Rod pumping unit and method of operation

A controller for operating a prime mover of a rod pumping unit includes a processor configured to operate the prime mover over a first stroke and a second stroke. The controller is further configured to compute a first motor torque imbalance value for the first stroke and engage adjustment of a counter-balance. The controller is further configured to estimate a second motor torque imbalance value for the second stroke. The controller is further configured to disengage adjustment of the counter-balance during the second stroke upon the second motor torque imbalance value reaching a first imbalance range.

BACKGROUND

The field of the disclosure relates generally to rod pumping units and, more particularly, to a rod pumping unit controller and method of operation for controlling a counter-balance during operation of the rod pumping unit.

Most known rod pumping units (also known as surface pumping units) are used in wells to induce fluid flow, for example oil and water. Examples of rod pumping units include, for example, and without limitation, linear pumping units and beam pumping units. Rod pumping units convert rotating motion from a prime mover, e.g., an engine or an electric motor, into reciprocating motion above the well head. This motion is in turn used to drive a reciprocating downhole pump via connection through a sucker rod string. The sucker rod string, which can extend miles in length, transmits the reciprocating motion from the well head at the surface to a subterranean piston, or plunger, and valves in a fluid bearing zone of the well. The reciprocating motion of the piston valves induces the fluid to flow up the length of the sucker rod string to the well head.

Typically, known rod pumping units impart continually varying motion on the sucker rod string. The sucker rod string responds to the varying load conditions from the surface unit, down-hole pump, and surrounding environment by altering its own motion statically and dynamically. The sucker rod string stretches and retracts as it builds the force necessary to move the down-hole pump and fluid. The rod pumping unit, breaking away from the effects of friction and overcoming fluidic resistance and inertia, tends to generate counter-reactive interaction force to the sucker rod string exciting the dynamic modes of the sucker rod string, which causes an oscillatory response. Traveling stress waves from multiple sources interfere with each other along the sucker rod string (some constructively, others destructively) as they traverse its length and reflect load variations back to the rod pumping unit. The resulting variable load on the rod pumping unit introduces inefficiencies in operating the rod pumping unit. For example, and without limitation, a variable load may introduce a torque imbalance on the prime mover, where a difference in peak torque values during an upstroke and a downstroke is non-zero. Such a torque imbalance, also referred to as a motor torque imbalance, is conventionally mitigated by a counter-balance.

BRIEF DESCRIPTION

In one aspect, a controller for operating a prime mover of a rod pumping unit is provided. The controller includes a processor configured to operate the prime mover over a first stroke and a second stroke. The controller is further configured to compute a first motor torque imbalance value for the first stroke and engage adjustment of a counter-balance. The controller is further configured to estimate a second motor torque imbalance value for the second stroke. The controller is further configured to disengage adjustment of the counter-balance during the second stroke upon the second motor torque imbalance value reaching a first imbalance range.

In another aspect, a method of operating a rod pumping unit is provided. The method includes operating a prime mover of the rod pumping unit over a first stroke and a second stroke. The method further includes computing a first motor torque imbalance value for the first stroke and engaging adjustment of a counter-balance. The method further includes estimating a second motor torque imbalance value for the second stroke. The method further includes disengaging adjustment of the counter-balance during the second stroke upon the second motor torque imbalance value reaching a first imbalance range.

In yet another aspect, a rod pumping unit is provided. The rod pumping unit includes a prime mover coupled to a ram within a pressure vessel. The rod pumping unit further includes a compressor, a bleed valve, and a rod pumping unit controller. The compressor and bleed valve are coupled to the pressure vessel. The compressor is configured to increase a pressure in the pressure vessel when the compressor is engaged. The bleed valve is configured to decrease the pressure in the pressure vessel when the bleed valve is engaged. The rod pumping unit controller is coupled to the compressor and the bleed valve, and is configured to operate the prime mover over a first stroke and a second stroke. The rod pumping unit controller is further configured to compute a first motor torque imbalance value for the first stroke and engage one of the compressor and the bleed valve to adjust a counter-balance. The rod pumping unit controller is further configured to estimate a second motor torque imbalance value for the second stroke. The rod pumping unit controller is further configured to disengage the compressor and the bleed valve during the second stroke upon the second motor torque imbalance value reaching a first imbalance range.

DETAILED DESCRIPTION

In the following specification and the claims, a number of terms are referenced that have the following meanings.

Further, as used herein, the terms “software” and “firmware” are interchangeable, and include any computer program stored in memory for execution by personal computers, workstations, clients and servers.

Embodiments of the present disclosure relate to a controller for a rod pumping unit. The controllers described herein, within a rod pumping unit stroke, estimate torque imbalance on the prime mover for that stroke based on measured torque imbalance for a previous stroke. The controllers use the estimated torque imbalance to engage or disengage an adjustment to a counter-balance in real-time within the stroke. Real-time engagement and disengagement of adjustments to the counter-balance facilitate the controllers operating the rod pumping unit such that torque imbalance on the prime mover efficiently converges to a desired range.

FIGS. 1 and 2are cross-sectional views of an exemplary rod pumping unit100in fully retracted (1) and fully extended (2) positions, respectively. In the exemplary embodiment, rod pumping unit100(also known as a linear pumping unit) is a vertically oriented rod pumping unit having a linear motion vertical vector situated adjacent to a well head102. Rod pumping unit100is configured to transfer vertical linear motion into a subterranean well (not shown) through a sucker rod string (not shown) for inducing the flow of a fluid. Rod pumping unit100includes a pressure vessel104coupled to a mounting base structure106. In some embodiments, mounting base structure106is anchored to a stable foundation situated adjacent to the fluid-producing subterranean well. Pressure vessel104includes a cylindrical or other appropriately shaped shell body108constructed of, for example, and without limitation, rolled steel plate, and further includes cast or machined end flanges110. Attached to the end flanges110are upper and lower pressure heads112and114, respectively.

Penetrating upper and lower pressure vessel heads112and114, respectively, is a linear actuator assembly116that includes a vertically oriented threaded screw118(also known as a roller screw), a planetary roller nut120(also known as a roller screw nut assembly), a forcer ram122in a forcer ram tube124, and a guide tube126. Pressure vessel104is coupled to a compressor148that compresses a fluid within pressure vessel104to build or increase a pressure that acts on forcer ram122as a counter-balance force. Pressure vessel104is further coupled to a bleed valve150that releases the fluid from pressure vessel104to relieve or decrease the pressure acting on forcer ram122, thereby reducing the counter-balance force. The fluid in pressure vessel104may include, for example, and without limitation, air.

Roller screw118is mounted to an interior surface128of lower pressure vessel head114and extends up to upper pressure vessel head112. The shaft extension of roller screw118continues below lower pressure vessel head114to connect with a compression coupling (not shown) of a motor130, i.e., the prime mover. Motor130is coupled to a variable speed drive (VSD)131configured such that the motor's130rotating speed may be adjusted continuously. VSD131also reverses the motor's130direction of rotation so that its range of torque and speed may be effectively doubled. Roller screw118is operated in the clockwise direction for the upstroke and the counterclockwise direction for the downstroke. Motor130is in communication with a rod pumping unit controller132. In the exemplary embodiment, pumping unit controller132transmits commands to motor130and VSD131to control the speed, direction, and torque of roller screw118.

Within pressure vessel104, the threaded portion of roller screw118is interfaced with planetary roller screw nut assembly120. Nut assembly120is fixedly attached to the lower segment of forcer ram122such that as roller screw118rotates in the clockwise direction, forcer ram122moves upward. Upon counterclockwise rotation of roller screw118, forcer ram122moves downward. This is shown generally inFIGS. 1 and 2. Guide tube126is situated coaxially surrounding forcer tube124and statically mounted to lower pressure head114. Guide tube126extends upward through shell body108to slide into upper pressure vessel head112.

An upper ram134and a wireline drum assembly136and fixedly coupled and sealed to the upper end of forcer ram122. Wireline drum assembly136includes an axle138that passes laterally through the top section of the upper ram134. A wireline140passes over wireline drum assembly136resting in grooves machined into the outside diameter of wireline drum assembly136. Wireline140is coupled to anchors142on the mounting base structure106at the side of pressure vessel104opposite of well head102. At the well head side of pressure vessel104, wireline140is coupled to a carrier bar144which is in turn coupled to a polished rod146extending from well head102.

Rod pumping unit100transmits linear force and motion through planetary roller screw nut assembly120. Motor130is coupled to the rotating element of planetary roller screw nut assembly120. By rotation in either the clockwise or counterclockwise direction, motor130may affect translatory movement of planetary roller nut120(and by connection, of forcer ram122) along the length of roller screw118.

FIG. 3is a force diagram for rod pumping unit100(shown inFIGS. 1 and 2). For clarity,FIG. 3depicts wireline drum assembly136, wireline140, polished rod146, pressure vessel104, and forcer ram122. When motor130drives forcer ram122upward, the load, Fscrew, on roller screw118includes the weight of wireline drum assembly136, Fassy, as well as the weight of the sucker rod string (not shown) suspended from polished rod146. The weight of the sucker rod string and the fluid is also referred to as the well load, Fwell, and acts doubly on roller screw118, because wireline140is attached at anchors142, providing a tension in wireline140equal and opposite the well load, Fwell. The load, Fscrew, on roller screw118also includes an inertial component for wireline drum assembly136. The load, Fscrew, on roller screw118is reduced by a counter-balance force, Fcbal. Counter-balance force, Fcbal, is a function of a surface area, A, of forcer ram122and the pressure in pressure vessel104. Counter-balance force, Fcbal, produces a counter-balance, or a counter-balance effect, for rod pumping unit100. For a downstroke, roller screw118acts against the counter-balance force, Fcbal. The load, Fscrew, on roller screw118is the sum of these forces and is represented by the following equation:
Fscrew(x)=2·Fwell(x)+massy·g+massy·{umlaut over (x)}−Fcbal(x),  Eq. (1)
where,massyis the mass of wireline drum assembly136,g is the acceleration of gravity,{umlaut over (x)} is the acceleration of wireline drum assembly136,massy·g represents the force, Fassy, produced by the weight of wireline drum assembly136, andmassy·{umlaut over (x)} represents the force produced by the inertia of wireline drum assembly136.

The well load, Fwell, varies over the course of a pump stroke due to various factors, including for example, and without limitation, well conditions and pump speed. The load variation contributes to the occurrence of force imbalance on roller screw118and the prime mover, which is motor130in rod pumping unit100. Force imbalance on roller screw118manifests as torque imbalance. The relationship between motor torque, Tmotor, and Fscrewis represented by the following equation:

Tmotor⁡(x)=Fscrew⁡(x)·γ2·π·η+Iscrew⁢α,Eq.⁢(2)
where,Fscrew(x) is the load on roller screw118as a function of stroke position, x,γ is the pitch of roller screw118,η is the efficiency of roller screw118,Iscrewrepresents the inertia of roller screw118, andα represents the angular acceleration of roller screw118.

Motor torque imbalance is defined as a difference in absolute values of peak torque values during an upstroke and a downstroke as a percentage of the maximum of the two, i.e., a greater value of the two. Rod pumping unit100operates most efficiently when the motor torque imbalance value is zero. In certain embodiments, a desired range of motor torque imbalance is defined around zero and, further, an acceptable range of motor torque imbalance may be defined around the desired range of motor torque imbalance. Motor torque imbalance is desirably maintained within the desired imbalance range, however, if motor torque imbalance increases in magnitude beyond the desired imbalance range, but still within the acceptable imbalance range, corrections are not necessary. If motor torque imbalance increases in magnitude beyond the acceptable imbalance range, corrections are made to bring the motor torque imbalance back within the desired imbalance range. In one embodiment, for example, and without limitation, the desired range of motor torque imbalance values is defined inclusively as −5% to 5%, and the acceptable range of motor torque imbalance values is defined inclusively as −10% to 10%. If motor torque imbalance is measured to be 7%, no corrections are made. If the motor torque imbalance is measured to be 12%, corrections are made to bring the motor torque imbalance within the −5% to 5% range. Motor torque imbalance for a single pump stroke is generally determined after the pump stroke is complete and peak torque values are measured and known. Motor torque imbalance is defined by the following equation.

Imbalance=Tpeak,up-Tpeak,downmax⁡(Tpeak,up,Tpeak,down)·100,Eq.⁢(3)
where, Tpeak,upand Tpeak,downare peak motor torques for the upstroke and the downstroke.

Given a variable well load, Fwell, the motor torque imbalance also varies over time and over one or more pump strokes. For example, the fluid in the system, such as air, may leak over time, contributing to an imbalanced system. Accordingly, the counter-balance effect of the counter-balance force, Fcbal, varies and is adjustable to control motor torque imbalance. The counter-balance in a linear pumping unit, such as rod pumping unit100, is adjustable by engaging compressor148or bleed valve150to increase or decrease the quantity of the fluid in pressure vessel104, affecting the pressure accordingly. Conventionally, when a motor torque imbalance outside an acceptable range is identified after a pump stroke is complete, an adjustment to the counter-balance is engaged and the motor torque imbalance is determined again after the next pump stroke. If the new motor torque imbalance is still outside a desired range, the adjustment remains engaged for another pump stroke. Otherwise, the adjustment is disengaged until another motor torque imbalance outside the acceptable range is identified after a subsequent pump stroke. Controlling adjustment of the counter-balance after motor torque imbalance is computed at the end of a stroke results in sub-optimal convergence on the desired imbalance range due to over-adjusting the counter-balance.

In rod pumping unit100, two imbalance conditions are possible: an under-balance and an over-balance. In an under-balance condition, where the motor torque imbalance is positive, the counter-balance force, Fcbal, is low and should be increased to converge the motor torque imbalance on zero. In an over-balance condition, where the motor torque imbalance is negative, the counter-balance force, Fcbal, is high and should be decreased to converge the motor torque imbalance on zero.

In alternative embodiments, such as a beam pumping unit, for example, a counter-balance mass may be shifted. In another alternative embodiment, such as an air-balanced beam pumping unit, for example, a similar configuration of pressure vessel104, compressor148, and bleed valve150is used as a counter-balance. Referring again to rod pumping unit100, the counter-balance force, Fcbal(x), is defined by the following equation:
Fcbal(x)=P(x)·A,Eq. (4)
where,A is the surface area of forcer ram122,Fcbal(x) is the counter-balance force as a function of stroke position, x, andP (x) is the pressure inside pressure vessel104as a function of stroke position,x, which is generally measurable or estimated in real-time.

FIG. 4is a block diagram of a control system400for use with rod pumping unit100(shown inFIGS. 1 and 2). Control system400includes a controller410that operates motor130and includes a processor420. Control system400further includes a position sensor430configured to measure stroke position, x, for rod pumping unit100, and generate and transmit a position signal432to controller410. In certain embodiments, position sensor430includes, for example, and without limitation, a linear transducer. In alternative embodiments, position sensor430includes, for example, and without limitation, an encoder on the prime mover, i.e., motor130. In certain embodiments, position is estimated based on RPMs of motor130. Control system400further includes a current sensor440configured to measure current supplied to motor130. In alternative embodiments, torque is measured by a torque sensor or any other suitable measurement for estimating torque. The current supplied to motor130is directly related to motor torque, Tmotor, which is further related to the load on roller screw118, Fscrew. Current sensor440is further configured to generate and transmit a load signal442to controller410. Control system400further includes a pressure sensor450configured to measure pressure, P, inside pressure vessel104. Pressure sensor450is further configured to generate and transmit a pressure signal452to controller410.

Control system400further includes a bleed valve460coupled to pressure vessel104. Bleed valve460is controlled by controller410using a valve control signal462transmitted to a valve controller470for bleed valve460. When bleed valve460is engaged by controller410, bleed valve460opens and decreases the fluid within pressure vessel104. Control system400further includes a compressor480coupled to pressure vessel104. Compressor480is controlled by controller410using a compressor control signal482transmitted to a compressor controller490for compressor480. When compressor480is engaged by controller410, compressor480increases the fluid within pressure vessel104. When compressor480and bleed valve460are disengaged, the amount of fluid in pressure vessel104is maintained. In certain embodiments, the fluid within pressure vessel104changes over time even when compressor480and bleed valve460are disengaged. Typically, the fluid changes slowly. In such embodiments, controller410is configured to assume the amount of fluid remains constant from one stroke to the next when compressor480and bleed valve460are disengaged. If the fluid changes substantially within a stroke or other short period of time, such a change could induce errors in computations.

The pressure, P, within pressure vessel104changes as a function of stroke position, because the volume of pressure vessel104changes as forcer ram122translates on each upstroke and each downstroke. Controller410is configured to treat the compression of the fluid in pressure vessel104as a polytropic process, which is described by the following equation:
P(x)·V(x)n=C,Eq. (5)
where,P(x) is the pressure within pressure vessel104as a function of stroke position, x,V(x) is the volume of pressure vessel104as a function of stroke position, x,n is a polytropic index, andC is a constant for the compression of a fixed quantity of fluid.

Controller410is configured to model volume, V(x), based on known physical dimensions of pressure vessel104and stroke position, x. The polytropic index, n, is generally constant. Controller410, in certain embodiments, is configured to estimate polytropic index, n, when neither of compressor480and bleed valve460are engaged, i.e., when the amount of fluid in pressure vessel104is constant. When compressor480or bleed valve460are engaged, controller410is configured to use a last-estimated value for polytropic index, n. Polytropic index, n, is estimated using a recursive least square estimator, or any other suitable estimator, including, for example, and without limitation, a Kalman filter, with a forgetting factor based on the equation below:
log(P(x))=−n·log(V(x))+log(C),  Eq. (6)

a0, a1, a2, etc. are estimated using the recursive least square estimator or other suitable estimator,

a0varies with the amount of fluid, and

During operation of rod pumping unit100, controller410is configured to receive position signal432, load signal442, and pressure signal452. During a first stroke, controller410computes a first motor torque imbalance using load signal442and Eq. 3. The first motor torque imbalance is a function of a peak motor torque for the upstroke, TU1, and a peak motor torque for the downstroke, TD1, which are computed using Eq. 1 and Eq. 2. When the first motor torque imbalance is outside an acceptable imbalance range, adjustment of a counter-balance is engaged. In an under-balance condition, controller410engages compressor480by transmitting compressor control signal482to compressor controller490. Compressor480increases the fluid in pressure vessel104and increases pressure, P. In an over-balance condition, controller410engages bleed valve460by transmitting valve control signal462to valve controller470. Bleed valve460decreases the fluid in pressure vessel104and decreases pressure, P.

Controller410is configured to determine stroke positions at which peak motor torques, TU1and TD1, occur during the first stroke. Peak motor torque TU1occurs at peak motor torque stroke position XU1. Peak motor torque TD1occurs at peak motor torque stroke position XD1. Controller410is further configured to determine peak pressures at positions XU1and XD1, referred to as P(XU1) and P(XD1). Controller410is configured to use peak motor torque stroke positions for the first stroke as estimated peak motor torque stroke positions during the following stroke. Actual peak motor torque values and actual peak motor torque stroke positions are determinable for a given stroke once the stroke is complete.

During a second stroke, which may immediately follow the first stroke, or may be one or more strokes later, controller410is configured to estimate a second motor torque imbalance for the second stroke. To estimate the second motor torque imbalance, controller410is configured to measure a counter-balance component at a current stroke position based on pressure signal452. In rod pumping unit100, the measured counter-balance component is pressure, P. Controller410is configured to then use the counter-balance component at the current stroke position to estimate a counter-balance force at peak motor torque stroke positions in the second stroke. Based on the polytropic compression described in Eq. 5 and peak motor torque stroke positions XU1and XD1, pressures in pressure vessel104are estimated at peak motor torque stroke positions XU1and XD1for the second stroke. The estimated pressures, P(XU1) and P(XD1), which are used as surrogate estimates for P(XU2) and P(XD2), are determined using the following equivalencies based on Eq. 5:
P(x)·V(x)n=P(XU1)·V(XU1)nEq. (8)
P(x)·V(x)n=P(XD1)·V(XD1)nEq. (9)

In certain embodiments, such as those using the polynomial relationship described in Eq. 7, pressures are estimated according to the following equation:
P(XD1)=(P(x)−a1x−a2x2)+a1XD1+a2XD12Eq. (10)

The estimated pressures, P(XU1) and P(XD1), are then used to estimate peak motor torques, TU2and TD2, for the second stroke using Eq. 1, Eq. 2, and Eq. 4, as shown below, collectively referred to as Eq. 11, where Fcbalvaries between strokes and other terms are assumed to remain constant. For TU2:

Likewise, the computations, collectively referred to as Eq. 11, are repeated for TD2.

The estimated peak motor torques, TU2and TD2, are then used to estimate a second motor torque imbalance for the second stroke using Eq. 3, in real-time during the second stroke.

When the estimated second motor torque imbalance, during the second stroke, is in a desired imbalance range, adjustment of the counter-balance is disengaged by disengaging both bleed valve460and compressor480. If motor torque imbalance goes outside the acceptable imbalance range again, adjustment of the counter-balance is engaged until motor torque imbalance is back inside the desired imbalance range.

FIG. 5is a flow diagram of an exemplary method500of operating controller410(shown inFIG. 4). Referring toFIGS. 4 and 5, the method begins at a start step510. At an operating step520, controller410operates the prime mover of rod pumping unit100, i.e., motor130, over multiple pump strokes, including a first stroke and a second stroke. When the first stroke is complete, controller410is configured to compute a first motor torque imbalance for the first stroke at a computing imbalance step530. The first motor torque imbalance is computed based on a load signal442from a sensor, such as current sensor440. Controller410uses load signal442to identify peak torque values, TU1and TD1, for the upstroke and downstroke of the first stroke, and then uses the peak torque values to compute the first motor torque imbalance based on Eq. 3.

When the first motor torque imbalance indicates an imbalance outside an acceptable imbalance range, controller410engages adjustment of a counter-balance at an engaging adjustment step540. Engaging adjustment includes engaging compressor480or bleed valve460to increase or decrease the fluid in pressure vessel104, thus increasing or decreasing the pressure that contributes to the counter-balance force. Compressor480is engaged by transmitting compressor control signal482to compressor controller490. Bleed valve460is engaged by transmitting valve control signal462to valve controller470.

During the second stroke, stroke position and pressure are measured using position sensor430and pressure sensor450. At an estimating imbalance step550, controller410estimates a second motor torque imbalance for the second stroke. Controller410uses a current pressure and a current stroke position, during the second stroke, to estimate pressures, P(XU1) and P(XD1), based on Eq. 5. The estimated pressures, P(XU1) and P(XD1), are then used to estimate peak motor torques, TU2and TD2, for the second stroke using Eq. 1, Eq. 2, and Eq. 4. The estimated peak motor torques, TU2and TD2, are then used to estimate the second motor torque imbalance for the second stroke using Eq. 3, in real-time during the second stroke.

When the second motor torque imbalance, during the second stroke, is in a desired imbalance range, adjustment of the counter-balance is disengaged at a disengaging adjustment step560by disengaging both bleed valve460and compressor480. If motor torque imbalance goes outside the acceptable imbalance range again, adjustment of the counter-balance is engaged until motor torque imbalance is back inside the desired imbalance range. Method500ends at an end step570.

The above described controllers for rod pumping units, within a rod pumping unit stroke, estimate torque imbalance on the prime mover for that stroke based on measured torque imbalance for a previous stroke. The controllers use the estimated torque imbalance to engage or disengage an adjustment to a counter-balance in real-time within the stroke. Real-time engagement and disengagement of adjustments to the counter-balance facilitate the controllers operating the rod pumping unit such that torque imbalance on the prime mover efficiently converges to a desired range.

An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) estimating torque imbalance on the prime mover for a stroke within that stroke, (b) engaging and disengaging of counter-balance adjustments in real-time based on estimated torque imbalance, (c) reducing under-shoot and over-shoot of counter-balance force, (d) improving torque imbalance convergence, and (e) improving operating efficiency of rod pumping units due to improved torque imbalance convergence.

Exemplary embodiments of methods, systems, and apparatus for rod pumping unit controllers are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other non-conventional rod pumping unit controllers, and are not limited to practice with only the systems and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications, equipment, and systems that may benefit from reduced cost, reduced complexity, commercial availability, improved reliability at high temperatures, and increased memory capacity.