Patent Publication Number: US-6209641-B1

Title: Method and apparatus for producing fluids while injecting gas through the same wellbore

Description:
DESCRIPTION 
     1. Technical Field 
     The present invention relates to a method and apparatus for producing through a wellbore while simultaneously injecting gas into a subterranean formation through the same wellbore and in one aspect relates to a method and apparatus for injecting gas into a formation wherein the gas is mixed with a carrier fluid at the surface which, in turn, is flowed down a well where at least a portion of the gas is separated and injected into the formation while fluids are being produced through the wellbore. 
     2. Background 
     It is well known that many hydrocarbon reservoirs produce extremely large volumes of gas along with crude oil and other liquids. In producing fields such as these, it is not unusual to experience gas-to-oil ratios (GOR) as high as 25,000 standard cubic feet per barrel (scf/bbl.) or greater. As a result, large volumes of gas must be separated out of the liquids before the liquids are transported to storage or for further processing. Where the production sites are near or convenient to large markets, this gas is considered a valuable asset when demands for gas are high. However, when demands are low or when the producing reservoir is located in a remote area, the produced gas can present major problems since production may have to shut-in or at least drastically reduced if the gas can not be timely and properly disposed of. 
     In areas where substantial volumes of the produced gas can not be marketed or otherwise utilized, it is common to “reinject” the gas into a suitable, subterranean formation. For example, it is well known to inject the gas back into a “gas cap” zone which often overlies a production zone of a reservoir to maintain the pressure within the reservoir and thereby increase the ultimate liquid recovery therefrom. In other applications, the gas may be injected into a producing formation through an injection well to drive the hydrocarbons ahead of the gas towards a production well. Still further, the produced gas may be injected and “stored” in an appropriate, subterranean permeable formation from which it can later be recovered when the situation dictates. 
     To reinject the gas, large and expensive separation and compression surface facilities must be built at or near the production site. A major economic consideration in such facilities is the relatively high costs of the gas compressor train which is needed to compress the large volumes of produced gas to the pressures required for injection. As will be understood in this art, significant cost savings can be achieved if the gas compressor requirements can be down-sized or eliminated altogether. To achieve this, however, it is necessary to either raise the pressure at the surface by some means other than mechanical compression or else reduce the pressure required at the surface for injection of the gas downhole. 
     Recently, several techniques have been proposed for disposing of at least a part of the produced gas by separating the gas at the surface and then injecting it through the production wellbore into a subterranean formation. For example, in co-pending and commonly assigned U.S. patent application Ser. No. 09/072,657, filed May 5, 1998 a method and apparatus are disclosed wherein gas, which has been separated from a production stream, is mixed with a with a carrier liquid (e.g. brine) to form a dense mixture which, in turn, is flowed downward in a wellbore of an injection well. The mixture flows through a downhole separator in the wellbore to thereby separate at least a portion of the gas from the mixture. The separated gas is then injected into a formation adjacent the wellbore while the carrier liquid and any unseparated gas is returned to the surface. By mixing the gas with a carrier liquid, the need for expensive gas compressors at the surface is eliminated. However, this method requires a dedicated injection well. 
     Another similar method for disposing of produced gas is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 09/073,215, filed May 5, 1998 wherein a common wellbore is used for both producing an oil and gas stream and for re-injecting at least a portion of the produced gas. The production stream passes through a separator as it is being produced to the surface to thereby separate at least a portion of the gas from the production stream which, in turn, is then flowed to the surface. After a production cycle, the well is shut-in and the separated gas is the flowed down the same wellbore to be injected into an formation different from the production formation. 
     The gas is first mixed with a carrier fluid, e.g. brine, at the surface to form a dense mixture which, in turn, is flowed through the downhole separator in the wellbore wherein a portion of the gas is separated from the mixture. The separated gas is then injected into the formation with remainder of the mixture being returned to the surface. Again, by using this method, no gas compressors are needed at the surface but the production well must be shut-in during the injection of the gas. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for producing an oil and gas production stream from a first subterranean zone through a wellbore while simultaneously injecting gas into a second subterranean zone through the same wellbore. The injected gas can be that from the production stream or may be from a separate source. Basically, the production stream is produced to the surface where gas is separated therefrom. Gas from the production stream and/or gas from a separate source is then mixed with a carrier fluid at the surface to form a carrier fluid-gas mixture which, in turn, is flowed back down the wellbore and a downhole separator to separate at least a portion of the gas from the carrier liquid-gas mixture. This separated gas is then injected into the second subterranean zone while the carrier fluid and any unseparated gas is mixed and commingled with the production stream before the combined streams are flowed to the surface. 
     More specifically, the present invention provides a method and an apparatus for producing an oil and gas stream from a first subterranean zone through a wellbore while simultaneously injecting gas into a second subterranean zone through said wellbore wherein the wellbore has a string of production tubing positioned therein which forms an annulus between the production tubing and the wellbore. An oil and gas stream is produced to said surface through the production tubing where it is separated into its gas, oil, and water components. The separated gas is mixed with a carrier fluid (e.g. water, brine, etc.) at the surface to form a carrier fluid-gas mixture which is then flowed down the annulus in the wellbore while the well is still being produced through the tubing string. 
     As the carrier fluid-gas mixture approaches the bottom of the annulus, it flow through a downhole separator (e.g. an inverted hydrocyclone separator) wherein at least a portion of the gas is separated from the carrier fluid-gas mixture. The separated portion of the gas is then injected into the second subterranean zone which lies adjacent the wellbore. The mixture of the carrier fluid and any unseparated gas is mixed with the production stream at the outlet of the hydrocyclone separator by means of a jet pump or the like to thereby mix and commingle the streams before they are flowed to the surface through the production tubing. 
     By forming a mixture with a dense, carrier fluid, the gas does not need to be compressed at the surface before it is injected down the wellbore. As will be appreciated, by reducing or eliminating the need for gas compressors, the costs involved in disposing of excess gas through injection are substantially reduced. Further, since the gas can be injected while the well is being produced, the well does have to be shut-in in order to disposed of excess gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The actual construction, operation, and apparent advantages of the present invention will be better understood by referring to the drawings which are not necessarily to scale and in which like numerals refer to like parts and in which: 
     The FIGURE illustrates a well which is being produced while gas is simultaneously being injected into a subterranean formation through the same wellbore in accordance with the present invention. 
    
    
     BEST KNOW MODE FOR CARRYING OUT THE INVENTION 
     Referring more particularly to the drawing, the FIGURE discloses a production well  10  having a wellbore  11  which extends from the surface  12 , through an injection or second zone  13 , and into a production or first zone  13   a . As will be understood in the art, zones  13  and  13   a  may lie in the same subterranean formation or may be separate formations as shown. As illustrated, wellbore  11  is cased with a string of casing  14  to a point slightly above the upper or injection zone  13 . A liner  15  or the like has openings  16  (e.g. perforations or slots) adjacent injection zone  13  and a second set of openings  16   a  adjacent production zone  13   a.    
     Liner  15  is suspended from the lower end of casing  14  and is closed at its lower end by cement plug  15   a  or the like. A packer  17  is provided near the top of liner  15  to block flow from around the outside of the liner into casing  14 . While this is one well-known way to complete a well, it will be recognized that other equally as well-known techniques can be used without departing from the present invention: e.g., wellbore  11  may be cased throughout it entire length and then perforated adjacent both zones  13  and  13   a  or it may be completed “open-hole” adjacent these zones, etc. 
     A string of tubing  18  is positioned within casing  14  and extends from the surface substantially throughout the length of casing  14  and into liner  15 . Packer  19  is positioned near the lower end of tubing  18  to block any flow in the annulus  20  between tubing  18  and casing  14  at that point. A separator (e.g. hydrocyclone  25 ) is positioned within tubing  18  at the lower end thereof. As seen in the FIGURE, hydrocyclone separator  25  is in an inverted position from that in which would normally be operated at the surface but could also be operated in other orientations, if desired. 
     Separator  25  can be affixed within tubing  18  and lowered therewith or, as will be understood, it can be lowered into the tubing on a wireline, coiled-tubing, or the like (not shown) and landed on a landing nipple or the like (not shown) within the tubing after the tubing  18  has been positioned within the wellbore. Hydrocyclone separator  25 , as shown, is basically comprised of a housing  26  having inlet ports  27  and an outlet  28  at one end and a vortex finder  29  at the other end thereof. The inlet ports  27  in housing  26  align with ports or openings (unnumbered for the sake of clarity) in tubing  18  for a purposed described below. Seal means (e.g. packer  30 ) seals between the housing  26  and the inside of tubing  18 . 
     As will be understood in the art, a hydrocyclone is a separator which is designed to impart a swirling or cyclonic motion to a flow stream as the stream flows through the hydrocyclone. This motion forces the denser or heavier component of the stream (oil, water, etc.) outward against the wall of the separator housing while squeezing the less dense or lighter component (gas) toward the vortex at the center of the housing. Generally speaking, the less dense or lighter component (gas) is taken off through the vortex finder  29  while the denser or heavier component (oil-water) is taken off through outlet  28 . 
     A production sub or conduit  31  extends from the lower end of tubing  18  and terminates substantially adjacent production zone  13   a . Conduit  31  has a reduced diameter portion  32  which extends through vortex finder  29  and substantially the entire length of housing  26 , terminating substantially adjacent outlet  28 . One or more openings (e.g. nozzles  33 ) are positioned near the upper end of reduced portion  32  to provide fluid communication between the inside and outside of conduit  31 . A check valve  35  is positioned in the lower end of conduit  31  which permits flow only in an upward direction for a purposed described below. 
     In operation, an oil and gas production stream  40  (e.g. may contain water) is produced from a first or production zone  13   a  through perforations  16   a  into liner  15  and flows upward through check valve  35  into production conduit  31 . Production stream  40  exits through nozzles  33  at the upper end of conduit  31 , flows up to the surface through tubing  18 , and into a three-phase separator  41  where stream  40  is separated into gas, oil, and water. The oil is removed through line  42  for further processing. 
     In accordance with the present invention, the effective density of the gas to be injected (e.g. gas from separator  41 ) is increased at the surface before it is fed down wellbore  11 . This is done by blending a dense, carrier fluid (e.g. liquid) with the gas at the surface to form a mixture having a bulk density between that of the carrier fluid and that of the gas. Dense carrier fluids may include any fluids which will suspend the gas in the mixture but will then allow separation of at least a part of the gas as the mixture passes through hydrocyclone separator  25 . For example, such carrier liquids may include water; water-based liquids with added/dissolved densifying materials (e.g. produced water such as that from separator  41 , seawater, drilling muds, “well-kill” brines, etc. with or without corrosion inhibitors); oil-based liquids such as drilling muds or the like; petrochemicals such as glycol; stabilized or volatile crude oils; or esoteric fluids such as a “heavy media”, i.e. suspensions of fine particles of metal or the like (e.g. a suspension of fine iron filings in water). The gas and the carrier liquid are mixed so that the density of the mixture, when flowed under pressure (i.e. pumped) down wellbore  11 , will overbalance the bottom-hole pressure within injection zone  13 , as will be more fully discussed below. 
     Again, referring to the FIGURE, gas from separator  41  is supplied to mixer  43  through line  44 . In some instances, it may be desirable to supply additional gas from a separate source (not shown) through line  44   a . As shown, the carrier liquid is the produced water which has been separated from the production stream  40  within separator  41  and is supplied to mixer  43  through line  45 . Additional water or other carrier liquid can be supplied through line  45   a , if needed or desired. The carrier liquid is pumped under pressure by pump  46  through line  47  into mixing chamber  43  or other mixing device to form a carrier liquid-gas mixture. A foaming agent (e.g. low concentrations of sulphonates, polysulphonates, long-chain alcohols) may be added to the mixture (e.g. within mixer  43 ) to prevent “slugging” as the mixture flows downward in tubing  18  as will be understood in the art. 
     This mixture (arrows  50 ) flows down tubing  18  and through tangential inlets  27  into hydrocyclone separator  25  where centrifugal force (i.e. cyclonic force) of the tangential flow within the hydrocyclone separator  25  causes at least a portion of the gas to separate from the mixture  50  as explained above. The separated gas (arrows  60 ) migrates toward the center of hydrocyclone  25  and exits through vortex finder  29 . Packers  17  and  19  block any substantially upward flow of gas so it can only flow through openings  16  and into zone  13  as the gas accumulates and the pressure increases in within liner  15 . 
     The dense carrier liquid plus any remaining gas mixture (i.e. carrier liquid-gas mixture, arrows  70 ) flows along the outside separator housing  26  and exits through outlet  28 . The pressure of the carrier liquid-gas mixture  70  will normally be greater than the pressure of production stream  40  which could result in backflow of the carrier liquid-gas mixture within the hydrocyclone  25 . However, the jet nozzles  33  act as a venturi jet pump which allows the carrier fluid-gas stream  70  to readily mix with production stream  40  so that the combined streams  40 ,  70  can be commingled and produced to the surface. Upon reaching the surface, the combined streams will flow into separator  41  where the separating and gas injection process is effectively repeated. It should be understood that carrier liquid may be added or removed from the circuit through line  45   a  as a particular situation may dictate. 
     In order to inject gas into zone  13 , the pressure of the separated gas in liner  15  has to be greater than the pressure within zone  13 . Accordingly, the pressure of the carrier liquid-gas mixture at the surface must be sufficient to overbalance the well pressure thereby allowing the mixture to flow down the wellbore  11 . This pressure is dictated by the pressure of the gas supply. Given that pumping liquid is easier than compressing gas, the pressure of the liquid in line  45  is substantially matched to the available gas pressure. The liquid pressure is generated at the surface primarily by pump  46  as it pumps the carrier-liquid to mixer  43 . By generating the necessary injection pressure through the pumping of liquid, gas compression is substantially reduced or eliminated thereby significantly reducing the costs involved in the gas injection operation.