US 7066972 B2
The invention provides an apparatus for online and on-site tracer generation for tagging natural gas stored in underground storage fields wherein feedstock is drawn from a feedstock source. The feedstock undergoes initial analysis to determine hydrocarbon levels. The feedstock then undergoes reaction to produce tracers such as ethylene, propylene, acetylene hydrogen and carbon monoxide. The feedstock is then analyzed to determine post reaction tracer concentration. The feedstock including generated tracers is then introduced back into the feedstock stream. Tracer levels in the pre-reaction or initial analysis of feedstock are compared with tracer levels in the post-reaction feedstock and the rate of flow of feedstock through the system is adjusted to achieve a predetermined level of tracer concentration. The level of tracer concentration will then be used to identify the particular natural gas charge in a storage field.
1. Apparatus for the generation and introduction of tracer into a carbonaceous feedstock comprising,
a. A carbonaceous feedstock source,
b. A means for fluidly directing said carbonaceous feedstock through said apparatus, said carbonaceous feedstock source fluidly connected to said means for fluidly directing said carbonaceous feedstock,
c. At least one means for converting carbonaceous feedstock into said tracer, said means fluidly connected to said means for fluidly directing said carbonaceous feedstock.
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This application is a continuation of application Ser. No. 10/235,180 filed Sep. 4, 2002 now U.S. Pat. No. 6,793,699 which claims the benefit of PPA Application No. 60/317,702 with a filing date of Sep. 5, 2001.
This invention relates to an on-site, continuous method of tracer generation that can be utilized to tag natural gas. Natural gas is composed primarily of methane but contains lesser proportions of many compounds. Notable among those compounds are ethane, propane, and higher hydrocarbons. Although this invention finds application in tagging natural gas feedstock, it can be used to tag many other carbonaceous compounds including pure methane. Feedstock as used in this application encompasses natural gas, pure methane, the components of natural gas such as ethane, or any other carbonaceous substance in either liquid or gaseous form.
Most of the natural gas that is used in North America is produced either in the Gulf Coast region or in Northwestern Canada. Yet, most of the gas is used in the Northeast, the Midwest, and the northwestern United States. Therefore, large pipelines crisscross the country to transport natural gas from the producing areas to areas where the gas is used. Natural gas is frequently a byproduct of oil production. To produce oil, one often must also produce natural gas. Thus natural gas is produced year round in oil producing areas. However, there are also areas, which produce only natural gas, without oil. In those areas it is necessary to produce gas continuously, at a controlled rate, to maximize the productivity of a gas field. Further, if gas or oil is produced too rapidly, it can result in groundwater being drawn into the well and can seriously damage or even destroy a well.
Because gas is produced throughout the year but used primarily during the winter months, it is necessary to store natural gas until the months of peak usage. The most common method of storing natural gas is in underground storage reservoirs. Many of these storage reservoirs are areas where natural gas was produced years before. Because these reservoirs were demonstrated to have contained natural gas for millions of years, they provide a natural storage mechanism. Underground storage fields generally consist of porous rocks that are overlain by non-porous and non-permeable rocks. The porous rocks generally have the pore space filled with water. If one drills through the non-porous overlaying rock, or cap rock, one can pump gas into the pore space of the underlying reservoir unit, displacing the water.
There are over 350 such underground storage fields in North America in which gas is pumped underground during the warmer months of the year, and then withdrawn when additional gas is needed during cold periods. Some of these reservoirs are near the producing areas and others are near the end markets, sometimes in populated areas. Although underground storage reservoirs are designed to contain the gas, leakage of gas from these reservoirs does sometimes occur, resulting in a loss to the owner.
There are many scenarios in which identification of gas that has leaked or has been removed from a storage reservoir is critical. For example, if gas migrates to the surface it can enter shallow groundwater, used for drinking water supplies, and can even come to the surface, enter buildings, and result in explosions. Whenever natural gas is detected in the near-surface environment, over or near a gas storage reservoir, it becomes critical to determine if it is naturally occurring, native gas, or if it is gas leaking from the storage reservoir.
Another setting in which gas identification is critical is when there are producing oil and/or gas wells near gas storage fields. There are numerous situations throughout North America where this is the case. Although a gas company may attempt to define and describe the limits of the underground storage reservoir, the natural variations in the earth structure make it extremely difficult to be precise. Thus when gas is produced from a horizon above or adjacent to a gas storage field, the question frequent arises as to the ownership of that gas. If the gas occurs naturally within the rocks, it is the property of the producer. However, if the gas has migrated from a gas storage field, depending upon local laws, it may remain the property of the gas company. There have been numerous disputes throughout the country over the ownership of natural gas.
Thus, the ability to tag natural gas and the consequent capability of identifying the owner of the gas, is of significant value. To identify the source of natural gas, a tracer (like a fingerprint) may be added to the stored natural gas. By detecting the tracer contained in the gas under investigation, one could trace it back to its source. To qualify, the tracer has to satisfy several criteria: a). it must not normally exist in natural gas; b). it should not segregate from stored natural gas; c). it should not decompose rapidly or react with any other components; d). it should not be absorbed by the aquifer; and e). the detection limit should be low (that is the resolution should be high), so that the amount of added tracer can be low.
Natural gas within distribution pipelines in the country is tagged by adding an odorant. This is generally a sulfur bearing mercaptan. Because these mercaptans do no normally exist within natural gas, the presence of a mercaptan within the gas identifies it as pipeline gas. In gas storage reservoirs, mercaptans cannot be used effectively as tracers because, among other reasons, they are very reactive with the rocks. The gas may contain mercaptans when it is injected into a reservoir, but that mercaptan can quickly disappear and not remain with the gas. There are no existing methods of tagging gas prior to gas storage that are simple enough and inexpensive enough to be used on a routine basis as is done for pipeline gas distribution systems.
Many tracers have been tried, including ethylene, propylene, hydrogen, carbon monoxide, and others. Ethylene (C2H4) is one of the best tracers among all the tested tracers because it satisfies all the requirements of a good tracer. Pure ethylene generated offsite and shipped to the storage field has been used. Since the amount of natural gas to be stored is huge, in the range of billions of cubic feet, the use of pure ethylene is too expensive if it is used on a regular basis. Furthermore, commercially available quantities of ethylene are either too large or too small and are thus not suited to continuous use in tagging natural gas storage fields. This invention produces ethylene and other potential tracers at a low cost and in quantities ideal for tagging natural gas with this tracer.
Although there have been several other tracers developed which can be utilized in gas reservoir studies for various purposes, there are none without serious limitation. For example, U.S. Pat. No. 4,551,154 to Malcosky describes an approach where the chemical sulfur hexafluoride and/or chloropentafluoroethane is injected into gas fields to determine ownership. Field tests have indicated that the two compounds were not fully recovered whereas as tracers such as ethylene, were fully recovered. The two tracers appeared to be less mobile than ethylene. Low permeability structures could restrict the migration of these compounds. Further, this system utilizes very expensive chemicals and specialized analytical equipment. Other authorities have determined that sulfur hexafluoride was not deemed to be a suitable tracer in this application due to its instability and reactivity under long-term field conditions and its differing dispersion behavior relative to methane, while yet other authorities maintain that sulfur hexafluoride may have toxicity problems that may preclude its extensive utilization.
The invention uses materials to generate the tracer that are all readily available and inexpensive, i.e., the primary components of natural gas itself. Most of the processes that are used to generate ethylene or propylene from natural gas use only heat (pyrolysis), or at most, oxygen or water as the other reactant. Oxygen is of course readily available from air. Therefore, the invention does not involve transporting reactants from some great distance and is not hindered by commercially available quantities. With the use of the proper reactor, the only other thing needed to generate a tracer from natural gas is energy, which can even be supplied by combustion of a small amount of the natural gas itself.
Pure ethylene can be used as a tracer, but because the amount of natural gas to be stored is huge, the use of pure ethylene is too expensive if it is used on regular basis. A new technology, which could produce ethylene and other potential tracers at a low cost is needed. The invention described herein, provides an apparatus whereby tracer can be added to natural gas continuously, and at very low cost. All current methods of adding tracers to natural gas involve transporting pure or manufactured products to the point where they can be introduced into the gas line. This invention allows on-site generation of tracer.
The process generates compounds that are not normal constituents of natural gas and that have been previously verified as usable tracers within the gas storage industry. More specific tracers can be generated by utilizing water that is enriched in deuterium, tritium, oxygen-18, or other isotopic species. The process, being either pyrolysis or the catalytic reaction of air, carbon dioxide or water with natural gas, is such that the necessary, commercially available equipment can be made transportable for easy movement from one site to another.
The cost of this process is so low that it will be possible to routinely and continuously tag all of the gas injected into a storage reservoir eliminating many of the problems associated with existing tracer technology. Currently there are no tracers for gas that is stored in underground reservoirs that can be economically utilized on a long term, continuous basis.
The analytical equipment and methods necessary for analysis of the basic tracers are those present in most laboratories capable of carrying out routine analysis of natural gas, further adding to the economic benefits of this process.
This invention is based on the discovery of a method of utilizing a feedstock, itself, to generate identifying tracers through either a pyrolytic process or a reaction process in the presence of certain catalysts. Ethylene is the primary tracer generated, however, other tracers such as propylene, acetylene, H2, CO, are also generated in the reaction process or other tracers such as deuterated water and isotopically labeled hydrocarbons can be introduced and can serve singly as tracers.
Accordingly, these tracers can be used in combination to produce readily identifiable tracer mixtures that serve as unique markers. The invention not only creates the tracers but creates the tracers in predetermined concentrations. Feedstock tagged with predetermined concentrations can also serve as unique identifiers.
A further aspect of the invention is the on-site capability of tracer generation. This allows entire storage fields to be continuously tagged at the time the fields are initially filled or injected eliminating the need to acquire tracer in commercially reasonable amounts and transporting those tracers to the field injection well.
This invention utilizes several processes to generate ethylene tracer and other secondary tracers. The processes are the oxidative coupling of methane (OCM) in natural gas process and pyrolysis of ethane, a constituent of natural gas. For pyrolysis, both atmospheric pressure and high-pressure conditions were studied. These two technologies allow a cost-effective on-site and online process for underground gas storage use on a regular basis. Furthermore, the process may also employ oxidative pyrolysis, chloropyrolysis, steam and/or carbon dioxide reforming and partial oxidation of natural gas and natural gas conversion using electric arc or plasma to generate such tracers as acetylene, carbon monoxide, hydrogen, and isotopically labeled hydrocarbons
An experimental reaction system was designed for the OCM and pyrolysis experiments. Separate sources for CH4, natural gas and air were fed into a central line through individual flow meters. The central line then led to a heat source surrounding the reactor. In the atmospheric pressure experiments, a quartz tube (7 mm ID) was used as the reactor with a heating zone approximately 30 cm long. In the pressurized pyrolysis, a stainless steel tube (0.04 inch ID and ¼–16 inch OD) was used. Here the heating zone was also 30 cm long. In the latter system, a pressure release valve was used to keep the system pressure at 850 psi. Actual pipeline gas was used but pure methane was tested for comparison purposes. Table 1 illustrates the composition of methane and the pipeline gas used.
In the OCM process, methane, the major component of natural gas, is used as feedstock to generate higher hydrocarbon compounds. The simplified chemistry of OCM process is as follows: 2CH4+O2══C2H4+2H2O. The oxygen can be from air or pure oxygen gas. For the purposes of the invention, air is easier and cheaper to obtain. The OCM process will utilize a catalyst that results in the production of ethylene as one of the major C2 products when the reaction is properly controlled. Since the OCM reaction is very fast and strongly exothermic, only low oxygen concentrations can be applied. Thus the concentration of ethylene in the product stream is usually low. It should be noted that low concentration of product, added to the high cost of separating ethylene from the product stream are factors that hinders the commercialization of OCM process for ethylene production, but are not factors for the on-site production of tracer.
One catalyst studied was Mn/Na2WO4/SiO2. Table 2 illustrates the yield of ethylene in one sample of pure methane and one sample of natural gas (NG), both in the presence of the Mn/Na2WO4/SiO2 (LICP-1) catalyst.
These test results show the yield of ethylene from natural gas in the catalytic process increased by more than two percent as compared to that observed for natural gas in the non-catalytic process.
Ethane pyrolysis is a well-established process. However, reaction kinetics have been studied primarily with pure ethane (with steam) pyrolysis and at atmospheric pressure. In order to obtain more realistic data, pyrolysis of real pipeline gas (NG) was conducted at a total pressure of 1 atmosphere. Table 3 illustrates the results of ethylene production at standard pressures using pipeline gas.
The results showed that at 900° C. about 70% of the ethane in the pipeline gas is converted to ethylene. A small amount of acetylene is also formed, which can also be used as a tracer. The results are in agreement with the results from theoretical prediction. It can be seen in Table 3 that, as predicted by thermodynamics, higher temperature favors the ethane pyrolysis reaction.
Since pipeline gases are usually pressurized and the pressure of gas to be stored underground is even higher, it would be desirable to convert ethane at an elevated pressure, especially at or above the transportation pressure of pipeline gas. Most of the pipeline gas has a pressure range from 600 psi to 850 psi, and 850 psi was chosen as the test pressure. Table 4 illustrates the results of ethylene production at elevated pressures similar to those seen in natural gas pipelines.
The ethylene concentration in the product stream produced at high pressure was lower than the ethylene concentration produced in the atmospheric system. This can be explained by the effect of partial pressure of ethane in the system. Total pressure adversely affects the equilibrium constant for ethane conversion. Increasing pressure decreases the ethylene concentration. At 850° C. and at 850 psi, about 30% of ethane that existed in pipeline natural gas is converted to ethylene, compared with 70% for the atmospheric process. This is in agreement with the thermodynamics. At 850° C., and under optimized residence time, the maximum ethylene concentration is about 30% of the ethane concentration in the feedstock. In this case ethane concentration in feedstock is around 3.6 and the highest ethylene concentration in the test is 1.2%. Ethane partial pressure in the pressurized system is around 3.6%*850=30 psi, which is approximately 2 atm and is close to the pressure used in commercial processes. It should be noted as illustrated in the last column, that propylene is also generated and this too can serve as a tracer. Controlling the ethylene/propylene ratio provides a way of generating different “signatures” in different gas streams. It is interesting to note that the optimized conditions for maximizing ethylene concentration could be very close to the optimization conditions for maximizing propylene concentration.
All mechanisms tested generated ethylene in sufficient quantities to allow a tracer concentration of 50 to 100 parts per million to be generated in the post pyrolysis feedstock to be introduced into the feedstock stream designated for injection.
Additional tracers can be generated post-pyrolysis by reforming reactions using water and/or carbon dioxide or partial oxidation using air. Reforming reactions involving the addition of heat, would follow the general formula 2H2O+C2H6==2CO+5H2 or 2CO2+C2H6==4CO+3H2. Oxidation reactions would follow the general formula O2+C2H6==2CO+3H2. CO is not present in natural gas and can provide additional tracer functions.
De-coking can also be accomplished by the addition of water, carbon dioxide and air, pre-pyrolysis. The basic reactions would be as follows: H2O+C══CO+H2, or CO2+C==2CO, and finally O2+C==2CO.
Drawing feedstock from the feedstock source is accomplished by second line 3 that exits the storage field compressor and enters first flow meter 4 that measures the flow rate within the feedstock source. A transducer in flow meter 4 will transmit data, through first data line 52 to computer control 55 indicating the volume of feedstock passing through flow meter 4. Twenty-sixth line 5 exist flow meter 4 and enters the storage field. Third line 6 establishes fluid communication with the feedstock source and removes feedstock under pressure to flow control and pressure reduction valve 7, also fluidly connected to third line 6. Regulating flow and pressure thorough the fluid communication is flow control and pressure reduction valve 7. Valve 7 is controlled through second data line 53, which is connected to the computer control 55 and controls the quantity and pressure of the gas passing valve 7. The flow control and pressure reduction valve also will serve to reduce the variations in pressure, which may be induced by the storage field compressor and is controlled by computer control 55, again through second data line 53. Fourth line 8 then delivers feedstock to a collector 9 that cools the feedstock within the fluid communication. Collector 9 is designed to cryogenically precipitate certain classes of compounds such as butanes and pentanes, which contribute to coking later in the process. Fifth line 10 then exits the collector 9 and enters second flow meter 11. Second flow meter 11 measures the flow rate within the fluid communication at this stage. Second flow meter 11 contains a transducer, which transmits data, through third data line 54, to computer control 55, reporting the effects, on the feedstock, of flow control and pressure reduction valve 7. Sixth line 12 exits second flow meter 11 and enters heat exchanger 13. Heat exchanger 13 utilizes heat from downstream feedstock exiting from a reaction zone to allow preheating of the feedstock within the fluid communication which then enters the reaction zone of the reactors. Preheating in heat exchanger 13 saves energy and reduces the time necessary for the feedstock to remain within the reaction zone. Seventh line 14 exits heat exchanger 13 and enters first three-way valve 15. First three-way valve 15 directs the feedstock to either first primary reactor 32 or second primary reactor 33. In
The post reaction analysis of the feedstock to determine trace levels is accomplished when thirteenth line 45 diverts a sample of feedstock from twelfth line 44 into third three-way valve 45 a. Third three-way valve 45 a then diverts feedstock in thirteenth line 45 into fourteenth line 47 and consequently into analyzer 48. Thus a fluid communication with post reaction feedstock is established. Introduction of the post reaction feedstock into the analyzer is accomplished allowing the measure of tracer levels. Analyzer 48, in this configuration, would be a gas analyzer such as a gas chromatograph, mass spectrometer, infrared spectroscope or other analyzer of similar capability. Analyzer 48 measures the level of tracer and transmits that information to computer control 55 through fourth data line 50. Data establishing the desired level of tracer concentration is introduced into the computer control 55 that has been programmed to adjust the system to achieve a predetermined desired tracer concentration. Computer control 55 consequently transmits flow and pressure regulating data within the fluid communication and adjusts the flow rate through flow control and pressure reduction valve 7 by transmitting data instructions through second data line 53. Adjusting the rate of draw of feedstock into the system is initiated if the analysis reveals that tracer levels are falling, computer control 55 then increases the amount of feedstock flowing through flow control and pressure reduction valve 7 and, consequently, a greater amount of tracer is generated bringing the tracer level up to the desired value. Three-way valve 45 a also will allow a sample to be taken through fifteenth line 46 of the feedstock in second line 3 emanating from the storage field compressor. Thus a fluid communication with pre reaction feedstock is established. Introduction of the pre reaction feedstock into the analyzer is accomplished allowing the measure of tracer levels at that point in the system. Tracer levels within the post reaction feedstock and pre reaction feedstock are compared with the predetermined desired tracer concentration. Software that could be utilized could be programs such as “The Gas Flow Control System” by Zin Technologies or the combined use of “Lookout” by National Instruments and “TLC Momentum from Modocom Instruments.
Sixth dataline 51 connects third three-way valve 45 a and computer control 55. Computer control 55 will cause three-way valve 45 a to continuously and alternately draw samples from fourteenth line 45 and fifteenth line 46. As stated, fourteenth line 45 draws product gas from first line 1, however, fifteenth line 46 will draw pre pyrolysis feedstock from second line 3. Feedstock from second line 3 is continuously analyzed to determine the level of tracer that has been introduced through fourteenth line 45 into first line 1.
Introducing the feedstock into a reaction zone is accomplished by first three-way valve 15 being set to direct the feedstock flow from seventh line 14 into seventeenth line 16 and into first primary reactor 32. After remaining in the reaction zone for a predetermined period of time, where the tracer is generated. Feedstock then exits through eighteenth line 24 and into second three-way valve 36, which is set to accept feedstock from eighteenth line 24 passing it on through to tenth line 42. In this way, the reaction zone may be shifted from second primary reactor 33 to first primary reactor 32, thereby taking second primary reactor offline to allow decoking. In this manner, second primary reactor 33 and first primary reactor 32 may be alternately taken off line for maintenance, component replacement and decoking. Decoking of the second primary reactor may be accomplished by adjusting first three-way valve 15 and second three-way valve 36 to place first primary reactor 32 online. Then, first valve 19 is closed and second valve 22 is opened. This will allow compressed air from compressed air source 20 to flow into nineteenth line 21 and subsequently into twentieth line 27 and then into second primary reactor 33 allowing coke burn off. At the same time third valve 26 is closed and fourth valve 29 is open. Then the decoking product stream exits second primary reactor 33 via ninth line 34, then enters twenty-first line 35, then into through fourth valve 29, into twenty eighth line 30 and exits the system through vent 31.
Alternatively, first three-way valve 15 and second three-way valve 36 may be set to allow the redirecting of the feedstock into second primary reactor 33. Second valve 22 is closed and first valve 19 is open. Thus, allowing compressed air to pass into nineteenth line 21 and on into twenty second line 17, then into first primary reactor 32. The combustion stream from decoking then exits first primary reactor 32 via eighteenth line 24, then enters twenty third line 25 passing through open third valve 26 entering line 30, then closed fourth valve 29 will direct the combustion product to vent outside the system through vent 31.
In order to facilitate decoking or to generate further secondary tracers, other reactants may be introduced under pressure through reactant source 38. Reactant source 38 and the consequent introduction of reactants, is activated by computer control 55 through fifth data line 49. Should decoking be desired, compounds such as water, carbon dioxide and air may be introduced. In this case, those compounds would exit reactant source 38 into fourth three-way valve 28, which will be sent to empty into twenty third line 40, which will then transmit the decoking compounds through seventh line 14 into either the first primary reactor 32 or the second primary reactor 33. Alternatively, fourth three-way valve 28 could be configured to introduce reactants from reactant source 38 into twenty fifth line 37, which will then be transferred into secondary reactor 41.
An alternative embodiment would be the use of a mechanism to generate pressure differential such as a separate compressor, choke, or valve in place of the storage field compressor, to cause flow through the reactor. As shown in
Although the description above contains many detailed specifics, they should be viewed as illustrative and not as limiting the scope of the invention which should be determined by the claims and their legal equivalents.