Method for controlling an artificial lifting system and an artificial lifting system employing same

A method for lifting downhole fluid from a reciprocating downhole fluid lifting device to surface, comprising:

• setting up a first and a second target stop position;
• reciprocating a movable component of a linear actuator between said first and second target stop positions for driving the downhole fluid lifting device;
• determining a first actual stop position corresponding to said first target stop position and a second actual stop position corresponding to said second target stop position;
• determining a first drift being the difference between the first actual stop position and the first target stop position, and a second drift being the difference between the second actual stop position and the second target stop position;
• and
• automatically adjusting the reciprocating of the movable component to minimize for the first and second drifts;
• wherein the method further comprises an initialization process, comprising:
• determining an initial first stop position and an initial second stop position about the mid-point of the target top and bottom stop positions, the distance between the initial first stop position and the initial second stop position is a predefined percentage of the distance between the first and second target stop positions;
• moving the movable component to one of the initial first and second stop positions to reciprocate the movable component for n reciprocating cycle(s), wherein n≧1, and in each of the n reciprocating cycle(s), said control unit controls said power unit to expand the initial first and second stop positions toward the first and second target stop positions, respectively, by the first expansion step value;
• and
• when the distance between the first and second stop positions and the first and second target stop positions, respectively, is smaller than said first expansion step value, reciprocating the movable component for m reciprocating cycle(s), wherein m≧1, and in each of the m reciprocating cycle(s), said control unit controls said power unit to expand the initial first and second stop positions toward the first and second target stop positions, respectively, by a second expansion step value.

A lifting system for lifting downhole fluid from a downhole rod pump in a wellbore to surface, comprising:

• a linear actuator comprising a movable component moveable between a first and a second limit and driveably coupled to the downhole rod pump;
• a power unit coupled to said linear actuator for driving said movable component to reciprocate;
• the reciprocating of said movable component driving said downhole rod pump to pump downhole fluid to the surface;
• a sensor for detecting the position of said movable component;
• and
• a control unit coupled to said sensor and said power unit for:
• controlling the power unit for reciprocating said movable component between a first target stop position and a second target stop position, for moving said movable component uphole to stop at about said first target stop position, and for moving said movable component downhole to stop at about said second target stop position;
• determining, based on the position information received from said sensor, a first actual stop position and a second actual stop position;
• determining a first drift being the difference between the first actual stop position and the first target stop position, and a second drift being the difference between the second actual stop position and the second target stop position;
• and
• automatically controlling the operation of the power unit to minimize the first and second drifts;
• wherein said control unit controls said power unit to move the movable component towards the first target stop position at a first speed and to move the movable component towards the second target stop position at a second speed;
• and wherein said control unit receives a command from an operator indicating a change of at least one of the first and the second speeds, and in response to said command, initializes the operation of the lifting system by:
• determining an initial first stop position if the first speed is changed, said initial first stop position being intermediate to the first and second target stop positions with a distance to the first target stop position of (1−C1)SN/2, wherein SN is the distance between the first and second target stop positions and C1 is a predefined percentage;
• determining an initial second stop position if the second speed is changed, said initial second stop position being intermediate to the first and second target stop positions with a distance to the second target stop position of (1−C1)SN/2;
• determining at least a first expansion step value;
• determining at least a first number p of reciprocating cycles corresponding to said first expansion step value;
• and
• reciprocating the movable component for p reciprocating cycles, wherein
• in the first cycle of the p reciprocating cycles, said control unit controls said power unit to:
• move the movable component to the initial first stop position if the first speed is changed;
• move the movable component to the initial second stop position if the second speed is changed;
• and
• in the next (p−1) reciprocating cycles, said control unit controls said power unit to:
• expand the initial first stop position toward the first target stop position by the first expansion step value if the first speed is changed;
• and
• expand the initial second stop position toward the second target stop position by the first expansion step value if the second speed is changed.