MEMBRANE-BASED SYSTEM FOR GENERATING HIGH-PURITY NITROGEN

Abstract

A non-cryogenic system for producing high-purity nitrogen is connected to a coiled-tubing unit. The nitrogen is produced by passing compressed ambient air through two polymeric membrane modules connected in series. The output of the second module is a stream of high-purity nitrogen, which is conveyed into a coiled tube. The nitrogen can be used for inerting the interior of the tube, or the coiled tube can be inserted into an oil well, for delivering nitrogen into the well. The use of nitrogen in a coiled tube helps to prevent corrosion in the tube.

Claims

1. A method for providing nitrogen to a coiled-tubing system, the method comprising the steps of: a) conveying air to a first module containing a polymeric membrane capable of separating air into its components, wherein an output of said first module comprises nitrogen-enriched air, b) conveying the output of said first module into a second module containing a polymeric membrane capable of separating air into its components, wherein an output of said second module comprises substantially pure nitrogen, and c) directing the output of said second module into a coiled tube.

2. The method of claim 1, further comprising directing at least a portion of gas flowing through said second module into said first module.

3. The method of claim 2, wherein step (a) is preceded by the step of compressing air in a compressor, and wherein said portion of gas flowing through said second module is directed into an input of said compressor.

4. The method of claim 1, wherein the method is performed in a vicinity of an oil well.

5. A method of providing nitrogen for a coiled-tubing system, the method comprising the steps of producing nitrogen, having a high purity, by passing air through a polymeric membrane module, the module being capable of separating air into its components, wherein an output of the module is a stream of high-purity nitrogen, the method further comprising the step of directing the high-purity nitrogen into a coiled tube.

6. The method of claim 5, further comprising inserting the coiled tube into an oil well for delivery of nitrogen into the well.

7. The method of claim 5, wherein the air-passing step comprises passing air through two polymeric membrane modules connected in series.

8. The method of claim 7, wherein there are first and second membrane modules, and wherein the method further comprises the step of recycling a portion of gas in said second membrane module into said first membrane module.

9. Apparatus for providing nitrogen to a coiled-tubing system, comprising a means for directing ambient air to a polymeric membrane module capable of separating air into its components, wherein a product of said membrane comprises nitrogen, and means for conveying said nitrogen into a coiled tube.

10. The apparatus of claim 9, wherein there are two membrane modules, connected in series.

11. The apparatus of claim 9, further comprising a compressor, connected to receive the ambient air, an output of the compressor being connected to an input of the membrane module.

12. The apparatus of claim 10, further comprising a conduit for conveying a portion of air flowing through a second of said two membrane modules into said compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 provides a schematic diagram showing the system of the present invention.

[0027] FIG. 2 provides a perspective view of a coiled-tubing system, into which the output of the system of FIG. 1 is directed, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention comprises the use of membrane air separation technology in generating nitrogen of high purity, i.e. nitrogen in which there is about 0.1% residual oxygen, for use in oil or gas drilling and/or enhanced oil recovery, using coiled-tubing technology.

[0029] Coiled tubing technology comprises the use of coils of metal tubing to line wells while drilling or to pressurize existing drilled wells to promote oil production. Coiled tubing avoids the need to use fixed-length, straight tubing that is more cumbersome, and impossible to use for some directly drilled wells. Coiled tubing is particularly useful for angled drilling or directional drilling, where it is desired to provide a tube along a non-linear path. A coiled tube can be used to deliver gas to an existing well, and can later be retracted, stored on a reel, and used in the same well or in another well. The tube is typically formed of a flexible but strong metal, so the coiled tube can do essentially the same work as a conventional tube.

[0030] In applications in which coiled tubing is used, it is necessary to provide inert gases, having very low levels of oxygen, not only to avoid flammable conditions in the well, but to prevent rusting of the tubing. Avoidance of flammable conditions requires only that the oxygen content be in the range of about 3-7%, but avoidance of corrosion requires that the oxygen content be less than about 0.1%. Prevention of corrosion is important because it extends the useful life of the tubing.

[0031] To obtain high-purity nitrogen, having not more than 0.1% oxygen, the accepted practice of the industry is to produce nitrogen from air using conventional cryogenic separation techniques. Such nitrogen comes at a high cost, due both to the cost of the gas and the cost of transporting it to the oil field where it is needed.

[0032] Membrane processes typically produce enriched nitrogen by separating air, with a residual oxygen content of about 3-7%. In the present invention, however, the membrane system is operated to produce nitrogen having a purity of 99.9%, i.e. having only 0.1% oxygen, and in an economical manner.

[0033] The membrane system of the present invention is designed to operate in a series array, with a partial recycle loop, to minimize inefficiencies in the membrane processes that often prevent the membrane systems from producing the high-purity nitrogen in an economical way.

[0034] The present invention includes a method for making pure nitrogen, and a system for practicing that method. An example of the system is shown in FIGS. 1 and 2.

[0035] As shown in FIG. 1, ambient air enters the system through conduit 1, and is compressed in compressor 3. The compressed air may be filtered by passing it through filter 5. The compressed air then is directed through a gas-separation membrane module 7. The module includes a polymeric membrane material appropriate for separating air into its components. In the present example, the product gas is nitrogen, and the waste gas is oxygen. Oxygen-enriched air is therefore vented from the module through vent 9.

[0036] The product of module 7, which exits through conduit 11, comprises air in which the concentration of nitrogen is greater than ambient, and in which the concentration of oxygen is less than ambient. This stream then is the input to membrane module 13. The product of the module 13, appearing at outlet 15, is a stream of nitrogen having a high purity. The product of the module 13 comprises the output which is fed to a coiled-tubing system.

[0037] A portion of the stream being separated by module 13 may be conducted, by conduit 17, to the inlet of the compressor. This stream has a lower oxygen concentration than ambient, and using it as part of the inlet stream ultimately yields a product comprising nitrogen of very high purity. The recycling of the stream through conduit 17 is optional, however.

[0038] The output of the system shown in FIG. 1 is then conducted into a coiled-tube system, such as for inerting the contents of the coiled tube, or for other purposes associated with the operation of an oil well. A coiled-tube system is illustrated in FIG. 2, which shows tubing 22 wound around reels 20. The tubing can be readily unwound and fed into a well bore.

[0039] The advantage of the present invention is shown by the following Examples.

EXAMPLE 1

[0040] This Example provides a baseline for evaluation of the present invention, and represents the prior art. In this Example, a plurality of membrane modules are arranged in parallel. A simple parallel array produces a product stream having a nitrogen purity of 99.9% (with 0.1% oxygen), at a rate of 11.3 scfm (standard cubic feet per minute) per membrane element. The membrane module used in this Example was a Model 7200 manufactured by Generon IGS, Inc. of Houston, Tex. The module has nominal dimensions of 1072 inches. The module has a feed flow requirement of 190 psig compressed air at 45 C. and 85 scfm. This yields a product flow recovery of 13.3%. The recovery is defined as the ratio of product flow to feed flow.

[0041] The parallel arrangement of modules is the standard configuration for modules used in the oil and gas industry, for generating inert gas. The efficiency of the membrane system is low relative to ideal performance because of known variations in the modules that cause flow distribution issues when operated at product recoveries less than 30%. These flow distribution issues can greatly limit the operating efficiency of an individual module or groups of modules by not allowing for the uniform removal of oxygen from the feed stream in discrete parts of the module or systems of modules.

EXAMPLE 2

[0042] In this Example, a system using Generon modules was constructed with the modules arranged in series. This arrangement also produces a product stream having a purity of 99.9% nitrogen, with 0.1% oxygen. In this arrangement, the modules are operated in series so the individual product recoveries for the individual stages are above 30%, and they do not suffer from the flow distribution inefficiencies seen in Example 1.

[0043] The two-module series array, using the same modules used in Example 1, produces high-purity nitrogen at 31 scfm (15.6 scfm per module) when operated at 190 psig and 45 C., while requiring 175 scfm of feed (87 scfm/module). In this case, the modules have a net product recovery of 17.8%. This represents a 38% increase in product flow and a 34% increase in product recovery as compared with Example 1.

EXAMPLE 3

[0044] In this Example, a pair of modules were operated in series, with the addition of recycling part of the permeate stream from the second stage (module) to the inlet of the feed compressor. Thus, this Example uses the arrangement shown in the Figure.

[0045] The second stage permeate stream contains less than 7% oxygen and effectively lowers the oxygen level in the compressed feed stream to the first stage module(s) to around 17%.

[0046] Since there is less oxygen to remove, the operation is more efficient in producing the nitrogen of 99.9% purity. This type of operation, having two stages in series, with recycling of permeate, produces a product stream of 99.9% nitrogen (0.1% oxygen) at higher rates and efficiencies relative to Example 2.

[0047] In particular, with two modules connected in series, as shown in the Figure, the modules being the same as in Examples 1 and 2, the high-purity nitrogen stream is produced at 36 scfm (18 scfm per module) when operated at 190 psig and 45 C., while requiring 171 scfm of feed (86 scfm/module).

[0048] In this case, the modules have a net product recovery of 21.2%. This represents a 59% increase in product flow and in product recovery, as compared with Example 1. This translates directly to a lower cost for making high-purity nitrogen, in that both the capital cost (i.e. the number of modules) and the power cost (i.e. the amount of compressed feed air required) is only 62% of that needed for modules operated as in Example 1.

[0049] The above Examples show that with the improved performance achieved with the series arrangement used in Examples 2 and 3, it is economically feasible to provide high-purity nitrogen using a membrane-based system, for use with coiled tubing at the site of an oil well. The membrane-produced nitrogen is more advantageous than cryogenically-produced nitrogen, which would otherwise need to be transported to the well site in batches.

[0050] The use of membrane modules connected in parallel effectively provides a shorter path for gas. With membranes connected in series, there is a longer path, and a greater pressure drop. But this is not a major concern, because the pressure of the gas is often boosted before entry into the well, and most users of the gas will already have compressors available at the well site.

[0051] The series configuration, used in the present invention, allows each module to operate at higher recovery levels, where flow variability is less of a concern. The product gas is also allowed to mix between the two stages, even better to balance out the variable output from the individual fibers. Since the modules are in series and have higher pressure drops associated with them, problems with manifolding the feed and product flows are also minimized. The end result is that one can take quite dissimilar modules and get very reproducible performance when running them in series.

[0052] If a dedicated compressor is available, a series arrangement, with permeate recycling, as shown in the Figure, is the preferred choice for any purity level greater than 99%. Use of the arrangement of the present invention makes it practical to reduce the size of the compressor by 8-15%, and to reduce the module requirements by 5%.

[0053] The use of the series configuration, of the present invention, does have the disadvantage that, because the second stage permeate gas has less than 6% oxygen, it is not breathable. Such gas must be properly vented away or mixed with the permeate from the first stage to provide a breathable atmosphere. Therefore, the compressor used with this system must be dedicated to the membrane system, and must not be used inadvertently to provide compressed air for general purposes.

[0054] The present invention is not limited to use with coiled-tubing applications. Other applications, relating to the oil industry, which would benefit from the use of high-purity nitrogen produced by on-site membrane systems include managed pressure drilling, well unloading, enhanced oil recovery (EOR), and gas lifting. While some of these applications currently use nitrogen of only moderate purity, concerns about corrosion of process equipment will favor membrane systems that can produce nitrogen with lower oxygen content, just as has happened in the case of coiled-tubing technology.

[0055] The invention can be modified further, as will be appreciated by those skilled in the art. Such modifications should be considered to be within the spirit and scope of the following claims.