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

摘要

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.

主权项

1. 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; wherein adjusting the size of the variable orifice is a function, at least in part, of any change in differential pressure.