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A process and equipment for treating natural gas produced by a well that has recently been stimulated, and that contains an undesirably high concentration of the fracturing gas used to stimulate the well. The process involves treating the gas by membrane separation, and provides for control of treat

A process and equipment for treating natural gas produced by a well that has recently been stimulated, and that contains an undesirably high concentration of the fracturing gas used to stimulate the well. The process involves treating the gas by membrane separation, and provides for control of treatment parameters to compensate for the changing concentration of fracturing gas in the produced gas, as well as changes in gas flow rate.

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We claim: 1. A process for treating a natural gas stream produced by a well that has been recently stimulated using a fracturing gas, and that contains an undesirable amount of the fracturing gas, comprising: (a) providing a membrane unit comprising: (i) a membrane having a feed side and a permeate

We claim: 1. A process for treating a natural gas stream produced by a well that has been recently stimulated using a fracturing gas, and that contains an undesirable amount of the fracturing gas, comprising: (a) providing a membrane unit comprising: (i) a membrane having a feed side and a permeate side; (ii) automatic monitoring means positioned in a gas flow line to the membrane unit to monitor at least a concentration of the fracturing gas in the natural gas, a flow rate of the natural gas or a pressure of the natural gas; (iii) automatic control means connected in signal-transferring relationship to the automatic monitoring means and adapted to adjust at least one operating parameter of the membrane unit; (b) passing the natural gas stream through the gas flow line and across the feed side as a feed stream; (c) providing a driving force for transmembrane permeation; (d) withdrawing from the membrane unit a first treated gas stream enriched in the fracturing gas compared with the feed stream; (e) withdrawing from the membrane unit a second treated gas stream depleted in the fracturing gas compared with the feed stream; (f) adjusting the operating parameter in response to a change in at least one of the concentration, the flow rate and the pressure. 2. The process of claim 1, wherein the automatic monitoring means is further adapted to monitor a product concentration of the fracturing gas in the second treated gas stream, and wherein the automatic control means is further adapted to adjust at least a second operating parameter of the membrane unit to maintain the product concentration at least approximately at a target value. 3. The process of claim 1, wherein the fracturing gas is nitrogen. 4. The process of claim 1, wherein the fracturing gas is carbon dioxide. 5. The process of claim 1, wherein the second treated gas stream is a residue stream from the membrane unit. 6. The process of claim 1, wherein the second treated gas stream is a permeate stream from the membrane unit. 7. The process of claim 1, wherein the automatic monitoring means comprises a gas chromatograph. 8. The process of claim 1, wherein the automatic monitoring means comprises a flow monitor. 9. The process of claim 1, wherein the automatic control means comprises an automatic valve in signal transferring relationship with the automatic monitoring means. 10. The process of claim 1, wherein the operating parameter is a volume gas flow. 11. The process of claim 1, wherein the operating parameter is a transmembrane pressure difference. 12. The process of claim 1, wherein the membrane is divided between multiple membrane modules and the operating parameter is the number of on-stream membrane modules. 13. The process of claim 1, wherein the automatic controls means starts the process when the undesirable amount drops to a first predetermined value. 14. The process of claim 1, wherein the automatic control means stops the process when the undesirable amount drops to a second predetermined value. 15. The process of claim 1, wherein the membrane comprises silicone rubber. 16. The process of claim 1, wherein the membrane comprises a fluorinated polymer. 17. The process of claim 1, wherein the membrane is divided between two membrane banks arranged in a two-stage configuration. 18. The process of claim 1, wherein the membrane is divided between four membrane banks arranged in a two-step, two-stage configuration. 19. The process of claim 1, wherein the flow rate is variable and the automatic control means is adapted to adjust an internal recycle stream flow rate to compensate for the variable flow rate. 20. The process of claim 1, wherein the concentration is variable and the automatic control means is adapted to adjust an internal recycle stream flow rate to compensate for the variable concentration. 21. The process of claim 2, wherein the target value is less than about 15 vol %. 22. The process of claim 1, wherein the undesirable amount is at least about 30 vol %. 23. A process for treating a natural gas stream produced by a well that has been recently stimulated using a fracturing gas, and that contains a changing concentration of fracturing gas, comprising: (a) measuring the changing concentration at least periodically; (b) compressing the natural gas stream; (c) passing the compressed natural gas stream to a feed inlet of a membrane unit comprising a first membrane bank, a second membrane bank, a third membrane bank and a fourth membrane bank, the membrane banks being connected to allow gas flow at least from the first bank to the second and third banks, and from the third bank to the fourth bank; (d) processing the natural gas stream in the first membrane bank to produce a first permeate stream at lower pressure than the compressed natural gas stream and a first residue stream; (e) passing the first residue stream to the second membrane bank and there processing the first residue stream to produce a second permeate stream at lower pressure than the compressed natural gas stream and a second residue stream; (f) discharging the second residue stream from the process; (g) recompressing the first permeate stream; (h) passing the recompressed first permeate stream to the third membrane bank and there processing the first permeate stream to produce a third permeate stream at lower pressure than the compressed natural gas stream and a third residue stream; (i) discharging the third permeate stream from the process as a treated natural gas stream having a reduced concentration of fracturing gas; (j) passing the third residue stream to the fourth membrane bank and there processing the third residue stream to produce a fourth permeate stream at lower pressure than the compressed natural gas stream and a fourth residue stream; (k) recirculating the second permeate stream through a first control valve to step (b); (l) recirculating the fourth residue stream to step (c); (m) recirculating the fourth permeate stream through a second control valve to step (g); (n) measuring the reduced fracturing gas concentration at least periodically; (o) as the changing concentration changes, periodically adjusting a first setting of the first control valve to change the volume of gas flowing through the first control valve in response to a change in the changing concentration; (p) if the reduced fracturing gas concentration deviates from a target value, periodically adjusting a second setting of the second control valve to change the volume of gas flowing through the second control valve. 24. The process of claim 23, wherein: (A) the natural gas stream has a variable flow rate and the process further comprises measuring the variable flow rate at least periodically; (B) the second residue stream is discharged through a pressure-regulating valve; and (C) the process further comprises periodically adjusting a third setting of the pressure-regulating valve in response to a change in the variable flow rate to maintain a desired transmembrane pressure difference in the first and second membrane banks. 25. The process of claim 23, wherein steps (a), (n), (o) and (p) are performed by means of an automated process control system. 26. The process of claim 25, wherein the natural gas stream is not admitted to step (b) until the changing concentration as monitored in step (a) has reached a first chosen value. 27. The process of claim 25, wherein the process is automatically stopped when the changing concentration as monitored in step (a) has reached a second chosen value. 28. The process of claim 23, wherein the fracturing gas is nitrogen. 29. The process of claim 23, wherein the first chosen value is at least as high as 30 vol %. 30. The process of claim 23, wherein the target value is less than about 15 vol %. 31. The process of claim 23, wherein: (A) the first membrane bank comprises multiple first tubes arranged in parallel such that gas entering that bank may be split into multiple first sub-streams, each first sub-stream directed to one of the first tubes; (B) at least one of the first tubes has a first tube inlet line in which is positioned a first inlet control valve; (C) the third membrane bank comprises multiple third tubes arranged in parallel such that gas entering that bank may be split into multiple third sub-streams, each third sub-stream directed to one of the third tubes; (D) at least one of the third tubes has a third tube inlet line in which is positioned a third inlet control valve; (E) the process further comprises closing the first inlet control valve when the changing concentration drops below a first determined value; and closing the third inlet control valve when the changing concentration drops below a second determined value. 32. The process of claim 31, wherein the fracturing gas is nitrogen. 33. The process of claim 31, wherein steps (a), (n), (o), (p) and (E) are performed by means of an automated process control system. 34. Apparatus for treating a natural gas stream produced by a well that has been recently stimulated using a fracturing gas, and that contains a changing concentration of fracturing gas, comprising: (a) a raw gas inlet line, in which are positioned a feed control valve, a flow monitor and a concentration monitor; (b) a first compressor having a first suction side and a first exhaust side and connected on the first suction side to the raw gas inlet line so that gas may flow through the feed control valve into the first compressor; (c) a first compressor exhaust line, connected to the first exhaust side and to a first bank of membrane modules having a first feed side and a first permeate side, so that gas may flow from the first exhaust side to the first feed side; (d) a first residue line joining a first residue outlet of the first bank of membrane modules to a second bank of membrane modules, having a second feed side and a second permeate side, on the second feed side; (e) a first recycle line connecting the second permeate side to the first suction side, so that gas may be recirculated to the first compressor; (f) a first flow control valve positioned in the first recycle line and connected through an automated process control system to the concentration monitor; (g) a residue discharge line from the second bank of membrane modules to enable gas to be collected from the second feed side and discharged from the apparatus; (h) a back-pressure regulating valve positioned in the residue discharge line and connected through an automated process control system to the flow monitor; (i) a first permeate line connecting the first permeate side to a second compressor, having a second suction side and a second exhaust side, on the second suction side, so that gas may flow from the first permeate side to the second suction side; (j) a second compressor exhaust line, connected to the second exhaust side and to a third bank of membrane modules having a third feed side and a third permeate side, so that gas may flow from the second exhaust side to the third feed side; (k) a third residue line joining a third residue outlet of the third bank of membrane modules to a fourth bank of membrane modules, having a fourth feed side and a fourth permeate side, on the fourth feed side; (l) a second recycle line connecting the fourth permeate side to the second suction side, so that gas may be recirculated to the second compressor; (m) a second flow control valve positioned in the second recycle line and connected through the automated process control system to the concentration monitor; (n) a third recycle line connecting a fourth residue outlet of the fourth bank of membrane modules to the first compressor exhaust line, so that gas may be recirculated to the first feed side; (o) a permeate discharge line from the third permeate side to enable gas to be collected from the third permeate side and discharged from the apparatus.