Abstract:
An apparatus is provided for maintaining a steady flow rate and pressure of a carbon dioxide stream at high pressure when a low-pressure source of the carbon dioxide varies with time. Liquid level in an accumulator that is sized to accommodate variations in supply rate is controlled by sub-cooling of liquid entering the accumulator and heating in the accumulator, the sub-cooling and heating being controlled by a pressure controller operable in the accumulator.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to surface apparatus for processing carbon dioxide (CO2) to be injected into wells for enhanced recovery of crude oil. More particularly, apparatus and method are provided for decreasing flow rate variations (i.e., flow dampening) and supplying high-density carbon dioxide to a well at higher energy efficiency when carbon dioxide gas is sourced from a variable rate or intermittent source. 
         [0003]    2. Description of Related Art 
         [0004]    Injection of carbon dioxide into an oil reservoir to increase the recovery of crude oil from the oil reservoir is a proven technology. It has been practiced for more than 40 years. Carbon dioxide gas is injected into some wells, flows through rock containing crude oil, and is produced from other wells, along with oil and often a large volume of water. Variations of the process include injection of slugs of water with the carbon dioxide to improve sweep efficiency of the carbon dioxide. In some oil reservoirs, additional recovery of oil is primarily the result of the high solubility of carbon dioxide in the oil, which expands the oil phase and decreases the amount of oil left trapped in the rock. Carbon dioxide&#39;s effect in lowering the viscosity of crude oil is important in improving oil recovery from some reservoirs. Under other conditions a displacement zone between the crude oil and carbon dioxide may become miscible with the oil and carbon dioxide. 
         [0005]    The sources of carbon dioxide currently used for flooding of oil reservoirs are reservoirs containing high purity carbon dioxide and anthropogenic carbon dioxide. Anthropogenic carbon dioxide may be recovered from industrial plants or from power sources. Recently it was announced that carbon dioxide will be recovered from a refinery and used for injection into wells (Dallas Bus. J., May 10, 2013). Recovery of carbon dioxide from a nitrogen plant and planned recovery from an industrial plant are reported in the same source. 
         [0006]    Recovery of carbon dioxide from the atmosphere offers an almost limitless supply for injection underground, but the concentration of carbon dioxide in the atmosphere is low compared with industrial sources. Nevertheless, new processes using the atmosphere, engine exhaust, flue gas or other sources of carbon dioxide are being developed. One such process is described in U.S. Pat. App. Pub. No. 2013/0047664, which discloses removal of carbon dioxide from the atmosphere by a combination of drying with a desiccant, adsorption of carbon dioxide from the dry air, releasing the carbon dioxide from the adsorbent by decreasing pressure to a vacuum and solidifying the carbon dioxide on a cold surface in a vacuum chamber. U.S. Pat. App. Pub, No. 2013/0025317 discloses a process for removing carbon dioxide from a gas stream by de-sublimation, vaporization and liquefaction. U.S. Pat. App. Pub. No. 2011/0252828 discloses a carbon dioxide recovery method using cryo-condensation. U.S. Pat. App. Pub. No. 2013/0025317 discloses an auto-refrigerated process for de-sublimation of a flue gas. Of course, carbon dioxide may be separated from other gases by well-known cryogenic processes (liquefaction, distillation), but they are expensive and not practical as a stand-alone recovery process for carbon dioxide from gases containing low concentrations of carbon dioxide. 
         [0007]    The output of carbon dioxide from some of the processes disclosed above and other possible processes varies with time. Output pressure may be low and output rate may be intermittent, as from a batch process, or not at a steady rate, as from any carbon dioxide recovery process that requires regeneration. For use in enhanced oil recovery (EOR) carbon dioxide gas is injected for months or years at pressures usually in the range from 1200 psi to 3000 psi, requiring high compression ratios from a low-pressure source. A steady rate is needed, because conventional methods of pressurization are negatively affected by problems associated with intermittent flow. 
         [0008]    Equipment and methods are needed for providing a more energy-efficient method for pressurizing CO2 and providing the fluid at a steady rate from processes that supply carbon dioxide at a varying rate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Carbon dioxide (CO2) gas from a source at or above the triple-point pressure is cooled by a heat pump to a sub-cooled liquid and sprayed into a surge vessel or accumulator containing two phases. The amount of heat added in a heating coil in the lower part of the accumulator and the temperature of the sub-cooled liquid are controlled by a pressure controller in the accumulator, such that the level of the dense phase in the accumulator moves between two levels (forming an “accumulator volume”), while pressure in the vessel is maintained near constant as dense CO2 is pumped out of the bottom of the accumulator at a constant rate and input rate of CO2 from the source varies with time. The accumulator volume in the accumulator is sized to account for variations in output rate of the particular source. A carbon dioxide pump, with speed controlled by the average flow rate from the source, is used to pump the more dense CO2 phase in the bottom of the accumulator to the pressure needed for injection into wells for enhanced oil recovery or into a pipeline (often in the range from 1200 psi to 3000 psi) or for other uses. Additional cooling may be used immediately upstream of the pump to insure adequate suction pressure and prevent cavitation in the pump. The heat pump process for the two-phase vessel may use a conventional heat pump with propane or other fluids or mixtures of heat pump fluid selected for maximum efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates one embodiment of apparatus used to decrease variations of flow rate of carbon dioxide supplied for pumping to high pressure for injection into wells, a pipeline or other uses. 
           [0011]      FIG. 2  shows a flow chart of the disclosed method for maintaining a steady stream of carbon dioxide from a source having variations in flow rate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to  FIG. 1 , variable-rate or intermittent carbon dioxide source  10  uses a batch process, regeneration process or other process that results in varying output rates of carbon dioxide. Source  10  may be based on adsorption-desorption, de-sublimation-sublimation, or other processes. The pressure of CO2 from source  10  is greater than, or is compressed to be equal to or greater than, the triple point pressure (75.12 psia). Preferably, the pressure is less than the critical pressure, but the pressure may be as high as about 2000 psi. Intermittent flow isolation device  11  may be used to prevent backflow to source  10 . This device may be a throttle, check or snap acting valve or it may be controlled by pressure controller  11   a . The CO2 may be any in any combination of phases (solid, liquid and gas). Heat exchanger  12  may be a shell and tube, counter-flow or any type heat exchange device. The CO2 may be cooled or heated (depending on the phases of CO2 from source  10 ) in heat exchanger  12  to liquefy CO2 or densify any supercritical CO2 and sub-cool the liquid, using external heat pump  16 . The heat pump may include a compressor and condenser and may use a refrigerant selected to optimize the vaporization and liquefaction of CO2 at any application-specific pressure. The refrigerant supply is controlled by temperature control valve  13   b   2 . Alternatively, the heat pump may include heat sinks and heat sources from outside processes, such as adsorption and desorption separation of CO2 to supply source  10 . The outside processes may be synchronized to accommodate the need for alternating heat flux in the disclosed apparatus. Alternatively, a heat storage device may be used to provide a thermal capacitance suitable for specific application alternating heat flux requirements. 
         [0013]    Sub-cooled liquid (below saturation temperature) from heat exchanger  12  passes to accumulator  13 , where it flows (preferably as a spray through mister system  13   a ) into the vapor space. The level of heavier phase carbon dioxide may vary between  13   a   1  and  13   a   2 , which define the bottom and top of the accumulator volume in accumulator  13 . Accumulator volume is selected to accommodate the variations in output rate of source  10 . Level controls  13   a   1  and  13   a   2  may be used to shut-down an upset condition and/or to adjust to more gradual changes to average flow of source  10 . Level controls  13   a   1  and  13   a   2 , pressure controller  13   b,  coil  19  and sub-cooled liquid flowing into accumulator  13  are used to maintain the liquid level between level controls  13   a   1  and  13   a   2 . Pressure controller  13   b,  which may work in conjunction with temperature controller  12   b,  controls heat flux of sub-cooled liquid by valve  13   b   2  and heat flux through coil  19  by valve  13   b   1 . Heat medium fluid or refrigerant enters coil  19  at  16   a.  The heat flux may be supplied from heat pump  16  or another source, such as a CO2 recovery process using adsorption and desorption (not shown). Pressure controller  13   b  throttles valve  13   b   2  such that sub-cooled fluid flowing through mister system  13   a  cools the vapor in  13 , liquefying enough vapor to offset the volume of net positive influx of liquid into accumulator  13 . Pressure controller  13   b  throttles heat flow into the saturated liquid section of accumulator  13  to vaporize sufficient liquid to offset the net negative liquid influx. If there is a net positive flow of CO2 into accumulator  13 , pressure is maintained in accumulator  13  by cooling vapor to liquefy a portion of the vapor to offset the reduction of the vapor space volume (rising liquid level). If there is a net negative flow of CO2 into accumulator  13 , pressure is maintained by heating the saturated liquid section such that sufficient liquid is vaporized to offset the increase in vapor space volume (falling liquid level). 
         [0014]    Pump  15  may be a conventional pump, such as a multistage centrifugal pump. It may be used to pump liquid CO2 to a pipeline or well or other use. The CO2 may be further densified at heat exchanger  14 , which may use refrigerant from heat pump  16 , ambient air or other means, to increase the Net Positive Suction Head to prevent cavitation or increase efficiency of pump  15 . Temperature control is provided at valve  14   b,  controlled by temperature controller  14   a.  Further cooling may be provided at heat exchanger  17  to increase the efficiency of a downstream pipeline or injection well. Equipment may be industry-standard. One of the important features of the apparatus described herein is the ability to pump dense or liquid carbon dioxide from the apparatus at a steady rate and without the inefficiency and high cost of compression of gas while avoiding problems of control and wear caused by cycling of the CO2 pump. 
         [0015]    Referring to  FIG. 2 , the steps of the method for supplying carbon dioxide at a steady rate from a source producing carbon dioxide at a varying or intermittent rate are shown. An intermittent or varying rate source of carbon dioxide at a pressure at or above its triple-point pressure is supplied. If the source originally does not produce CO2 at a pressure at or above the triple-point pressure, the CO2 pressure is increased to that pressure. The stream is then cooled or heated to a temperature sufficient to produce sub-cooled liquid carbon dioxide. The stream is then conveyed to an accumulator, where the temperature of the sub-cooled carbon dioxide is controlled by a pressure controller responsive to pressure in the accumulator. Heat flux may also be supplied to the accumulator by a fluid flowing through a conduit or coil in the accumulator at a rate controlled by the pressure controller responsive to pressure in the accumulator. A conduit may be any type of heat transfer device, including electric heaters and other conventional devices, with appropriate controls for the heat transfer device. A pump removes the dense or liquid carbon dioxide from the accumulator at a steady rate determined by the average flow rate of the stream entering the accumulator. 
         [0016]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.