RODSTAR has the capability to load RODDIAG or XDIAG files. This allows you to do several useful things that include:
To load a RODDIAG or XDIAG file into RODSTAR, select open file to see the Window shown in Figure 1. Then, click on the drop down arrow under "List files of type:" and select RODDIAG or XDIAG file type. This activates a file "filter" that displays only the files of the type you selected. Then, to select the file you want, simply double click on its name.
If all data in the RODIAG file is correct and the load cell was reading correctly, then you should be able to get a fairly close match between the predicted and actual cards. However, in some cases you may have to change the rod-tubing friction, especially if the actual well has more friction than normal due to paraffin or scale, or a crooked hole.
To do this, you go to the Tubing and pump input window in RODSTAR and select to enter your own rod-tubing friction coefficients. To get an idea of what is normal for the average well, make a run first after selecting to have RODSTAR calculate the rod-tubing friction for you and then go back and change it yourself. This way the program will display the default value of the rod-tubing friction coefficient for you. For example, if you see a default value of 0.5, then to double the amount of rod-tubing friction you can enter 1.0 and then run the program to see if this gives you a closer match with the measured dynamometer card. Then you can continue to adjust the rod-tubing friction coefficients for both upstroke and downstroke to try to get as close as possible to the measured card. When you are done, you will have a much better value for rod-tubing friction than letting the program estimate this for you.
If you are an XDIAG user, this is a little easier because XDIAG does this type of matching analysis for you and when you load an XDIAG file into RODSTAR, it has a "custom" value for rod-tubing friction that should get you a lot closer to the measured card. However, you can get even closer by making a few additional runs yourself as described above.
This capability also allows you to detect an out of calibration load cell that is reading too high or too low (this is a common problem especially if you are using pump off controllers).
If the overall match history match for the shape is good but one card is shifted up or down from the other one, this indicates a load calibration error. Actually, this does not happen if you have XDIAG since XDIAG corrects this problem for you.
However, RODDIAG does not and in that case you may see a comparison such as the one shown in Figure 2. This indicates that the load cell is reading too low.
Another clue that the load cell is out of calibration is whether the downhole pump dynamometer card shows too much negative or not enough negative loads. Normally, the downhole pump card should have a zero load that is about 1/3 from the bottom of the card for steel rods and up to the middle of the downhole pump card for fiberglass rods.
If the zero load on the downhole pump card is too high or too load as compared to where it should be, that indicates that the load cell is reading too high or too low.