Patent Publication Number: US-2005133270-A1

Title: Oil recovery using non-cryogenically produced nitrogen and off-gas recycling

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to the field of recovery of oil from oil wells, and provides an improved method for enhanced oil recovery.  
      Enhanced oil recovery means the recovery of oil with the assistance of a compressed fluid which is pumped into a well so as to force the oil out. Such a technique is appropriate where the oil in the reservoir exists naturally in liquid form, but where the pressure of the oil is too low to cause the oil to exit the reservoir spontaneously. Such reservoirs typically include a mixture of liquid oil and natural gas, and may have some oxygen mixed therewith.  
      In the prior art, it has been known to use a relatively inert gas, such as nitrogen or nitrogen-enriched air, to force the oil out of a reservoir. An inert gas, or relatively inert gas, is preferred, so as to prevent the fluid from reacting with the contents of the reservoir. A stream of compressed nitrogen, or other relatively inert gas, effectively pressurizes the reservoir, and enables the recovery of oil that would otherwise not readily flow out.  
      Nitrogen for conducting an enhanced oil recovery process can be supplied by transporting it to the well site in cylinders or tanks, or by vaporizing liquid nitrogen that has been brought to the well site or produced there and stored. But providing gaseous nitrogen in cylinders is cumbersome and expensive, especially where the well site is remote. It is also expensive to produce, handle, and store cryogenic liquids at a well site. It has therefore been proposed, in the prior art, to use non-cryogenically produced nitrogen, at the site of a well, for the purpose of enhancing recovery of oil. Examples of patents which deal with enhanced oil recovery using non-cryogenically produced nitrogen are U.S. Pat. Nos. 5,749,422, 5,862,869, 6,041,873, 6,206,113, and 6,443,245, the disclosures of all of which are incorporated by reference herein.  
      In an enhanced oil recovery, the pressurized inert gas displaces not only liquid oil, from the reservoir, but also one or more “off-gases”. The term “off-gas” means any gas that comes out of the production well with the recovered oil. Typically, the off-gas includes methane, and/or some heavier hydrocarbons, and it may also eventually include some of the nitrogen that was pumped into the well and which may have become mixed with the liquid oil.  
      The off-gases withdrawn from a well have traditionally been considered waste, and have been vented to the atmosphere. The present invention provides a method whereby such off-gases are used productively, so as to reduce the load on the membrane system, or other system, for producing the nitrogen. The present invention therefore substantially improves the efficiency of an enhanced oil recovery process.  
     SUMMARY OF THE INVENTION  
      The present invention comprises an improved process for enhanced oil recovery. A relatively inert gas, such as nitrogen, or nitrogen-enriched air, is generated by a non-cryogenic process, at or near the site of an oil-bearing formation. This inert gas is compressed and injected into the formation, displacing liquid oil in the formation, and causing the liquid oil to flow out. Also exiting the formation are one or more off-gases. The off-gases are separated from the liquid oil, so that the liquid oil can be recovered as the final product.  
      Meanwhile, the separated off-gases are compressed and re-injected into the formation. Preferably, the off-gases are conveyed to the same conduit used to inject the inert gas. The off-gases therefore become mixed with the inert gas while being injected into the formation. Recycling of the off-gases in this manner reduces the requirement for inert gas. Essentially, the system need only supply “make-up” inert gas, thus reducing the size and power consumption requirements of the equipment for generating the inert gas.  
      The inert gas may be produced by a membrane unit or by a pressure swing adsorption (PSA) system. Ambient air is compressed, dried, heated, and filtered, and then passed through either the membrane unit or the PSA unit, to produce a nitrogen-enriched stream used as the inert gas.  
      The invention therefore has the primary object of providing an improved process for enhanced oil recovery.  
      The invention has the further object of providing an enhanced oil recovery process in which the size and power requirements for a non-cryogenic gas generator are reduced.  
      The invention has the further object of providing an enhanced oil recovery process, in which off-gases withdrawn from an oil-bearing formation are used in the recovery process and are not discarded.  
      The invention has the further object of enhancing the efficiency of oil recovery from a formation.  
      The invention has the further object of improving the economics of recovery of liquid oil from a formation in which the liquid oil is at a relatively low pressure.  
      The invention has the further object of reducing environmental harm caused by an enhanced oil recovery process.  
      The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawing, the detailed description of the invention, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      The FIGURE provides a schematic diagram of an apparatus used for practicing the process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In this specification, the terms “oil well”, “formation”, and “reservoir” are used interchangeably to identify a natural underground source of liquid oil.  
      The FIGURE provides a schematic diagram of the components of a system used to practice the present invention. The system is preferably physically located at or near the site of an oil well.  
      Air compressor  1  takes ambient air and compresses it. The compressed air passes through receiver  2  and moisture separator  3 . The air may then pass through an optional air dryer  4 . The air passes through coalescing filters  5  and  6 , and heater  7 . Air leaving the heater may pass through an optional carbon bed  8 , and then through particulate filter  9 .  
      Air leaving the particulate filter enters air separator  10 , which may be either a membrane system or a pressure swing adsorption (PSA) unit. The air separator converts the incoming air into two streams, one which is oxygen-enriched and the other which is oxygen-depleted (nitrogen-enriched). In the extreme case, the separator may produce one stream that is virtually all, or nearly all, oxygen, and another stream which is almost all nitrogen. In the more general case, the separation of the air is less than complete.  
      The oxygen-depleted (nitrogen-enriched) output of the air separator  10  is conveyed to compressor  11 , where it is compressed, and then injected into an oil well or reservoir, as indicated. The oxygen-enriched stream is not used in the specific process of the present invention, and is therefore not shown in the FIGURE, but could be collected and used for other purposes.  
      The product of the reservoir, comprising a mixture of condensed liquid oils and off-gases, is directed through conduit  12  into gas/liquid separator  13 . The gas/liquid separator produces two streams, one being the liquid oil, which exits through conduit  14 , and the other being the off-gases, which exit through conduit  15 . The conduit  14  therefore carries the final product of the oil recovery process. Meanwhile, the off-gases are compressed in compressor  16 , and conveyed into the well or reservoir, preferably through the same conduit used to inject the original nitrogen.  
      The present invention therefore recycles the off-gases withdrawn from the oil well, and uses these gases to assist in the physical displacement of oil from the reservoir. The recycling of the off-gases reduces the amount of fresh nitrogen needed. In fact, after the process is in continuous operation, the membrane or PSA unit need only provide “make-up” inert gas, i.e. inert gas needed to compensate for losses during the oil recovery process. Thus, the size and power consumption of the membrane or PSA unit can be reduced, as compared to what would be required if the off-gases were vented to the atmosphere. Since the membrane or PSA unit operates with minimal manpower, the efficiency of the oil recovery process is further enhanced.  
      The invention also has a further environmental benefit, because the off-gases are not vented to the atmosphere, but are compressed and recycled.  
      The process is preferably operated continuously. In theory, off-gases from the well may be recycled through the well many times, as part of the compressed gas stream used to displace the liquid oil.  
      The advantage of the present invention may be quantified by the following example. For a system producing about two barrels of oil per day, one may need an inert gas flow (inert gas being defined as a gas having less than 5% oxygen) of 10 barrels per day of gas at 3000 psig (about 15000 scfd). A membrane system alone would require about 15000 scfd of inert gas from a compressor feed flow of about 30000 scfd. But if one recycles 7500 scfd of off-gas that came with the liquid oil, then the required flow of inert gas from the membrane could be reduced by half, to about 7500 scfd, and the throughput of the feed compressor for the membrane process would also be reduced by half. The extent of feed air pretreatment, and the number of membrane elements, would also be reduced by one-half, to produce the same amount of oil. The membrane system generally operates at less than 350 psig, and must have the inert gas pressure boosted to a higher injection pressure. Less compression may be required for the off-gas from the product stream, which may be at a higher pressure than the 350 psig from the membrane system.  
      The invention may be modified in various ways. Different conduit arrangements for mixing and injecting the inert gas and the off-gases may be provided.. The components used to treat of the air being separated can be modified, some of the components being indicated in the FIGURE as optional. These and other modifications, which will be apparent to the person skilled in the art, should be considered within the spirit and scope of the following claims.