If you want a concise summary of what a Dynamometer is and how it works, click HERE to go back to the Services Page.
Example Dyno Card Videos
Click on the Screen-Shot image below, scroll left/right through the example Dyno Cards and then click the
"Go to Link" (below the image title when the image is expanded) to see the video of the example Dyno cards.
Example Dyno Survey Reports (Echometer reports related to the Dyno Videos above):
4. Worn Pump
5. Tag on Pump
Page 2 of my Sucker Rod Pumping Short Course (see the Resources page to download).
How a Dynamometer Works:
A Dynamometer is a diagnostic device used on Sucker Rod Pumped Wells that measures the load on the top rod (Polished Rod) and plots this load in relation to the polished rod position as the pumping unit moves through each stroke cycle. The plot of the polished rod Load vs Position is known as the Surface Card.
The load on the polished rod is a summation of every load occurring downhole and is thus a function of: the fluid load on the pump, the rod-string design and the buoyant rod weight, the acceleration forces (which are a function of the SPM x SL, pumping unit geometry and the direction of rotation), the mechanical friction of the rods-on-tubing (and the rods on paraffin, etc), and the viscous frictional forces generated between the rods and fluid. With the Surface Card being affected by so many different factors, there is no "standardized" shape to Surface Cards—which greatly increases the difficulty of interpreting what is happening downhole. For example, compare the shapes of the "flying duck" Surface Card (from an all steel string) at the top of the page with the Surface Card shown in the top left corner of the sheet above (50% Fiberglass rod string).
Although there is some value to be gather from the Surface Card, oil operators are much more interested to know what is happening downhole at the pump. After all, the only reason for having any of the surface or downhole equipment is to effectively actuate the downhole pump so new fluid will be drawn into the tubing and lifted to surface.
Thankfully, a very intelligent man by the name of Dr. Sam Gibbs [Lufkin's well known "Sam POC" was named in his honor] provided a solution to this problem when he developed the Wave Equation. The Wave Equation models the elastic behavior of the rod string in its dynamic motion—which allows the surface measurements acquired from the polished rod to be “waved” downhole to the pump to generate a plot of the Load vs Position on the pump's plunger. This plot is known as the downhole Pump Card. The beauty of the Wave Equation is that it allows surface data to be acquired (which is easily accessible and inexpensive) and then mathematically filters the data to illustrate the downhole pump performance each stroke. By filtering out the effects of the rod-string and everything else occurring above the pump's plunger, the Wave Equation essentially standardizes the various shapes the Pump Card can take and greatly improves the ability of the pump performance to be properly diagnosed. However, interpretation can still be difficult at times due to the simultaneous occurrence of multiples factors affecting the shape of the Pump Card.
The Wave Equation is also employed in Predictive Rod Design Programs, with the best known programs being RODSTAR (by Theta) & SRod (by Lufkin). These programs give the rod designer a great deal of control and versatility in simulating various downhole conditions—allowing for the prediction of mechanical stresses on each rod taper and the expected producing performance of the pumping system. These programs can be quite pricey. For those who do not need to perform many complicated design simulations, it is worth noting that Echometer provides a completely free software that solves the Wave Equation—known as QRod. Compared to other programs, QRod is much more simplified with fewer design controls, but the accuracy of its computations is comparable with the other two programs for vertical wellbores. You can download QRod for free at www.echometer.com.
In addition to acquiring the Surface & Pump Cards over the well's pumping cycle, Dynamometers are also used to quantify the amount of Fluid Slippage leaking by the pump's plunger. When both upper corners of the Pump Card are rounded-off Fluid Slippage is indicated—but the Pump Card in itself does not provide a means of determining how much slippage is occurring. In order to quantify this value a Traveling Valve Test is performed—where the pumping unit is stopped at various points along the up-stroke and the dynamometer records how quickly the fluid load falls off the plunger. Based off the speed of load-loss, the fluid slippage is calculated and displayed in BFPD (barrels of fluid per day). This test is very valuable when trying to determine how worn a pump is and whether or not that pump wear warrants performing a pump change. Similarly, a Standing Valve Test can be performed by stopping the rods during the down-stroke to see if the pump's Standing Valve is leaking. If the Standing Valve is leaking (which allows fluid to drain out of the tubing), the Dynamometer will measure a load increase during the test. As fluid leaks out of the Pump Chamber past the Standing Valve, the Traveling Valve ball will seat and the rods will begin to carry the fluid load—which is the reason the Dynamometer records a load increase during the test.
In the lower-left corner of the page displayed above, common examples of Pump Card shapes are displayed. The ideal card is represented by a rectangle: indicating the fluid load is rapidly picked up and held throughout the duration of the up-stroke, and then it is instantaneously released (onto the tubing) as the plunger begins the down-stroke. Any deviation from this shape indicates inefficiencies and lost pump stroke. As discussed on the above sheet, it is vitally important to understand that the only part of the downhole Stroke Length that contributes to the "pumping action" (or the displacement of new fluids into the tubing) is the Effective Plunger Travel. Effective Plunger Travels only occurs in the center portion of the Pump Card when the Traveling Valve is open and the Standing Valve is closed, or vice versa. This area is highlighted on the Gas Interference sample card (above).
On many wells in the Permian Basin, the bottom portion of the tubing is not anchored (for fear of "planting" the TAC by setting it below fracked sand-producing formations)—but this is the only inefficiency that an oil operator will actually design into the system. This producing inefficiency is deemed to be acceptable when compared with the enormous potential costs (and risks) of a tubing fishing job. Other than this, all the other pumping inefficiencies that reduce the Effective Plunger Travel should be avoided by the proper design of the downhole equipment and the operation of the well.
The main factors affecting the Pump Card are: incomplete pump fillage, gas interference, tubing movement ("tubing breathing"), and excessive fluid slippage (indicating a worn pump). The height of the Pump Card is a function the fluid load on the plunger, and the fluid load is a function of the plunger diameter and the hydrostatic Net Lift. The area of the Pump Card is equivalent to the amount of work being performed by the pump.
Additional information that also can be observed on a Pump Card include: pump tagging (bottom or top), erratic valve action (due to solids/trash or a worn/pitted ball & seat), plunger sticking or excessive friction (due to solids in-between the plunger and barrel), and a few other anomalies.
Two General Categories of Dynamometers—Horseshoe Load Cells vs Quick-Install PRT Strain Gauges
A) Horseshoe Dyno being installed between the Polished Rod Clamp & the Carrier Bar (Bridle).
B) Echometer's PRT Dyno clamped onto the Polished Rod (below the PR-Clamp)
It is important to note the distinguishing features of the two general categories of oilfield dynamometers:
Horseshoe (or Donut) Load Cell Dynamometer: These load cells are placed between the Polished Rod Clamp and the Bridle—which requires the rod string first be stacked out on the wellhead to create separation for the dynamometer to be inserted (as shown in the image where the stack-off "suitcase" is resting on the stuffing box to support the rod load). With the Dyno being installed and the well returned back to pumping, the load cell acts like an ordinary bathroom scale and measures the true load on the polished rod. A Donut Load-Cell operates similarly but it is used for continuous dynamometer acquisition with POC's (Pump Off Controllers) while the Horseshoe is made for temporary installation by a Well Tech. Understandably, a horseshoe dynamometer is more accurate than a PRT because it takes direct measurements of the polished rod load. However, horseshoe dyno's are not used as frequently as the PRT for routine diagnostic analysis due to many issues related with its installation (described below).
PRT (Polished Rod Transducer) Dyno: This is Echometer's quick-install Dynamometer and is the most commonly used. The PRT does not directly measure the Polished Rod Load. Rather it is a clamp-on strain-gauge that is screwed onto the polished rod and it measures the minute changes in diameter of the polished rod along the stroke length. The rods carry the fluid load on the up-stroke but not on the down-stroke. The added fluid load adds stress to the rod string and that axial stress causes a proportional radial diameter change. Using Hooke's Law (which is a relationship between the radial strain caused by an axial stress) the minute changes in polished rod diameter are used to back-calculate the axial load causing the diameter change: i.e. the fluid load. Adding the fluid load to the calculated buoyant rod weight, the Surface Dyno is computed. From this point on, the Wave Equation is used to calculate the Pump Card in the same fashion as the Horseshoe Load Cell. Comparative studies performed by Echometer analyzing the accuracy of the PRT found it is within 2-3% of the loads recorded by the Horseshoe Load-Cell as long as accurate input data is used and there is no bending of the Polished Rod due to vertical misalignment (effects described below).
So....while Load Cell Dyno's are more accurate, their installation process creates other problems. Usually Well Techs will only use the clamp-on PRT Dyno unless specific circumstances warrant more accurate information—at which point the Horseshoe Dyno can be installed.
These reasons illustrate the installation issues associated with the Horseshoe Dyno:
In order to install the Horseshoe, the well must have a good functioning brake. This is needed when stacking the rods out on the wellhead.
It takes much more time to install: A 2nd (stack-out) Polished Rod Clamp must be installed on the Polished Rod (shown in above image), lubricators or any other non-load bearing devices (if present) on top the stuffing box must be removed, & the stuffing box bolts must be tightened to protect the rubber packing before stacking out the rod weight. After recording the data, these steps much be back-tracked to restore the well back to its original condition.
Due to stacking out the rod-string and the other steps involved, the liability risks of injury to the operator and/or well equipment are greatly increased.
The well is turned off during the installation process—which disturbs the well's normal run-time cycles causing the fluid level to build up and thus changes the downhole conditions. This disturbance usually necessitates a Well Tech acquire longer periods of data so the well can re-stabilize (or risk acquiring unrepresentative data). Also, this disturbance to the pumping cycles interferes with the ability to properly calibrate the time-clock to the well's production.
The disadvantages associated with raising the rod-string. The rod-string is raised 3" by the addition of the Horseshoe underneath the polished rod clamp and this is disadvantageous for two reasons:
It changes the pump spacing. This reduces the pump's compression ratio and can aggravate gas interference (making the acquired data look worse than it actually is during normal operation). Or it can hide the fact that the well actually has a 2-3" tag on the pump during normal operation (which will be removed when recording the dyno).
Raising the rods 3" higher equally raises the plunger 3" higher relative to the pump barrel. At the top of the upstroke, this will cause the plunger to move into an unswept area of the barrel—which can be problematic if the pump has been downhole a while and built-up scale deposits exist here.
In contrast, the installation of the PRT Dyno is extremely easy and can even be performed while the well is pumping (which is necessary if there is no working break and unit is counter-weight heavy to where the horse-head always settles up when the unit is turned off).
Now, lets evaluate the downsides of the PRT Dynamometer:
PRT Data Accuracy: The PRT data is not as accurate as the Horseshoe and is (slightly) more qualitative in nature when compared to the Horseshoe. However, the PRT is sufficiently accurate for 95% of most work unless more precise measurements are needed for a particular situation, for example: when performing a scientific analysis on the acquired data or when greater clarity in the numbers are needed for a well that is difficult to accurately diagnose.
Polished Rod Bending: if the polished rod is out of vertical alignment with the wellhead (or hits the horse-head at the top of the up-stroke), bending stresses will result in the polished rod. These stresses will cause additional changes in the rod diameter—which the PRT naturally interprets as being "real" load changes occurring downhole. If the polished rod is out of vertical alignment—the effect will cause the Pump Cards to slightly slant. If the polished rod hits the horse head at the top of the up-stroke—there will be some form of anomalous load change during that side load (a good operator must recognize this while at the well site). The distortions to the pump card are often subtle, but not always. Thankfully, however, they usually do not prevent the accurate diagnosis of the downhole pump performance. Sometimes these distortions can even be removed by repositioning the PRT in relation to the angle of bending.
Load/Temperature Drift: The internal strain-gauge in the PRT is extremely precise in measuring minute changes in diameter but temperature changes on the strain gauge itself cause the gauge to expand or contract. The good gospel news is: the PRT recalibrates itself to zero each stroke (so the temperature change will cause little disturbance...unless your SPM is really really slow and there is a significant, sudden temperature change). The problem with Temperature Drift is more pronounced during Valve Checks or performing counter-balance tests (where the zero recalibration process is not occuring as frequently). Good operator practices can usually minimize this small annoyance.
Downhole Diagnostic has both types of Dynamometers and can employ either one as needed.
Rod Pumping Optimization: a few extracted slides from a training course...
What the Dynamometer Survey Can Answer:
The Pump Card—in conjunction with the Fluid Level Survey—form the backbone of Well Diagnostic Analysis. By determining if liquid resides above the pump & how effectively the pump is filling and displacing that liquid—the well's producing & lifting efficiency can be quantified and analyzed. Based off the finding, the well can be optimized by making immediate operational changes (to the run-time, pump spacing or surface equipment) and downhole equipment changes can be planned for and made the next time the well is pulled.
The information operators are most frequently interested to find from a Dynamometer Analysis include:
Is the pump full of fluid throughout the run time?
If not, at what point during the run cycle does incomplete fillage begin?
During incomplete fillage, does the Pump Card indicate Fluid Pound or Gas Interference? And does the Pump Intake Pressure from the fluid level shot agree with the Pump Card shape and size?
What is the appropriate run-time for the well to get full production without damaging the equipment?
How effectively is the downhole gas separator working?
Is the well tagging bottom? Or does severe gas interference warrant spacing the pump closer to bottom?
How worn is the pump and is it causing the well to lose production?
How much barrels of fluid per day does the Dyno interpret the pump is displacing?
Are any anomalies or anything else unusual indicated (pump sticking, excessive friction in the pump or rods, etc.)?
And are the sucker rods operating within mechanical limitations?
Incomplete Pump Fillage and Destructive Pumping Practices:
No dynamometer discussion would be complete without addressing the 3 most destructive pumping practices: Fluid Pound, Gas Interference, & Pump Tagging. If there is complete liquid fillage in the pump, the rod string is being stroked more than necessary. For example, if the average pump fillage is 50%—the well is stroking twice as much as necessary. Instead of running 80% of the day, it could produce that volume in 40% of the day with a full pump. Let's take a look at all 3 in detail.
Three Destructive Pumping Practices:
Fluid Pound: each impact load imparted to the pump’s plunger causes: the rods to buckle and hit the tubing; high load impacts which beat out the Traveling Valve ball & seat; causes the pump's valve rod to buckle creating side-loading of the plunger onto the barrel; the stresses imparted in the rod drastically increase the Modified Goodman Rod Loading and reduce the rod life; and the impact load eventually transmits all the way up the rod-string and damages the tightly inter-locking teeth in the gearbox.
Gas Interference: While Gas Interference (or "Gas Pound") is in many regards very similar to Fluid Pound, it does not create nearly the same type of impact loads—however, volumetrically: it is just as inefficient. And due to the inefficiency, the additional strokes required are the most damaging to the equipment. Also, gas that enters the pump eventually makes its way up the tubing string and this is the most common cause of stuffing box leaks (due to gas heading the top of the tubing dry). Gas Interference keeps Pumpers on their toes because the well's pump performance is often erratic.
Pump Tagging: Light pump tags do have their place in the oilfield but should be avoided if possible. All too often, a pumper reports to the office that a well has a "light" tag. It might feel "light" at surface, but how light is that tag 2-miles downhole at the pump??? And when not periodically monitored, a "light" tag today often gets a little heavier a couple weeks down the road. Since Fiberglass rod strings absorb much of the tag and do not transmit the shock wave vertically as effectively as steel does, a "light" tag on a FG Rod well might be twice as hard as "light" tag on an all steel rod string. Usually the well is put on tag due to pumping problems related to gas interference but a much better way to resolve the situation is to design the gas interference out (either in the downhole gas separator or handle the gas in the pump's design). Tagging is not only destructive to the pump, rods and tubing, but tagging also reduces the downhole Plunger Travel. A 2" tag is 2-inches of lost downhole stroke. Look at the example Gas Interference Dyno Report at the top of the page: due to the small pumping unit, this well as loosing 7% of its total downhole stroke-length as a result of the tag. Also, pump tagging tends to hammer the pump into the seating nipple (making it more difficult to unseat next pull) and the large impact forces can also turn the previously-set TAC loose (leading to additional inefficiencies).