MARSH, Peter Andrew (Kesslerpark 1, GS Rijswijk, NL-2288, NL)
| C L A I M S 1. A method for processing and recycling CO2 in a CO2 enhanced oil recovery method from a formation containing crude oil and Ci-C6 hydrocarbons, wherein: a) CO2 is injected into the formation; b) a mixture of CO2, crude oil and Ci-C6 hydrocarbons is produced from the formation; c) the mixture is separated into a substantially liquid fraction comprising crude oil and a substantially gaseous fraction comprising CO2 and Ci-C6 hydrocarbons; d) the substantially gaseous fraction is separated into a first fraction which is enriched in C5-C6 hydrocarbons and a second fraction which is enriched in CO2 and Ci-C4 hydrocarbons ; e) at least part of the first fraction is mixed with the substantially liquid fraction comprising crude oil; and f) at least part of the second fraction is recycled into the formation. 2. The method of claim 1, wherein steps (b)-(f) are cyclically repeated. 3. The method of claim 1 or 2, wherein in step (d) the content of C3-C4 hydrocarbons in the second fraction is controlled such that the ratio between the Ci and the C3-C4 hydrocarbon molar fractions in the second fraction is maintained above a selected threshold value in order to ensure full or partial miscibility of the stream with the oil in the reservoir. 4. The method of claim 1, wherein the crude oil and at least some other components of the mixture produced in accordance with step (b) of claim 1 are used as feedstock for producing a fuel, lubricant and/or a chemical product . 5. A system for processing and recycling CO2 in a CO2 enhanced oil recovery method from a formation containing crude oil and Ci-C6 hydrocarbons, the system comprising: a) a first separation assembly for separating a mixture of CO2, crude oil and Ci-C6 hydrocarbons produced from the formation into a substantially liquid fraction comprising crude oil and a substantially gaseous fraction comprising CO2 and Ci - C6 hydrocarbons; b) a second separation assembly for separating the substantially gaseous fraction into a first fraction which is enriched in C5-C6 hydrocarbons and a second fraction which is enriched in CO2 and Ci-C4 hydrocarbons; c) a first outlet conduit connected to the second separation assembly for injecting at least part of the first fraction into the substantially liquid fraction comprising crude oil; and f) a second outlet conduit for recycling the second fraction into the formation. 6. The system of claim 5, comprising control means for maintaining the ratio between the C3-C4 and the Ci hydrocarbon molar fractions in the second fraction above a selected threshold value. 7. The system of claim 6, wherein said control means comprise a mixing conduit through which a selected amount of C4-C6 hydrocarbons from the first fraction is injected into the second fraction. 8. The system of claim 6, wherein said control means comprise a Joule-Thompson valve, a Low Temperature Separation (LTS ) vessel and a stabilization column for separating relatively heavy and liquefied C5+ hydrocarbons from relatively light and substantially gaseous CO2 and Ci-C4 with a selected fraction of C3-C4 sufficient to compensate the detrimental effect of Ci on the miscibility of CO2 in crude oil. |
The invention relates to a minimal gas processing scheme for recycling carbon dioxide (CO 2 ) in a CO 2 enhanced oil recovery method.
It is known to enhance crude oil production by injecting CO 2 into the formation such that the CO 2 mixes with the crude oil. The thus formed CO 2 and crude oil mixture has a lower density and viscosity than the crude oil, resulting in higher oil mobility and therefore higher oil recovery factors. If the crude oil containing formation also comprises natural gas then the natural gas may comprise various components that form after condensation Natural Gas Liquids (NGL's), and which may include Liquefied Natural Gas or LNG (which predominantly comprises methane or (Ci or CH 4 ), Ethane (C 2 or C 2 H 6 ), Liquefied Petrol Gas or LPG (which predominantly comprises propane and butane or C3 and C 4 ) and condensate (which predominantly comprise Cs + fractions) .
In this specification and accompanying claims the various natural gas and/or Natural Gas Liquid (NGL) fractions are hereinafter identified by the abbreviations Ci, C 2 , C3, C 4 , C 5 and C 5+ . Fractions with more carbon atoms per molecule than Ci, C 2 , C3, C 4 , C 5 are identified as Ci + , C 2+ , C3 + , C 4+ , C 5+ . A first reason for separating the C02 from the hydrocarbon gas is that some of the components in the hydrocarbon gas have a negative impact on the miscibility of the CO 2 i.e. increase the required reservoir pressure for the CO2 to be miscible with the oil and ultimately lower the incremental oil recovery of the miscible flood. It is known that methane (CH 4 or Ci) has a negative impact on miscibility, ethane (C 2 H 6 or C 2 ) and carbon dioxide (CO 2 ) have comparable miscibility, whereas onwards through the hydrocarbon family have a progressively more positive impact on miscibility i.e. decrease the required reservoir pressure for the CO 2 to be miscible with the oil and ultimately increase the incremental oil recovery of the miscible flood.
A second reason for separating the CO 2 from the hydrocarbon gases is when the capital and operating expenditure required for the gas treatment plant is more than compensated for by the increased revenue from the gas and NGL products from the gas processing facilities. The associated gas from a miscible CO 2 EOR flood is often miscible with the oil in the reservoir without any treatment because the positive benefit of the heavier hydrocarbon product counteracts the negative impact of the methane and nitrogen (if present) . As such, gas side economics are more likely to drive the ultimate decision to process or not.
The gas processing required for separating the associated gas from a miscible CO 2 EOR flood is generally very complicated and expensive because there is a three phase vapour/liquid/solid region in the phase diagram when separating methane (CH 4 ) and CO 2 . The solid CO 2 prevents the use of conventional fractionation.
Furthermore there is an azeotrope formed between ethane (C 2 H 6 ) and CO 2 . An azeotrope is a constant composition boiling mixture where vapour and liquid have the same composition, which makes fractionation of ethane (C 2 H 6 ) and CO 2 impossible. A typical method of overcoming these problems is to use an extractive distillation whereby a third or different component is added to the system to "break" the azeotrope. One such process, which has been widely used for separating CO 2 from hydrocarbon within CO 2 miscible flood associated gas is known as the Ryan Holmes process, which utilises a C 4+ recycle within the process to facilitate separation of the CO2 from the hydrocarbons. The Ryan Holmes process is described in International patent applications WO8301294 and WO9005755, European patent 0129704, US patent application 200304792 and US patents 4318723, 4383842, 4654062, 4675032 and 5335504. The Ryan Holmes process can produce sales grade gas and condensate streams, a liquid CO 2 stream, and a C 2 -C 4 LPG stream.
The Ryan Holmes process is equipment intensive, highly energy intensive (in the form of heat), operationally complex and has large inventories of LPG ' s . The Ryan Holmes process is, therefore, not suitable for offshore CO2 miscible flood applications.
Currently, the most widely employed alternative method for separation of CO 2 from hydrocarbon within miscible CO 2 floods utilizes membranes. Membranes rely on the fact that CO 2 permeates faster through a membrane than hydrocarbons.
However, membranes do not produce a sharp separation and it is normal to utilise more than one stage of membranes to effect a reasonable separation. Membranes require significant gas processing upstream to ensure that there are no contaminants (e.g. water, glycol, C 6+ ) in the gas, which would destroy the separating capability of the membranes. This gas treatment is generally expensive . Furthermore, the C02, and other compounds, which permeate across the membranes, drop in pressure considerably across the membranes, thereby requiring expensive compression to raise the pressure back up either for reinjection or for further treatment within the process.
Membranes are often used in conjunction with a secondary process downstream (e.g. amines) in order to obtain reasonably pure CO 2 as the membranes do not perform this last stage of separation very efficiently. The membrane process can produce a sales grade gas stream, a vapour CO 2 stream, and a C 2 -C 4+ NGL stream. Membranes are generally light in weight, which makes them preferred (particularly for offshore applications), and are operationally simple.
However the process is still energy intensive due to the requirement for large amounts of compression.
From the above it is obvious that the existing first choice processes for gas treatment within CO 2 EOR schemes are generally expensive and energy intensive. They also do not take account of the comparative product values of the various products produced within the process in order to optimise the value for every CAPEX (Capital Expenditure) dollar spent. It is an object of the present invention to provide a CO 2 EOR minimal gas processing scheme for recycling CO 2 in a C02 enhanced oil recovery process which considers the values of the products within the associated gas from a CO 2 EOR flood and concentrates on recovering the high value products while ensuring minimal impact on the miscibility of the CO 2 stream being reinjected in the CO 2 flood. It is a further object of the present invention to provide a method and system, which purify CO2 recycled into a crude oil and Ci-C 6 hydrocarbons in a more efficient, reliable and optimum way than the known Ryan Holmes and membrane CO 2 purification methods, which are complex, have a high CAPEX and/or are highly energy intensive .
In accordance with the invention there is provided a method for processing and recycling CO 2 in a CO 2 enhanced oil recovery method from a formation containing crude oil and Ci-C 6 hydrocarbons, wherein: a) CO 2 is injected into the formation; b) a mixture of CO 2 , crude oil and Ci-C 6 hydrocarbons is produced from the formation; c) the mixture is separated into a substantially liquid fraction comprising crude oil and a substantially gaseous fraction comprising CO 2 and Ci-C 6 hydrocarbons; d) the substantially gaseous fraction is separated into a first fraction which is enriched in Cs-C 6 hydrocarbons and a second fraction which is enriched in CO 2 and Ci-C 4 hydrocarbons ; e) at least part of the first fraction is mixed with the substantially liquid fraction comprising crude oil; and f) at least part of the second fraction is recycled into the formation.
It is preferred that in step (d) the content of C3- C 4 hydrocarbons in the second fraction is controlled such that the ratio between the Ci and the C3-C4 hydrocarbon molar fractions in the second fraction is maintained above a selected threshold value in order to ensure full or partial miscibility of the stream with the oil in the reservoir . In accordance with the invention there is further provided a system for processing and recycling CO2 in a CO2 enhanced oil recovery method from a formation containing crude oil and Ci-C 6 hydrocarbons, comprising: a) a first separation assembly for separating a mixture of CO 2 , crude oil and Ci-C 6 hydrocarbons produced from the formation into a substantially liquid fraction comprising crude oil and a substantially gaseous fraction comprising CO 2 and Ci-C 6 hydrocarbons; b) a second separation assembly for separating the substantially gaseous fraction into a first fraction which is enriched in C 5 -C 6 hydrocarbons and a second fraction which is enriched in CO 2 and Ci-C 4 hydrocarbons; c) a first outlet conduit connected to the second separation assembly, such as a stabilizer column, for injecting at least part of the first fraction into the substantially liquid fraction comprising crude oil; and f) a second outlet conduit for recycling the second fraction into the formation. It is preferred that the system comprises control means for maintaining the ratio between the C 2 -C 4 and the Ci hydrocarbon molar fractions in the second fraction above a selected threshold value in order to ensure full or partial miscibility of the stream with the oil in the reservoir, such as a mixing conduit through which a selected amount of C 5 -C 6 hydrocarbons from the first fraction is injected into the second fraction.
Said control means may furthermore comprise a Joule- Thompson valve, a Low Temperature Separation (LTS) vessel with temperature control means and a stabilization column with a variable reboiler duty for separating relatively heavy and liquefied C 5+ hydrocarbons from relatively light and substantially gaseous CO 2 and Ci-C 4 with a selected fraction of C3-C4 sufficient to compensate the detrimental effect of Ci on the miscibility of CO2 in crude oil.
These and other features, embodiments and advantages of the method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of preferred embodiments disclosed in the accompanying drawings in which reference numerals are used which refer to corresponding reference numerals that are shown in the drawings.
Figure 1 depicts a flow scheme of the method and system according to the invention;
Figure 2 depicts the value of various hydrocarbons Ci-C 5 and crude oil on a barrel of oil equivalent (boe) basis; Figure 3 shows the impact of various components in associated gas on miscibility with crude oil in an Enhanced Oil Recovery (EOR) process;
Figure 4 shows a first alternative flow scheme of the method and system according to the invention; Figure 5 shows a second alternative flow scheme of the method and system according to the invention;
Figure 6 shows a third alternative flow scheme of the method and system according to the invention; and
Figure 7 shows a fourth alternative flow scheme of the method and system according to the invention.
A flow scheme of the method and system according to the invention is shown in Fig.l. In accordance with the invention the values of the products within the associated gas from a CO 2 EOR flood are taken in consideration and a balance is provided between recovering the high value products while ensuring minimal impact on the miscibility of the CO 2 stream being reinjected in the CO 2 flood. Figure 2 is a plot in which the values of the various hydrocarbon products Ci-C 5 and crude oil are compared on an equal basis. This equal basis is based on energy equivalence or, calculation of the value of the product on a per Barrel of Oil Equivalent (BOE) basis. It is clear from Figure 2 that the value of the C 3 , C 4 and C 5+ (condensate) elements of the associated gas have substantially the same BOE value as crude oil, whereas the Ci and C2 products generally have a product value significantly lower (sometimes almost an order of magnitude lower depending on the local gas market) than the C 3+ products.
This observation that methane and ethane have significantly lower values is a relevant insight for the CO 2 assisted EOR method and system according to the invention. The above descriptions of the Ryan Holmes and membrane CO 2 purification methods indicate why these known methods for separating CO 2 from associated natural gas are so complex and expensive, methane and ethane are the very components which are hardest to separate from the CO 2 yet they generally have a significantly lower value than the other components in the hydrocarbon gas.
An insight that forms a background of the method and system according to the invention is that it is not optimal to separate out the methane and ethane from associated gas. These components can be left in the CO 2 re-injected within the flood as long as sufficient heavier components are also left in the CO2 in order to ensure that the CO 2 is sufficiently miscible. Figure 3 shows the impact of various components in associated gas on miscibility with crude oil in an Enhanced Oil Recovery (EOR) process. It shows that Nitrogen (N 2 ) and methane (CH 4 or Ci) reduce miscibility, ethane (C2H 6 or C2) and carbon dioxide (CO2) have comparable miscibility, whereas propane (C3) , Hydrogen Sulphide (H 2 S), butane and higher condensates (C 5+ ) have an increasingly positive impact on miscibility. In accordance with the invention the negative impact of methane (Ci) on miscibility is compensated by adding a sufficient amount of C 3 -C 4 compounds to the CO 2 and hydrocarbon gas (Ci-C 4 ) containing gas mixture that is recycled through the formation to preserve miscibility of the mixture with the crude oil in the formation.
Figure 1 depicts a process scheme of the method according to the invention. The method according to the invention utilizes standard process unit operations, which are arranged in a novel manner. The novel part of the method and system according to the invention is shown within the dotted line 1 in Figure 1. The method according to the invention uses a standard NGL recovery system comprising a Joule-Thomson valve 2, a Low Temperature Separator [LTS] 3 and cross heat-exchanger 4 in order to drop the temperature of the associated gas in order to drop out the heavier components C 5+ from the associated gas. In the LTS 3 a substantially gaseous stream 12 comprising mainly CO 2 and Ci-C 4 is separated from a condensed liquid stream 12, which has a reduced CO 2 and Ci-C 4 content. The condensed liquid from the from the LTS 3 is routed to a stabilization column or stabilizer 5 to separate the remaining substantially gaseous stream 14 of CO 2 and lighter hydrocarbons Ci-C 4 condensed by the JT valve from the substantially liquid stream 14 of NGL ' s . A reboiler 16 is arranged at the bottom of the stabilization column 5 to separate a stream 18 of C 5+ from a stream 17 of C 4 _ hydrocarbons, which latter stream 17 is recycled into the stabilization column 5. The stabilization column 5 is often necessary to reduce large recycles in the system. The condensate contains high percentages of CO 2 , which if spiked directly into the main oil/gas/water separators (lower operating pressure) simply recycles through the associated gas compression train resulting in large power demand and unstable process operation. The stabilization column 5 removes the majority of the CO 2 and lighter hydrocarbons and spikes mainly C 5+ hydrocarbons (the high value products) into the oil export stream 6. The CO 2 being routed to re-injection contains sufficient C 3 and C 4 (positive impact on miscibility) to balance the Ci (negative impact on miscibility) . As such, the process according to the invention maximizes recovery of high value products while having no impact on the miscibility of the injected CO 2 .
Figure 1 furthermore shows that a mixture 7 of crude oil, CO 2 and Ci-C 6 hydrocarbon fractions is fed from one or more oil production wells (not shown) to a bulk separator 8 in which the substantially liquid crude oil fraction 6 is separated from the substantially gaseous CO 2 and Ci-C 6 hydrocarbon fractions, and that the stream 10 comprising the substantially gaseous CO 2 and Ci-C 6 hydrocarbon fractions are compressed in a gas compressor 9 and is then fed to the cross heat exchanger 4.
Benefits of the method and system according to the invention are that they:
-Maximize the recovery of the high value products within the gas; -Do not separate the harder to separate, lower value components within the associated gas; -Have a significantly lower CAPEX than the existing Ryan Holmes and membrane C02 purification schemes. Therefore the method and system according to the invention would have a significantly reduced CAPEX compared with the existing Ryan Holmes and membrane C02 purification options.
Minimal processing equipment would make the method and system according to the invention particularly attractive for offshore applications but also very attractive for onshore applications. A further advantage of the method and system according to the invention is that they utilize existing field proven technology and require no technical step-out using unproven and therefore unreliable technologies. The method and system according to the invention would be simple to operate.
The gas treatment process would not introduce any significant Health Safety and Environment (HSE) issues that would not already be present within the main oil/gas/water separation and compression trains. It will be understood that possible variations to the process scheme shown in Figure 1 are that: -It may be possible to replace the stabilizer with a heated flash in some circumstances, further reducing the cost of the method and system according to the invention. -It may be possible to separate out more LPG's (C 3 and C 4 ) from the gas by e.g. providing a condenser at the top of the column in order to produce a liquid LPG stream. This option may be pursued if there was: a. high product value for the LPG's in the intended development location; and/or b. the resulting CO2 was still easily miscible within the reservoir, thereby having no impact on oil recovery from the EOR flood. Some possible variations to the process scheme shown in Figure 1 are shown in Figures 4-7.
In Figures 1 and 4-7 similar components of the alternative embodiments of the system according to the invention are identified by similar references numerals.
Figure 4 shows a process scheme wherein the stabiliser 5 of Figure 1 is replaced by a heated flash assembly 40-43, further reducing the cost of the process. In this alternative process scheme, the condensed liquid stream 13 from the LTS 3 is cross exchanged in a heat exchanger 40 with a warm NGL stream 44 from the heated flash assembly 40-43, and further heated by a heater 41 before being dropped in pressure across a JT valve 42. The two- phase stream is then routed to a separator 43 where a CO 2 rich gas stream 45 is separated and recycled back to the CO 2 and associated gas mixture 10 upstream of the compressor 9 at the start of the process. The C 5+ stream 44 is cross-exchanged with the incoming NGL stream 40 from the LTS 3 and is then routed to export or may also be spiked into the crude oil stream.
Figure 5 shows a second alternative embodiment of the method and system according to the invention wherein more LPG's (C3 and C 4 ) are separated out from the gas if there is: a. a high product value for the LPG's in the intended development location; and/or b. the resulting CO 2 still is easily miscible within the reservoir, thereby having no impact on oil recovery from the EOR flood. Figure 5 shows the processing scheme for this utilising the stabiliser column 5 design. The scheme is similar to Figure 1, however, the operating conditions are altered in order to drive C 3 and C 4 into the NGL liquid product 18. The product 18 is therefore not stabilised. This liquid product 18 could be further separated into sales grade LPG's and condensate in an NGL Fractionation plant (not shown) . Transport to such a plant could be separate to the oil or by spiking into the oil and separating the oil and NGL ' s at a separate location .
Further to the above it may also be possible to operate the alternative processing scheme in Figure 4 in order to drive C3 and C 4 into the NGL liquid product. This alternative scheme is shown in Figure 6.
Figure 6 depicts an alternative embodiment of the method and system according to the invention wherein a heated flash similar as shown in Figure 4 is applied and wherein LPG & Condensate (non-stabilised) 44 are exported or spiked into the exported crude oil Stream.
The embodiment shown in Figure 6 is attractive if it is possible to produce a separate LPG (C3 and C 4 ) export stream, again in the situation when there is: a. high product value for the LPG's in the intended development location; and b. the resulting CO 2 was still easily miscible within the reservoir, thereby having no impact on oil recovery from the EOR flood.
A possible processing scheme for this option is shown in Figure 7. In this scenario, the column design of the stabilizer 5 would be altered to a condensed/refluxed column i.e. column overhead is condensed in condenser 70 and separated in separator 71 in order to form a liquid product 73 and provide reflux 74 to the stabilizer column 5. Liquid C 3 /C 4 product is produced from the top of the stabiliser separately to the condensate product C 5+ stream 18 from the bottom of the stabiliser column 5.