As those of you with pump-off controllers may have noticed, the downhole dynamometer card shapes calculated from Pump Off Controller (POC) dynos are much more difficult to interpret than downhole dynamometer cards from stand-alone dynamometer systems. As a matter of fact, downhole dynamometer cards from POCs in many cases are impossible to interpret correctly because of severe distortions. Up to this point, the cause of these distortions was unknown.
After researching the cause of the bad data from POCs we discovered the reason behind these errors and how to correct them in XDIAG and XSPOC.
The ability to correct data from POCs results in downhole dynamometer cards that are more accurate and much easier to interpret correctly.
This significant new capability has been added to the latest versions of XDIAG and XSPOC.
We found out that there are four sources of error in POCs:
Position errors can be significantly different from actual positions when a position switch is used because the POC fills in the position data with simulated data by assuming a sine wave or modified sine wave. Or, the position data may be erratic because of a bad position transducer or because of vibration when an accelerometer is used.
The top of stroke setting, which is required by some POCs, can also cause the position data to be phase-shifted if not done correctly. This results in downhole dynamometer cards that are “rotated” to the right or the left depending on the error in setting the top of stroke on the POC as the figure at right shows.
The phase shift problem causes distortions on the downhole pump card that are similar to the top of stroke setting errors. However, the root cause of this problem is different. Dynamometer data for downhole calculations needs to be collected at equal time increments to calculate an accurate downhole dynamometer card.
However, we found out that most POCs do not record load and position data at equal time increments. This is caused by delays due to slow processors and data filtering that can cause delays of up to 400 milliseconds between load and position measurements. In other words, if the load point is recorded first, the corresponding position point is not recorded until 400 ms later, instead of simultaneously. This phase-shift causes significant distortions that make the downhole pump card very difficult to interpret.
The next figure shows a downhole pump card with phase-shift problems. As you can see, this downhole pump card is difficult to interpret and can result in unnecessary pulling jobs or pump replacement.
Load cells that are out of calibration can also cause large distortions that make the downhole pump card unrecognizable and all the results of the diagnostic software useless. Large load cell errors are especially prevalent with beam mounted load cells that many POC systems use. Polished rod load cells can also get out of calibration or the load zero setting can drift causing large errors.
Many times those errors are combined and result in downhole pump cards that are impossible to analyze, or they can be misinterpreted as severe pump problems. This is why in the latest versions of XDIAG and XSPOC contain logic and pattern recognition enhancements that allow the software to detect and correct these errors. This can be done either automatically by the software, or manually by the user for each well.
The best way to correct these errors is to set the necessary corrections for each well individually, based on comparisons of dynamometer cards from the POCs versus measured dynamometer cards from an accurate stand-alone dynamometer system. However, this is time consuming and may not be practical if you have hundreds or thousands of wells.