ENVIRONMENTALLY FRIENDLY METHOD FOR GENERATING ENERGY FROM NATURAL GAS

Abstract: The invention relates to an environmentally friendly method for generating energy from natural gas. Through the combustion of the natural gas with a mixture comprising substantially oxygen and CO2, a number of advantages are achieved with regard to the emission of CO2 and NOx. According to the invention, the CO2-rich product gas is injected into a natural gas reservoir from which natural gas has been extracted. By the underground replacement of natural gas by CO2, a pressure drop in the natural gas reservoir can be controlled, so that subsidence is prevented. Additionally, in this manner, the emission of CO2 is reduced or even wholly prevented.

Description: Title: Environmentally friendly method for generating energy from natural gas The invention relates to a method for producing energy from natural gas coming from a natural gas reservoir. The invention further relates to the use of CO 2 from combustion gases as injection medium in natural gas reservoirβ. The extraction and subsequent combustion of natural gas for generating energy has a number of undesired effects on the environment. The extraction of natural gaβ has as a result that the preββure in the natural gas reservoir decreaβes. This may lead to so-called compaction of the reservoir rock, which may cause undesired soil subsidence. The degree of compaction and the subsequent soil subsidence is highly dependent on the soil conditions between the reservoir and the surface, the properties of the reservoir rock and the preββure decrease caused by gas extraction. Depending on the location of the natural gaβ reservoir, however, minor subsidence may already result in major damage to the environment, including any buildings present therein. Additionally, in the combustion of natural gas, waβte products, in particular water, CO2 and nitrogen oxides (NOx) are formed. The carbon dioxide iβ an undesired product, since this gas is considered one of the main causative agents of the so-called greenhouse effect. NOx iβ noxious because it can lead to acid rain and smog formation. Moreover, the decrease of the preββure in the natural gas reservoir has as a result that, during exploitation, in time, the preββure falls to the extent where it is no longer profitable to further exploit the reservoir. Although in some caβeβ the natural affluence of water iβ such that the decrease in the preββure caused by the extraction of natural gas is to some extent compensated for, this compensation is often not sufficient. In the state of the art, for thiβ reason inter alia, use is made of injection of gases or water to maintain the pressure in the natural gas reservoir. It has been proposed, for instance, to use an inert gaβ, such as nitrogen, as a replacement for the extracted natural gas. A drawback of the use of nitrogen is that it is expensive and therefore this method is often economically not profitable. In natural gaβ extraction, injection of air iβ not attractive because of the reactivity of oxygen with natural gaβ. As an alternative, use is sometimes made of injection of water. However, this has as a disadvantage that with this method, the natural gas iβ replaced with the fluid water. The reservoir then becomes filled with water, so that the pressure distribution in the reservoir changes, which can lead to disruptions at the earth's surface. Another result of pressure drop in combination with affluence of and/or injection of water is that a part of the natural gas is entrapped in the subterranean rock. If this rock becomes submerged in water, the extraction of the gaβ trapped therein is rendered more difficult. Also, the economic life of the reservoir iβ shortened because the water content in the extracted natural gas increases as a result of the injected water. If the water content is too high, further exploitation of the natural gas reservoir iβ no longer attractive. The object of the present invention is to provide a method for generating energy from natural gas, which method removes the above-mentioned disadvantages at least partly. It haβ been found that a method in which the combustion of natural gaβ takes place in the presence of substantially oxygen and CO 3 , and in which the CO a originating from the combustion gases of the natural gaβ combustion plant is injected into the natural gas reservoir, resolves the above-mentioned problems at least partly. Therefore, the invention is characterized by a method for producing energy from natural gas, comprising the steps of; a) converting natural gaβ coming from a natural gaβ reservoir, in the presence of substantially oxygen and COa, into a flow comprising COg and water, whereby energy is generated; b) removing at least a part of the water from the flow comprising COa and water, so that a product flow iβ obtained; and c) injecting at leaβt a part of the product flow into a natural gaβ reservoir from which natural gaa has been extracted. Such a method does not possess, or does so to a limited extent only, the disadvantages entailed in the known methods. Owing to the natural gas being combusted in the presence of substantially oxygen and COa, a substantial reduction in the production of NOx is achieved. The fact is that in the usual combuβtion processes, NOx is formed to a considerable extent from nitrogen coming from the air. Although according to the invention total absence of nitrogen is not required, it will be clear that the use of a gaβ mixture with a nitrogen content lower than in air will lead to a lower production of NOx. Preferably, the natural gaβ iβ combusted in the presence of substantially no other gaβeβ than oxygen and COa and any gaβeβ present as contaminant in the natural gaa. The generation of energy in step a) occurs by combusting the natural gaβ in a suitable combuβtion unit, known per se. In the combustion of natural gas, which comprises mainly methane and additionally, poββibly, other light hydrocarbons, and, according to the invention, a varying content of COa, in any case heat iβ released which can be usefully employed, for instance by • applying heat exchangerβ. Another important advantage of the invention iβ that by injecting at least a part of the product flow referred to into the natural gaa reservoir, geothermal energy can be recovered. This means that the injected gas flow warms aβ a result of the terrestrial heat. This iβ advantageous because the produced natural gas flow does not have to be heated, or less so, before being converted according to the invention, which, for instance for some gas engines, is desired or necessary. Also, if heating the natural gaβ flow before converting it in step a) is not necessary or desired, this heat of geothermal origin can also be recovered with a heat exchanger and be usefully employed. The possibility of recovering geothermal energy renders the process even more attractive economically. The magnitude of the geothermal temperature gradient iβ highly dependent on the geographical location and varies considerably worldwide. For instance, in a Dutch field of natural gaβ, this can be approximately 3°C per 100 meters. The depth of fields of natural gas also varies considerably worldwide. A typical depth for the Netherlands is, for instance, 3 - 5 km. Thus, for instance, for a field of a depth of 3 km, a COa-containing gaβ of 20°C and at a pressure of 60 - 80 bar can be injected, this CO 2 -containing gaβ being, for instance, obtained by cooling the combuβtion gaβeβ of step a) to 20°C. Depending on the depth of the reservoir and the geothermal gradient, natural gas can then be obtained at the production side of the field at, for instance, 100°C with a pressure of 150 - 200 bar. As said, these values strongly depend on the depth of the reservoir, the prevailing geothermal temperature gradient and other geographically strongly differing factors. It ia also possible to generate work, for inβtance by having the combuβtion take place in a gaβ engine. The work thuβ produced can, for instance, be uβed for the production of electricity. An important difference between the method according to the invention and conventional methods is that the combustion takes place in the presence of substantially COa and oxygen instead of air. Nevertheless, customary means can be applied for combusting natural gaβ in the presence of COa and oxygen and thuβ generating energy. Therefore, a gas engine for use in the method according to the invention does not need to differ essentially from a conventional gas engine, i.e. a gaβ engine suitable for use with natural gas and oxygen coming from air. Generally, natural gas can be combusted without many problems in the presence of substantially oxygen and CO2 instead of in the presence of substantially oxygen and nitrogen coming from air, aβ iβ customary. When used in a gaβ engine, possibly adaptations known per se to the skilled person can be sufficient to have this gas engine function with optimum efficiency. Particular advantage is obtained by generating in step a) both heat and work by using a so-called total energy plant (WKK plant). By generating energy, the natural gaβ flow iβ converted in the preβence of oxygen and CO 3 into a .'low mipriβing substantially water and COa. Additionally, usual combustion products, such as carbon monoxide and unburned hydrocarbons can be present in a minor amount. These additional combuβtion productβ form, in principle, no impediment to carrying out the method according to the invention. According to the invention, this flow of combuβtion products is first, at least partly, stripped of the water. This can be done, for an important part, by cooling the flow so that the water condenses and can be separated. The heat which is thereby abstracted from the gaβ flow can also be usefully employed. In customary methodβ for generating energy from natural gaβ, the off- gases of the combustion step are usually exhausted to the atmosphere. For this reason, the ee gases need to have a sufficiently high temperature to enable exhaustion by meanβ of, for instance, a chimney. Because the product flow formed according to the method does not have to be exhausted into the atmosphere, it can be cooled deeper, for instance to 20 β C, in step b) and therefore more heat can be recovered from the off-gases of step a) so as to be usefully applied, which offers a clear advantage. Subsequently, the obtained product flow, or a part thereof, ia injected according to the invention in step c) into a natural gaβ reservoir from which natural gas has been extracted. To that end, customary means are applied. The part of the product flow to be injected is brought to the proper pressure with the aid of a compressor. This compressor can, for instance, be driven with the energy generated in step a). Advantageously, however, this compressor iβ coupled to an expansion unit. This expansion unit serves to bring the extracted natural gaβ to a pressure desired for further processing. In the expansion unit, according to this embodiment, the energy which is released when expanding the extracted natural gas iβ used for driving the compressor. If a part of the product flow is not injected, it can be exhausted. In view of the above-mentioned COa issue, however, the product flow iβ for a considerable part, and preferably entirely, injected into a natural gaβ reservoir. It is preferred to keep the preββure of the natural gas reservoir as constant as possible. As mentioned above, undesired subsidence can be prevented in this manner. This eans that for each unit of volume of natural gas which iβ extracted from the reservoir, preferably a more or leββ equal unit of volume of product gaβ iβ injected in its place. To keep the pressure in the reservoir constant, it may be necessary to supplement the product flow with COs or another suitable gaβ or poββibly water, which is obtained from elsewhere. Although in this connection the above-mentioned disadvantages are associated with the use of water, theβe disadvantages naturally apply to a leββer extent if not exclusively water iβ used as an injection medium. The water can also originate from the conversion step a), in which case, accordingly, leas water needs to be abstracted from this flow in step b). The oxygen flow or oxygen-rich flow necessary for obtaining the proper COa/oxygen ratio in step a) can be obtained from elsewhere. Particular advantage can be obtained, however, when this oxygen is made by separation of air, for instance by means of techniques known per βe, such aβ cryogenic separation techniques and/or membrane separation techniques. The energy which is necessary for this separation can very advantageously be obtained from the energy generating βtep a), for instance in the form of electricity. Aβ an additional advantage, in this manner a gaβ flow can be obtained which, in relation to air, has an increased nitrogen content. Depending on the intended UBΘ and the operating conditions of the method, the purity of this nitrogen-rich flow can be adjusted. When this nitrogen-rich flow is, for instance, applied as an additional injection medium, aβ mentioned hereinabove, it must poββeββ a sufficiently low concentration of oxygen to prevent explosions hazards. According to another preferred exemplary embodiment, the oxygen applied in βtep a) is obtained by converting water into hydrogen and oxygen, while the water can be obtained from elsewhere. Preferably, this conversion iβ carried out by means of electrolysis of the water. In this manner, in addition to a flow of oxygen or oxygen-rich gas, a flow of hydrogen or hydrogen-rich gaβ iβ obtained. The produced hydrogen can be deployed in different applications known to the skilled person. For instance, the produced hydrogen can be combusted, βo aβ to generate energy, which energy can for instance contribute to the energy need of the step for producing the oxygen flow. The proper ratio between COa and oxygen can be achieved by supplying these gases, for instance in the form of a previously prepared mixture, to the combuβtion unit, The proportion of COa and oxygen in βuch a mixture depends on the COa content in the natural gaβ. The COg/oxygen mixture can be obtained by mixing the oxygen flow or oxygen-rich flow with a COa flow or COa-rich flow. Although the COa can be obtained from elsewhere, the present invention specifically contemplateβ the uβe of COa originating from the product flow mentioned. To this end, it iβ neceββary that the product flow iβ sufficiently stripped of water to enable the combustion unit to function properly. The COs oxygen mixture is supplied to the combustion unit as a continuous flow. Aβ stated, the composition of this flow is adjuβted to the composition of the natural gas flow, especially in relation to the COa present in the natural gas flow. As a result of the injection of (part of) the product flow into the natural gaβ reservoir, the extracted natural gas, in a continuous implementation of the method, will have a COa concentration which increases over time. To enable a continuous implementation of the method, it ie necessary that per time unit a molar flow of methane (and possibly other light hydrocarbonβ) adjuβted in magnitude to the combuβtion unit, and a molar flow of oxygen adjusted thereto are supplied to the combuβtion unit. The amount of CO2 which is to be applied to obtain a suitable COa/oxygen mixture therefore depends inter alia on the composition of the extracted natural gas. The natural gaβ flow can be sampled with cuβtomary means to determine the COa content. Further, with cuβtomary regulating means the COa supply can be regulated such that a suitable COz oxygεn mixture is obtained. Assuming that, for a good combustion of methane, a mixture with 80 vol. % COa and 20 vol. % oxygen is suitable, this means that a natural gas flow which contains up to approximately 89 vol. % CO 2 can still be combusted with conventional techniques by admixing a flow containing exclusively oxygen. If further exploitation of the natural gaa reservoir iβ desired, i.e. exploitation wherein the COa level increases to above approximately 89 vol. %, the combuation unit haβ to be adapted to βuch natural gaβ mixtures. Also, it is possible to separate COa from the natural gaβ with cuβtomary techniques before leading this to the combuβtion unit in βtep a). Where in this specification and claims reference iβ made to a natural gas reservoir, it will be clear to the βkilled person that the method according to the invention iβ not limited to extraction from and injection into one and the same natural gaa reservoir. Extraction can for instance advantageously proceed from different natural gaβ reservoirs, and the respective flows can be combined so that these can jointly be led to the combuβtion unit in step a). Injection of the product flow can take place in one or more of these reservoirs, or in other natural gas reservoirs. By carrying out the method according to the invention, a number of advantages are obtained. Through the injection of the product flow, the emission of COa can be limited or even eliminated. Additionally, the uβe of an oxygen source in which less or no nitrogen iβ present leads to a decreased NOx emission into the atmosphere and it can even be eliminated without having to provide means to remove the NOx, as iβ often required in conventional, comparable methods. Further, by underground replacement of natural gaβ with COa, pressure drop in the natural gaβ reservoir can be controlled BO that subsidence can be prevented. The COa does not have to be separated from natural gas production (up to a total field production of approximately 89% COa). If the method ia uβed for operating a power station, an increaβed efficiency ia obtained in that flue gas can be completely cooled and the heat present therein can be usefully employed. The method also offers the possibility of extra revenues through the production of a nitrogen flow. In Fig. 1, a preferred embodiment of the method according to the invention is represented. In it, in an air separation plant an air flow is separated into a flow containing substantially oxygen and a flow containing substantially nitrogen. The energy neceββary for this separation βtep *mn be supplied by the energy generating βtep represented. The oxygen flow iβ passed to the energy generating βtep together with COa-containing natural gaa. In the example, the CO 2 content iβ 89 vol. % COa- Such a flow of natural gaβ iβ typically obtained at the end of the production period of a natural gaβ field when exploited with a method according to the invention. In the energy generating step, heat and optionally electricity is produced. The product gaβeβ from the energy generating step are led to a compressor. The energy for operating this compreββor iβ preferably at least partly obtained from an expansion unit coupled therewith. Subsequently, the compressed COa iβ injected into the natural gaβ reβervoir via an injection well. The natural gas flow iβ extracted from a production well and passed to the expansion unit. The invention will now be elucidated in and by the following example. Example In Table 1, feed gaβ mixtures are compared which are suitable for combustion in a conventional gaβ turbine. By making, according to the invention, a feed gas mixture in which the nitrogen is replaced by COa, it iβ possible to use natural gas containing 8 parts of CO 2 and 1 part of methane. This corresponds to a content of 88.9 vol. % of COa and 1.1 vol. % of methane which can be present in the extracted natural

Claims: CLAIMS 1. A method for producing energy from natural gaβ, comprising the βtepβ of: a) converting natural gas originating from a natural gaβ reservoir, in the presence of substantially oxygen and COa, into a flow comprising COa and water, whereby energy is generated; b) removing at least a part of the water from the flow compriβing COa and water, βo that a product flow iβ obtained; and c) injecting at leaβt a part of said product flow into a natural gaβ reservoir from which natural gas has been extracted. 2. A method according to claim 1, wherein step c) iβ carried out auch that the preββure in the natural gas reservoir remains substantially constant. 3. A method according to any one of the preceding claims, wherein the energy in βtep a) ia generated in the form of heat and electricity. 4. A method according to any one of the preceding claims, wherein the generated energy is at leaβt partly uβed for obtaining a flow of oxygen-rich gas. 5. A method according to claim 4, wherein said flow of oxygen-rich gaβ iβ obtained by converting water into a flow of oxygen-rich gas and a flow of hydrogen-rich gaβ, preferably by means of hydroly is of the water. 6. A method according to claim 4, wherein said flow of oxygen-rich gas iβ obtained by separating a flow of air into a flow of nitrogen-rich gaβ and a flow of oxygen-rich gaβ. 7. A method according to any one of claims 4 - 6, wherein the flow of oxygen-rich gas is used as source of oxygen in step a). 8. A method according to any one of the preceding claims, wherein the product flow iβ at least partly used as source of CO 2 in βtep a). 9. A method according to any one of the preceding clams, wherein in step a) approximately 18 vol. % of oxygen ia present. 10. Uβe of COa from combustion gaβes aβ injection medium for preventing pressure drop in natural gaβ reservoirs.

Inventors: VAN DE WAAL WOUTER WILLEM, VAN DE WAAL, WOUTER, WILLEM
Applicants: DROPSCONE CORP N V, WAAL WOUTER WILLEM VAN DE, DROPSCONE CORPORATION N.V., VAN DE WAAL, WOUTER, WILLEM
Application year: 2000
Main classification: F02B43
IPC codes: E21B43, F02B43