Method for autonomous control of a chemical injection system for oil and gas wells

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to chemical injection systems for oil and gas wells. More particularly, it concerns autonomous control systems for the injection of liquid phase chemical treatment agents into subsea wells.

2. Description of Related Technology including information disclosed under 37

CFR 1.97 and 1.98

A variety of chemical agents are injected into hydrocarbon wells to control corrosion, hydrates, asphaltenes, paraffins, fouling and the like. Typically, these chemical agents are in the liquid phase and are pumped into the well at a selected rate using a chemical injection system. For subsea wells, the chemical supply and pump can be located on a production platform and are commonly connected to the wellhead via an umbilical cable. If the measurement of the chemical is performed only on the surface, any leakage in the umbilical cable or its connectors will give an erroneous indication of the amount of chemical that is injected into the well. In addition, each subsea well may require its own injection system on the platform and umbilical cable connection.

Certain prior art metering systems employ a variable orifice and an adjustable orifice that allows remote flow control in each well. Other prior art metering systems rely on pressure compensated flow control - an adjustable pressure regulator and a fixed orifice can maintain a constant flow in each well.

Frequent flow measurement over a wide range is often required over the life of the well. Orifice measurement is limited relative to range and is subject to film coverage, clogging and differentiated fluid properties.

Particulate contamination in long chemical injection lines is inevitable and can clog the small holes required for measurement and control. Inline filters are a further complication that affects system reliability, increasing capital costs and requiring periodic maintenance (which increases operating costs).

U.S. Patent 6,973,936 to Richard R. Watson discloses a fluid injection system that controls fluid distribution 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 for moving 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 determines the free piston travel time for selected positions and selectively adjusts a variable valve opening to adjust flow, alternates between the first and second positions and periodically increases the opening. valve for cleaning.

SUMMARY OF THE INVENTION

The present invention may be incorporated into a control program for a positive fluid displacement measurement system that measures the time required for a free piston displacement in a known volume cylinder to determine an average flow during a full piston stroke. . The system can also measure and record the inlet and outlet pressures of the differential pressure between the fluid inlet and outlet. The controller can take the flow commands from a customer subsea control capsule and adjust the flow rate by partially opening a four-way valve each time the valve is reversed.

The control program precisely positions a four-way valve that can function as an adjustable metering orifice in response to the measured average flow and / or changes in inlet and outlet pressures to achieve the desired flow. At the conclusion of each stroke, the four-way valve is repositioned to reverse the flow of liquid through the metering cylinder.

The system can review stored valve position settings for both direct stroke and reverse stroke based on the measured time required for a full stroke at a currently stored valve position. In this way, the system iteratively compensates for all changes in fluid properties and fluid pressure. Certain embodiments of the invention further comprise an optional cleaning cycle that gradually opens the valve gradually and may, if necessary, fully open the valve in both directions of fluid to clear a valve orifice plug. Cut-off seal or gate-valve construction have been found to be the most suitable designs for reliable operation when high pressure fluids are contaminated with rigid particulate material. The standard operating procedure for clearing such a blocked valve is to move it to the fully open and fully closed positions. This allows the accumulated particulates to pass through and the sealing elements to be swept away or cut off all remaining obstructions. The use of this valve construction for the two-way four-way valve and valve actuator control in response to cylinder stroke time results in precise flow control with excellent contamination resistance.

The actuation of the two-way four-way control valve can be performed with a conventional stepper motor that drives a ball screw to convert rotation into linear motion. This combination has been found to give very high accuracy to the linear position of the valve. This precision allows the valve to be partially opened, thus creating a precision hole each time the valve is moved.

As the valve is displaced to allow flow to the first side and then to the other side of the cylinder piston, flow is regulated by the precision orifice created by the partially open valve. In certain embodiments, a contact time is included after the cylinder piston has completed its displacement and flow has been interrupted. This provides precise control of the volume of chemicals injected into the continuous flow of oil or gas well production over a certain period of time.

A system according to the invention may be designed to accommodate all chemicals currently used for the control of corrosion, hydrates, asphaltenes, paraffins and scale in hydrocarbon wells. Changes in fluid viscosity or density do not require recalibration of the positive displacement measurement system. Verification data may be transmitted to the client subsea control capsule.

The use of a plurality of systems according to the invention can provide assurance of treatment chemical flow to multiple subsea wells from a single umbilical cable.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a schematic diagram of a prior art chemical injection apparatus which may be controlled by the method of the present invention.

Figure 2 is a cross-sectional view of the control valve used in the apparatus of Figure 1 in a first position.

Figure 3 is a cross-sectional view of the control valve shown in Figure 2 in a second position.

Figure 4 is an enlarged cross-sectional view of a portion of the valve shown in Figure 2.

Figure 5 is a schematic diagram of a modified chemical injection system for use with the present invention.

Figure 6 is a flow chart depicting the steps of a method according to an embodiment of the invention.

Figure 7 is a flow chart produced by a particular representative control valve as a function of the number of steps taken by a stepper motor that drives the valve actuator.

Figure 8 is a flow chart depicting the steps of a method according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically illustrates details of a metering body 12 interconnected with a control system 14 and a multi-position valve 16 actuated by actuator 45 in a chemical injection system 10. Measuring body 12 has a bore 20 for containing chemical fluid from be distributed in a well. An axially movable free piston 22 in bore 20 divides the metering body 12 into first and second chambers 24, 26 with variable volume. Free piston 22 seals metering body 12 with a sealing member, such as O-ring 25. Conventionally, metering body 12 and free piston 22 comprise a cylinder and piston assembly as shown. The first and second inlet-outlet ports 28, 30 are provided for passing fluid in and out of the first and second chambers 24, 26. Supply line 33 supplies high-pressure chemical fluids through multi-position valve 16 to metering body 12.

In a first valve position shown in Figure 1, conceptually illustrated by the 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 orifice input - output 30, and into chamber 26. As fluid passes into chamber 26, fluid pressure pushes free piston 22 toward the end 34 of metering body 12, decreasing the volume of first chamber 24 and displacing fluid through the bore port. inlet - outlet 28. Fluid exiting through port 28 passes through line 27, back through valve 16, and out through line 31 to an injection point in the well.

In a second position (not shown), which can be conceptually visualized by sliding the cross flow lines 15 in valve 16 to the left to align with lines 31 and 33, fluid passes from supply line 33 through the multi-position valve. 16, line 27, inlet - outlet 28, and into chamber 24. As fluid passes into chamber 24, fluid pressure pushes free piston 26 toward end 26 of metering body 12, decreasing volume of chamber 26 and displacing fluid through the inlet port - outlet 30. Fluid exiting through 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 repeatedly reversing the direction of the multifunction valve 16 after the free piston 22 has reached a selected position, fluid can continuously flow from line 33 to line 31 to the injection point in the well.

Position sensors 38 and 40 are included to detect free piston position

22 Position sensors 38, 40 are in communication with control system 13 as represented by dashed lines 39, 41 by conventional device such as a wired, fiber optic or wireless signal. When free piston 22 reaches selected positions, position sensors 38, 40 signal control system 14 in response to which control system 14 can selectively reverse the position of multi-position valve 16 to reverse the direction of free piston travel. 22

Because the selected positions are known, the relative displacement of free piston 22 is also known, corresponding to a known volumetric displacement of metering body fluid 12, computed as the product of free piston displacement 22 and cross sectional area of ​​the piston. hole 20. Control system 14 includes an internal timer for synchronizing free piston travel 22 between selected positions, as signaled by position sensors 38, 40. Therefore, a volumetric flow rate is also known which can be computed as divided volumetric travel. by the travel time. The multi-position valve 16 includes a variable valve opening discussed below in conjunction with figures 2-4 to control the flow between supply line 33 and metering body 12. Control system 14 selectively adjusts the variable valve opening. valve in response to free piston travel time 22. If the travel time is too long, indicating a flow less than the desired flow, the control system 14 may increase the variable valve opening to increase the flow. Conversely, if the travel time is too short, indicating a flow greater than the desired flow, the control system 14 may selectively decrease the valve opening to reduce the flow. In this way, the flow of fluid distribution in the well is controlled.

As shown in Figure 1, preferably the selected positions of free piston 22 are the positions of free piston 22 having reached both ends 34, 36 of metering body 12. The selected positions of free piston 22 may alternatively be any place along the free piston travel range 22, and need not be at the ends 34, 36 of the metering body 12. In typical embodiments, as illustrated, position sensors 38, 40 are substantially in 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, conceivably, position sensors may not need to be axially aligned with the selected positions. A position sensor may additionally comprise an optional pressure transducer 49 or a flow transducer 42. Optional orifice valves, such as may comprise sealing elements 43, 44 on free piston 22, may be included to seal inlet ports - outlet 28, 30 when the free piston reaches the ends 34, 36. This may more dramatically decrease the pressure on line 31 and thereby provide a more distinct indication that the free piston 22 has reached the end of its travel. Such an indication may provide a substitute for confirming or replacing position sensors 38 and 40.

The terms "first position" and "second position" with respect to valve 16 generally refer to the direction resulting from the flow rather than a fixed position of the valve components 16 because there is generally a degree adjustability in each of the two positions, such as to adjust the flow. Figure 2 shows a detailed view of the preferred embodiment of multi-position valve 16 in the first valve position, partially open to limit flow through the valve. Figure 3 shows a detailed view of the preferred valve 16 in the second partially open valve position. Figure 4 shows in closer detail a portion of port-type valve 16 in the first position of the valve of figure 2.

Particularly with respect to FIG. 2, the schematic diagram of FIG. 1 and the closest view of FIG. 3, the multi-position valve is generally indicated at 16 with a body 17. A port 50 is positioned in a cavity 52 in the FIG. body 17. Port 50 has a hole 54 which, in the position shown, is in communication with an inlet port 32 and a first flow passage 56 extending through body 17 to a first exchange port 57. Thus, in this position, chemical fluid supplied by the above discussed supply line 33 flows into the body 17 through the inlet port 32, through the port hole 54 and through the first flow passage 56, exiting through the first exchange port 57. to line 29. As discussed, fluid passes through line 29 into metering body 12, and other fluid passes from metering body 12 through line 27 back to valve 16. Then, the flow passes back into the body 17, through the second exchange hole 59, into a second flow passage 58, passes around the port 50 into an outlet passage 53, and outwards. through an outlet port 55. Finally, outflow through outlet port 55 passes through line 31 to the injection point in the well as exposed.

In Figure 3, the port hole 54 is positioned, in contrast, in communication with the inlet port 32 and the second flow passage 58. Thus, the flow of line 33 passes through the inlet port 32 into the passageway. 51, through the port hole 54, and through the second flow passage 58, exiting through the second exchange orifice 59 to line 27. As stated, fluid passes through line 27 into metering body 12, and other fluid passes from metering body 12 through line 29 back to valve 16. Then fluid passes back into valve 12. body 17 through the first exchange hole 57, into the first flow passage 56, into the outlet passage 53, and out through the outlet port 55. Finally, outflow through outlet port 55 passes through line 31 to the injection point in the well. Thus, as described with respect to Figure 1, the flow between valve 16 and metering body 12 may be reversed by moving the valve between the first and second valve positions shown respectively in Figure 2 and Figure 3, but in In each case, the net flow is from line 33 to line 31 to the injection point in the well.

In the embodiment shown in Figs. 2 - 4, stepper motor 45 drives a ball screw 48 to axially move port 50 in cavity 52, adjusting the size of the flow path defined between port hole 54 and the first flow passageway. 56 thereby adjusting the flow to a desired flow rate. Gate 50 may be moved axially to switch between the first valve position of FIG. 2 and the second valve position of FIG. 3. Those skilled in the art perceive alternative devices for moving the gate, different from the stepper motor 45.

Hall effect devices using motion perception and motion limit switches can offer greater reliability in extreme environments. Since there are no moving parts involved in the sensor or magnet, typical life expectancy is increased compared to traditional electromechanical switches. Additionally, the sensor and magnet may be encapsulated in a suitable protective material. The Hall effect device, when properly packaged, is immune to dust, dirt, mud and water. These features make the Hall effect device particularly preferred in a system according to the present invention for piston position perception compared to alternative devices such as optical and electromechanical perception.

Flow control at a few gallons a day at several hundred pounds per square inch pressure drop requires a very small orifice of only a few thousandths of an inch. The valve used in a particular preferred embodiment of the invention is a gate type shut-off valve with a bounce rate of 2,880: 1. This valve provides the required small orifice and reverses the flow for each positive displacement load to prevent clogging.

Certain prior art systems used filters to prevent clogging of small particle flow control holes, but typically these filters need to undergo subsea maintenance, which is highly costly. Prior art devices also used larger area capillary holes for a given flow restriction, and these can be made to adjust their bore diameter by a tapered screw thread so that flow rates can be changed, and A temporary larger opening can be used to provide contamination cleaning. With each of these solutions, measuring a wide flow range is a separate necessary task that requires costly flow instrumentation. Flow cannot be accurately measured by pressure loss through an unknown restriction orifice, as in the situation with partial particulate obstruction. The present invention may include a device for creating the small orifice fitted with a four-way gate valve that is constantly displaced to prevent clogging, and may also be fully opened to allow the particulate to pass through. What's more, the system provides very accurate measurement of flow that is immune to partial obstruction or changing fluid properties or film orifice coverage - all conditions that are present and can destroy conventional meters that are based on a drop in pressure through a hole.

Pressure transducers 49 and 49 'can provide the controller with more information establishing the valve opening degree, but they cannot measure or verify flow. Flow measurement and verification is provided by synchronization circuits and position sensors in the positive displacement cylinder.

If the travel cylinder fails to travel in the expected time, a condition that indicates an obstruction, the controller can drive the four-way valve to the fully open position to allow debris to pass.

An orifice cannot be used as a reliable subsea flow metering device because it is subject to clogging and film covering (coating) by chemicals passing through it. Chemicals that are measured in a chemical injection system for a hydrocarbon well may have film coating characteristics as a desired trait. Common prior art flow measurement devices use a pressure loss measurement through a hole to indicate flow. If a cheap pressure drop measurement system cannot be used, conventional alternatives are costly. Additionally, no other measuring device, such as turbine, ultrasonic, vortex or thermal mass type, can meet the range of a displacement cylinder in accordance with the present invention. All of them are limited to the total range from 100: 1 to 200: 1. This means they can measure precisely 1 gallon per day (GPD) (3.79 L per day) to about 200 GPD (757.09 L per day). A system according to the present invention can measure 1 GPD (3.79 L per day) to over 3,000 GPD (11,356.24 L per day). Also, many well treatment chemicals are non-Newtonian fluids, ie their viscosities change with pressure in a nonlinear manner, a feature that makes accurate flow measurement more challenging for most measurement technologies. prior art, but has no effect on a system employing a positive displacement cylinder.

Referring now to Figure 6, a particular preferred embodiment of the invention is disclosed in the form of a flow chart representing certain steps of a method for controlling a valve in a chemical injection system. The chemical injection system may comprise a processor, and a method may be implemented as processor instructions that may be stored on storage media.

As shown in figure 6A, the process can start at manual input

100 with an operator inserted the desired flow rate of the chemical to be injected. The flow may have the dimensions of volume unit per unit of time. Flow rates for chemical injection systems used in conjunction with oil and gas wells in the domestic power industry are often expressed in gallons per day (GPD). In certain embodiments, insertion of the desired flow rate may be performed by an operator located on an offshore production platform, and the command may be transmitted to or near the controller at the head of the underwater well through an umbilical cable. . The command can also be transmitted via a telemetry system from a offshore facility or another offshore unit.

In a particular preferred embodiment, system initialization includes driving the valve actuator to a mechanical limit by commanding a stepper motor that drives the actuator to walk numerous steps in a direction that exceeds the number of steps previously determined to correspond to all actuator travel. One or more actuator inversions, followed by attempted “direct” travel in excess of inverted travel, may be used to ensure that the actuator is firm with respect to the mechanical limit. Thus, although the initial position of the valve may be unknown upon system initialization, an initialization routine may be used to move the valve to a known position. At block 105, the system can determine initial valve settings (number of steps) for both forward and reverse valve position from the desired flow rate entered at 100, a stored flow curve 115, and valve actuation position data 120 - that is, the number of steps from the closed position of the valve to the point at which the valve orifice begins to open in a certain direction. In a particular preferred embodiment, initialization includes moving the valve actuator from the mechanical limit position to a “central” closed position defined to be the midpoint between the “direct” valve actuation position and the valve actuation position. "Inverted". By way of example, using the flow curve of figure 7, if the desired flow rate is 40 GPD (151.42 L per day) and the direct valve actuation position is 33 steps from the “central” closed position, then The initial direct valve setting will be 171 steps from the center (138 + 33). The valve actuation position has been found to be valve dependent and may vary from valve to valve and / or change after maintenance on the valve or valve actuator. Forward and reverse valve settings can be loaded into registers designated for this function.

Flow curve data 115 may be in the form of a digitized flow curve, such as the curve shown in Figure 7. In a particular preferred embodiment, the flow data is tabulated for each step of a valve motor actuated valve. step. In other embodiments, the flow curve data may be in the form of a mathematical representation - for example, slope and intercept values ​​for a

<img file="BRPI0804350B1_D0001.tif" />

substantially linear flow curve. In the case of modalities using digitized curves, the system may comprise device for interpolation of data points using conventional curve fitting techniques.

In certain embodiments (not shown), initial system inputs may include selecting a particular flow curve that may be associated with a particular chemical or mixture of chemicals to be injected or a certain property of the chemical. fluid to be injected, for example the specific gravity of the fluid, the viscosity of the fluid, the concentration of an active ingredient (s) in a solvent, or the like. In still other embodiments, the initial input may include a correction factor that the system may use to modify a general purpose flow curve previously stored for use with a specific chemical or chemical property, that is, the flow curve. Stored water may be for diluting aqueous solutions, and a correction factor provided allows the system to adapt the curve to a fluid with substantially different rheological properties. However, it is understood that the system according to the present invention will automatically compensate for fluids with different properties, and adapting the flow curve to a specific fluid provides an advantage only in the initial valve position adjustments and in the first few computations of flow corrections. valve adjustment.

Upon system startup, the position of free piston 22 in bore 20 may be unknown. This way, upon initialization, the system can configure the valve to drive the piston in a known location. At decision diamond 125, the system can first test the actuation of the direct limit switch (indicating that piston 22 is at the end of forward stroke travel). If switch actuation is detected, the process can proceed to block 130 for the reverse stroke sequence. If the direct limit switch is not actuated (branch N in diamond 125), the system proceeds to block 140 with a direct stroke sequence (see figure 6B).

After system initialization, the normal flow process of direct and reverse alternating cycles can begin. A representative direct course sequence is illustrated in the flowchart of figure 6B, and a corresponding inverted stroke sequence is shown in figure 6C.

Referring now to Fig. 6B, the direct stroke sequence begins at block 200 with the current direct stroke valve adjustment (which may be in steps of the actuator limit, center (closed) position, or most valve position being loaded from register 202. In block 204, the control valve is driven by the stepper motor to the most current direct valve setting, and a timer is started (block 205). In this condition, the system is now measuring fluid through control valve 16 from supply line 33 to chamber 26 through line 29. As fluid is pumped into chamber 26, piston 22 moves (to 5) moving fluid in chamber 24, which flows through line 27, valve 16 and line 31 to the well injection point. The fluid pressure in the injection line 31 may be measured by the pressure transducer 49, while that in the supply line 33 is measured by the pressure transducer 49 '.

The program may include one or more routines that test piston movement. For example, a straight stroke sequence (Figure 6B) is usually entered from the completion of an inverted stroke sequence signaled by actuation of the reverse limit switch 40. Piston movement 22 away from the inverted stroke limit should disable the reverse limit switch 40. This condition can be tested on diamond 208. If the switch remains activated (ie the piston is still within the limit switch operating range), the system may wait for a selected time interval before taking corrective action. In the illustrated embodiment, the system waits (at rhombus 210) for an interval equal to 50% of the expected stroke time (cylinder displacement volume divided by the selected flow) and, if the inverted limit switch remains activated, the valve may be opened. 20 additional steps (in block 212). Similarly, the system can now expect (at rhombus 216) an additional time interval which, in the illustrated embodiment, is equal to the expected stroke time (now cumulatively 150% of the expected stroke time) for the key- reverse limit to disable (diamond 214). As before, if the piston does not move sufficiently to deactivate the reverse limit switch, the valve opens in an additional 20 steps (in block 218). In the illustrated embodiment, progressive valve opening in the event of no piston movement may be repeated on lozenges 220 and 222 with additional valve opening in block 224. If the reverse limit switch remains activated (branch N in lozenge 226) and cumulative time since valve opening has reached 350% of the expected stroke time (branch S in diamond 228), a flush cycle (described more fully below) can be started (in block 230). However, if the reverse limit switch is disabled (branches S in lozenges 208, 214, 220, or 226), the system proceeds to normal straight-stroke sequence block 232.

While the fluid is flowing, the pressure transducer outputs 49 and 49 'may be sampled periodically and a differential pressure (ΔΡ) may be stored by controller 14. In a particular preferred embodiment, a running average ΔΡ is stored by controller 14. together with the three most recent ΔΡ values ​​in a PEPS stack. Additional filtering algorithms may be applied to eliminate or reduce the influence of pressure increases that may be encountered during a stroke. This process can be implemented as shown in Fig. 6B in block 232, whose subroutine is executed at a preselected interval measured at diamond 238.

In the normal course of events, fluid flow continues until piston 22 reaches the end of its direct stroke (left wall of cavity 24 in figure 5) activating limit switch 38, the activation of which is detected in diamond 234. As shown In Figure 6C, the stopwatch is interrupted at block 270, and the accumulated time on the stopwatch counter is the total time it takes for piston 22 to travel a full stroke. Since a complete stroke displaces a known volume of fluid (as determined by the physical dimensions of cylinder 12 and piston 22), this volume divided by the accumulated time produces the average fluid flow during that particular direct stroke. At block 272, the average flow rate measured for the stroke is compared to the desired flow rate that was entered by the operator at 100.

Corrections (if any) in direct stroke valve adjustment are computed in block 274. In a particular preferred embodiment, the difference between the measured flow rate and the desired flow rate is equated in a number of steps from the stored flow curve. at 115. The correction can be taken directly from the curve or computed from the first derivative of the curve. As stated, in certain embodiments, the system may interpolate the data points to determine the correction.

As shown in block 276, the valve adjustment correction can be further refined by a factor related to a change in the average ΔΡ of the previous direct stroke. In certain embodiments, the correction factor ΔΡ may be a function (in whole or in part) of the selected ΔΡ values, for example the three most recent ΔΡ values ​​in the PEPS stack of the illustrated embodiment. Especially at relatively slow flow rates, a change in ΔΡ just before the end of the stroke may be more indicative of the ΔΡ likely to be encountered during the next straight stroke.

The Δ correção correction can be derived from empirically determined flow values ​​at various differential pressures. In other embodiments, the ΔΡ correction can be calculated from a function that relates flow (or valve actuator motor steps 45) to ΔΡ.

It should be noted that the process of the present invention will function without ΔΡ data, ie the absence or failure of a pressure sensor 49 will not disable the system. The corrections computed in block 274 will compensate for changes in ΔΡ. Using the ΔΡ information (in block 276) enables the system to make better predictions of the valve adjustment required to produce the desired flow rate. However, the iterative process will “zero” in the correct setting even without this data.

At block 278, the revised valve setting to be used for the next direct stroke is stored in the register (or other storage device) designed for this purpose, and the process proceeds to the reverse stroke sequence as shown in block 282. (and figures 6D and 6E). Optionally, at block 280, data relating to the newly completed stroke sequence may be recorded before proceeding to the reverse stroke sequence. Examples of log data include actual stroke time, and additional valve openings time and number (e.g., blocks 212, 218, 224, 240, and / or 246), and whether a discharge stroke (blocks 230 or 256) was required. Any other parameters detected by the system can also be recorded at this stage of the process.

Since the full displacement of free piston 22 displaces a known volume of fluid, the time that must be required for a full stroke of piston 22 at the desired flow rate can be calculated to produce an expected stroke time. As shown in block 236, the elapsed travel time can be compared to the expected travel time and, if the elapsed travel time exceeds the expected travel time by a selected margin (100% in the illustrated example), the system can initiate corrective action - progressive valve opening in 20-step increments at 200% of expected stroke time (diamond 236) and again at 300% of expected stroke time (diamond 244). If the time exceeds 400% of the expected stroke time (branch S in diamond 254), a flush cycle (block 256) is started in the illustrated embodiment. In each of these routines, ΔΡ readings can be taken and stored (blocks 250 and 260) at a selected and repetitive time interval (diamonds 248 and 258).

Controlling an “inverted” stroke of movement - that is, a stroke where the control valve is positioned such that the flow path 15 is active and fluid flows into chamber 24 via line 27 and is expelled by chamber 26 and within line 29 as piston 22 moves from left to right in figure 5 - is illustrated in figures 6D and 6E. The process is analogous to that illustrated for a "straightforward" course in Figures 6B and 6C and discussed earlier. Reference numbers for corresponding elements in Figures 6B and 6C differ by a value of 100 from those of Figures 6D and 6E. The current reverse stroke valve setting can be stored in register 302 and loaded on the controller in block 300. Fixes computed for the reverse stroke valve setting in block 374 and (optionally) in block 376 can be stored in register 305 in block 378 and used for the next inverted stroke. Upon completion of an “inverted” stroke, the process returns to the straight stroke sequence (at block 382). In this way, the system continuously iterates forward and reverse valve settings to provide the requested fluid flow.

As shown in blocks 230 and 256 (figure 6B) and blocks 330 and 356 (figure 6D), the system may initiate a flush cycle in case the elapsed stroke time exceeds the expected stroke time by a selected margin. . A possible cause of lower than expected fluid flow is debris that obstructs or partially obstructs a hole in control valve 16. The system may act to discharge debris from the control valve (or an associated fluid conduit). A discharge cycle according to a preferred embodiment of the invention is illustrated in the flow chart of figure 6F. In this cycle, the control valve is first actuated to its fully open position in one direction (block 460) and then to its fully open position in the opposite direction (block 465). It has been found in practice that this cycle is almost always successful in discharging obstructive debris from the chemical injection system. Following a flush cycle, the system can perform a boot sequence (block 470).

Figure 8 is a flowchart illustrating an alternative embodiment of the invention wherein a known fixed volume of chemicals is injected within a predetermined time interval. This mode has particular advantage in those applications where controlling the total volume of injected chemicals over a certain period of time is more important than injecting the chemical at a constant rate.

The process can start at manual input 500 with operator selection of an average flow. Since a full stroke of piston 22 displaces a known volume of chemicals in block 510, the system can compute the time required to offset the volume of injected chemicals during a full stroke at the selected flow rate. In block 520, the system may be initialized as described above in order to position piston 22 at the beginning of a stroke. Using the stored flow curve, the valve setting that should produce the selected flow rate is determined in block 530 from the stored flow curve in 535. In block 540, the valve is opened to provide a somewhat larger orifice than required to achieve the selected flow rate. The absolute value of the overflow can be a selected percentage increase at the selected flow rate (eg X + 10% GPD), a selected incremental increase at the flow rate (eg X + 5 GPD) or a preselected number of additional steps of the flow. stepper motor that positions the valve (eg position computed from the flow curve + 15 steps).

A timer can be started at block 545 and then the system can test piston movement (at rhombus 550) by sensing the previously activated limit switch deactivation. If piston movement is not detected (N branch 550) in the embodiment illustrated, the valve is opened in an additional 20 steps. This process can be repeated (branch A at diamond 557) at selected time intervals and, if no piston movement is detected after a selected cumulative time ( branch B at 557), a discharge cycle can be started at block 559 to clear any obstruction in the valve bore.

Once piston movement has been detected (S branch 550), the system can expect (by 560) that the limit switch signals that the piston has reached the end of a stroke and that the known volume of a full stroke has been injected. in the well. The valve position set at 540 should result in a full stroke that is completed before the 510 time interval has elapsed - that is, the system must wait for a “contact time” to elapse before starting another stroke. . In diamond 565, sis15

<img file="BRPI0804350B1_D0002.tif" />

theme tests the end of the computed time interval before the limit switch is acted upon. If true (branch S at 565), an error condition exists (block 570) and the system can take corrective action by correcting the valve setting used in block 530. If the limit switch is not yet activated after a selected interval (diamond 572), the corrective action may include a flush cycle (block 573) as shown in relation to figure 6F, and / or an upward position adjustment. stored from the valve.

However, in the normal course of events, the piston will reach the end of a stroke (thereby acting as the limit switch) before the end of the time interval computed at 510 (branch S at 560). The system can store the limit switch actuation time at 575 and then wait (at 580) for the end of the time period at diamond 580.

The time it takes the piston to take a full stroke (recorded at 575) can be used to compute and store a revised valve setting in block 585. Then, this revised setting can be used by the system for the next stroke in the same direction. In this way, the system continuously refines the valve setting to compensate for any parameter changes that may affect flow - for example, feed pressure, viscosity, density, etc.

At block 590, the system orders a corresponding process for a stroke in the opposite direction (which can start at block 530) and the system alternates between “forward” and “inverted” cycles, repeating the required valve adjustments.

While the invention has been described in detail with respect to certain preferred embodiments, variations and modifications exist in accordance with the scope and spirit of the invention described and defined in the following claims.