Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. provisional application Ser. No. 62/028,091, filed on Jul. 23, 2014. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The 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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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 INVENTION 
       [0007]    In 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. 
         [0008]    The method may further comprise feeding a supplemental nitrogen supply into the oil field. 
         [0009]    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, 
         [0010]    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. 
         [0011]    The recovered oil may be addressed in a manner of way but is typically fed to a storage facility. 
         [0012]    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. 
         [0013]    The recovered nitrogen is fed to an intermediate pressure compressor before being fed to the high pressure compressor. 
         [0014]    A mixture of nitrogen and methane is recovered from said nitrogen rejection unit and is fed to the high pressure compressor. 
         [0015]    Alternatively, the C2+ hydrocarbons may be fed along with the nitrogen and methane to the high pressure compressor. 
         [0016]    The recovered C2+ hydrocarbons may be fed to a recycle or fractioning unit. 
         [0017]    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. 
         [0018]    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. 
         [0019]    The source of nitrogen is a primary air separation unit. The supplemental source of nitrogen is a supplemental air separation unit. 
         [0020]    The nitrogen rejection unit operates at high pressure of 30 bar. 
         [0021]    The nitrogen rejection unit separates a gas mixture or nitrogen, natural gas and C2+ hydrocarbons. 
         [0022]    The system further comprises an intermediate pressure compressor. 
         [0023]    The system further comprises a high pressure compressor. 
         [0024]    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. 
         [0025]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic of the method for nitrogen rejection according to the invention. 
           [0027]      FIG. 2  is a schematic of a nitrogen rejection unit according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1  is a schematic of an enhanced oil recovery operation. An air separation unit (ASU) of  32  million standard cubic feed per day (MMSCFD) A provides nitrogen through line  1  to line  4 . This nitrogen is fed through line  4  to a high pressure compressor B which is capable of compressing the nitrogen to  6700  pounds per square inch (PSI). 
         [0029]    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. 
         [0030]    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. 
         [0031]    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. 
         [0032]    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 . 
         [0033]    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. 
         [0034]      FIG. 2  is a schematic of a plant featuring a nitrogen rejection unit. Feed stock from a gas plant is fed through line  23  and open valve V 1  through the main heat exchanger J and into a flash separator L. The flash separator will separate out the C2+ hydrocarbons from the feed stock and will return the C2+ hydrocarbons with the assistance of pump O back through the main heat exchanger J where they are recovered through line  21 . The remainder of the feed stock which consists mainly of methane and nitrogen is fed from the top of the flash separator through line  24  to the pressurized nitrogen rejection column M which will act to separate the nitrogen from the natural gas and other components of the gas plant feed stock. 
         [0035]    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. 
         [0036]    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. 
         [0037]    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. 
         [0038]    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 V 2  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. 
         [0039]    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. 
         [0040]    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,