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The invention relates to a system and method of treating a flow back fluid exiting a well site following stimulation of a subterranean formation. The invention utilizes a two-stage membrane process during the period that the gas contains high concentrations of CO2 by volume, and allows for separatio

The invention relates to a system and method of treating a flow back fluid exiting a well site following stimulation of a subterranean formation. The invention utilizes a two-stage membrane process during the period that the gas contains high concentrations of CO2 by volume, and allows for separation of CO2 from the natural gas components, providing pipeline-quality natural gas (approximately 5% CO2 by volume) to the gas collection system.

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1. A two stage membrane process for treating a flow back gas exiting a well site following the CO2 stimulation of a subterranean formation, wherein said process comprises: pretreating the flow back gas exiting the well site;processing said flow back gas in a first stage membrane unit to separate wat

1. A two stage membrane process for treating a flow back gas exiting a well site following the CO2 stimulation of a subterranean formation, wherein said process comprises: pretreating the flow back gas exiting the well site;processing said flow back gas in a first stage membrane unit to separate water and water vapor from the flow back gas, forming a water-rich permeate stream and a water depleted flow back stream;processing said water depleted flow back stream in a second stage membrane unit downstream from said first stage membrane unit to separate out CO2 from said water depleted flow back stream forming a carbon dioxide rich permeate stream and a carbon dioxide depleted retentate flow back stream rich in hydrocarbons. 2. The process of claim 1 wherein the pretreatment comprises one or more of adjusting the pressure of the flow back gas, adjusting the temperature of the flow back gas, and/or removing any one of water, solid particulates, liquid hydrocarbons, hydrogen sulfides or a combination thereof from said flow back gas. 3. The process of claim 1 wherein said flow back gas is processed in said first stage membrane unit until the temperature of the retentate stream (13) leaving the second stage membrane unit is above the temperature at which solids may form, after which first stage membrane unit 1 is bypassed and the flow back gas exiting the well is routed directly to said second stage membrane unit 2. 4. The process of claim 3 wherein said flow back gas is processed in said first stage membrane unit until the temperature of the retentate stream (13) leaving the second stage membrane unit is above the freezing point of water, after which first stage membrane unit 1 is bypassed and the flow back gas exiting the well is routed directly to said second stage membrane unit 2. 5. The process of claim 1 wherein said flow back gas is processed in said second stage membrane unit until the CO2 concentration in said flow back exiting the well site has dropped to a range of from about 2 to about 10% CO2 by volume, after which processing of the flow back gas is terminated and all flow back gas is sent directly to a gathering system. 6. The process of claim 1 additionally comprising processing said carbon dioxide depleted flow back stream rich in hydrocarbons exiting said second stage membrane unit in a natural gas separator configured to separate any condensed liquids from said stream, forming a condensed liquid stream and a gas stream rich in hydrocarbons. 7. The two stage membrane process of claim 1 wherein said first stage membrane unit or said second stage membrane unit or both of said first and second stage membrane units comprise separate outlet passages for liquid and gas retentate. 8. The process of claim 6 wherein the condensed liquids are subjected to one or more stabilization treatments to remove CO2 and lower methane content to meet Y-grade specifications for commercial transport and use. 9. The process of claim 1 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a membrane material resistant to the effects of condensed natural gas liquids. 10. The process of claim 9 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a membrane material selected from Selective Dense or Asymmetric Polymer Membrane, Facilitated Transport Membrane, Liquid Support Membrane, Fixed Support Membrane, Membrane Gas Absorption Membrane; Inorganic Membrane; Mixed Matrix Membrane; and combinations thereof. 11. The process of claim 10 wherein the Selective Dense or Asymmetric Polymer Membrane is selected from the following membrane materials: cellulose acetate, polyimides, polyamides, perfluoropolymers, polysulfone, polycarbonates, polyetherimides, polyether ether ketone, fluoropolyimides, poly(amide-imide), poly(dimethylsiloxane), Polyethylene oxide, poly(phenylene oxide) with Nylon 6, polyaniline, polyaramides, polyarylates, polyarylene ether ketone, polyether ketone, polyetherimide, polyethersulfone, polyketone, polyphenylene sulphide, polypyrrolones, polysemicarbazides; and combinations thereof. 12. The process of claim 11 wherein said membrane material is selected from the group consisting of cellulose acetate, polyimides, polyamides, perfluoropolymer, polysulfone, polycarbonates, polyetherimides, and poly(ether ether ketone). 13. The process of claim 12 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a polyether ether ketone (PEEK) membrane. 14. A system for processing flow back gas exiting a well site following the CO2 stimulation of a subterranean formation, wherein said process comprises: a first stage membrane unit configured to separate water and water vapor from the flow back gas, forming a water-rich permeate stream and a water depleted flow back stream;a second stage membrane unit downstream from said first stage membrane unit configured to receive said water depleted flow back stream and separate out CO2 from said water depleted flow back stream forming a carbon dioxide rich permeate stream and a carbon dioxide depleted retentate flow back stream rich in hydrocarbons, wherein said flow back gas is processed in said first stage membrane unit until the temperature of the retentate stream (13) leaving the second stage membrane unit is above the freezing point of water, after which first stage membrane unit 1 is bypassed and the flow back gas exiting the well is routed directly to said second stage membrane unit 2. 15. The system of claim 14 further comprising a pretreatment unit configured for pretreating said flow back gas prior to processing said flow back gas in said first stage membrane unit. 16. The system of claim 15, wherein the pretreatment unit is configured to adjust the pressure and/or temperature of the flow back gas. 17. The system of claim 16 wherein said pretreatment unit is configured to remove at least one of water, solid particulates, liquid hydrocarbons, hydrogen sulfides or a combination thereof from the feed stream. 18. The system of claim 14 wherein said flow back gas is processed in said first stage membrane unit until the temperature of the retentate stream (13) leaving the second stage membrane unit is above the freezing point of water, after which first stage membrane unit 1 is bypassed and the flow back gas exiting the well is routed directly to said second stage membrane unit 2. 19. The system of claim 14 wherein said flow back gas is processed in said second stage membrane unit until the CO2 concentration in said flow back gas exiting the well site has dropped to a range of from about 2 to about 10% CO2 by volume, after which the system processing is terminated and all flow back gas is sent directly to a gathering system. 20. The system of claim 14 further comprising a natural gas liquids separator downstream of said second stage membrane unit to receive the carbon dioxide depleted flow back stream rich in hydrocarbons and separate any condensed liquids from said stream, forming a condensed liquid stream and a gas stream rich in hydrocarbons. 21. The system of claim 20 which further comprising a product separation unit downstream of said natural gas liquids separator to receive said condensed liquid stream and said gas stream rich in hydrocarbons, wherein said product separator optionally comprises a heater for heating said high gas stream rich in hydrocarbons and is operated at a predetermined pressure allowing any condensed hydrocarbons to recombine with said gas stream rich in hydrocarbons forming a natural gas product stream. 22. The system of claim 21 which additionally comprises a gas gathering system wherein said predetermined pressure of said heater is the pressure of the gas gathering system. 23. The system of claim 14 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a membrane material resistant to the effects of condensed natural gas liquids. 24. The system of claim 23 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a membrane material selected from Selective Dense or Asymmetric Polymer Membrane, Facilitated Transport Membrane, Liquid Support Membrane, Fixed Support Membrane, Membrane Gas Absorption Membrane; Inorganic Membrane; Mixed Matrix Membrane; and combinations thereof. 25. The system of claim 24 wherein the Selective Dense or Asymmetric Polymer Membrane is selected from the following membrane materials: cellulose acetate, polyimides, polyamides, perfluoropolymers, polysulfone, polycarbonates, polyetherimides, polyether ether ketone, fluoropolyimides, poly(amide-imide), poly(dimethylsiloxane), Polyethylene oxide, poly(phenylene oxide) with Nylon 6, polyaniline, polyaramides, polyarylates, polyarylene ether ketone, polyether ketone, polyetherimide, polyethersulfone, polyketone, polyphenylene sulphide, polypyrrolones, polysemicarbazides; and combinations thereof. 26. The system of claim 24 wherein said membrane material is selected from the group consisting of cellulose acetate, polyimides, polyamides, perfluoropolymer, polysulfone, polycarbonates, polyetherimides, and poly(ether ether ketone). 27. The system of claim 14 wherein at least one of said first stage membrane unit and second stage membrane unit comprises a polyether ether ketone (PEEK) membrane. 28. The system of claim 14 wherein the entire system or parts of the system are mobile. 29. The system of claim 14 wherein said first stage membrane unit or said second stage membrane unit, or both of said first and second stage membrane units comprise separate outlet passages for liquid and gas retentate. 30. The system of claim 15 comprising a flow meter and flow control valve (XV-3) to establish gas flow rate entering the membrane units, wherein said flow rate is fixed, or can vary automatically in response product gas (18) CO2 concentration within the minimum and maximum flow constraints of the system. 31. A system for processing flow back gas exiting a well site following the CO2 stimulation of a subterranean formation, wherein said system comprises: a pretreatment unit for processing flow back gas from a well site, wherein said pretreatment unit is configured to adjust feed pressure and/or temperature, and/or to remove any one of water, solid particulates, liquid hydrocarbons, hydrogen sulfides or a combination thereof from said flow back gas,a first stage membrane unit downstream of said pretreatment unit to receive the pretreated flow back gas therefrom and separate water vapor from the flow back gas, forming a water-rich permeate stream and a water depleted flow back stream;a second stage membrane unit downstream from said first stage membrane unit to receive said water depleted flow back stream and separate out CO2 from said water depleted flow back stream forming a carbon dioxide rich permeate stream and a carbon dioxide depleted retentate flow back stream rich in hydrocarbons;a natural gas liquids separator downstream of said second stage membrane unit to receive the carbon dioxide depleted flow back stream rich in hydrocarbons, wherein said separator is configured to separate any condensed liquids from said carbon dioxide depleted flow back stream, forming a condensed liquid stream and a gas stream rich in hydrocarbons;a product separation unit downstream of said natural gas liquids separator to receive said condensed liquid stream and said gas stream rich in hydrocarbons, anda gas gathering system, wherein said product separation unit is operated at a pressure required by the gas gathering system, allowing any condensed hydrocarbons to recombine with said gas stream rich in hydrocarbons forming a natural gas product stream. 32. The system of claim 31 wherein said flow back gas is processed in said first stage membrane unit until the temperature of the retentate stream (13) leaving the second stage membrane unit is above the freezing point of water, after which first stage membrane unit 1 is bypassed and the flow back gas exiting the well is routed directly to said second stage membrane unit 2. 33. The system of claim 32 wherein said flow back gas is processed in said second stage membrane unit until the CO2 concentration in the flow back gas exiting the well site has dropped to a range of from about 2 to about 10% CO2 by volume, after which the system processing is terminated. 34. The process of claim 8 wherein said stabilization treatments include at least one of dehydration, desulfurization, heating, distillation, vapor/liquid separation, and addition of gas hydrate inhibitors.