Patent Publication Number: US-6336504-B1

Title: Downhole separation and injection of produced water in naturally flowing or gas-lifted hydrocarbon wells

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally directed to a method and system for the downhole separation and injection of water contained in produced mixtures from a production zone of a hydrocarbon well. 
     2. Background 
     In many hydrocarbon wells, there is a high percentage of water (referred to as water cut) in the produced fluid mixture. In typical practice, the produced fluid is lifted to the surface and the water is separated from hydrocarbon at the surface. Surface separated water is subsequently treated and disposed of on the surface or re-injected into a subterranean formation for disposal or as part of an enhanced reservoir recovery program. This process is not always entirely satisfactory because of the energy needed to lift the water to surface, and costs involved in separation of the water and hydrocarbon fluid, and re-injection of the water. 
     In many cases, it might be more economical to separate the water downhole in the same wellbore and re-inject it into a suitable zone accessible through the same wellbore. Examples of methods for the downhole separation and re-injection of water contained in fluids produced from hydrocarbon wells have been described in the patent literature: WO86/03143, U.S. Pat. Nos. 4,805,697, 5,296,153, 5,456,837, 5,711,374, and 5,730,871. These approaches describe various means to achieve downhole separation of oil and water components of produced fluids with subsequent lifting of separated oil to the wellhead. These approaches rely on downhole pumps to re-inject the water component into a suitable zone, and to bring the oil to surface. 
     In order to drive a downhole pump, some form of power, be it mechanical, electrical or hydraulic, must be transmitted from surface to the pump. Hydrocarbon wells are often located in places where providing for power for such functions is not convenient. 
     In offshore wells, gas-lift systems are often preferred due to the simplicity and reliability of their associated downhole components. In such techniques, compressed gas is commingled downhole with the produced fluids, thereby reducing the density of the produced fluids until the weight of the column of the gasified fluids becomes less than the pressure exerted on the body of fluids in the well, and flow of produced fluids to the surface is facilitated. Examples of the gas-lift technique are described in U.S. Pat. Nos. 5,217,067, 4,251,191, and 3,718,407. 
     U.S. Pat. No. 5,857,519 describes an approach for the downhole disposal of the water component of produced fluids while using gas lift techniques to lift the oil component to the surface. The oil and water components are separated downhole by gravity in an annular space located between a production tubing and the wellbore casing. Pressurized gas is used to drive a downhole pump that re-injects downhole-separated water, and exhaust gas from the downhole pump is used to assist in the lifting of oil to the wellhead. 
     Using present technology, downhole separation and disposal of water in a wellbore require downhole pumps. The present technology is therefore, inherently plagued with two main problems: 1) complex completions associated with providing power from surface to drive downhole pumps, and 2) poor reliability of the downhole pumps. 
     SUMMARY OF THE INVENTION 
     What is required is a method and system for the downhole separation and injection of water contained in produced mixtures from hydrocarbon wells that does not require downhole pumps. Accordingly, the present invention concerns a method and system for separating and injecting downhole, the water contained in the produced mixture of a hydrocarbon well while lifting hydrocarbon contained in the produced mixture to surface without the use of a downhole pump. 
     According to an aspect of the present invention, there is provided a method for the downhole separation and injection of a predominately water component of a production fluid comprising at least some water and at least some oil from a production zone of a hydrocarbon well comprising the steps of separating downhole, at a position elevated with respect to an injection formation, the production fluid into a predominately water component and a predominately hydrocarbon component and delivering the predominately water component to the downhole injection formation, wherein the separating step is conducted at a sufficiently elevated location with respect to the injection formation to permit the predominately water component to be delivered to the downhole injection formation under the force of gravity. In accordance with a preferred embodiment of the invention, the method further comprises injecting gas into the production fluid to deliver the production fluid to the elevated position in the well. In another preferred embodiment, the injected gas is delivered downhole through a gas-lift string that extends from the head of the well. 
     In accordance with yet another preferred embodiment, the production fluid is delivered to the elevated position by way of a conduit that extends from the production formation to the elevated position. In accordance with yet another preferred embodiment, the production fluid is delivered to the elevated position by way of an annular space within the well. 
     In a preferred embodiment of the invention, the percentage of water in the production fluid is at least 20%. 
     In accordance with yet another preferred embodiment of the invention, the production fluid contains gas. In accordance with yet another preferred embodiment, gas is separated from the production fluid and this step optionally precedes the step of separating the production fluid into a predominately water component and a predominately hydrocarbon component. In yet another preferred embodiment of the invention, the separated gas is delivered to the surface. 
     In accordance with yet another preferred embodiment of the invention, the mostly hydrocarbon component is transported to the surface. In accordance with yet another preferred embodiment of the invention, the separated gas and the predominately hydrocarbon component are combined and delivered to the surface. In a preferred embodiment of the present invention, a mixing device is used to combine gas and the mostly hydrocarbon component of the production fluid. 
     In accordance with another aspect of the invention, there is provided a system for the downhole separation and injection of a predominately water component of a production fluid comprising at least some water and at least some oil from the production formation of a hydrocarbon well. The system comprises an oil-water separator located downhole at a position elevated with respect to an injection formation, a first passage to provide fluid communication between the production formation and an inlet of the separator, and a second passage to provide fluid communication between the water outlet of the separator and a downhole injection formation. The separator is located at a sufficiently elevated location with respect to the injection formation to permit the mostly water component emerging from the water outlet to be delivered to the downhole injection formation under the force of gravity. 
     In a preferred embodiment, the oil-water separator comprises at least one cyclone. 
     In another preferred embodiment of the present invention, the system further comprises means for injecting gas into the production fluid in order to deliver the production fluid to the separator such as a conduit extending between the head of the well and the production formation. 
     In yet another preferred embodiment, the system includes a gas-liquid separator located at an elevation at least as high as the oil-water separator and having a gas-liquid inlet in fluid communication with the production fluid for receiving the production fluid as well as an outlet for passage of liquid from the gas-liquid separator to the oil-water separator. In a preferred embodiment, the gas-liquid separator comprises at least one cyclone. In another preferred embodiment, the gas-liquid separator comprises at least one auger. In yet another preferred embodiment, the gas-liquid separator comprises a combination of at least one cyclone and at least one auger connected in series or in parallel. In yet another preferred embodiment, the cyclone incorporates a swirl generator. 
     In yet another preferred embodiment, the system includes a third passage that extends between the oil outlet of the oil-water separator and the head of the well. 
     In yet another preferred embodiment, the system includes means for injecting gas into the third passage to promote flow of the mostly hydrocarbon component of the production fluid from the oil outlet to the head of the well. Means can include a conduit for providing fluid communication between a gas outlet of the gas-liquid separator and the third passage. 
     In accordance with yet another aspect of the invention, there is provided a method of completing a well for production of hydrocarbon from an underground formation comprising installing an oil-water separator downhole at a position elevated with respect to the injection formation, providing a first passage for fluid communication between the production formation and an inlet of the separator, providing a second passage that is isolated from the first passage for fluid communication between the water outlet of the separator and the injection formation, and locating the separator at a sufficiently elevated location with respect to the injection formation to permit fluid emerging from the water outlet to be delivered to the downhole injection formation under the force of gravity. 
     In a preferred embodiment of the present invention, providing an oil-water separator comprises installing at least one cyclone. 
     In another preferred embodiment of the present invention, the method further comprises providing means for injecting gas into the production fluid in order to deliver the production fluid to the separator. In a preferred embodiment, a conduit extending between the head of the well and the production formation is provided to provide means for injecting gas. 
     In yet another preferred embodiment, the method further comprises providing a gas-liquid separator located at an elevation at least as high as the oil-water separator and having a gas-liquid inlet in fluid communication with the production fluid for receiving the production fluid as well as an outlet for passage of liquid from the gas-liquid separator to the oil-water separator. In a preferred embodiment, the gas-liquid separator comprises a cyclone. In another preferred embodiment, the gas-liquid separator comprises an auger. 
     In yet another preferred embodiment, the method further comprises providing a third passage that extends between the oil outlet of the oil-water separator and the head of the well. 
     In yet another preferred embodiment, the method further comprises providing means for injecting gas into the third passage to promote flow of the mostly hydrocarbon component of the production fluid from the oil outlet to the head of the well. Means include a conduit for providing fluid communication between a gas outlet of the gas-liquid separator and the third passage. 
     With the present method and system, there does not need to be a downhole pump to inject the downhole-separated water component of produced fluids. The separator is located in a position in the wellbore so as to produce the predominately water component at a sufficient pressure so that it may be injected downhole without the use of a pump. This variable position of the separator can also lead to a reduction in gas-lift requirements. The lower the injection pressure needed to inject the water, the lower the location of the separator which in turn results in reduced artificial lift requirements. Also, with the present system, the produced mixture can be lifted to the separator in either a dedicated tube or annular space. This arrangement leads to a variable tubing configuration for optimizing flow of fluids in the wellbore. Potential benefits include increased production rates in wells currently production limited due to existing tubular and surface facilities, reduction of water handling (both processing and disposal) at the surface, elimination of surface infrastructure for powering downhole pumps, reduced gas-lift usage, reductions in the cost of running high water cut hydrocarbon wells, improved system reliability and environmental benefits from reduced discharge of produced water. As well, gas separated from produced fluids downhole can be commingled and brought to surface with downhole separated oil to reduce tubing requirements in the well. 
     Other and further advantages and features of this invention will be apparent to those skilled in the art from the following detailed description thereof, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the present invention are more fully set forth in the following description of illustrative embodiments of the invention. The description is presented with reference to the accompanying drawing in which: 
     FIG. 1 is a schematic representation of an embodiment of the present invention in which the total produced mixture is delivered to a gas-liquid separator by way of a conduit extending from an underground production zone to the separator; 
     FIG. 2 is a schematic diagram of the gas-liquid separator and oil-water separator of the FIG. 1 embodiment; 
     FIG. 3 is a schematic representation of an alternate embodiment of the present invention in which production fluid is delivered to the gas-liquid separator by way of an annular space located within the wellbore; 
     FIG. 4 is a schematic diagram of the gas-liquid separator and oil-water separator of the FIG. 3 embodiment; and 
     FIGS. 5 a  to  5   d  are more detailed schematic representations of types of gas-liquid separators illustrated in FIGS. 1 to  4 : FIG. 5 a  illustrates a gas-liquid separator that includes a cyclone with a combined swirl intake/gas outlet; FIG. 5 b  illustrates a gas-liquid separator that includes a cyclone with swirl intake and a gas segregation finder; FIG. 5 c  illustrates a gas-liquid separator that includes a cyclone with a combined swirl intake/gas outlet and an auger; and FIG. 5 d  illustrates a gas-liquid separator that includes a cyclone with a combined swirl intake/gas outlet and an auger, with the auger gas outlet extending into the combined swirl intake/gas outlet. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The description which follows, and the embodiments described therein, are provided by way of illustration of an example, or examples of particular embodiments of the principles of the present invention. These examples are provided for the purpose of explanation, and not limitation, of those principles and of the invention. The examples include a description of the best mode of practising the invention currently known to the inventors. 
     With reference to FIG.  1  and FIG. 3, there is shown hydrocarbon production well  10  having wellbore casing  12  that penetrates at least one production formation  14  and at least one injection formation  16 . Production perforations  18  in the wellbore casing are provided in the area of the production formation  14  to allow for inflow of the produced mixture from production formation  14 . Injection perforations  20  in the wellbore casing are provided in the area of injection formation  16  to allow for injection of water into injection formation  16 . Injection formation  16  may be above or below production formation  14 . Lower annular sealing packer  22  isolates production formation  14  from injection formation  16 . Separator  24 , to separate water, gas and hydrocarbon contained in the produced mixture, is located within wellbore casing  12  above production formation  14 . In FIGS. 1 through 4, separator  24  has been illustrated as a simple schematic and one skilled in the art can appreciate that the separator is more complicated. Also, in FIG.  1  and FIG. 3, separator  24  is located near the head of the well. In other embodiments, its location may be lower in the well. In other embodiments, its location may be higher in the well. 
     In FIG. 1, total production conduit  26  extends within wellbore casing  12  from production formation  14  to separator  24  for flow of the total produced mixture in the direction indicated by arrow  28 . In this embodiment, gas-lift is provided through one or more gas-lift valves  30  spaced along the length of total production conduit  26  that extends into the wellbore to aid in lifting the produced mixture up the well. Alternative embodiments of the gas lift system will be apparent to those skilled in the art. For example, a continuous gas lift system may be used. An intermittent gas lift system may also be used. In wells where the eruptive force of the well is sufficient to lift the produced fluids up the well naturally, gas-lift may not be required. Upper annular sealing packer  32  isolates the production formation from annular space  34  in the well. Means for introducing lift gas (not shown), flowing in the direction indicated by arrows  36 , is provided for on the surface. Separator  24  includes, in this embodiment, gas-liquid separator  38  and oil-water separator  40 . Gas-liquid separator  38  reduces the fraction of free gas in the produced mixture entering oil-water separator  40 . The produced mixture from the production formation can contain gas, oil and large amounts of water in the oil, as well as other impurities. In a preferred embodiment, there is a high water cut, for example 80% water cut, in the produced fluids. In other preferred embodiments, the water cut is higher or lower. This mixture flows from production formation  14  to separator  24 , shown in FIG. 2, through total production conduit  26  and enters the upper portion of gas-liquid separator  38 , through production fluid inlet  42 . Accordingly, gas is separated from the total produced mixture by gas-liquid separators of the types shown schematically in FIGS. 5 a  to  5   d , and free gas, travelling in the direction indicated by arrow  44 , exits gas-liquid separator  38  through upper port  46  of gas collection conduit  48 . Alternative embodiments of the gas-liquid separators illustrated in FIGS. 5 a  to  5   d  will be apparent to one skilled in the art. The gas-depleted produced mixture, travelling in the direction indicated by arrow  50 , exits gas-liquid separator  38  through liquid outlet  52  and enters oil-water separator  40 . Oil-water separator  40  includes separation chamber  56  wherein gas depleted production fluid is separated into a predominately hydrocarbon component and a predominately water component using cyclone separator  58 . Alternative embodiments of the oil-water separator to separate the produced mixture into a predominately hydrocarbon component and a predominately water component will be apparent to one skilled in the art. For example, one or more cyclones can be housed in one or more separators, which, in turn, can act in series or in parallel, to separate produced fluids. The predominately hydrocarbon component, travelling in the direction indicated by arrow  60 , exits separation chamber  56  and travels through oil concentrate conduits  62  which extend up the wellbore to conduit  66 , which, in turn, extends to the head of the well. Gas collection string  48  is connected in this embodiment to conduit  62  through junction  68 , so that free gas travelling in the direction indicated by arrow  44  and hydrocarbon travelling in the direction indicated by arrow  67  are lifted to the wellhead commingled. In another embodiment, a pressure drop device such as an orifice can be utilized to commingle the predominately hydrocarbon component and gas. The predominately water component, travelling in the direction indicated by arrow  70 , exits oil-water separator  40  into water disposal string  74 . Water disposal string  74 , preferably equipped with adjustable downhole choke  76 , passes through lower annular sealing packer  22  and extends from the bottom of separator  24  to injection formation  16 . The predominately water component flows in the direction indicated by arrow  78 , to injection formation  16 . 
     In FIG. 3, another embodiment of this invention is disclosed. Elements previously described have been given the same reference number. The total produced mixture flows in the direction indicated by arrow  28 , up the wellbore to separator  24  through annular space  34  located in the wellbore. In this embodiment, annular space  34  is formed between the casing of the well and water disposal string  74 . Using an annular space for the flow of the produced mixture can allow for larger flow area and higher capacity that using a dedicated tubing for flow of the produced mixtures. Gas-lift string  80  traverses down the wellbore casing  12  from the head of the well to the lowest desired gas injection point. In this embodiment, the desired location is above the production formation. In general, the gas-lift string extends to a location below the wellhead but above the production formation. To assist in lifting total production fluid to separator  24 , gas, flowing in the direction indicated by arrow  36 , is provided through gas-lift string  80  having one or more gas-lift valves  30  spaced along the length of gas-lift string  80 . Production fluid enters the upper portion of gas-liquid separator  38 , shown in detail in FIG. 4, through one or more inlets  82 . Accordingly, gas is separated from the total produced mixture by gas-liquid separators of the types shown schematically in FIG.  5 . Free gas, flowing in the direction indicated by arrow  44 , exits gas-liquid separator  38  through upper port  46  of gas collection string  48 . The gas-depleted produced mixture, flowing in the direction indicated by arrow  50 , exits gas-liquid separator  38  through liquid outlet  52  and enters oil-water separator  40  shown in FIG.  4 . Oil-water separator  40  includes separation chamber  56  wherein the gas depleted produced mixture is separated into a predominately hydrocarbon component and a predominately water component using cyclone separator  58 . Alternative embodiments of the oil-water separator to separate the produced mixture into a predominately hydrocarbon component and a predominately water component will be apparent to one skilled in the art. For example, one or more cyclones can be housed in one or more separators, which, in turn, can act in series or in parallel, to separate produced fluids. The predominately hydrocarbon component, flowing in the direction indicated by arrow  60 , exits separation chamber  56  through oil concentrate conduits  62  which in turn extend up the wellbore to conduit  66 , that extends to the surface. The predominately water component, flowing in the direction indicated by arrow  70 , exits oil-water separator  40  into water disposal string  74 . Water disposal string  74 , preferably equipped with adjustable downhole choke  76 , passes through lower annular sealing packer  22  and extends from the bottom of separator  24  to injection formation  16 . Water flows in the direction indicated by arrow  78  to injection formation  16 . 
     Referring now to FIGS. 5 a  to  5   d , schematics of various types of the gas-liquid separator component of the present invention are shown. FIG. 5 a  shows the gas-liquid separator of the present invention which includes a cylindrical cyclone  83  with a combined swirl intake/gas outlet  84  and vortex breaker  91 . FIG. 5 b  shows the gas-liquid separator of the present invention, which includes a cylindrical cyclone  83  with swirl intake  84 , gas segregation finder  86  and vortex breaker  91 . FIG. 5 c  shows the gas-liquid separator of the present invention, which includes cylindrical cyclone  83  and combined swirl intake/gas outlet  89  and auger  88 . FIG. 5 d  shows the gas-liquid separator of the present invention which includes cyclone  83  with combined swirl intake/gas outlet  90 , and auger  88 , with the auger gas outlet extending into the combined swirl intake/gas outlet  90 . 
     While the invention has been described with reference to certain embodiments, it is to be understood that the description is made only by way of example and that the invention is not to be limited to the particular embodiments described herein and that variations and modifications may be implemented without departing from the scope of the invention as defined in the claims hereinafter set out.