Abstract:
A water stream is treated to remove oxygen before injecting the water into an underground hydrocarbon reservoir. The water-oxygen mixture is separated by centrifugal action using fixed spin blades ( 20 ). Oxygen is removed from the center of the conduit through a gas pipe ( 30 ) coupled to a vacuum pump ( 50 ). Nitrogen may be introduced upstream of the centrifuge to dilute the water-gas solution to improve the efficiency of the gas-liquid separation.

Description:
CROSS-REFERENCE 
   Applicant claims priority from U.S. Provisional Application Ser. No. 60/526,677 filed 3 Dec. 2003. 

   BACKGROUND OF THE INVENTION 
   When hydrocarbons are produced from underground hydrocarbon reservoirs (which may also lie under a sea), the pressure and production rate tends to fall unless a fluid such as water is injected into the reservoir. Sea water is probably the most common injected fluid used in the production of hydrocarbons from undersea reservoirs, although water produced along with hydrocarbons from a reservoir may be reinjected. Sea water generally has about 10 ppm (parts per million) of dissolved oxygen. Once the water is pumped to high pressurize for reservoir injection, oxygen in the water can cause rapid corrosion of many of the steels commonly used in the construction of the system. The oxygen also feeds undesirable biological activity in the reservoir. As a result, it is common to reduce the amount of oxygen before it is pressurized and injected. 
   One way to remove the oxygen is to reduce the pressure of the water so that dissolved gases break out of solution, and to then separate these two phases under normal gravity separation in a vertical tower filled with packing. This equipment is comparatively large. An apparatus and method that reduces the overall size and weight of the necessary equipment would be of value for the offshore oil industry where provisions for space and weight have a significant cost. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the present invention, applicant provides an apparatus and method for separating gas from liquid, and for reducing oxygen in water that is to be injected into a hydrocarbon reservoir. A fluid stream which is a combination of water and dissolved gases, passes into a main conduit where its pressure is reduced to cause dissolved gases to break out of solution. The resulting mixture of water and gases is centrifuged to move gas to the center of the conduit and water to the periphery. A gas pipe inlet portion lies at the center of the water conduit at a location closely downstream of the centrifuge, to remove gases from the fluid and pass them through the gas pipe and a vacuum pump to the atmosphere. 
   The amount of oxygen in the water to be injected may be further reduced by injecting nitrogen into the fluid stream before the centrifuge separation, to cause nitrogen to be dissolved in the water and displace some of the other gases that include oxygen. When the water pressure is reduced, more gas breaks out of solution and the oxygen content is further reduced. 
   The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified view of a gas/water separation system of one embodiment of the present invention. 
       FIG. 2  is a simplified view of a gas/water separation system of another embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a system  10  with a main conduit, or water conduit,  12  with an inlet  14  that receives fluid  16  that is water that may have been filtered and that has dissolved gases. In the example to be described, the water is sea water (water with dissolved salts) with dissolved gases that consist primarily of air (21% oxygen, 78% nitrogen, 1% argon and trace amounts of other gases). It is the oxygen, which corrodes steels used in the construction of the water injection system, and which results in undesirable biological activity in a hydrocarbon reservoir, that is to be removed from the sea water. Where the fluid consists primarily of water, it sometimes will be referred to herein simply as “water.” 
   The system of  FIG. 1  applies a vacuum to water at the entrance  14 . This can be accomplished by a valve  18  upstream of the conduit. The restriction in flow caused by the valve is adjustable to regulate water pressure downstream of the valve. The reduced water pressure results in gases in the water coming out of solution and forming gas bubbles. The system of  FIG. 1  includes a centrifuge device  20  formed by fixed blades that direct the incoming gas-liquid mixture to rotate while continuing to flow. Immediately downstream of the centrifuge device, the spinning fluid separates into gas at the center of the conduit and liquid at the outer or peripheral portion of the conduit. A gas removal pipe  30  has a gas pipe inlet portion  32  lying downstream of the centrifuge to remove gas from the mixture. The gas pipe inlet portion  32  is spaced slightly downstream from the centrifuge blades to allow the spinning fluid to separate. The remaining portion of the fluid mixture  34 , which has been treated so most of the gas has been removed, passes downstream along the conduit portion  36 . 
   In many cases the treated fluid, or water  34  passes through an exit blade device  40  that removes the spin. According to the law of conservation of energy, the removal of spin recovers some pressure, so that a higher vacuum lies upstream of the device  40 . The water that has been treated to deoxygenate it, usually passes through a downstream pump  46  that injects it into an undersea hydrocarbon reservoir to maintain the pressure therein. Such pump is positioned below the conduit so the vertical water column in a downward portion  42  of the conduit provides a head or pressure at the pump to meet the required minimum suction conditions of the pump. This vertical height is typically 4 to 8 meters and depends on the pump design. 
   In accordance with one aspect of the present invention, applicant applies a gas-drawing mechanism such as a vacuum pump  50  to the gas pipe  30 . The gas pipe could be provided with a simple hole at its upstream end  35  to take out gas. However, applicant prefers to use a perforated gas inlet portion  32  that withdraws gas along a distance in the water pipe that is greater than the diameter at the upstream gas pipe end. Although the diameter of the gas-containing region at the center of the water pipe may be larger than the diameter of the gas inlet  32 , all of the gas in the gas-containing region can readily flow into the gas pipe because of the vacuum being applied and the perforations in the side of the gas pipe inlet portion. It is possible to vary the level of the vacuum (the pressure below atmospheric, or below the pressure in the water pipe at a location upstream from the gas pipe inlet) applied by the vacuum pump  50  to remove a high proportion of the gas that is present while removing a minimum of the liquid. 
   Some of the fluid drawn into the gas pipe inlet includes water droplets. The water should be removed before the fluid reaches the vacuum pump  50 .  FIG. 1  shows a separator  52  located along the gas pipe  30 . The separator includes a chamber  60  that receives the fluid that has passed into the gas pipe. The fluid which is primarily gas, is preferably routed downward through gas pipe end  54  to chamber  60  so that the vacuum in the conduit is improved by an amount equal to the static head of fluid in the pipe  30 . The gas and liquid separate under gravity in the chamber  60 . Gas in the chamber is removed from the upper portion  56  of the chamber to avoid a pressure buildup in the chamber.  FIG. 1  shows a water pipe  64 , pump  66  and a valve  68  through which the liquid is removed. A level transmitter  70  senses when the level of the liquid  62  drops below a certain level, and this signal is fed into a controller to maintain a fixed liquid level. The controller may regulate the water pump speed or regulate the opening of valve  68  in the water outlet line, so gas is not removed through the liquid removal pipe. The removed water passes along a path  74  that can lead to an atmospheric disposal location which may be a drain system or the open sea, or that can be otherwise handled. 
   Sea water commonly contains 50 ppm (parts per million) of air which includes 10 ppm of oxygen. The water may be close to saturation with dissolved gases, so a reduction in pressure can lead to gas being released and forming gas bubbles. Atmospheric air consists of 21% oxygen, 78% nitrogen, 1% argon and trace amounts of other gases. Nitrogen normally does not react with hydrocarbons or steel and argon is inert, so they do not affect a reservoir of hydrocarbons. Thus, the reduction in the oxygen content of fluid injected into the reservoir is the major goal of the system.  FIG. 2  illustrates a system  100  of the invention that further reduces the amount of oxygen in the injected fluid. 
   In the system of  FIG. 2 , nitrogen gas is injected from a source  102  into the water-gas fluid that lies in or that enters the water conduit. The nitrogen gas is mixed into the water and air mixture  16  and some of the nitrogen is dissolved in the water. The nitrogen dissolved in the water saturates the water with gas, and causes more of the gas to come out of solution when the water pressure is reduced. Some of the additional gas that comes out of the solution with water is additional oxygen. After gas bubbles are removed by the gas pipe inlet  32 , the fluid mixture  112  in the water conduit  12  now contains gas with a smaller amount of oxygen per cubic meter, although with the same or even a greater overall amount of gas in the water, and passes along a conduit portion  114 . Nitrogen can be obtained by liquefaction of air at moderate cost, and typically produces gas with 98% nitrogen. Other means of supplying nitrogen include pressurized bottles shipped to the site, and the reverse osmosis process. 
   Instead of injecting the nitrogen into the fluid initially entering the water pipe, it is possible to inject nitrogen into the fluid after much of the air has been removed. In  FIG. 2  applicant indicates in phantom lines that instead of a spinner he may use a second centrifuge  120  and a second gas pipe  122  with a second gas pipe inlet portion  124  connected to a second pump  126 . The second centrifuge is a barrier to water flow, and results in a vacuum in and downstream of the second centrifuge. Nitrogen may be injected only at location  130  which lies downstream of the first gas pipe inlet  32 , as by an injection pipe  132 . At the nitrogen injection location  130 , a considerable percent of the air has already been removed, so a high percent of the injected nitrogen will become dissolved in water and as a result of the two stages a higher percent of oxygen will be removed from the water. However, this requires establishing a vacuum at two locations ( 110  and  130 ). 
   Thus, the invention provides a system and method for the separation of gas from a gas/liquid mixture, and which is especially useful in the removal of oxygen from water that is to be injected into a hydrocarbon reservoir. Water with air in it is flowed though a water conduit, through a conduit region of reduced water pressure so gas comes out of solution, and is rotated in the conduit region to cause air bubbles to move to the middle of the conduit from which they are removed. The mechanical separation efficiency of the gas withdrawal may be adjusted by varying the gas volume fraction to an optimum value depending on system operating pressure and specific geometry. The efficiency of gas withdrawal is increased by injecting nitrogen into the water conduit and mixing the nitrogen with the air before the water is centrifuged. The nitrogen increases the gas content of the water so more gas comes out of solution where the water pressure is reduced. It also is possible to first remove much of the air, then inject nitrogen to increase the amount of dissolved gas in the water, and then remove the gas again. To remove much of the remaining oxygen in water at location  42 , an oxygen scavenger chemical an be injected in the water at  43 . Such chemical can reduce the oxygen contact to zero by chemically reacting with the oxygen in a manner similar to that in a conventional tower deoxygenation system. 
   Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.