Hydraulic oil well pumping system, and method for pumping hydrocarbon fluids from a wellbore

A method of pumping oil from a wellbore, the wellbore having a bore extending into an earth surface, and the method comprising:

• providing an elongated hydraulic cylinder over the bore, with the hydraulic cylinder being part of a well head;
• providing a piston that is movable between upper and lower rod positions within the hydraulic cylinder;
• mechanically connecting the piston to a rod string such that the rod string extends downwardly from the piston and the well head and into the bore;
• providing a hydraulic pump that is powered by a prime mover;
• connecting the hydraulic pump and the hydraulic cylinder with an oil line that transmits hydraulic fluid from the pump to the cylinder;
• providing a directional control valve that moves between upstroke and neutral flow conditions;
• providing a fluid reservoir for containing hydraulic fluid to be supplied to the pump;
• providing a vent line configured to return any hydraulic fluid that leaks past the piston back to the fluid reservoir or the oil line;
• providing a downstroke control valve configured to choke a flow of fluid from the cylinder back to the fluid reservoir to limit a rate of flow of hydraulic fluid when the directional control valve is in its neutral flow condition;
• providing a pressure sensor in pressure communication with the oil line and configured to transmit differential pressure signals;
• using an electronic control system, controlling movement of the piston as it moves between the upper and lower rod positions by cycling the directional control valve between (i) its “upstroke” condition wherein the pump is pumping hydraulic fluid through the directional control valve, through the oil line and into the hydraulic cylinder to move the piston to its upper rod position, and (ii) its “neutral” condition wherein the pump is no longer pumping fluid into the hydraulic cylinder, but is allowing hydraulic fluid to flow back through the oil line in response to gravitational fall of the rod string and connected piston, wherein the piston moves to its lower rod position;
• providing a pressure cut-off switch, wherein the hydraulic pump is turned off by the pressure cut-off switch when a pressure reading along the oil line during a piston upstroke exceeds a designated limit;
• reciprocating the piston and mechanically connected rod string in order to pump fluid from the wellbore;
• wherein the electronic control system is programmed to:
• receive the differential pressure signals, and calculate flow rates of the hydraulic fluid through the oil line based upon the differential pressure signals, and convert the calculated flow rates into volumetric measurements to determine a location of the piston in the hydraulic cylinder in real time, and without reference to position sensors along the wellhead;
• change a position of the directional control valve in order to cycle movement of the piston between upstrokes and downstrokes;
• adjust a position of the downstroke control valve to control a rate of descent of the rod string and connected piston when the directional control valve is in its neutral condition in response to determining the location of the piston within the cylinder;
• reset the volumetric measurements to “0” when a pressure signal in the oil line has reached a value of substantially 0, indicating the piston is at a mechanical bottom of the down stroke;
• and
• change an operating speed of the prime mover in order to control a rate of ascent of the piston within the cylinder when the directional control valve is in its upstroke condition in response to changes in load on the rod string;
• and wherein the hydraulic pumping system is pre-calibrated to identify a top-of-stroke position, and various data samples correlating differential pressures with the top-of-stroke piston position are loaded into the electronic control system.