METHOD FOR AUTONOMOUS CONTROL OF A CHEMICAL INJECTION SYSTEM FOR OIL AND GAS WELLS

• US 20090114391A1
• Filed: 11/02/2007
• Published: 05/07/2009
• Est. Priority Date: 11/02/2007
• Status: Active Grant

• Current Assignee: National Coupling Company, Inc. (Hunting PLC)
• Original Assignee: National Coupling Company, Inc. (Hunting PLC)
• Inventors: SMITH, ROBERT E. IV, Slot, Maurice, Watson, Richard R.
• Granted Patent: US 9,255,465 B2
• Time in Patent Office: Days
• US Class Current: 166/305.100
• CPC Class Codes: E21B 34/16 Control means therefor being outside the borehole control systems for submerged well heads E21B33/0355
• CPC Class Codes: E21B 37/06 using chemical means for preventing or limiting , e.g. eliminating, the deposition of paraffins or like substances
• CPC Class Codes: E21B 41/02 in situ inhibition of corrosion in boreholes or wells
• CPC Class Codes: G05B 15/02 electric
• CPC Class Codes: G05D 7/0635 by action on throttling means G05D7/0688, G05D7/0694 take precedence
• CPC Class Codes: Y10T 137/0368 By speed of fluid
• CPC Class Codes: Y10T 137/0379 By fluid pressure
• CPC Class Codes: Y10T 137/7759 Responsive to change in rate of fluid flow
• CPC Class Codes: Y10T 137/776 Control by pressures across flow line valve
• CPC Class Codes: Y10T 137/7761 Electrically actuated valve

Abstract

A control program for a positive displacement metering system measures the time required for the travel of a free piston in a cylinder of known volume to determine an average flow rate during a full stroke of the piston. The system may also measure and record the inlet and outlet pressures or the differential pressure between the fluid inlet and outlet. The control program positions a four-way valve which may function as an adjustable metering orifice in response to the measured average flow rate and/or changes in the inlet and outlet pressures to achieve the desired flow rate. At the end of each stroke, the four-way valve is repositioned to reverse fluid flow through the metering cylinder. The system may revise the valve position settings for both forward and reverse strokes based on the measured time required for a full stroke at a certain valve position. In this way, the system automatically and iteratively compensates for changes in fluid properties and fluid pressure. A cleaning cycle which progressively opens the valve stepwise and may culminate in full open valve travel in both fluid flow directions is provided in the event of an obstruction of the valve orifice.

Claims

1. A method for injecting a liquid phase chemical into a well comprising:

inputting a certain desired fluid flow rate;

selecting a variable orifice position from data relating orifice position to flow rate;

opening a variable orifice to the selected position to dispense a known volume of fluid;

timing the period required to dispense the known volume of fluid;

computing the average fluid flow rate during the timed period;

comparing the computed average fluid flow rate to the desired flow rate;

adjusting the size of the variable orifice to minimize the difference between the computed flow rate and the desired flow rate.

2. A method as recited in claim 1 wherein the known volume of fluid is dispensed by moving a barrier in a fluid container a known distance.

3. A method as recited in claim 2 wherein the fluid container comprises a cylinder and the fluid barrier comprises a free piston in the cylinder.

4. A method as recited in claim 2 wherein the barrier divides the container into a first compartment and a second compartment and moving the barrier comprises displacing the barrier by pumping fluid into the first compartment.

5. A method as recited in claim 1 further comprising storing a predetermined curve of fluid flow rate versus a value related to orifice size.

6. A method as recited in claim 5 wherein opening a variable orifice to dispense a known volume of fluid comprises selecting a value related to orifice size from the stored curve.

7. A method as recited in claim 5 wherein selecting a value related to orifice size comprises interpolating between points on the stored curve.

8. A method as recited in claim 6 wherein adjusting the size of the variable orifice comprises using the stored curve to select a value related to orifice size that corresponds to the difference between the computed average flow rate the desired flow rate.

9. A method as recited in claim 1 further comprising storing a function that equates fluid flow rate to a value related to orifice size.

10. A method as recited in claim 9 wherein opening a variable orifice to dispense a known volume of fluid comprises selecting a value related to orifice size using the stored function.

11. A method as recited in claim 9 wherein adjusting the size of the variable orifice comprises using the stored function to select a value related to orifice size that corresponds to the difference between the computed average flow rate the desired flow rate.

12. A method as recited in claim 1 further comprising computing an expected value of the time period required to dispense the known volume of fluid and increasing the size of the variable orifice if the time elapsed from the opening of the orifice exceeds the expected value by a pre-selected margin.

13. A method as recited in claim 1 further comprising computing an expected value of the time period required to dispense the known volume of fluid and opening the variable orifice fully if the time elapsed from the initial opening of the orifice exceeds the expected value by a pre-selected margin.

14. A method as recited in claim 1 further comprising:

computing an expected value of the time period required to dispense the known volume of fluid;

increasing the size of the variable orifice if the time elapsed from the opening of the orifice exceeds the expected value by a first pre-selected margin; and

,

opening the variable orifice fully if the time elapsed from the initial opening of the orifice exceeds the expected value by a second pre-selected margin.

15. A method as recited in claim 14 wherein the orifice is a variable valve having a first position which allows fluid flow in a first direction and a second position which allows fluid flow in a second direction and opening the variable orifice fully comprises sequentially opening the valve fully in the first direction and opening the valve fully in the second direction.

16. A method for dispensing a liquid phase chemical from a pressurized supply into a well injection line comprising:

inputting a certain desired fluid flow rate;

opening a variable orifice to dispense a known volume of fluid;

timing the period required to dispense the known volume of fluid;

periodically sampling and recording the differential pressure between the pressurized supply and the injection line during the period required to dispense the known volume of fluid;

computing the average fluid flow rate during the timed period;

comparing the computed average fluid flow rate to the desired flow rate;

adjusting the size of the variable orifice to minimize the difference between the computed flow rate and the desired flow rate.

17. A method as recited in claim 16 wherein adjusting the size of the variable orifice is a function, at least in part, of any change in differential pressure.

18. A method as recited in claim 16 further comprising computing a running average of the sampled differential pressures.

19. A method as recited in claim 17 wherein differential pressures sampled later in the period required to dispense the known volume of fluid are preferentially weighted by the function.

20. A method as recited in claim 16 wherein sampling and recording the differential pressure between the pressurized supply and the injection line during the period required to dispense the known volume of fluid comprises computing and storing a running average of differential pressures in a memory device and storing a plurality of differential pressures in a FIFO stack in a memory device.

21. A method for injecting a liquid phase chemical into a well comprising:

inputting a certain desired fluid flow rate;

computing the time required to dispense a known volume of fluid at the desired fluid flow rate;

opening a variable orifice sufficiently to dispense the known volume of fluid in less than the time computed;

timing the period required to dispense the known volume of fluid;

waiting the computed time required to dispense a known volume of fluid at the desired fluid flow rate before dispensing additional fluid; and

,

adjusting the size of the variable orifice to ensure that the time required to dispense the known volume is less than the computed time required to dispense the known volume at the desired flow rate.

22. A method as recited in claim 21 further comprising:

periodically sampling and recording the differential pressure between a pressurized supply of liquid phase chemical and a well injection line during the period required to dispense the known volume of fluid; and

,

adjusting the size of the variable orifice based on changes in the differential pressure to ensure that the time required to dispense the known volume is less than the computed time required to dispense the known volume at the desired flow rate.

23. A method for injecting a liquid phase chemical into a well comprising:

inputting a certain desired fluid flow rate;

selecting a variable orifice position from data relating orifice position to flow rate;

opening a variable orifice to the selected position to dispense a known volume of fluid by moving a barrier in a fluid container a known distance;

sensing movement of the barrier;

timing the period required to dispense the known volume of fluid;

computing the average fluid flow rate during the timed period;

comparing the computed average fluid flow rate to the desired flow rate;

adjusting the size of the variable orifice to minimize the difference between the computed flow rate and the desired flow rate.

24. A method as recited in claim 23 wherein the fluid container comprises a cylinder and the fluid barrier comprises a free piston in the cylinder.

25. A method as recited in claim 23 wherein the barrier divides the container into a first compartment and a second compartment and moving the barrier comprises displacing the barrier by pumping fluid into the first compartment.

26. A method as recited in claim 23 further comprising opening the variable orifice an additional amount if barrier movement is not sensed within a pre-selected time interval.

27. A method as recited in claim 26 wherein the pre-selected time interval is a function of the inputted desired fluid flow rate.