Abstract:
A metering body for a chemical injection system comprises a free piston having a pair of mechanically-actuated poppet valves which may individually open to permit fluid to pass from one side of the free piston to the other. The free piston slides within a cylinder having cylinder heads at opposing ends. Input/output ports are provided in the cylinder heads. In a preferred embodiment, the poppet valves have actuators that extend beyond the opposing faces of the free piston. If one face of the piston moves to within a predetermined distance of a cylinder head, the actuator contacts the cylinder head and further movement of the piston causes the corresponding poppet valve to open, permitting fluid to flow through the piston. In this way, a fault-tolerant system may be implemented. If a power interruption or other failure of the controller occurs, the system will continue to supply fluid at the most recently selected flow rate. When power is restored (or the fault is corrected), the controller causes a valve to reverse the flow of fluid through the metering body and the spring-loaded poppet valve will close as the free piston moves away from the cylinder head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    None 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    None 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to chemical injection systems for oil and gas wells. More particularly, it relates to a positive-displacement volumetric device for use in systems for injecting liquid-phase chemical treatment agents into undersea wells. 
         [0005]    2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
         [0006]    A variety of chemical agents are injected into hydrocarbon wells for the control of corrosion, hydrates, asphaltenes, paraffins, scale and the like. These chemical agents are typically in the liquid phase and are pumped into the well at a selected rate using a chemical injection system. For undersea wells, the chemical supply and pump may be located on a production platform and are most commonly connected to the wellhead via an umbilical line. If metering of the chemical agent is performed only at the surface, any leak in the umbilical or its connectors will give an erroneous indication of the quantity of chemical agent being injected into the well. Moreover, each subsea well may require its own injection system on the platform and connecting umbilical line. 
         [0007]    Certain metering systems of the prior art employ a variable orifice—an adjustable orifice that allows remote control of flow at each well. Other metering systems of the prior art rely on pressure-compensated flow control—an adjustable pressure regulator and a fixed orifice can maintain a constant flow at each well. 
         [0008]    Metering flow over a large range is often necessary over the life of the well. Orifice metering is limited in range and subject to filming, clogging and differing fluid properties. 
         [0009]    Particulate contamination in long chemical injection lines is unavoidable and can clog the small orifices needed for metering and control. Filters on the lines are an added complication affecting system reliability, increasing capital costs and requiring periodic service (which increases operating costs). 
         [0010]    U.S. Pat. No. 6,973,936 to Richard R. Watson (the disclosure of which is hereby incorporated by reference in its entirety) discloses a fluid injection system that controls the distribution of fluid from a supply line to a selected well at an adjustable rate. A free piston divides a cylinder into first and second chambers. A multi-position valve comprises a first position for passing fluid from the supply line into the first chamber to displace fluid from the second chamber back through the valve to an injection point, and a second position for passing fluid from the supply line to the second chamber to displace fluid from the first chamber back through the valve to the injection point. A control system in communication with a position sensor times displacement of the free piston to selected positions, and selectively adjusts a variable valve opening to adjust flow rate, switch between the first and second positions, and periodically increase the valve opening for cleaning. 
         [0011]    The system disclosed in U.S. Pat. No. 6,973,936 may be characterized as a “fail closed” system—i.e., if power or control signals to the multi-position valve are interrupted, the system will continue to inject fluid into the well only until the free piston reaches the limit of its current stroke, at which point the flow of fluid will cease. 
         [0012]    The present invention provides a “fail-as-is” state for a chemical injection system of the type disclosed in U.S. Pat. No. 6,973,936 in the event of a loss of power or control signals to the reversing valve. In a chemical injection system according to the invention, interruption of power or control signals to the valve results in a fluid flow rate substantially equal to the most-recently selected value. In this way, chemical treatment of the well can continue in the interim between the onset of the fault and its discovery and repair. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    A volumetric metering body to which the present invention may be applied comprises a displacement cylinder divided into two chambers by a free piston. Fluid to be metered enters a first chamber which causes the free piston to move in a direction which increases the volume of that chamber and decreases the volume of the second chamber. Fluid in the second chamber is displaced by the movement of the free piston and exits the metering body. Since the chambers have known dimensions, a known volume of fluid (which may be injected into a well) is dispensed with each cycle of the free piston. 
         [0014]    In a metering body according to the present invention, the free piston is provided with two, mechanically-actuated valves which may be poppet valves. When open, the poppet valves permit fluid to flow from one side of the free piston to the other—i.e., fluid may flow from one chamber of the displacement cylinder to the other chamber. During normal operation of the metering body, the valves remain closed. However, if a fault occurs in the system which prevents the flow of fluid in the metering body to reverse at the end of the piston&#39;s stroke, at least one of the valves will open when the free piston comes within a preselected distance from a mechanical stop. In one particular preferred embodiment, the interior face of a cylinder head comprises the mechanical stop. With the valve open, pressurized fluid can continue to flow through the metering body at the last selected flow rate. In this way, a fault-tolerant system may be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0015]      FIG. 1  is a schematic diagram of a chemical injection system of the prior art which comprises a displacement cylinder for measuring the volume of fluid injected. 
           [0016]      FIG. 2  is a cross-sectional view of a displacement cylinder according to the prior art. 
           [0017]      FIG. 2A  is an enlarged view of the portion indicated in  FIG. 2 . 
           [0018]      FIG. 3  is an end view of a displacement cylinder according to the invention. 
           [0019]      FIG. 4  is a cross-sectional view of the displacement cylinder shown in  FIG. 3  taken along line  4 - 4 . 
           [0020]      FIG. 4A  is an enlarged view of the portion indicated in  FIG. 4 . 
           [0021]      FIG. 5  is a cross-sectional view of a free piston according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  schematically illustrates details of a metering body  12  interconnected with a control system  14  and a multi-position valve  16  in a chemical injection system  10 . The metering body  12  has a bore  20  for containing chemical fluid to be delivered to a well. An axially movable free piston  22  in bore  20  divides metering body  12  into variable-volume first and second chambers  24 ,  26 . Free piston  22  seals with metering body  12  with a sealing member such as O-ring  25 . Metering body  12  and free piston  22  conventionally comprise a cylinder and piston assembly, as shown. First and second input-output ports  28 ,  30  are provided for passing fluid into and out of first and second chambers  24 ,  26 . Supply line  33  supplies chemical fluids at high pressure through multi-position valve  16  to metering body  12 . 
         [0023]    In a first valve position shown in  FIG. 1 , illustrated conceptually by alignment of parallel line segments  18  with lines  31  and  33 , fluid passes from supply line  33 , through multi-position valve  16 , line  29 , and input-output port  30 , and into chamber  26 . As fluid passes into chamber  26 , fluid pressure urges free piston  26  toward end  34  of metering body  12 , decreasing the volume of first chamber  24  and displacing the fluid out through input-output port  28 . Fluid exiting port  28  passes through line  27 , back through valve  16 , and out through line  31  to an injection point in the well. 
         [0024]    In a second position (not shown), which may be visualized conceptually by sliding the crossed flow lines  15  in valve  16  to the left to align with lines  31  and  33 , fluid passes from supply line  33 , through multi-position valve  16 , line  27 , input-output port  28 , and into chamber  24 . As fluid passes into chamber  24 , fluid pressure urges free piston  26  toward end  36  of metering body  12 , decreasing the volume of chamber  26  and displacing the fluid out through input-output port  30 . Fluid exiting port  30  passes through line  29 , back through valve  16 , and out through line  31  to the same injection point in the well. Thus, by reversing the direction of multi-function valve  16  each time free piston  22  reaches a selected position, the fluid may be continually passed from line  33  to line  31  to the injection point in the well. 
         [0025]    Position sensors  38  and  40  are included for sensing the position of free piston  22 . Position sensors  38 ,  40  are in communication with control system  14  as represented by dashed lines  39 ,  41  through conventional means, such as by wire, optical fiber or wireless signal. When free piston  22  reaches selected positions, position sensors  38 ,  40  signals control system  14 , in response to which control system  14  may selectively reverse the position of multi-position valve  16  to reverse the direction of free piston  22 . 
         [0026]    Because the selected positions are known, relative displacement of free piston  22  is also known, corresponding to a known volumetric displacement of fluid from metering body  12 , computed as the product of displacement of free piston  22  and cross-sectional area of bore  20 . The control system  14  includes an internal timer for timing displacement of free piston  22  between the selected positions, as signaled by position sensors  38 ,  40 . A volumetric flow rate is therefore also known, which may be computed as the volumetric displacement divided by displacement time. The multi-position valve  16  includes a variable valve opening discussed below in conjunction with  FIGS. 2-4 , for controlling flow between supply line  33  and metering body  12 . The control system  14  selectively adjusts the variable valve opening in response to displacement time of free piston  22 . If the displacement time is too long, indicating a flow rate less than a desired flow rate, control system  14  may increase the variable valve opening to increase the flow rate. Conversely, if the displacement time is too short, indicating a flow rate more than the desired flow rate, control system  14  may selectively decrease the valve opening to reduce the flow rate. The flow rate of the fluid delivery to the well is thereby controlled. 
         [0027]    As shown in  FIG. 1 , the selected positions of free piston  22  are preferably the positions of free piston  22  having reached either end  34 ,  36  of metering body  12 . The selected positions of free piston  22  could alternatively be anywhere along the range of travel of free piston  22 , and need not be at ends  34 ,  36  of metering body  12 . In typical embodiments, as illustrated, position sensors  38 ,  40  are at substantially the same axial position as the selected positions. Conventional position sensors such as spring-loaded pins or magnetic or infrared proximity sensors may be used. In other embodiments, the position sensors conceivably may not need to be axially aligned with the selected positions. A position sensor may further comprise an optional pressure transducer  49  or a flow transducer  42 . These types of position sensors may sense position implicitly, such as when there is a sudden drop of pressure in line  31  as the free piston reaches ends  34 ,  36  of metering body  12 . Optional port valves such as might comprise sealing members  43 ,  44  on free piston  22  may be included for sealing input-output ports  28 ,  30  when free piston reaches ends  34 ,  36 . This may more dramatically decrease pressure in line  31 , and thereby provide a more distinct indication that free piston  22  has reached the end of its travel. Such an indication may provide a backup to confirm or substitute for position sensors  38  and  40 . 
         [0028]    Hall effect devices used in motion sensing and motion limit switches can offer enhanced reliability in extreme environments. As there are no moving parts involved within the sensor or magnet, typical life expectancy is improved compared to traditional electromechanical switches. Additionally, the sensor and magnet may be encapsulated in an appropriate protective material. Hall effect devices when appropriately packaged are immune to dust, dirt, mud, and water. These characteristics make Hall effect devices particularly preferred in a system according to the present invention for piston position sensing compared to alternative means such as optical and electromechanical sensing. 
         [0029]    If the displacement cylinder fails to stroke in the expected time, a condition that indicates clogging, the controller can drive the 4-way valve to the full open position to allow debris to pass. 
         [0030]      FIG. 2  shows a metering body  112  of the prior art. Metering body  112  comprises a cylinder  114  having bore  120  and capped at opposing ends by cylinder heads  116  and  117  which may be in treaded engagement with cylinder  114 . Seals  118  and  119  may be provided to ensure a fluid-tight seal between cylinder  114  and cylinder heads  116  and  117 , respectively. In one particular preferred embodiment, seals  118  and  119  are O-ring seals. 
         [0031]    Piston  122  slides within bore  120  between end  134  of cylinder head  116  and end  136  of cylinder head  117 . Piston  122  divides bore  120  into variable displacement chambers  124  and  126 . As may be more clearly seen in the enlarged view of  FIG. 2A , piston  122  may comprise one or more seals on its outer circumference for sealing to the inner wall of cylinder  114 . In the particular metering body illustrated in  FIG. 2 , the sealing members comprise primary O-ring seal  125  at the centerline of piston  122  and flanking backup O-ring seals  152  and  153 . Additionally, supplementary radial seals  154  and  155  and supplementary circumferential seal  156  may be provided. In one particular preferred embodiment, O-ring seals  125 ,  152  and  153  are fabricated using an elastomeric polymer and supplementary seals  154 ,  153  and  156  are fabricated from polyetheretherketone (PEEK), a semi-crystalline thermoplastic material. 
         [0032]    The opposing faces of piston  122  may have ring-shaped magnets  150  and  151  embedded therein for actuating position sensors  138  and  140  in cylinder heads  116  and  117 , respectively, as described below. 
         [0033]    In operation, fluid enters and exits chamber  124  via first input/output port  128  and fluid enters and exits chamber  126  via second input/output port  130 . Ports  128  and  130  may be in fluid communication with optional bleed ports  131  and  132 , respectively. Bleed ports  131  and  132  may be provided in order to give operators a visual indication of a leaking connection at ports  128  and  130 , respectively. 
         [0034]    Position sensors  138  and  140  may be located within cylinder heads  116  and  117 , respectively. In the metering body illustrated in  FIG. 2 , the position sensors  138  and  140  comprise Hall-effect switches which are spring-biased against the bottom of a blind hole in the cylinder head. As piston  122  approaches face  134  of cylinder head  116 , magnet  150  actuates the Hall-effect switch of position sensor  138  which signals the controller ( 14  in  FIG. 1 ) that piston  122  is at the end of a stroke. Likewise, as piston  122  approaches face  136  of cylinder head  117 , magnet  151  actuates the Hall-effect switch of position sensor  140  which signals the controller ( 14  in  FIG. 1 ) that piston  122  is at the end of the opposing stroke. 
         [0035]    When piston  122  is sufficiently close to face  134  of cylinder head  116  to actuate position sensor  138 , controller  14  (see  FIG. 1 ) may signal actuator  45  to position valve  16  such that fluid, under pressure, is admitted to chamber  124  by way of I/O port  128 . The fluid pressure in chamber  124  acts to urge free piston  122  towards face  136  of cylinder head  117  which displaces fluid within chamber  126  said fluid exiting metering body  112  via I/O port  130 . This action continues until free piston  122  is sufficiently close to face  136  to actuate position sensor  140  at which point controller  14  signals valve actuator  45  to supply fluid under pressure to I/O port  130  and exhaust fluid via I/O port  128 . The process then repeats with fluid being dispensed from port  128 . The volume of fluid displaced by a full stroke of free piston  126  is a known quantity—either by calculation or empirical measurement. Thus, each time piston  122  completes a stroke (as determined by deactivation of one position sensor followed by actuation of the opposite position sensor), a known volume of fluid has been dispensed by metering body  112 . 
         [0036]    It will be appreciated by those skilled in the art that the above-described system may be characterized as a “fail-closed system”—i.e., in the event of a failure of controller  14 , valve actuator  45  or valve  16 , the system would dispense fluid until piston  122  reached the end of its current stroke (assuming a continuing supply of fluid to the inlet) at which point the flow of fluid would cease. Without piston movement, metering body  112  acts as a closed valve, interrupting the flow of fluid. Inasmuch as controller  14  and valve actuator  45  require power in order to operate, an interruption of power to the system will cause the flow of fluid to cease, even if a pressurized source of fluid remains available. 
         [0037]    Referring now to  FIG. 4 , a metering body  212  according to the present invention is shown in cross section. Metering body  212  comprises a cylinder  214  having bore  220  and capped at opposing ends by cylinder heads  216  and  217  which may be in treaded engagement with cylinder  214 . Seals  218  and  219  may be provided to ensure a fluid-tight seal between cylinder  214  and cylinder heads  216  and  217 , respectively. In one particular preferred embodiment, seals  218  and  219  are O-ring seals. 
         [0038]    Piston  222  slides within bore  220  between end  234  of cylinder head  216  and end  236  of cylinder head  217 . Ends  234  and  236  may be concave. Piston  222  divides bore  220  into variable displacement chambers  224  and  226 . As may be more clearly seen in  FIG. 4A , piston  222  may comprise one or more seals on its outer circumference for sealing to the inner wall of cylinder  214 . In the particular preferred embodiment illustrated in  FIG. 4 , the sealing members comprise primary O-ring seal  225  at the centerline of piston  222  and flanking backup O-ring seals  252  and  253 . Additionally, supplementary radial seals  254  and  255  and supplementary circumferential seal  256  may be provided. In one particular preferred embodiment, O-ring seals  225 ,  252  and  253  are fabricated using an elastomeric polymer and supplementary seals  254 ,  253  and  256  are fabricated from polyetheretherketone (PEEK), a semi-crystalline thermoplastic material. 
         [0039]    The opposing faces of piston  222  may have ring-shaped magnets  250  and  251  embedded therein for actuating position sensors  238  and  240  in cylinder heads  216  and  217 , respectively, as described below. 
         [0040]    In operation, fluid enters and exits chamber  224  via first input/output port  228  and fluid enters and exits chamber  226  via second input/output port  230 . Ports  228  and  230  may be in fluid communication with optional bleed ports  231  and  232 , respectively. Bleed ports  231  and  232  may be provided in order to give operators a visual indication of a leaking connection at ports  228  and  230 , respectively. 
         [0041]    Position sensors  238  and  240  may be located within cylinder heads  216  and  217 , respectively. In the embodiment illustrated in  FIG. 4 , the position sensors  238  and  240  comprise Hall-effect switches which are spring-biased against the bottom of a blind hole in the cylinder head. As piston  222  approaches face  234  of cylinder head  216 , magnet  250  actuates the Hall-effect switch of position sensor  238  which signals the controller ( 14  in  FIG. 1 ) that piston  222  is at the end of a stroke. Likewise, as piston  222  approaches face  236  of cylinder head  217 , magnet  251  actuates the Hall-effect switch of position sensor  240  which signals the controller ( 14  in  FIG. 1 ) that piston  222  is at the end of the opposing stroke. 
         [0042]    When piston  222  is sufficiently close to face  234  of cylinder head  216  to actuate position sensor  238 , controller  14  (see  FIG. 1 ) may signal actuator  45  to position valve  16  such that fluid, under pressure, is admitted to chamber  224  by way of I/O port  228 . The fluid pressure in chamber  224  acts to urge free piston  222  towards face  236  of cylinder head  217  which displaces fluid within chamber  226  said fluid exiting metering body  212  via I/O port  230 . This action continues until free piston  222  is sufficiently close to face  236  to actuate position sensor  240  at which point controller  14  signals valve actuator  45  to supply fluid under pressure to I/O port  230  and exhaust fluid via I/O port  228 . The process then repeats with fluid being dispensed from port  228 . The volume of fluid displaced by a full stroke of free piston  226  is a known quantity—either by calculation or empirical measurement. Thus, each time piston  222  completes a stroke (as determined by deactivation of one position sensor followed by actuation of the opposite position sensor), a known volume of fluid has been dispensed by metering body  212 . 
         [0043]    Piston  222  comprises a first passageway connecting the opposing faces of the generally cylindrical piston and a second passageway which also connected the opposing faces of the piston. Each passageway is closed by a valve  240 ,  242 . In the embodiment illustrated in  FIGS. 4 and 5 , the valves  240  and  242  are spring-loaded poppet valves and are disposed in opposite directions. Valves  240 ,  242  comprise mechanical valve actuators  244  and  246 , respectively, which project from the face of piston  222  and which cause their respective valves to open against the force of the valve spring when depressed. 
         [0044]    If, for any reason, there is a failure of either or both position sensors  238 ,  240  or a failure of the valve actuator  45  or of controller  14 , piston  222  will continue to be driven towards one of the cylinder head faces  234 ,  236 . For the purposes of this discussion, it will be assumed that a failure in one of the above-referenced components or a power interruption occurs while fluid is being admitted into chamber  226  of metering body  212  via port  230  and fluid is being dispensed from port  228  as fluid is displaced from chamber  224 . 
         [0045]    When the advancing face of piston  222  moves sufficiently close to surface  234  to achieve contact between surface  234  and valve actuator  246 , further movement of piston  222  in that direction will cause valve  240  to open, permitting fluid to flow from chamber  226  through valve  240  in piston  222 , into chamber  224  and out via port  228 . If the face of piston  222  is generally planar, concave surface  234  will prevent piston  222  from blocking the interior end of port  228 —i.e., chamber  224  will at all times have a sufficient volume to permit the flow of fluid to continue through it. 
         [0046]    In certain embodiments, the force constant of the valve springs may be chosen to allow the valve to open if the fluid pressure differential across the piston  222  exceeds the nominal working pressure of the metering body. In this way, an additional fluid passage through the piston may be opened if the reciprocating movement of the piston is interrupted. 
         [0047]    It will be appreciated by those skilled in the art that the above-described system may be characterized as a “fail-as-is system”—i.e., in the event of a failure of controller  14 , valve actuator  45  or valve  16 , the system would continue to dispense fluid at the most recently selected flow rate (assuming no movement of valve  16 ). Inasmuch as controller  14  and valve actuator  45  require power in order to operate, an interruption of power to the system will not cause the flow of fluid to cease so long as a pressurized source of fluid remains available. 
         [0048]    It will also be appreciated by those skilled in the art that a metering body  112  of the prior art may be retrofitted to practice the present invention by replacing piston  122  with a piston  222  as shown in  FIG. 5 . 
         [0049]    Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.