Theta Newsletter - June 1999
How to use RODDIAG and XDIAG to estimate pump friction for RODSTAR
RODSTAR 2.34 allows you to enter a pump friction. This number is used to calculate the resistive force on the plunger on the downstroke. The pump friction can cause buckling of the rods above if a sufficient number of sinker bars are not included to counteract this resistance.
Pump friction is a function of many factors including pump plunger speed, fluid viscosity, plunger clearance, plunger length, and plunger configuration. Because it is difficult to get an accurate number for pump friction, most people use RODSTAR’s default value of 200 lbs.
However, there is a way that pump friction can be estimated from a downhole pump card calculated by a diagnostic program such as RODDIAG or XDIAG.
The Effect of Rod-Tubing Friction
When a wave equation diagnostic program (RODDIAG or XDIAG) calculates the downhole pump dynamometer card, it uses the rod-tubing friction coefficient to remove rod-tubing frictional forces from the surface card as it calculates the downhole pump card.
If you enter a rod-tubing friction coefficient that exactly represents the actual friction between rods and tubing, you will get a downhole pump card that only shows how much fluid load is being picked up by the plunger (the distance between the top and bottom of the card). If you enter a smaller value for rod-tubing friction than is actually there, you will get a downhole pump card that is taller than it should be. This is because the program did not remove all the frictional forces from the surface dynamometer card when it calculated the downhole pump card from it.
If you enter a rod-tubing friction coefficient that is too high, then the diagnostic software will remove not only all the friction, but also part of the true fluid load from the downhole pump card. This will distort the downhole pump card shape and should be avoided.
To be able to accurately estimate the pump friction, you must enter the correct rod-tubing friction coefficient for the system you are analyzing. This requires some expertise. However, if the correct rod-tubing coefficient is used, and you select not to include buoyancy effects then the software will calculate the pump friction accurately by calculating how much of the downhole pump card is negative. If everything else is correct, then the negative load on the pump card is only due to pump friction.
How to determine the correct rod-tubing friction coefficient
To determine the correct rod-tubing coefficient in RODDIAG or XDIAG, you need to make several runs. You may want to start by using the default rod-tubing friction. This is usually less than the actual friction. Then take a look at the shape of the downhole pump card. If the downhole pump card has a rounded shape or if it has a thick "nose", then this indicates that the rod-tubing friction coefficient you entered is too low. An example of this is shown in Figure 1.
Figure 1 -- Not enough rod-tubing friction
When the correct rod-tubing friction is used, then the downhole pump card shape is more square-looking and the nose becomes almost a line. An example of this is shown in Figure 2.
Figure 2 -- Correct rod-tubing friction
If you use too much friction then the card will have a concave look and the nose part of the card will show a "crossover" of the upstroke and downstroke lines. An example of this is shown in Figure 3.
Figure 3 -- Too much rod-tubing friction