FR2881482A1 - Mechanical energy producing method, involves extracting carbon-di-oxide at supercritical vapor state from underground storage tank and sending vapor through turbine till pressure less than critical pressure of carbon-di-oxide - Google Patents

Abstract

The method involves extracting carbon-di-oxide at supercritical vapor state from an underground storage tank. The vapor is sent through a turbine until a pressure less than the critical pressure of the carbon-di-oxide. The mechanical energy is collected on a shaft of the turbine. The expanded carbon-di-oxide vapor is liquefied and the liquid carbon-di-oxide vapor is injected in the storage tank.

Description

The present invention relates to the production of mechanical energy from

of geothermal energy.

  Geothermal energy is an energy resource with many advantages. It avoids the use of exhaustible fossil fuels. Moreover, it does not produce and avoid sending CO2 to the atmosphere. In addition, geothermal energy does not have the intermittent nature of solar or wind energy. So, from this point of view, it is well adapted for a production base electrical.

  One difficulty, however, is that for depths commonly achieved by drilling, except in regions where the geothermal gradient is particularly high, geothermal heat is available at relatively low temperatures, not exceeding 150 ° C.

  In such a case, it is not possible to generate water vapor directly in the basement. A circulation of hot water is therefore used, using a geothermal doublet formed by an injection well and a production well: the hot water is produced by a first well and after heat exchange, it is injected into the subsoil by a second well.

  This has many disadvantages: It must circulate very large flows of water, and for a certain extracted power, we can limit the flow by increasing the difference between the temperature produced and the temperature of the water reinjected . As a result, the efficiency of the energy production cycle decreases.

  - In most cases, an indirect cycle is required, in which the Rankine cycle fluid is vaporized by heat exchange, which further lowers the vaporization temperature, due to the temperature difference in the exchanger heat. In the case of geothermal energy at low or medium temperature, it is necessary to use an organic fluid, with the disadvantages that this represents: fluid availability, risks related to its handling, complexity and increased cost of the installation.

  Hot water that rises under pressure solubilizes salts present in the rock and this especially as the temperature is high. The brine circulating in the geothermal doublet is therefore very corrosive.

  US 6,668,554 B1 proposes using carbon dioxide as a fluid to recover geothermal heat by re-injecting it in the supercritical state. This exploitation is carried out in the context of geothermal dry rock, carbon dioxide being used to fracture the rock and then circulating in the fractures. The process disclosed in US 6,668,554 B1 is well suited for high temperature operations, preferably between 150 and 500 C.

  The present invention proposes to produce mechanical energy from geothermal energy which makes it possible in particular to operate in a powerful and economical way the geothermal deposits at low or medium temperature.

  The invention consists in using as thermodynamic fluid CO2, which is injected into the liquid phase in the subsoil, is recovered in the form of supercritical steam at relatively high pressure and at high temperature, is expanded, cooled by exchange with an ambient fluid of cooling, and recycled.

  In general, the present invention relates to a method for producing mechanical energy from geothermal energy, in which the following steps are carried out: a supercritical gaseous fluid is extracted from an underground reservoir, the fluid comprising at least 50% CO2, the fluid is expanded through a turbine, mechanical energy is recovered from the turbine shaft, the expanded fluid is liquefied, for example by cooling, the liquid fluid in the underground reservoir.

  According to the invention, the fluid can be expanded to a pressure below the critical pressure of CO2. In addition, the fluid can be expanded to a pressure greater than the CO 2 condensation pressure, this condensing pressure being taken at the liquefaction temperature of the expanded fluid.

  In addition, an additional flow of CO2 can be injected for storage in the underground reservoir.

  The underground reservoir may be an aquifer and / or a hydrocarbon reservoir. The underground reservoir can be a medium formed by a rocky or carbonate rock.

  According to the invention, the extraction and injection of the fluid can be carried out through wells having walls coated with a layer of thermally insulating material.

  It is possible to liquefy said fluid by cooling it by heat exchange with at least one of the following elements of the ambient air, water, a cooling fluid produced by a refrigeration circuit.

  Before injection, the fluid can be stored in liquid form. The recovered mechanical energy can be transformed into electrical energy.

  The fluid may comprise less than 10% water. More specifically, the fluid may comprise more than 90% of CO2.

  Other features and advantages of the invention will be better understood and will become clear upon reading the description given below with reference to the drawings, of which: FIG. 1 illustrates the principle of the invention, FIG. A thermodynamic diagram of CO2, FIG. 3 represents an embodiment of the invention, FIG. 4 represents a variant of the principle of the invention.

  In Figure 1, the CO2 is in an underground reservoir R in supercritical vapor state at a temperature and a pressure, which depend on the depth and the geothermal gradient. For example in the case of a depth of between 2000 m and 3000 m, the pressure is usually between 200 bar and 300 bar, but may be higher in the case of a geo-pressurized reservoir. The temperature of the reservoir may be between 70 C and 150 C.

  The supercritical CO2 vapor goes back to the supercritical state by the PR production well. The CO vapor which rises can be saturated with water, but in the case of an average thermal gradient of the order of 30 C per kilometer, the water content does not exceed a few percent. In the case of a high geothermal gradient the proportion of water in the CO vapor phase, which goes up through the PR production well can become relatively large and in some cases exceed 10%. In other words, the pressure of the CO inside the PR well is greater than the critical pressure of CO2, that is to say greater than 75 bars. The rise of CO2 in the PR well causes a drop in pressure. Going up supercritical CO vapor limits the weight of the hydrostatic column, and thus limits the pressure drop during CO 2 extraction. For example, if the fluid is initially at 200 bars and 150 C, at a depth of 2000 m in the tank R, the rise in the well results in a pressure drop of the order of 60 bars.

  It is advantageous to operate this ascent by keeping the temperature as high as possible. For this, the upwell is preferably thermally insulated. This minimizes the enthalpy drop of the CO 2 vapor.

  CO, a non-salt vapor, avoids the major problems of corrosion and solid deposition associated with high temperature brine circulation.

  At the output of the PR production well, the supercritical vapor phase is sent through line 1 to the expansion device Ti. The vapor phase is then expanded in the expansion device Ti, producing mechanical energy, which can be used to drive an electric generator. The vapor phase is expanded to a pressure below the critical pressure of CO2, for example at a pressure below 70 bar. Preferably, the vapor phase is expanded to a pressure close to the condensation pressure. Then, the expanded CO 2 is evacuated from the expansion device Ti via the duct 2. Thus, for example, the CO 2 is expanded to approximately 20 ° C. and approximately 60 bar, conditions which correspond to a CO dew point.

  The CO, expanded is condensed in the heat exchanger Cl. For example, the CO is cooled by indirect heat exchange with water from a water table, which is for example at a temperature of 12 C.

  The liquid CO2 is evacuated from the heat exchanger C1 through the conduit 4, and is stored in the storage tank B1.

  At the outlet of the storage tank B1, the CO is taken up by the pump P1 and injected via the pipe 5 into the injection well PI.

  In addition, a supplement of CO2 arriving via the conduit 3 can also be injected into the injection well PI to be stored in the reservoir R. This allows the injection of CO2 into the subsoil, which causes the There is now a growing interest in limiting CO2 emissions into the atmosphere. Indeed, it has recently been recommended to inject CO2 into the liquid phase and the conditions necessary to perform such an injection in the liquid phase have been specified. The method according to the invention can thus be coupled to a device for injecting CO2 into the subsoil for the purpose of storing CO2. In this case, a flow of CO2 is circulated in the injection well which is the sum of the injected flow rate to be stored and the re-circulating flow rate.

  The injection well PI is preferably isolated, to maintain the fluid stream at a temperature as low as possible and thus maximize the rise in pressure during the descent of the fluid in the injection well PI into the reservoir R. CO2 in liquid form can be easily reinjected into a well. Indeed the weight of the hydrostatic column of liquid CO2 facilitates the injection. In addition, by injecting the CO2 in liquid form, the weight of the hydrostatic column in the well PI is greater than the weight of the hydrostatic column of CO2 in the state of supercritical vapor in the well PR. This makes it possible to produce CO2 at the top of the PR well at a pressure greater than the CO2 injection pressure in the PI well. The injection pressure and / or the CO2 injection temperature can be selected in the well P1 in order to produce CO2 at the top of the PR well in the supercritical vapor state. For example, the injection pressure of the CO2 can be adjusted by means of the pump P1 and the injection temperature of the CO2 can be adjusted by means of the heat exchanger Cl. In addition, the pressure can be controlled and adjusted. of CO2 production by means of a valve arranged at the top of the production well PR.

  Thus, according to the invention, the fact of injecting the CO2 in liquid form makes it possible to produce CO2 in the supercritical vapor state, then to directly produce mechanical energy by expansion of this CO2 vapor. According to the invention, the CO2, because of its thermodynamic characteristics, is particularly well suited to the implementation of the method according to the invention for producing mechanical energy from geothermal energy from a low or medium reservoir. temperature, for example between 70 C and 150 C.

  The numerical example presented below in relation to the thermodynamic diagram of FIG. 2 serves to illustrate the invention. FIG. 2 represents the thermodynamic equilibrium curves of the CO2 in a diagram indicating the enthalpy on the abscissa and the pressure on the ordinate.

  The CO2 is stored in an underground reservoir at a depth of about 2000 m, at a pressure of about 260 bar (point (1) on the diagram) and at a temperature of 130 C. At the head of the well the pressure is of the order of 200 bar and the temperature is about 120 C (point (2) on the diagram). After expansion in the turbine T1, the pressure is close to 60 bar (point (3) in the diagram). The isentropic efficiency of the expansion turbine being 90%, the mechanical work produced is 54 kJ / kg of CO2. Thus, if one million tons of CO2 are circulated per year, the mechanical power produced in the Ti turbine is about 6.75 MW.

  The CO2 is then condensed at 20 ° C. and leaves the condenser C1 at 60 bars and 20 ° C. (point (4) in the diagram). It is then injected into the injection well, in which the pressure rises under the effect of gravity. When the CO2 circulates in the tank R, its temperature increases until reaching the temperature of the tank R (point (1) on the diagram).

  The reservoir R can be any type of subsoil forming a porous and permeable medium, which can serve as a storage volume for the injected carbon dioxide.

  This porous and permeable medium may be formed for example by a layer of sandstone or by a carbonate medium, surmounted by an impermeable cover, such as clay or shale.

  The porous medium is typically occupied by an aquifer. The depth of the aquifer into which the carbon dioxide is injected may be, for example, between 100 m and 3000 m deep. The temperature may for example be between 70 ° C. and 150 ° C.

  The porous and permeable medium can also be initially occupied by a hydrocarbon and in particular by a crude oil. In this case, the CO2 injection can contribute to improving the recovery of the oil contained in the reservoir formed by the porous medium.

  The vaporized carbon dioxide expansion energy production is preferably combined with a storage of carbon dioxide in the subsoil. In storage period, the flow injected into the basement is greater than the flow withdrawn.

  With reference to FIG. 3, each of the production and injection wells may comprise several parts (4 and 6, 7 and 9), corresponding to successive drilling phases. Each well is cemented and isolated. The CO 2 in the supercritical vapor state is evacuated by the production well PR, through the production tube 10. The tube 10 is placed in a well comprising the casings of wells 7 and 9 of decreasing section with the depth, the it is drilled in successive phases with tools of decreasing diameter. In the lower part of the casing 7, the casing is held by sealing means. The cooled CO2 is injected into the injection well PI. It circulates in the tube 3, placed in a well PI comprising well casings 4 and 6 of decreasing section with the depth.

  When the temperature of the external cooling fluid does not allow the CO2 to be cooled below its critical temperature and to condense it in the liquid phase, the CO can be cooled so as to condense it by using a refrigerating cycle, which is entrained in taking, for example, a fraction of the mechanical power produced by the expansion machine. FIG. 4 represents a part of the process of FIG. 1, in which exchanger C1 has been replaced by exchanger E1. With reference to FIG. 4, the CO, vapor leaving the expansion turbine Ti is cooled in the exchanger E 1 by a refrigerant, which is vaporized in the exchanger E1, compressed in the compressor K1, and then condensed in the condenser C, by exchange with the ambient cooling fluid water or air available, then expanded in the expansion valve Vl and recycled to the exchanger El.

  Various provisions may be adopted in the context of the invention. For example, it is possible to use several production wells associated with one or more injection wells. Different distances between the wells and different geometries in the arrangement of the wells can be used, without departing from the scope of the present invention.

  It is also possible to use different geometries of wells: deviated or horizontal, to make the best use of the geothermal resource.

  After relaxation, it is also possible to cool the CO, by heating an installation or a residential complex.

  Different materials and different types of equipment can be used without departing from the scope of the invention. The casings of the wells can be made of steel, receiving an anti-corrosion coating. The exchangers used may be tube-calender or plate type. The expansion device is preferably constituted by a turbine, but may also be constituted by a piston machine.

Claims (12)

  1) A method for producing mechanical energy from geothermal energy, in which the following steps are carried out: a supercritical gas-state fluid is extracted from an underground reservoir, the fluid comprising at least 50% CO2, the fluid is expanded through a turbine, mechanical energy is recovered from the turbine shaft, the expanded fluid is liquefied, the liquid fluid is injected into the underground reservoir.   2) The method of claim 1, wherein the fluid is expanded to a pressure below the critical pressure of CO2.   3) Process according to claim 2, wherein the fluid is expanded to a pressure greater than the condensation pressure of CO2.   4) Method according to one of the preceding claims, wherein is injected, in addition, an additional flow of CO2 to be stored in the underground reservoir.   5) Method according to one of the preceding claims, wherein the underground reservoir is selected from one of the following tanks: an aquifer and a hydrocarbon reservoir.   6) The method of claim 5, wherein the underground reservoir is a porous and permeable medium formed by one of the following media: rock sands and carbonate rock.   7) Method according to one of the preceding claims, wherein the extraction and the injection of the fluid are performed through wells having walls coated with a layer of thermally insulating material.   8) Method according to one of the preceding claims, wherein is liquefied by cooling said fluid by heat exchange with at least one of the following elements: ambient air, water, a refrigerant produced by a refrigeration circuit.   9) Method according to one of the preceding claims, wherein before the injection, the fluid is stored in liquid form.   10) Method according to one of the preceding claims, wherein the recovered mechanical energy is converted into electrical energy.   11) Method according to one of the preceding claims, wherein said fluid comprises less than 10% water.   12) Method according to one of the preceding claims, wherein said fluid comprises more than 90% of CO2.