Gravity-assisted multiphase geothermal re-injection system and method

Abstract

The system comprises a liquid-gas separator (31) having an inlet (31.1), a gas outlet (31.3) and a liquid outlet (31.2). The system further includes a first vessel (33), a second vessel (35), a pumping unit (37) having a suction side (37.1) and a delivery side (37.2); and a fluid connection (41.4) to the geothermal re-injection well (7). The suction side (37.1) of the pumping unit (37) is adapted to be fluidly coupled selectively with a lower side of the liquid-gas separator (31), a lower side of the first vessel (33) and a lower side of the second vessel (35). The delivery side (37.2) of the pumping unit (37) is adapted to be fluidly coupled selectively with the first vessel (33) and the second vessel (35). The gas outlet (31.3) of the liquid-gas separator (31) is adapted to be fluidly coupled selectively with an upper side of the first vessel (33) and an upper side of the second vessel (35). The fluid connection (41.4) to the geothermal re-injection well (7) is adapted to be fluidly coupled selectively with the upper side of the first vessel (33) and the upper side of the second vessel (35).

Description

GRAVITY- ASSISTED MULTIPHASE GEOTHERMAL RE-INJECTIONSYSTEM AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure concerns systems and methods for re-injection of spent geothermal fluid in a geothermal facility.BACKGROUND ART

[0002] Geothermal fluid with a high enthalpy content is often used as a source of energy for various kinds of facilities, in particular, but not exclusively, for the production of electric or mechanical power. Geothermal fluid, either in the form of steam or as a two-phase fluid containing steam and hot water, usually also contains a certain amount of non-condensable gases.

[0003] Non-condensable gases are mixed within the geothermal water steam coming from production wells and remain in gas phase after the steam is condensed when delivering its energy to the power plant. In other cases, when the geothermal fluid coming from production wells is in liquid phase (brine), it can be associated with non- condensable gasses that may require compression to be re-injected with the spent liquid brine into the re-injection wells. Non-condensable gases can include, among others, carbon dioxide (CO2), methane (CH4), hydrogen (H2), ammonia (NH3), hydrogen sulfide (H2S).

[0004] The common approach in the treatment of non-condensable gases from geothermal fluid has been to separate the non-condensable gases from the geothermal resource and to treat the toxic components (such as H2S, for instance) to avoid release thereof in the environment. However, most of the non-condensable gases releases in a geothermal plant are non-toxic, greenhouse gases, such as CH4 and CO2. In the past greenhouse gases generated by the exploitation of geothermal energy were simply released in the atmosphere.

[0005] However, the growing concern about the environmental impact and awareness about the effects of greenhouse gases in terms of climate changes, has recently led to the need to reduce or eliminate the emission of these gases into the environment. New geothermal power plants must be zero-emission to be considered renewable resources and to have access to higher feed-in tariffs.

[0006] Technologies for re-injecting two-phase spent geothermal fluid containing non-condensable gases are now under investigation (see Vlasios Leontidis, et. al., “Modelling reinjection of two-phase non-condensable gases and water in geothermal wells”, in Applied Thermal Engineering; 223 (2023) 120018, available at https: / / doi- org / 10.1016 / j . applthermal eng.2023.120018).

[0007] Reciprocating compressors, which are commonly used for reinjecting non- condensable gases from spent geothermal fluid into geothermal reinjection wells, are cumbersome and costly pieces of equipment. Reciprocating compressors may increase to 10% the total plant CAPEX and (due to the power required to run the compressors) reduce by about 5% the net output power, compared with traditional plants where green-house non-condensable gases are not re-injected, but simply released in the environment. Due to the high cost of reciprocating compressors, providing redundant compressors for re-injection purposes is not practical. Consequently, the availability of the plant is reduced by up to 4%. The maintenance and operating costs are increased by up to 20%.

[0008] These factors make the exploitation of geothermal energy less attractive.

[0009] A need therefore exists to reduce the drawbacks connected to the reinjection of non-condensable gases in a geothermal power plant or similar facilities exploiting geothermal fluid.SUMMARY

[0010] According to one aspect, a system for re-injecting spent geothermal liquid and non-condensable geothermal gases in a geothermal re-injection well is disclosed. The system comprises a liquid-gas separator having an inlet, a gas outlet and a liquid outlet. The system further includes a first vessel, a second vessel, a pumping unit having a suction side and a delivery side; and a fluid connection to the geothermal re-injection well. The suction side of the pumping unit is adapted to be fluidly coupled selectively with a lower side of the liquid-gas separator, a lower side of the first vessel and a lower side of the second vessel. The delivery side of the pumping unit is adapted to be fluidly coupled selectively with the first vessel and the second vessel. The gas outlet of the liquid-gas separator is adapted to be fluidly coupled selectively with an upper side of the first vessel and an upper side of the second vessel. The fluid connection to the geothermal re-injection well is adapted to be fluidly coupled selectively with the upper side of the first vessel and the upper side of the second vessel.

[0011] The two vessels are alternatively filed and emptied with liquid by the pumping unit through suitably controlled valves. The variable volume above the liquid in each vessel is used to suck-in, compress and dispatch non-condensable gases from the liquid-gas separator into the geothermal re-injection well. Intermittently liquid is also pumped to the re-inj ection well to form a liquid column or which acts in the geothermal re-injection well as a piston which compresses further the gas pocket previously formed in the geothermal re-injection well, with the aid of gravity. As the liquid column and the gas pocket move deeper into the geothermal re-injection well the pressure and the mixing of liquid and gas leads to the formation of a single-phase flow when teaching the geological formation: Variable gas composition and gas / liquid ratios can be accommodated with a suitable timing of the different pumping phases, as will be described in detail below with reference to non-limiting embodiments.

[0012] According to another aspect, a method is disclosed herein, for re-injecting geothermal liquid and geothermal non-condensable gases from a flow of exhaust geothermal fluid into a geothermal re-injection well, the method comprising the following steps:(i) pumping a pocket of pressurized non-condensable gases in the geothermal reinjection well;(ii) on top of the pocket of pressurized non-condensable gases, pumping a column of pressurized liquid in the geothermal re-injection well;(iii) repeating steps (i) and (ii).

[0013] According to some embodiments, a method for re-injecting geothermal liquidand geothermal non-condensable gases from a flow of exhaust geothermal fluid into a geothermal re-injection well is provided, which includes the following steps: delivering a flow of spent geothermal fluid, containing a liquid phase and non-condensable gases, in a liquid-gas separator; delivering non-condensable gases from a gas outlet of the liquid-gas separator selectively into at least a first vessel and a second vessel; pumping liquid from the liquid-gas separator into at least one of said first vessel and second vessel; pumping liquid from one of said first vessel and second vessel into the other of said first vessel and second vessel; delivering from at least one of said first vessel and second vessel into the geo-thermal re-injection well sequentially a pocket of pressurized non-condensable gases and a column of liquid.

[0014] According to embodiments disclosed herein, a method for re-injecting geothermal liquid and geothermal non-condensable gases from a flow of exhaust geothermal fluid into a geothermal re-injection well is provided. The method includes the following steps: delivering a flow of spent geothermal fluid, containing a liquid phase and non-condensable gases, in a liquid-gas separator having an inlet, a gas outlet and a liquid outlet; separately feeding liquid from a liquid outlet of the liquid-gas separator, and non-condensable gases from a gas outlet of the liquid-gas separator, in a pumping system comprising: at least a first vessel and a second vessel adapted to be fluidly coupled selectively with the liquid-gas separator and with the geothermal re-injection well; and a pumping unit for pumping liquid from one of said first vessel and second vessel to the other of said first vessel and second vessel and into the geothermal re-injection well; pumping from the system into the geothermal re-injection well sequentially a pocket of pressurized non-condensable gases and said a column of liquid from said first vessel and second vessel.BRIEF DESCRIPTION OF THE DRAWINGS

[0001] Reference is now made briefly to the accompanying drawings, in which:Fig. l is a schematic of a geothermal power plant using a Rankine cycle powered by high-enthalpy geothermal fluid for the power generation purposes;Figs 2A, 2B, 2C show a reinjection system and a sequence of the operation thereof in one embodiment;Fig.3 shows a schematic of a re-injection well with non-condensable gas pockets and columns of liquid therein, formed using a system according to the present disclosure;Fig.4 shows a further embodiment of a re-injection system according to the present disclosure;Figs. 5A, 5B, 5C and 5D show a sequence of operation of the system of the present disclosure in a further embodiment;Figs. 6A, 6B, 6C, 6D, and 6E show a sequence of operation of the system of the present disclosure in a further embodiment;Fig. 7 shows a further embodiment of a re-injection system according to the present disclosure.DETAILED DESCRIPTION

[0002] According to the present disclosure, re-injection of spent geothermal fluid containing a liquid phase and non-condensable gases is facilitated by compressing non-condensable gases in a re-injection well and forming therein a column or pocket of pressurized non-condensable gases, on top of which a column of pressurized liquid is pumped. Due to gravity, the pressurized column of liquid facilitates further compression of the non-condensable gases towards the bottom of the well in the re-inj ection well. A novel system is also disclosed for performing the above process. The system includes a liquid-gas separator and a plurality of vessels, which are fluidly coupled to one another and to the liquid-gas separator through a piping which further includes a pumping unit. By selectively connecting the vessels to the liquid outlet and gas outlet of the liquid-gas separator, to the pumping unit and to the well, liquid and non-condensable gases can be delivered separately from one another in the well in the form of a sequence of gas pockets and columns of liquid.

[0003] The novel re-injection system and method can be used in any facility wherein power is extracted from a high-enthalpy geothermal fluid and the spent liquid and noncondensable gases shall be re-injected in a re-injection well.

[0004] Turning now to the drawings, Fig.1 illustrates an exemplary and non-limiting geothermal plant, in which the re-injection system and method of the present disclosure can be used. In the exemplary embodiment of Fig.1 the geothermal plant is a power generation plant, wherein a thermodynamic cycle is used to convert thermal energy contained in the pressurized, high-temperature geothermal fluid into mechanical and electric power. Those skilled in the art will understand that the same re-inj ection system and method can be beneficial also in combination with geothermal plants of different kinds, such as plants used for heating or other purposes. The method and system disclosed herein can be beneficial in those situations where non-condensable gases released by a geothermal fluid require to be re-injected in a re-injection well, to prevent or reduce emissions of noxious or polluting gases, including greenhouse effect gases into the environment.

[0005] The geothermal plant of Fig.1 is globally labeled 1 and can include a thermodynamic cycle 3, a geothermal production well 5, wherefrom hot and pressurized geothermal fluid is extracted, as well as a re-injection geothermal well 7, wherein the spent geothermal fluid, including a liquid phase and a non-condensable gaseous phase (containing one or more non-condensable gases) can be re-injected. Reference 9 indicates a system for re-injecting the spent geothermal fluid, containing liquid and non- condensable gases, into the re-injection well 7. The system 9 is referred to here below shortly as “re-injection system”. Embodiments of the re-injection system 9 and methods of operating the same will be disclosed in detail below, reference being made to Figs. 2 to 7.

[0006] In the embodiment of Fig.1 the thermodynamic cycle 3 includes a Rankine cycle. In some embodiments, the thermodynamic cycle 3 can be an organic Rankine cycle (shortly ORC) using an organic fluid as a process fluid, such as carbon dioxide, ammonia, pentane, cyclopentane, HFC (hydrofluorocarbons) refrigerants, or the like. In other embodiments, the thermodynamic cycle can be a Rankine cycle using steam (water vapor) as a process fluid, rather than an organic fluid. The thermodynamic cyclecan include a recuperator with or without multiple pressure levels.

[0007] The thermodynamic cycle 3 comprises a heat exchanger section 11, which can include a pre-heater 11.1 and an evaporator 11.2. A pressurized process fluid flows through the cold side of the pre-heater 11.1 and of the evaporator 11.2 and is heated and evaporated in heat exchange with geothermal fluid from the geothermal production well 5, which flows in the hot side of the evaporator 11.2 and of the pre-heater 11.1.

[0008] The thermodynamic cycle 3 further comprises a turbine 13, wherein the evaporated process fluid expands, generating mechanical power which is available on a turbine shaft 15. The turbine shaft 5 can be drivingly coupled to a load, such as a compressor or another rotary machine, in some embodiments, the load can include an electric generator 17, which converts mechanical power into electric power, which is delivered to an electric power distribution grid 19, for instance.

[0009] The spent process fluid from the turbine 13 is condensed in a condenser 20 and pumped by a pump 21 towards the pre-heater 11.1. The thermodynamic cycle can include a recuperator 22, where spent process fluid from the turbine 13 can be cooled against a flow of condensed process fluid delivered by the pump 21.

[0010] Non-condensable gases can be either dissolved in the geothermal liquid from the production well or contained in gaseous form in the steam or two-phase (liquid and steam) mixture from the production well 5. The non-condensable gases are separated from the liquid phase directly into a portion of the pre-heater 11.1, for instance, or in a dedicated separator, as described in more detail below, and re-injected in the reinjection well 7 using the system 9 as follows.

[0011] Figs.2A, 2B, 2C show a schematic of the re-injection system 9 in one embodiment and in different steps of the re-injection process. The re-injection system 9 comprises a liquid-gas separator 31. As understood herein, a “liquid-gas separator” can be any piece of equipment in which an inlet flow containing a liquid and non-condensable gases is divided into a liquid stream and a gaseous stream, containing non-condensable gases, such as carbon dioxide, methane and other species separating from the geothermal fluid produced by the production well 5. For instance, the liquid-gas separator caninclude a heat exchanger which also performs other functions and operations within the plant 1, such as exchanging heat against the process fluid of the thermodynamic cycle 3. As mentioned above, the liquid-gas separator can be part of the pre-heater 11.1, for instance.

[0012] The liquid-gas separator 31 comprises an inlet 31.1, a liquid outlet 31.2 and a gas outlet 31.3.

[0013] The re-injection system 9 further comprises a plurality of vessels adapted to receive gas and liquid from the liquid-gas separator 31 in a specific sequence for the purpose of re-injecting the liquid and non-condensable gases in the re-injection well 7. In the embodiment of Fig.2 the re-injection system 9 comprises a first vessel 33 and 35. The two vessels 33, 35 can be identical or different from one another, for instance, the two vessels 33, 35 can have the same capacity, or different capacities.

[0014] The re-injection system 9 further comprises a pumping unit 37. In some embodiments, the pumping unit 37 can include a turbo-pump, for instance a centrifugal pump. Pumping unit 37 includes a suction side 37.1 and a delivery side 37.2. The pumping unit 37 is fluidly coupled with the liquid-gas separator 31, the first vessel 33 and the second vessel 35 through a piping system 39. Specifically, through the piping system 39 the suction side 37.1 of the pumping unit 37 can be fluidly connected with a lower part of the liquid-gas separator 31 and, more specifically with the liquid outlet 31.2 thereof. For this purpose, the piping system 39 includes a first connection 39.1 with a control valve 39.2. The control valve 39.2 can be selectively opened or closed to selectively open or close a fluid connection between the liquid outlet 31.2 of the liquid-gas separator 31 and the suction side 37.1 of the pumping unit 37.

[0015] The suction side 37.1 of the pumping unit 37 can further be fluidly coupled selectively with a lower side of the first vessel 33 and with a lower side of the second vessel 35. In the schematic of Fig.2, a duct 39.3 extends from the lower side of the first vessel 33 to the suction side 37.1 of the pumping unit 37, and a duct 39.4 extends from the lower side of the second vessel 35 to the suction side 37.1 of the pumping unit 37. A valve arrangement, can be used to fluidly couple the suction side 37.1 of the pumping unit 37 selectively with the liquid outlet 31.2 of the liquid-gas separator 31, with the lower side of the first vessel 33 and with the lower side of the second vessel 35.The valve arrangement includes the valve 39.2 and further two-way valves or a three- way valve, for instance. In the embodiment of Fig.2 the valve arrangement comprises a three-way valve 39.5.

[0016] The piping system 39 further comprises ducts adapted to fluidly couple the delivery side 37.2 of the pumping unit 37 electively to the first vessel 33 and to the second vessel 35. Specifically, in the embodiment of Fig.2, a duct 39.6 is provided for fluidly coupling the delivery side 37.2 of the pumping unit 37 to the first vessel 33. Moreover, a duct 37.7 is provided for fluidly coupling the delivery side 37.2 of the pumping unit 37 to the second vessel 35. A valve arrangement is further provided, to establish a fluid connection of the delivery side 37.2 with the first vessel 33 and the second vessel 35, selectively. In the embodiment of Figs. 2A, 2B, 2C the valve arrangement includes a three-way valve 39.8.

[0017] In the embodiment of Figs. 2A, 2B, and 2C the ducts 39.3 and 39.6 merge in a common port at the bottom or lower side of the first vessel 33; and the ducts 39.4 and 39.7 merge in a common port at the bottom or lower side of the second vessel 35. In a different embodiment, the ducts 39.6 and 39.3 can connect to the first vessel 33 in different positions, for instance in an intermediate position between the top and the bottom of the first vessel 33, and at the lower side or at the bottom of the first vessel 33. Similarly, in a different embodiment, the ducts 39.7 and 39.4 can connect to the second vessel 35 in different positions, for instance in an intermediate position between the top and the bottom of the second vessel 35, and at the lower side or at the bottom of the second vessel 35.

[0018] A piping system 41 is provided at the top of the liquid-gas separator 31, of the first vessel 33 and of the second vessel 35. The piping system 41 comprises a fluid connection 41.1 between the gas outlet 31.3 of the liquid-gas separator 31 and the first vessel 33 and second vessel 35. A valve arrangement, for instance including a three- way valve 41.2, is provided along the fluid connection 41.1 and is adapted to connect the gas outlet 31.3 of the liquid-gas separator 31 with the first vessel 33 and with the second vessel 35, respectively.

[0019] The piping system 41 further comprises a fluid connection 41.3 adapted to selectively connect the top or upper part of the first vessel 33 and of the second vessel35 with a fluid connection 41.4 to the geothermal re-injection well 7. A valve arrangement, which can include a three-way valve 41.5, is combined to the fluid connection 41.3, to establish a fluid coupling between either the first vessel 33 or the second vessel 35 and the re-injection vessel 7 through the fluid connection 41.4.

[0020] Non-retum valves, or check valves 45, 47 can be provided between the fluid connection 41.3 and the first vessel 33 and second vessel 35.

[0021] The re-injection system 9 and described so far can be operated according to sequential operations shown in Figs. 2A, 2B and 2C. In the various operating phases, ducts which are open and through which a fluid flows are shown in solid lines; ducts which are closed are shown with dashed lines.

[0022] In a first phase (Fig.2A), the suction side 37.1 of the pumping unit 37 is fluidly coupled with the lower part of the first vessel 33 through the three-way valve 39.5 and the duct 39.3. The delivery side 37.2 of the pumping unit 37 is fluidly coupled to the second vessel 35 through the three-way valve 39.8 and the duct 39.7. The gas outlet 31.3 of the liquid-gas separator 31 is fluidly coupled through duct 41.1 and three-way valve 41.2 to the first vessel 33.

[0023] The first vessel 33 contains liquid in the lower part and non-condensable gases in the upper part thereof. The pumping unit 37 pumps liquid from the first vessel 33 into the second vessel 35 such that the liquid level in the second vessel 35 increases. Non-condensable gases contained in the second vessel 35 are pressurized and pushed through the duct 41.3 and the three-way valve 41.5 in the fluid connection 41.4 and therefrom into the re-injection well 7 (Fig.l). The delivery pressure of the non-condensable gases in the fluid connection 41.4 can range for instance between 15 and 20 barA.

[0024] Since the liquid level in the first vessel 33 drops, non-condensable gases are delivered by suction from the gas outlet 31.3 into the first vessel 33.

[0025] In this phase, a pocket or column of non-condensable gases is formed in the re-injection well 7. The non-condensable gases are pressurized into the re-injection well by the pumping unit 37 which processes the spent geothermal liquid, pumping it from the first vessel 33 into the second vessel 35. The reduction of volume occupiedby non-condensable gases in the second vessel 35 is filled with liquid from the first vessel 33 with consequent suction of new non-condensable gases from the liquid-gas separator 31.

[0026] Spent geothermal fluid containing a liquid phase and non-condensable gases is meanwhile continuously fed through the inlet 31.1 into the liquid-gas separator, such that the liquid level inside the liquid-gas separator 31 increases, while the spent non- condensable gases escape through the gas outlet 31.3 towards the first vessel 33.

[0027] At some point in time, when the liquid level in the second vessel 35 has increased, even until the second vessel 35 is full of liquid, the operating condition of the pumping unit 37 is switched by switching the three-way valves 39.5 and 39.8, as shown in Fig. 2B, and a second operating phase starts. In this phase liquid is pumped from the bottom of the second vessel 35 through duct 39.4, three-way valve 39.5, pumping unit 37, three-way valve 39.8 and duct 39.6 into the first vessel 33.

[0028] Non-condensable gas which has been collected during the previous phase in the first vessel 33, is pushed under pressure, e.g., at 20-30 barA, through the three-way valve 41.5 into the fluid connection 41.4 and therefrom into the re-injection well 7. The pocket of gas in the re-injection well 7 continues growing in the re-injection well 7 and penetrates towards the bottom thereof. Non-condensable gases are sucked through the gas outlet 31.3, duct 41.1 and three-way valve 41.2 into the second vessel 35.

[0029] Pumping of liquid from the second vessel 35 into the first vessel 33 continues until a low-level of liquid is reached in the second vessel 35. At this point in time, the system is switched in the condition shown in Fig. 2C. During the phase shown in Fig. 2B, liquid continues to collect at the bottom of the liquid-gas separator 31, while non- condensable gases escape through the gas outlet 31.3 into the second vessel 35.

[0030] In the phase shown in Fig.2C, the suction side 37.1 of the pumping unit 37 is fludly coupled with the liquid outlet 31.1 of the liquid-gas separator 31. Liquid is pumped from the liquid outlet 31.1 and delivered to the first vessel 33 and therefrom into the re-injection well 7. The liquid pressure in the first vessel 33 and therefore in the fluid connection 41.3 can be around 15-30 barA, for instance. During this phasethe second vessel 35 can be isolated.

[0031] The liquid pumped into the re-injection well 7 during this phase forms a column of liquid, i.e., a column of liquid in the re-injection well 7. The pumping pressure and the weight of the column of liquid presses on top of the previously formed gas pocket, which is pushed further down into the re-injection well, until the non-conden- sable gases are dissolved in the liquid or absorbed in the solid structure at the bottom of the re-injection well.

[0032] When the liquid level in the liquid-gas separator 31 has dropped to the bottom of the liquid-gas separator 31, or if a column of liquid of sufficient height has been formed in the re-injection vessel 7, the system 9 is switched in the operating condition of Fig.2A again, and the cycle described above is repeated.

[0033] In some embodiments, switching from the phase of Fig. 2A to the phase of Fig.2B, from the phase of Fig.2B to the phase of Fig.2C, and from the latter to the phase of Fig.2A again can be controlled by suitable level sensors. In the embodiment of Figs. 2A, 2B, 2C a first level sensor 40 is associated with the first vessel 33, a second level sensor 42 is associated with the second vessel 35 and a third level sensor is associated with the liquid-gas separator 31. The level sensors 40, 42, 44 can be adapted to detect a minimum level threshold of the liquid in the respective vessel or separator, at which the system switches from one operating condition to another.

[0034] In some embodiments, the system switches from the operating condition of Fig.2A to the operating condition of Fig.2B when the level of the liquid in the first vessel 33 reaches a minimum threshold value detected by the first sensor 40. Indeed, in the condition of Fig.2A liquid is pumped out from the first vessel 33 and therefore the liquid level therein drops. Pumping from the first vessel 33 is interrupted before gas is sucked into the pumping unit 37. In phase 2B liquid is pumped from the second vessel 35. Suction of liquid from the second vessel 35 is interrupted when the liquid level in said second vessel 35 reaches the minimum threshold detected by the second sensor 42. The signal from the second sensor 42 causes the system to switch from the operating condition of Fig. 2B to the operating condition of Fig. 2C. This third operating condition lasts until the liquid level in the liquid-gas separator 31 reaches the minimum threshold detected by the third sensor 44.

[0035] In other embodiments, level sensors can be dispensed with. The system switches from one operating condition to the other when gas starts entering the suction side of the pumping unit 37. This can be detected indirectly, for instance by detecting a drop in the power absorbed by the motor driving the pumping unit 37.

[0036] In yet further embodiments, switching can be performed based on time and flowrate measurement, once the capacity (inner volume) of the liquid-gas separator 31, of the first vessel 33 and of the second vessel 35 are known.

[0037] Fig.3 schematically shows what happens inside the re-injection vessel 7 during the above-described operating cycle. A first column of liquid LSI formed in a preceding phase is shown in the bottom of the re-injection well 7. In the embodiment shown, the injection well has a depth of 3000 m, but it shall be understood that this value is by way of example only and shall not be construed as limiting the scope of the present disclosure. The first column of liquid LSI is shown as having a height of more than 1000m, again merely by way of example. On top of the first column of liquid LSI, a gas pocket GP1 of non-condensable gases is formed. A second column of liquid LS2 is forming on top of the gas pocket GPL By way of example, the pressure at the interface between the first column of liquid LSI and the gas pocket GP1 is around 110 barA and the pressure at the interface between the second column of liquid LS2 and the gas pocket GP1 is around 90barA.

[0038] These pressure values are generated by the delivery pressure of the pumping unit and by the weight of the column of liquids being formed in the re-injection well 7.

[0039] The high pressure achieved in the first column of liquid LSI causes non-con- densable gases, and in particular carbon dioxide (CO2) to dissolve in the liquid, which penetrates the solid structure surrounding the re-injection well 7. As a matter of fact, as the column of liquid and the gas pocket move towards the bottom of the re-injection well 7, the pressure and the mixing leads to formation of a single-phase flow when reaching the geological formation surrounding the bottom of the re-injection well.

[0040] An efficient re-injection process of non-condensable gases in a two-phase (liquid-gas) system is achieved using a centrifugal pump, rather than a reciprocatingcompressor.

[0041] Variable gas composition and variable gas / liquid ratios can be accommodated with a suitable timing of the different phases disclosed above.

[0042] In some embodiments, the centrifugal pump of the pumping unit 37 can be a variable speed pump driven by a variable speed electric motor, for instance. The rotary speed of the pump can vary as a function of the delivery pressure in order to optimize the pump efficiency. For instance, the rotary speed of the centrifugal pump can be slowed down as the height of the column of liquid increases.

[0043] Fig.4 illustrates a further embodiment of the re-injection system 9. The same reference numbers of Figs. 2A, 2B and 2C are used to designate the same or equivalent components, which will not be described again. The main difference between Fig. 4 and Figs. 2A, 2B, 2C is an additional fluid coupling 51 which connects the upper parts of the first vessel 33 and second vessel 35 to one another. A valve 52 is provided in the fluid coupling 51 to selectively connect and disconnect the two vessels 33, 35 from one another. The valve 52 and the fluid coupling 51 equalize the pressure in the two vessels during the phases shown in Figs. 2A and 2B. With this arrangement, some energy of the compressed gas released by the liquid can be recovered and transferred to the vessel which is in the compression phase, rather than reducing the useful volume of the vessel that is sucking-in new low-pressure gas.

[0044] With continuing reference to Figs 1 to 4, Figs. 5A, 5B, 5C, 5D illustrate a different operating sequence of the re-injection system 9 described above. The structure of the re-injection system is the same as shown in Figs. 2A, 2B, 2C and will not be described again. The same reference numbers designate the same elements shown in Figs 2A, 2B, 2C.

[0045] In Fig.5 A the pumping unit 37 pumps liquid from the first vessel 33 to the second vessel 35. Non-condensable gases are sucked from the top of the liquid-gas separator 31 into the first vessel 33 and non-condensable gases contained in the upper part of the second vessel 35 are pushed into the re-injection well forming a gas pocket.

[0046] In the next phase (Fig.5B) the first vessel 33 is inoperative and the pumping unit 37 pumps liquid from the bottom of the liquid-gas separator 31 into the secondvessel 35 and therefrom into the re-injection well 7, forming a liquid column.

[0047] In Fig.5C the operation of the pumping unit 37 is switched again and liqid ins pumped by the pumping unit 37 from the second vessel 35 into the first vessel 33, while non-condensable gases are pushed from first vessel 33 into the re-injection well 7 and sucked from the liquid-gas separator 31 into the second vessel 35. In Fig.5D the re-injection system 9 is again in the operating condition of Fig. 5 A.

[0048] With continuing reference to Figs 1 to 5D, Figs. 6A, 6B, 6C, 6D and 6E illustrate a different operating sequence of the re-injection system 9 described above. The structure of the re-injection system is the same as shown in Figs. 2A, 2B, 2C and will not be described again. The same reference numbers designate the same elements shown in Figs 2A, 2B, 2C.

[0049] In Fig.6A the pumping unit 37 pumps liquid from the first vessel 33 into the second vessel 35 and non-condensable gases are pushed thereby from the second vessel 35 into the fluid connection 41.4 and therefrom into the re-injection well 7, forming a gas pocket.

[0050] In the next step (Fig.6B) the first vessel 33 is inoperative and can be full of non-condensable gases. The pumping unit 37 sucks liquid from the liquid-gas separator 31 and pumps the liquid through the second vessel 35 and therefrom into the reinjection well 7.

[0051] In Fig.6C the pumping unit 37 has been switched to a further operation mode, in which liquid is sucked from the lower part of the second vessel 35 and pumped into the first vessel 33. Consequently, pressurized non-condensable gases are pushed from the first vessel 33 into the re-injection well 7 through the fluid connection 41.4. Non- condensable gases are further sucked from the top of the liquid-gas separator 31 into the second vessel 35.

[0052] In the next steph (Fig.6D) the second vessel 35 is inoperative and the pumping unit 37 pumps liquid from the bottom of the liquid-gas separator 31 through the first vessel 33 and therefrom into the re-injection well 7, forming a column of liquid on top of the gas pocket previously formed therein.

[0053] Finally (Fig.6E) the re-injection system 9 is switched again in the operating mode of Fig. 6 A.

[0054] With continuing reference to Figs.l to 6E, a further embodiment of the reinjection system 9 is shown in Fig.7, wherein the same reference numbers indicate the same or equivalent components already described in connection with Figs 2A, 2B, 2C, and which will not be described again.

[0055] In the embodiment of Fig.7, the re-injection system comprises three vessels 33, 35 and 36. A different number of vessels, for example two vessels as in Figs. 2A- 6E) or even more than three vessels in parallel can be used in alternative embodiments.

[0056] The delivery side 37.2 of the pumping unit 37 is adapted to be fluidly coupled selectively with one or the other of the paralleled vessels 33, 35, 36. the connection between the delivery side 37.2 and one or the other of the paralleled vessels 33, 35, 36 can be established by selectively opening and closing respective valves 63, 65, 67 and through a line 61.

[0057] The suction side 37.1 of the pumping unit 37 is in turn selectably connectable with one or the other of the vessels 33, 35, 36 placed in parallel, or with the liquid-gas separator 31. A valve 39.2 is positioned along the first connection 39.1, between the liquid outlet 31.2 and the suction side 37.1 of the pumping unit 37, as described above. A connection line 69 is further provided, which connects the suction side 37.1 of the pumping unit 37 with the three vessels 33, 35, 36 through respective valves 71, 73, 75. Connection between the suction side 37.1 of the pumping unit 37 and one or the other of the vessels 33, 35, 37 and the liquid-gas separator 31 is selectively established by switching the valves 39.2, 71, 73, 75 accordingly.

[0058] Non-condensable gases from each vessel 33, 35, 36 can be delivered into the re-injection well 7 through non return valves 45, 47, 48 and fluid connection 41.3, quite in the same way as shown in Figs. 2A, 2B, 2C. Non-condensable gases from the gas outlet 31.3 of the liqud-gas separator 31 can be delivered to each vessel 33, 35, 37 selectively, through fluid connection 41.1 and selectively activated valves 77, 79, 81.

[0059] The re-injection system 9 of Fig.7 can operate in substantially the same manner as the re-injection system described with reference to Figs. 2A-6E. By switchingthe operation mode of the pumping unit 37 in sequence and using the vessels 33, 35, 36 to collect non-condensable gases and liquid from the liquid-gas separator 31, gas pockets and liquid columns can be formed in sequence in the re-injection well 7.

[0060] While in the above exemplary embodiments a single power plant served by a single re-inj ection system is provided, in other embodiments multiple power plants can be served by a single re-injection system, or vice versa, multiple re-injection systems can serve one power plant.

[0061] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A system for re-injecting spent geothermal liquid and non-conden- sable geothermal gases in a geothermal re-injection well; the system comprising: a liquid-gas separator having an inlet, a gas outlet and a liquid outlet; a first vessel; a second vessel; a pumping unit having a suction side and a delivery side; and a fluid connection to the geothermal re-injection well; wherein: the suction side of the pumping unit is adapted to be fluidly coupled selectively with a lower side of the liquid-gas separator, a lower side of the first vessel and a lower side of the second vessel; the delivery side of the pumping unit is adapted to be fluidly coupled selectively with the first vessel and the second vessel; the gas outlet of the liquid-gas separator is adapted to be fluidly coupled selectively with an upper side of the first vessel and an upper side of the second vessel; and the fluid connection to the geothermal re-injection well is adapted to be fluidly coupled selectively with the upper side of the first vessel and the upper side of the second vessel.

2. The system of claim 1, wherein the first vessel and the second vessel vessel and the pumping unit are configured such that first vessel and the second vessel are alternatively filed and emptied with liquid by the pumping unit through suitably controlled valves, wherein the variable volume above the liquid in the first vessel and the second vessel is used to suck-in, compress and dispatch non-condensable gases from the liquid-gas separator into the geothermal re-injection well.

3. The system of claim 1 or 2, wherein the pumping unit comprises a dynamic pump, such as a centrifugal pump.

4. The system of claim 3, wherein the dynamic pump is a variable speed dynamic pump.

5. The system of any one of the preceding claims, further comprising a pressure equalizing connection between the first vessel and the second vessel.

6. The system of any one of the preceding claims, wherein a delivery side and a suction side of the pumping unit are connected to the lower side of the first vessel through a single duct, a suction control valve arrangement on the suction side and a delivery control valve arrangement on the delivery side of the pumping unit.

7. The system of claim 6, wherein a delivery side and a suction side of the pumping unit are connected to the lower side of the second vessel through a single duct, said suction control valve arrangement on the suction side and said delivery control valve arrangement on the delivery side of the pumping unit.

8. The system of claim 6 or 7, wherein the suction control valve arrangement comprises a three-way valve.

9. The system of claim 6, 7 or 8, wherein the delivery control valve arrangement comprises a three-way valve.

10. The system of any one of the preceding claims, wherein the upper side of the first vessel and the upper side of the second vessel are connected to the fluid connection to the geothermal re-injection well through a three-way valve.

11. The system of any one of the preceding claims, wherein the gas outlet of the liquid-gas separator is connected to the upper side of the first vessel and the upper side of the second vessel through a three-way valve.

12. A geothermal plant comprising: a geothermal fluid intake; a unit adapted to extract heat from the geothermal fluid;a system according to any one of the preceding claims.

13. The geothermal plant of claim 12, wherein the unit comprises a thermodynamic cycle adapted to convert heat extracted from the geothermal fluid into mechanical power.

14. The geothermal plant of claim 13, wherein the thermodynamic cycle comprises a turbomachine and an electric generator, drivingly coupled to the turbomachine, to covert mechanical power generated by the turbomachine into electric power.

15. The geothermal power plant of claim 13 or 14, wherein the thermodynamic cycle comprises: a closed thermodynamic circuit adapted to circulate a process fluid and comprising: a heater adapted to heat compressed process fluid; a power-generating turbomachine adapted to expand the compressed and heated process fluid; a cooler adapted to cool the expanded process fluid; a pumping unit adapted to pressurized the cooled process fluid; and a heat exchanger adapted to receive a flow of high-pressure and high-temperature geothermal fluid and to transfer heat from the geothermal fluid to the process fluid in the closed thermodynamic circuit.

16. A method for re-injecting geothermal liquid and geothermal noncondensable gases from a flow of exhaust geothermal fluid into a geothermal re-inj ection well, the method comprising the following steps:(i) pumping a pocket of pressurized non-condensable gases in the geothermal reinjection well;(ii) on top of the pocket of pressurized non-condensable gases, pumping a column of pressurized liquid in the geothermal re-injection well;(iii) repeating steps (i) and (ii).

17. The method of claim 16, comprising the following steps:delivering a flow of spent geothermal fluid, containing a liquid phase and non-condensable gases, in a liquid-gas separator having an inlet, a gas outlet and a liquid outlet; separately feeding liquid from a liquid outlet of the liquid-gas separator, and non-condensable gases from a gas outlet of the liquid-gas separator, in a pumping system comprising: at least a first vessel and a second vessel adapted to be fluidly coupled selectively with the liquid-gas separator and with the geothermal re-injection well; and a pumping unit for pumping liquid from one of said first vessel and second vessel to the other of said first vessel and second vessel and into the geothermal re-injection well; pumping from the system into the geothermal re-injection well sequentially said pocket of pressurized non-condensable gases and said column of liquid.

18. The method of claim 17, wherein the step of delivering from the system into the geothermal re-injection well sequentially said pocket of pressurized non- condensable gases and said column of liquid comprises the steps of: pumping liquid from one of said liquid-gas separator, first vessel and second vessel into the other of said first vessel and second vessel, and pushing non-condensable gases from the other of said first vessel and second vessel into the geothermal reinjection well, forming said pocket of pressurized non- condensable gases in the geothermal re-injection well; pumping liquid from the other of said first vessel and second vessel into the one of said first vessel and second vessel and pushing non-condensable gases from the one of said first vessel and second vessel into the geothermal re-injection well; pumping pressurized liquid from one of said first vessel and second vessel into the geothermal re-injection well.

19. The method of claim 16, comprising the following steps:(a) delivering a flow of spent geothermal fluid, containing a liquid and non-condensable gas, to a liquid-gas separator, the liquid-gas separator having an inlet, a gas outlet and a liquid outlet;(b) delivering non-condensable gases from the gas outlet of the liquidgas separator into the first vessel and simultaneously pumping liquid from the first vessel into the second vessel and pushing non-condensable gases from the second vessel into the geothermal re-injection well and forming a pocket of pressurized non-condensable gases therein;(c) delivering non-condensable gases from the gas outlet of the liquidgas separator into the second vessel and simultaneously pumping liquid from the second vessel into the first vessel and pushing non-condensable gases from the first vessel into the geothermal re-injection well and forming a pocket of pressurized non-condensable gases therein;(d) pumping liquid from a liquid outlet of the liquid-gas separator into the first vessel, into the second vessel, or into the first vessel and into the second vessel, and therefrom into the geothermal re-injection well, forming a column of liquid therein on top of the pocket of pressurized non-condensable gases;(e) repeating the sequence of steps (b) to (d).

20. The method of claim 16, comprising the following steps:(a) delivering a flow of spent geothermal fluid, containing a liquid and non-condensable gas, in the liquid-gas separator;(b) delivering non-condensable gases from the gas outlet of the liquidgas separator into the first vessel and simultaneously pumping liquid from the first vessel into the second vessel and pushing non-condensable gases from the second vessel the geothermal re-injection well and forming a pocket of non-condensable gases therein;(c) pumping liquid from the liquid-gas separator into the second vessel and pushing liquid from the second vessel into the geothermal re-injection well forming a column of liquid therein on top of the pocket of pressurized non-condensable gases;(d) delivering non-condensable gases in the second vessel, pumping liquid from the second vessel into the first vessel and pushing non-condensable gases from the second vessel into the geothermal re-injection well forming a pocket ofpressurized non-condensable gases therein;(e) repeating steps (b) to (d).

21. The method of claim 16, comprising the following steps:(a) delivering a flow of spent geothermal fluid, containing a liquid and non-condensable gas, in the liquid-gas separator;(b) delivering non-condensable gases from the gas outlet of the liquidgas separator into the first vessel and simultaneously pumping liquid from the first vessel into the second vessel and pushing non-condensable gases from the second vessel into the geothermal re-injection well and forming a pocket of non-condensable gases therein;(c) pumping liquid from the liquid-gas separator into the second vessel and pushing liquid from the second vessel into the geothermal re-injection well;(d) sucking non-condensable gases in the second vessel, pumping liquid from the second vessel into the first vessel and pushing non-condensable gases from the second vessel into the geothermal re-injection well;(e) pumping liquid from the liquid-gas separator in the first vessel and pumping liquid from the first vessel in the geothermal re-injection well and forming a column of liquid therein;(f) repeating steps (b) to (e).

22. A method for re-injecting geothermal liquid and geothermal non- condensable gases from a flow of exhaust geothermal fluid into a geothermal re-inj ection well, the method comprising the following steps: delivering a flow of spent geothermal fluid, containing a liquid phase and non- condensable gases, in a liquid-gas separator; delivering non-condensable gases from a gas outlet of the liquid-gas separator selectively into at least a first vessel and a second vessel; pumping liquid from the liquid-gas separator into at least one of said first vessel and second vessel;pumping liquid from one of said first vessel and second vessel into the other of said first vessel and second vessel; delivering from at least one of said first vessel and second vessel into the geothermal re-injection well sequentially a pocket of pressurized non-condensable gases and a column of liquid.

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