METHODS FOR RECOVERING NATURAL GAS USING NITROGEN REJECTION UNITS
A method and system for enhanced oil recovery by performing the steps of feeding a mixture of nitrogen from a primary nitrogen supply and an optional supplemental nitrogen supply into an oil field; separating recovered oil from a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons; feeding the gas mixture to a nitrogen rejection unit operating at elevated pressures; and recovering the nitrogen, natural gas and C2+ hydrocarbons. A method for the recovery of natural gas is also described herein.
The present application claims priority from U.S. provisional application Ser. No. 62/028,091, filed on Jul. 23, 2014.
BACKGROUND OF THE INVENTIONThe present invention relates to the use of high pressure nitrogen rejection units along with supplemental air separation units in enhanced oil recovery operations to improve performance.
Nitrogen injection in mature oil fields maintains the field pressure and assists in increasing oil production. The nitrogen content of an oil field will increase depending upon its age and maturity. Nitrogen rejection units (NRUs) are used to separate nitrogen from natural gas thereby resulting in a saleable natural gas, The nitrogen that is separated is either vented or recompressed and fed back into the oil field for use in enhanced oil recovery operations.
Conventional NRUs are designed for nitrogen contents of approximately 20% in the feed gas (see for example, Arthur J. Kidnay, William R. Parrish, Fundamentals of Natural Gas Processing, 2006). Those NRUs operate at low pressures to provide saleable natural gas at economic operating conditions. The produced natural gas and the nitrogen will require significant compression power downstream of the NRU to feed the natural gas into a pipeline and the nitrogen back for enhanced oil recovery (FOR) operations, respectively. Some of the produced natural gas is also used to provide the required compression power.
However, mature oil fields may reach a nitrogen concentration that is greater than 80% nitrogen. This high nitrogen concentration makes the operation of the originally installed NRUs uneconomical or almost impossible to produce saleable natural gas. Oil plant owners may face the choice that it is more economic to bypass the IRU and recompress the nitrogen rich gas which still contains up to 20% hydrocarbons for EOR. In this instance the oil plant is not producing any more natural gas. To provide the required compression power for the EOR operations, the oil plant owner would need to actually buy natural gas to drive the compressors which increases their operation expenses significantly.
The present invention overcomes these difficulties by using a primary source of nitrogen and a supplemental source of nitrogen to assist in recovering oil and a gas mixture from the oil field. While the oil is recovered, a nitrogen rejection unit is then used to separate the gas mixture into component portions which can be utilized for example as a fuel source for the compressors used in the nitrogen feeding operation.
SUMMARY OF THE INVENTIONIn a first embodiment of the invention, there is disclosed a method for enhanced oil recovery comprising the steps of (a) feeding nitrogen from a primary nitrogen supply into an oil field; (b) recovering oil and a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons from the oil field; separating the oil from the gas mixture (d) feeding the gas mixture to a nitrogen rejection unit; and (a) recovering nitrogen, natural gas and C2+ hydrocarbons.
The method may further comprise feeding a supplemental nitrogen supply into the oil field.
The primary nitrogen supply is from an air separation unit and the supplemental nitrogen supply is from a supplemental air separation unit. The primary nitrogen supply is fed to a high pressure compressor which preferably operates at a pressure up to 6700 pounds per square inch,
The oil and the gas mixture are fed to an oil separation and sour gas removal unit, thereby separating the recovered oil from the gas mixture.
The recovered oil may be addressed in a manner of way but is typically fed to a storage facility.
The nitrogen rejection unit will separate the nitrogen from the natural gas and C2+ hydrocarbons. The nitrogen rejection unit will typically operate at elevated pressures of about 30 bar.
The recovered nitrogen is fed to an intermediate pressure compressor before being fed to the high pressure compressor.
A mixture of nitrogen and methane is recovered from said nitrogen rejection unit and is fed to the high pressure compressor.
Alternatively, the C2+ hydrocarbons may be fed along with the nitrogen and methane to the high pressure compressor.
The recovered C2+ hydrocarbons may be fed to a recycle or fractioning unit.
In a second embodiment of the invention, there is disclosed a method for the recovery of natural gas from an enhanced oil recovery operation comprising the steps of (a) feeding a primary nitrogen supply into an oil field; (b) recovering oil and a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons from the oil field; (c) separating the oil from the gas mixture; (d) feeding the gas mixture to a nitrogen rejection unit operating at elevated pressures; and (e) separating natural gas from nitrogen and C2+ hydrocarbons.
In a third embodiment of the there invention, there is disclosed a system for enhanced oil recovery comprising a source of nitrogen, a supplemental source of nitrogen, and a nitrogen rejection unit.
The source of nitrogen is a primary air separation unit. The supplemental source of nitrogen is a supplemental air separation unit.
The nitrogen rejection unit operates at high pressure of 30 bar.
The nitrogen rejection unit separates a gas mixture or nitrogen, natural gas and C2+ hydrocarbons.
The system further comprises an intermediate pressure compressor.
The system further comprises a high pressure compressor.
About 50% of the hydrocarbon rich stream recovered can be used for mixing with the existing fuel gas stream. This modification of the fuel gas stream enables adjustment of its energy content and the specific heating value to the specific compressor requirements for both intermediate and high pressure compressors. The remainder of the hydrocarbon rich stream is valuable due to its energy content and relatively high concentration of C2+ components and may be further purified to produce a saleable product.
The advantages achieved by the present invention include a reduction in overall operating expenses by reducing the amount of natural gas purchased as well as maintaining the recycle stream nitrogen pressure. Due to the separation of the nitrogen and hydrocarbons in the nitrogen rejection unit, the content and flow of recompressed hydrocarbons for enhanced oil recovery is significantly reduced. There is also less compression power required to recompress the nitrogen stream from the nitrogen rejection unit outlet pressure of approximately 30 bar to the enhanced oil recovery pressures of approximately 6800 psi compared to conventional low pressure nitrogen rejection units.
This high pressure nitrogen stream is fed through line 6 to the oil field E. A supplemental ASU D operating at 17.8 MMSCFD also produces nitrogen and fees this supplemental nitrogen into line 6 through line 5. The combined nitrogen streams are at 96% nitrogen purity and 103 MMSCFD when fed into the oil field E.
The enhanced oil recovery operations are undertaken at the oil field formation E and the output from the enhanced oil recovery operation is an oil and gas mixture which is fed through line 8 to an oil separation process, sour gas removal and chiller and deethanizer F. The gas that is recovered from this operation is primarily a mixture of nitrogen, natural gas and C2+ which is fed through line 9 to a nitrogen rejection unit (NRU) G operating at elevated pressures of approximately 30 bar. The nitrogen rejection unit will produce a 96% purity nitrogen feed at quantities of 53 MMSCFD and feed this through line 10 to an intermediate pressure compressor unit C.
The intermediate pressure compressor C receives the original feed from the existing ASU A through line 3 and an additional feed from the NRU G through line 10. The intermediate pressure compressor C will combine these two feed and will pressurize the nitrogen to 2250 psi. This pressurized combination can be fed through line 4 to the high pressure compressor B for ultimate entry into the oil field E.
The nitrogen rejection unit G operating at elevated pressures will further produce a stream of natural gas and nitrogen which is recovered through line 12 at a feed rate of 4.5 MMSCFD as well as a stream of greater than 85% purity C2+ hydrocarbons at a feed rate of 4.8 MMSCFD which is recovered through line 11.
This C2+ hydrocarbon stream is fed through line 11 to line 13 where it will be fed into a C2+ recycle or fractioning unit H. Alternatively, the C2+ hydrocarbons that are recovered can be fed through line 14 to the high pressure compressor B. This stream will be joined by the mixture of natural gas and nitrogen recovered through line 12 which can also be fed to line 14. The resulting mixture as fed through line 2 can be used to provide fuel to the high pressure compressor B and limit the amount of natural gas that must be purchased for use in feeding the high pressure compressor B.
The nitrogen from the separation process in the pressurized nitrogen rejection column is collected from the top of this column through line 22 and recovered after passing through the main heat exchanger J.
Gas for combustion purposes is supplied through line 20 and also passes through the main heat exchanger J and with the assistance of pump N is fed to the pressurized nitrogen rejection column.
A hydrocarbon heat pump compressor K will act to drive the reboiler R and condenser P and will pass a cooler hydrocarbon mixture which is mainly methane from the compressor K through line 26 where it will pass through the main heat exchanger J and pass cooling to the main heat exchanger J. The slightly warmer hydrocarbon mixture will continue through line 26 where it will provide some refrigeration to the reboiler R.
The bottoms from the pressurized nitrogen rejection column M, namely a mixture of natural gas and other hydrocarbons is fed through line 31 to the reboiler R. These hydrocarbons are fed to a subcooler Q through line 27 and then through open valve V2 and line 28 to a condenser P. The condensed hydrocarbons are returned through line 32 to the pressurized nitrogen rejection column where they will provide cooler temperatures to the column internals.
The warmer hydrocarbons that have been fed to the reboiler R will exit the reboiler through line 30 and pass through the main heat exchanger J where they will exchange heat and be fed through line 30 back to the hydrocarbon heat pump compressor K to provide some cooling to the compressor before being fed back through the main heat exchanger J through line 26 and cycled again through the reboiler R.
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention,
Claims
1. A method for enhanced oil recovery comprising the steps of (a) feeding nitrogen from a primary nitrogen supply into an oil field; (b) recovering oil and a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons from the oil field; (c) separating the oil from the gas mixture (d) feeding the gas mixture to a nitrogen rejection unit; and (d) recovering nitrogen, natural gas and C2+ hydrocarbons.
2. The method as claimed in claim 1 further comprising feeding a supplemental nitrogen supply into the oil field.
3. The method as claimed in claim 1 wherein the primary nitrogen supply is from an air separation unit.
4. The method as claimed in claim 2 wherein the supplemental nitrogen supply is from a supplemental air separation unit.
5. The method as claimed in claim 1 wherein the primary nitrogen supply is fed to a high pressure compressor.
6. The method as claimed in claim 1 wherein the high pressure compressor operates at a pressure up to 6700 pounds per square inch.
7. The method as claimed in claim 1 wherein the oil and the gas mixture are fed to an oil separation and sour gas removal unit, thereby separating the recovered oil from the gas mixture.
8. The method as claimed in claim 1 wherein the recovered oil is fed to a storage system.
9. The method as claimed in claim 1 wherein the nitrogen rejection unit will separate the nitrogen from the natural gas and C2+ hydrocarbons.
10. The method as claimed in claim 1 wherein the nitrogen rejection unit is operating at high pressures.
11. The method as claimed in claim 10 wherein the high pressure is 30 bar.
12. The method as claimed in claim 1 wherein the recovered nitrogen is fed to an intermediate pressure compressor before being fed to the high pressure compressor.
13. The method as claimed in claim 1 wherein a mixture of nitrogen and methane is recovered from said nitrogen rejection unit and is fed to the high pressure compressor.
14. The method as claimed in claim 9 further comprising feeding the C2+ hydrocarbons along with the nitrogen and methane to the high pressure compressor.
15. The method as claimed in claim 1 wherein the recovered C2+ hydrocarbons are fed to a recycle or fractioning unit.
16. A method for the recovery of natural gas from an enhanced oil recovery operation comprising the steps of (a) feeding a primary nitrogen supply into an oil field; (b) recovering oil and a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons from the oil field; (c) separating the oil from the gas mixture; (d) feeding the gas mixture to a nitrogen rejection unit operating at elevated pressures: and (e) separating natural gas from nitrogen and C2+ hydrocarbons.
17. The method as claimed in claim 16 further comprising feeding a supplemental nitrogen supply into the oil field.
18. The method as claimed in claim 16 wherein the primary nitrogen supply is from an air separation unit.
19. The method as claimed in claim 17 wherein the supplemental nitrogen supply is from a supplemental air separation unit.
20. The method as claimed in claim 16 wherein the primary nitrogen supply is fed to a high pressure compressor.
21. The method as claimed in claim 16 wherein the high pressure compressor operates at a pressure up to 6700 pounds per square inch.
22. The method as claimed in claim 16 wherein the oil and the gas mixture are fed to an oil separation and sour gas removal unit, thereby separating the recovered oil from the gas mixture.
23. The method as claimed in claim 16 wherein the recovered oil is fed to a storage system.
24. The method as claimed in claim 16 wherein the nitrogen rejection unit will separate the natural gas from the nitrogen and C2+ hydrocarbons.
25. The method as claimed in claim 16 wherein the nitrogen rejection unit is operating at elevated pressures.
26. The method as claimed in claim 25 wherein the high pressure is 30 bar.
27. The method as claimed in claim 16 wherein the recovered nitrogen is fed to an intermediate pressure compressor before being fed to the high pressure compressor.
28. The method as claimed in claim 16 wherein the natural gas is recovered from said nitrogen rejection unit and is fed to the high pressure compressor.
29. The method as claimed in claim 16 wherein the recovered C2+ hydrocarbons are fed to a recycle or fractioning unit.
30. A system for enhanced oil recovery comprising a source of nitrogen, a supplemental source of nitrogen, and a nitrogen rejection unit.
31. The system as claimed in claim 30 wherein the source of nitrogen is a primary air separation unit.
32. The system as claimed in claim 30 wherein the supplemental source of nitrogen is a supplemental air separation unit.
33. The system as claimed in claim 30 wherein the nitrogen rejection unit operates at high pressure.
34. The system as claimed in claim 30 wherein the high pressure is 30 bar.
35. The system as claimed in claim 30 wherein the nitrogen rejection unit separates a gas mixture or nitrogen, natural gas and C2+ hydrocarbons.
36. The system as claimed in claim 30 further comprising an intermediate pressure compressor.
37. The system as claimed in claim 30 further comprising a high pressure compressor.
Type: Application
Filed: Jul 22, 2015
Publication Date: Jan 28, 2016
Inventors: Joseph Naumovitz (Lebanon, NJ), Martin Kibili (Kleinaitingen)
Application Number: 14/805,776
