NITROGEN INJECTION

Abstract: A method for production of nitrogen for application at high pressure, preferably for pressure injection in submarine or subterranean oil or natural gas wells, is utilized in a system for combustion of natural gas in air for binding of the oxygen content of the air in carbon dioxide, which is removed from the flue gas by absorption in water, preferably in atomized form, whereas the remaining nitrogen is pressurized to a predetermined high pressure suitable for the intended application. In order to reduce the weight of the system the combustion air is compressed prior to the combustion to an intermediate pressure between the predetermined high pressure and the atmospheric pressure, and substantially above the latter pressure, whereupon the combustion takes place at said intermediate pressure. In practice the injection pressure is of the order of magnitude 100-500 bars, and an intermediate pressure of 10-60 bars, preferably 25-40 bars, is then suitably applied, in agreement with optimizing calculations in each case. The developed heat energy by the combustion is heat exchanged at the intermediate pressure with water for production of steam, which subsequently is utilized for mechanical work, preferably in steam turbines (TL, TH) for compression of the combustion air to the intermediate pressure and the produced nitrogen gas to the injection pressure, respectively. The residual oxygen in the flue gas is removed by addition of hydrogen to be combusted with the oxygen while the flue gas is guided past a catalyst at said intermediate pressure. The carbon dioxide is removed from the flue gas at said intermediate pressure by absorption in water, e.g. sea water and preferably in atomized form, and possibly in combination with the cooling of the flue gas.

Description: Nitrogen injection The present application is related to a method and a system for production of nitrogen for application at high pressure, preferably for pressure injection in submarine or subterran¬ ean oil or natural gas wells. With recovering of oil and gas, in particular from submarine wells, it is in certain cases of importance to perform a pressure injection of nitrogen gas into the well in question to increase the operational pressure and prevent the well from collapsing. Such a situation- is present for the Ekko- fisk field in the North Sea, and in order to generate the necessary large amounts of nitrogen gas in this connection, it is suggested to produce nitrogen by combustion of natural gas in air for binding of the oxygen content of ±he air in carbon dioxide, which then is removed from the flue gas by absorption, whereas the remaining nitrogen is pressurized to a predetermined high pressure suitable for said application. In the suggested system the combustion takes place partly in gas turbines and partly through direct combustion of gas in boiler systems at atmospheric conditions. The system is then very space-demanding, unpractical and heavy. This implies very great disadvantages when mounted on an off-shore platform, and it is then an object of the inven¬ tion to provide a method as stated above, but in which the indicated disadvantages are overcome. Thus, the invention concerns a method for production of nitrogen for application at high pressure, preferably for pressure injection in submarine or subterranean oil or natural gas wells, by combustion of natural gas in air, which is compressed prior to the combustion, for binding of the oxygen content of the air in carbon dioxide to be removed from the flue gas by absorption, whereas the remaining nitro¬ gen is pressurized to a predetermined high pressure suitable for said application. A compression of the combustion air for the present purpose is known in principle from British Patent Specification No. 1.440.476, and in view of this known prior art, the unique features of the method according to the invention are that the compression of the combustion air prior to the combustion is made to a preselected, operationally optimum intermediate pressure between said predetermined high pressure and the atmospheric pressure, and substantially above the latter pressure, the heat energy developed by the combustion being heat exchanged at the intermediate pressure with water for production of steam, which subsequently is utilized for mechanical work, preferably in steam turbines for compression to the intermediate pressure and maintenance of said pre¬ determined high pressure. As the injection pressure in this connection would be of the order of magnetude 100-500 bars, an immediate pressure of 10-60 bars, normally 25-40 bars, is preferably applied. The residual oxygen in the flue gas is preferably removed by addition of hydrogen for wet combustion with the oxygen while the flue gas is guided past a catalyst at said intermediate pressure. By compressing the combustion air to e.g. 25-40 bars before the combustion and performing the gas combustion and energy recovering at this pressure, the gas volumes under treatment are reduced in the corresponding proportion, i.e. with a factor at 1/25 - 1/40, and at the same time the heat transfer conditions are dramatically improved. The invention also concerns a system for production of nitro¬ gen by the method indicated above, the system comprising a combustion chamber for combustion of natural gas in air, which is compressed prior to the combustion, for binding of the oxygen content of the air in carbon dioxide to be removed from the flue gas in an absorption device, and an outlet compressor for pressurizing the remaining nitrogen to a predetermined high pressure suitable to said application. In view of prior art the system of the invention is then characterized by the feature that it further comprises an in¬ let compressor disposed and adapted for said compression of the combustion air to a preselected, operationally optimum intermediate pressure between said predetermined high pres¬ sure and the atmospheric pressure, and substantially above the latter pressure, as well as for feeding the pressurized air to the combustion chamber, which is adapted for com¬ bustion in the air at said optimum intermediate pressure, and a heat exchanger connected with the combustion chamber and adapted for heat exchange at said intermediate pressure between the developed flue gases and supplied water for steam production in a steam boiler, preferably for operation of steam turbines disposed for the driving of * said inlet and outlet compressors . The absorption device preferably comprises nozzles for in¬ jection and atomization of water in the flue gas in order to achieve the absorption of the carbon dioxide content of the gas at said intermediate pressure. Optionally, a further steam turbine may be included for ope¬ ration of an electrial generator when supplied with possible excess steam. A system of this type would be essentially more lightweight- ed, less costly, plainer and more reliable than the above- mentioned, suggested system for the same purpose. The invention will now be described in more detail by means of an exemplified embodiment with reference to the accompa¬ nying drawing, the only figure of which shows a schematical outline of a system according to the invention. The core of the illustrated system is a combustion chamber 1, which is supplied with methane as well as combustion air, which by means of an inlet compressor KI is compressed to an intermediate pressure of 25-40 bars, and then the combustion of methane in the combustion chamber 1 also takes place at this pressure. The quantity ratio between the supplied methane and the combustion air is generally adjusted in such a manner that essentially complete combustion is achieved at the intermediate pressure, which in practice means that less than 0,5% oxygen is present in the flue gases developed by the combustion. Under certain operative conditions, however, it may be appropriate to perform the combustion substoichio- metric. The flue gases are guided through a heat exchanger 2 for exchanging heat with supplied water for produciton of steam in a steam boiler D. This heat exchange then takes place very efficiently at the established -intermediate pressure of the magnetude 25-40 bars . From the heat exchanger the exhaust or flue gas is furthered to a combustion unit 3, in which the residual oxygen is re¬ duced to a proportion of 5 ppm by addition of hydrogen to the flue gas, which subsequently is made to pass a catalyst at a temperature of the order of magnitude 180 C. Combined cooling of the flue gas and removal of the carbon dioxide takes place in a cooling and adsorption device 4. During the treatment of the flue gas the pressure in this device is still 25-40 bars, and sea water in atomized form is then used as a CO_-absorbent. Under the prevailing condi¬ tions this device may in fact be included in a power-produc¬ ing cooling and absorption process developed by Norsk Hydro, as the expansion of CO- provides a further power contri¬ bution. After the removal of posible residual oxygen and the carbon dioxide content of the flue gas, the gas delivered from the device 4 would essentially consist of nitrogen at a pressure of 25-40 bars, corresponding to the established intermediate pressure according to the invention. This nitrogen gas is then supplied to an outlet compressor KU for increasing the gas pressure to the predetermined injection pressure, which in the present case is 277 bars. As mentioned above, the heat energy recovered from the heat exchanger 2 is utilized for production of steam in a steam bolier D, preferably for the operation of steam turbines. These turbines are primarely a high pressure turbine TH for the operation of the outlet compressor and a low pressure turbine for the operation of a inlet compressor, but beyond this the process would normally provide excess energy for operation of e.g. an electric generator G by means of a further steam turbine TM, which is inserted in the steam cir¬ cuit between the high pressure turbine TH and the low pressure turbine TL. On the outlet side of the turbine TL the exploited steam is conveyed to a condenser K, from which condensed water is again supplied to the heat exchanger 2 for steam production. As an example of operational parameters in the above-mention¬ ed system it may be indicated that a produced nitrogen volume of 17 millions m 3 per 24 hours corresponds the following values : Injected nitrogen volume 186 Nm 3 /s Injection pressure 277 bars Air volume for compression 235 Nm 3 /s Methane consumption 24,7 Nm 3 /s Energy conent of the methane 890 Mw Output from the steam turbines 330 Mw Required compression power 210 Mw Net power surplus 120 Mw As deduced from this example, the process gives a power sur¬ plus of 120 Mw. This excess may be utilized for production of electric energy, which advantageously may be used on the involved production platform for oil and gas recovery. This may substantially reduce the natural gas consumption of the platform, as gas turbine operated current generators with low thermal efficiency may be excluded. Combustion and heat transfer at 25-40 bars would to a high extent reduce the space requirements of a system for the pre¬ sent purpose. At the first heat surface of the heat exchang¬ er heat transfer of the order of magnetude 1 Mw/m 2 is achiev¬ able. Thus, the process outfit would be very compact. A combustion chamber with attached interconnected heat exchang¬ er and steam boiler having diameter 4 m and length 15 may be considered. With a flow velocity of 0,5 m/s through a cooling and absorption tower 4 its external "diamter would be 5 m. In each case a thorough optima ion calculation would be required in order to establish the most favourable intermed¬ iate pressure. It is possible that intermediate pressures up to 50 bars and higher would provide optimum operational con¬ ditions in certain cases. Apart from steam turbines, compressors and electric generator the outfit included in the present system would be of a low- cost type. Thus, a very reasonable and plain system for nitrogen injection would be achievable according to the present invention.

Claims: - Patent Claims 1. Method for production of nitrogen for application at high pressure, preferably for pressure injection in sub¬ marine or subterranean oil or natural gas wells, by combustion of natural gas in air, which is compressed prior to the combustion, for binding of the oxygen content of the air in carbon dioxide to be removed from the flue gas by ab¬ sorption, whereas the remaining nitrogen is pressurized to a predetermined high pressure suitable for said application, c h a r a c t e r i z e d i n that the compression of the combustion air prior to the combustion is made to a pre¬ selected, operationally optimum intermediate pressure between said predetermined high pressure and the atmospheric pres¬ sure, and substantially above the latter pressure, the heat energy developed by the combustion being heat exchanged at the intermediate pressure with water for production of steam, which subsequently is utilized for mechanical work, pre¬ ferably in steam turbines (TH, TL) for compression to the intermediate pressure and maintenance of said predetermined high pressure. 2. Method as claimed in claim 1, c h a r a c t e r i z e d i n that an intermediate pres¬ sure of 10-60 bars, preferably 25-40 bars is applied. 3. Method as claimed in claims 1 or 2 , c h a r a c t e r i z e d i n that the residual oxygen in the flue gas is removed by addition of hydrogen for combus¬ tion with the oxygen while the flue gas is guided past a catalyst. 4. Method as claimed in claims 1 - 3, c h a r a c t e r i z e d i n that the carbon dioxide is removed from the flue gas at said intermediate pressure by absorption in water, e.g. sea water and preferably in atomi¬ zed form, and possibly together with the cooling of the flue gas . 5. System for production of nitrogen for application at high pressure, preferably for injection at high pressure in submarine or subterranean oil or natural gas wells, and com¬ prising a combustion chamber (1) for combustion of natural gas in air, which is compressed prior to the combustion, for binding of the oxygen content of the air in carbon dioxyde to be removed from the flue gas in an absorption device (4), and an outlet compressor (KU) for pressurizing the remaining nitrogen to a predetermined high pressure suitable for said application, c h a r a c t e r i z e d i n that the system further comprises an inlet compressor (KI) disposed and adapted for said compression of the combustion air to a preselected, operationally optimum intermediate pressure between said pre¬ determined high pressure and the atmospheric pressure, and substantially above the latter pressure, as well as for feed¬ ing the pressurized air to the combustion chamber (1), which is adapted for combustion of natural gas in the air at said optimum intermediate pressure, and a heat exchanger (2) connected with the combustion chamber (1) and adapted for heat exchange at said intermediate pressure between the developed flue gases and supplied water for steam production in a steam boiler (D), preferably for operation of steam turbines (TH, TL) disposed for the driving of said inlet and outlet compressors (KI, KU) . 6. System as claimed in claim 5, c h a r a c t e r i z e d i n that it comprises a com¬ bustion unit (3) disposed for throughflow of the flue gas and adapted for supply of hydrogen and combustion of the same with the residual oxygen in the flue gas in association with a catalyst at said intermediate pressure. 7. System as claimed in claim 5 og 6 , c h a r a c t e r i z e d i n that the absorption device (4) comprises nozzles for injection and atomization of water in the flue gas for absorption of its carbon dioxide content at said intermediate pressure. 8. System as claimed in claims 5-7, c h a r a c t e r i z e d i n that it comprises a further gas turbine (TM) for the operation of an electric generator (G) by supply of possible excess steam from the steam boiler (D).

Inventors: GRAN HELGE GERHARD, HARTMANN GOTFRED, GRAN, HELGE, GERHARD, HARTMANN, GOTFRED
Applicants: NORSK ENERGI, KJELFORENINGEN-NORSK ENERGI
Application year: 1987
Main classification: C01B21
IPC codes: B01J19, C01B21, E21B41, E21B43