the invention, in a schematic and non-limiting manner. The injection process may conform to the flow diagram of the [ Fig 4 In these figures, identical references correspond to the same information and will therefore not necessarily be re-explained in each figure. Furthermore, these figures are constructed in the same way as the [ Fig 2 ].

[0121] There [ Fig 5a This corresponds to liquid injection only. Curves 1, 2, and 3 represent three successive points during liquid injection from the wellhead. Thus, the liquid level changes from H1 to H2 to H3 depending on the well's injectivity. It is observed that when the column reaches level H3, the pressure in curve 3, at depth DV1 (corresponding to the elevation of the first valve / check valve assembly), slightly exceeds the predefined pressure threshold PV1 of this first valve. Therefore, valve OGL1 is opened, and gas injection into the annular space can begin.

[0122] There [ Fig 5b ] shows the gas injection on curve 4. The slope of curve 4 is steeper than that of curves 1, 2 and 3 of the [ Fig 5a because the hydrostatic gradient of the gas is significantly lower than that of the liquid.

[0123] There [ Fig 5c Figure 1 shows the evolution of the curves after the start of gas injection into the liquid column. The former curve 3 (dotted line) has become curve 5. Indeed, during gas injection into the liquid, the density of the liquid / gas mixture is between that of the liquid and that of the gas. Consequently, the hydrostatic gradient of the mixture is also between that of the gas and that of the liquid. Thus, the slope of curve 5 changes, and the angle of this curve with respect to the vertical axis (corresponding to the depth axis D) lies between the angle of curve 4' and that of curve 3.

[0124] Curve 4' is roughly parallel to curve 4, but since the pressure at the injection point has increased with the column lightening, the gas annular pressure is increased to ensure the injection flow rate, and thus curve 4' is shifted to the right. Consequently, the injection pressure at the wellhead is increased accordingly.

[0125] In this figure, we observe a break at point DV1. Below this point, the pressure follows curve 5 and above it, it follows curve 3', parallel to curve 3. Indeed, during the injection of gas into the liquid, the column becomes lighter and thus, the level of the column rises.

[0126] Above DV1, the column is only liquid and thus the pressure follows the evolution 3' which has substantially the same hydrostatic gradient and therefore the same slope as curve 3.

[0127] Below this point DV1, the column is a liquid / gas mixture whose hydrostatic gradient is an intermediate value between the hydrostatic gradient of the liquid and that of the gas.

[0128] We observe that the pressure in the column (curves 3' and 5) is slightly higher than the PV2 pressure threshold of the second valve from the bottom, at the altitude DV2 of this second valve, which causes the opening of the second valve OGL2.

[0129] Furthermore, the gas injection pressure along curve 4' exceeds the mixture pressure inside the inner cylinder, as defined by curves 3' and 5, at the DV2 altitude of the valve. Therefore, valve CNR2 opens. Gas injection can then occur at this second level.

[0130] On the [ Fig 5d ], we observe that the gas pressure, following curve 4" becomes lower than the internal pressure of the internal cylinder, represented by curves 3" and 5", at the altitude DV1 of valve no. 1, which causes the closure of this non-return valve.

[0131] The stages of figures 5a à 5d These patterns are similarly reproduced for the higher levels of the valve / non-return valve assemblies. Furthermore, in these examples, gas injection control is managed by the injection flow rate, but it can similarly be managed by the gas pressure in the annulus.

[0132] An example of a geothermal application is detailed here. For this example, the principle of the [ system is applied] Fig 3a The system uses liquid, water, injected into the central volume, and non-condensable gases injected into the annular volume. The water comes from a process that recovers energy from underground steam extracted using producer wells. The water is then reintroduced into the underground formation, which is particularly necessary for geothermal applications. The non-condensable gases from the underground formation, recovered with the steam, can be toxic or greenhouse gases. To avoid polluting emissions, it is advantageous to reintroduce them into the underground formation.

[0133] The process according to the invention makes it possible to limit the reinjection power of the two fluids by ensuring the establishment of an extended and stable gas-liquid flow and therefore increases the performance of the geothermal system (energy recovery and injection of fluids into the underground formation).

[0134] The method and system according to the invention can also be used, in a non-limiting way, for the storage of CO2 in an aquifer or the simultaneous injection of gas and water for maintaining the pressure of oil tanks (assisted recovery). EXEMPLE

[0135] The characteristics and advantages of the process and system according to the invention will become apparent from the example below.

[0136] The example concerns the combined reinjection of condensed water and non-condensable gases, the characteristics of which are as follows: The outer cylinder has an external diameter of 168.3 mm and a thickness of 7.11 mm, reaching the reservoir cap at a depth of -2400 m and extending into an open-hole zone several tens of meters long. The inner cylinder has an external diameter of 114.3 mm and a thickness of 6.02 mm. The system includes a sealing device positioned between the inner and outer cylinders at a depth of -2000 m. This sealing device isolates the annular portion of the inner cylinder. Gas is injected from the annular portion into the inner cylinder via an orifice at -1300 m. Several orifices can be installed in the -2400 m to -1300 m zone to ensure the initiation of gas injection as defined in the invention. The nominal injection flow rate is 20 kg / s with 8%m of non-condensable gas, which gives a normal water flow rate of 18.4 kg / s and a non-condensable gas flow rate of 1.6 kg / s.The underground formation pressure is 65 bar with an injectivity of 1 kg / s / bar, resulting in a downhole pressure increase of 20 bar at the nominal flow rate. The injection temperature of both fluids at the wellhead is 80 °C.

[0137] The method of implementation corresponds to that of the [ Fig 3a ].

[0138] These gas reinjection conditions, according to the invention, are compared to a prior art case where the liquid and gas would be mixed only at the bottom of the well, just before being introduced into the underground formation, the other parameters being identical.

[0139] There [ Fig 6 ] illustrates the results of the pressure changes in this comparative study. The graph of the [ Fig 6 Figure 1 illustrates the evolution of pressure P (abscissa) as a function of the depth D considered in the well (ordinate). Curve 500 (dashed line) represents the evolution of gas pressure according to the prior art method. Curve 600 represents the evolution of gas pressure according to the method of the invention.

[0140] The required gas pressure at the bottom of the BHP well is 85 bar at the nominal injection flow rate.

[0141] According to the prior art process, the gas pressure required at the wellhead for direct gas reinjection G1 is 60.5 bar (curve 500 on the [ Fig 6 This well configuration consumes energy for gas compression at the surface and requires expensive and bulky compression equipment.

[0142] According to the process of the invention, a gas-liquid mixing zone is forced into the inner cylinder over 1100 m with safe hydrostatic recompression because the velocity conditions are favorable for gas entrainment by the liquid phase (surface gas and liquid velocities greater than 1 m / s). The required gas pressure at the wellhead G2 is then reduced to 31.5 bar (curve 600 of the [ Fig 6 ]).

[0143] The method according to the invention thus makes it possible to significantly reduce the required wellhead pressure, while ensuring the safety of the hydrostatic recompression zone in the inner cylinder. The lengths of the inner cylinder, the position of the injection points, and their number can be optimized according to the characteristics of the reinjection site and the desired reinjection flow rates.

Claims

1. Method for injecting a gas into a subterranean formation by means of a well, the well comprising two coaxial cylinders (10, 20) delimiting a central volume and an annular volume, the well comprising a wellhead (60), said wellhead (60) comprising a liquid inlet and a gas inlet, said well comprising a gas / liquid mixture outlet, said gas circulating from said gas inlet to said liquid / gas mixture outlet, said liquid circulating from said liquid inlet to said liquid / gas mixture outlet, said method comprising the following steps: a) injecting said liquid (IL) from said liquid inlet and said gas from said gas inlet separately, one into said central volume and the other into said annular volume, b) mixing said liquid and said gas by causing one of these two fluids to pass through said central cylinder radially by opening or closing at least one injection point among at least two injection points, said at least two injection points being distributed along said central cylinder (20), said central cylinder (20) extending substantially along the entire length of said well, and, c) causing a liquid / gas mixture thus obtained to emerge at said gas / liquid mixture outlet to transfer it to said subterranean formation.

2. Method for injecting gas into a subterranean formation according to Claim 1, wherein, in step b), the openings or closures of said at least two injection points follow a sequence of openings / closures.

3. Method for injecting gas into a subterranean formation according to Claim 1, wherein said sequence of openings / closures involves the opening of an injection point when the pressure of the fluid passing through said central cylinder is higher than the pressure of the other fluid, at said injection point, and when the pressure of said liquid is higher than a predetermined pressure threshold, at said injection point.

4. Method for injecting gas into a subterranean formation according to Claim 3, wherein said predetermined pressure threshold (PV1, PV2) is different for each of the injection points.

5. Method for injecting gas into a subterranean formation according to one of the preceding claims, wherein: a) said liquid is injected into said central volume and said gas into said annular volume, separately, b) said liquid and said gas are mixed by causing said gas to pass radially through said central cylinder from said annular volume to said central volume by opening at least one injection point among at least two injection points distributed along said central cylinder (20), c) said liquid / gas mixture thus obtained is caused to emerge at said gas / liquid mixture outlet, said liquid / gas mixture outlet being situated at the foot of said central volume.

6. Method for injecting gas into a subterranean formation according to one of Claims 1 to 4, wherein: a) said liquid is injected into said central volume and said gas into said annular volume, separately, b) said liquid and said gas are mixed by causing said liquid to pass radially through said central cylinder from said central volume to said annular volume by opening at least one injection point among at least two injection points distributed along said central cylinder (20), c) said liquid / gas mixture thus obtained is caused to emerge at said gas / liquid mixture outlet, said liquid / gas mixture outlet being situated at the foot of said annular volume.

7. Method for injecting gas into a subterranean formation according to one of Claims 1 to 4, wherein: a) said liquid is injected into said annular volume and said gas into said central volume, separately, b) said liquid and said gas are mixed by causing said liquid to pass radially through said central cylinder from said annular volume to said central volume by opening at least one injection point among at least two injection points distributed along said central cylinder (20), c) said liquid / gas mixture thus obtained is caused to emerge at said gas / liquid mixture outlet, said liquid / gas mixture outlet being situated at the foot of said central volume.

8. Method for injecting gas into a subterranean formation according to one of Claims 1 to 4, wherein: a) said liquid is injected into said annular volume and said gas into said central volume, separately, b) said liquid and said gas are mixed by causing said gas to pass radially through said central cylinder from said central volume to said annular volume by opening at least one injection point among at least two injection points distributed along said central cylinder (20), c) said liquid / gas mixture thus obtained is caused to emerge at said gas / liquid mixture outlet, said liquid / gas mixture outlet being situated at the foot of said annular volume.

9. Method for injecting gas into a subterranean formation according to one of the preceding claims, wherein the liquid and the gas are mixed through said at least two injection points that are regularly distributed on said central cylinder, along the well.

10. Method for injecting gas into a subterranean formation according to one of Claims 3 to 9, wherein the sequence of openings / closures comprises the successive openings of the injection points starting from the lowest injection point of the well to the highest injection point of the well.

11. Method for injecting gas into a subterranean formation according to Claim 10, wherein the sequence of openings / closures comprises the successive closures of the injection points starting from the lowest injection point of the well to the highest injection point of the well.

12. Method for injecting gas into a subterranean formation according to one of the preceding claims, wherein, during the step of said mixing of said liquid and said gas, the injection flow rate (AIG1, AIG2) of said fluid passing radially through said central cylinder (20) is progressively increased.

13. Method for injecting gas into a subterranean formation according to one of Claims 1 to 12, wherein, during the step of said mixing of said liquid and said gas, the injection pressure of said fluid passing radially through said central cylinder (20) is controlled.

14. Method for injecting gas into a subterranean formation according to one of the preceding claims, wherein the superficial displacement velocities of the fluids are greater than 1 m / s.

15. System for injecting gas into a subterranean formation for implementing the method according to one of the preceding claims, comprising a well, said well comprising two coaxial cylinders (10, 20) delimiting a central volume and an annular volume, the central cylinder (20) extending substantially along the entire length of said well, said well comprising a wellhead (60), said wellhead (60) comprising a gas inlet and a liquid inlet, said well comprising a gas / liquid mixture outlet, said system comprising a means for circulating said gas from said gas inlet to said liquid / gas mixture outlet, said system (100) comprising a second means for circulating said liquid from said liquid inlet to said liquid / gas mixture outlet, said system having a means for injecting liquid into said wellhead, to inject said liquid into one of the two volumes, and a means for injecting gas into the wellhead, to inject said gas into the other volume, said system (100) having at least two radial passage means (30, 35) to allow one of the fluids to pass through the central cylinder and to inject it into the other fluid, said at least two radial passage means (30, 35) being regularly distributed along said well, said system (100) being characterized in that each radial passage means (30, 35) comprises at least one valve (90) and one non-return check valve (70), said valve (90) comprising a system for opening the valve when the pressure of said liquid, at said valve, exceeds a predefined pressure threshold (PV1, PV2), said non-return check valve (90) having a means for opening the check valve when the pressure of the fluid passing through said central cylinder exceeds the pressure of the other fluid, at said non-return check valve (90).

16. System according to one of Claims 15, wherein said system has a shut-off means (40) at the foot of the volume where no liquid / gas mixture forms, preferably said shut-off means (40) is an annular element when the volume where said liquid / gas mixture forms is said central volume, or said shut-off means (40) is a cylindrical element when the volume where said liquid / gas mixture forms is said annular volume.

17. System according to Claim 15 or 16, wherein said system has a restriction means, said restriction means being positioned above said radial passage means (30, 35), in the volume where said liquid / gas mixture forms.

18. System according to one of Claims 15 to 17, wherein said valve (70) is calibrated by a gas load equal to said predetermined pressure threshold (PV1, PV2).

19. Use of the method or system according to one of the preceding claims for a geothermal or CO2 storage application.