First-Hand:Learning by Serendipity
Submitted by Sam Gibbs
Well analysis work has become very scientific. Some phases of it even involve solution of partial differential equations. Not many activities, even in so-called high tech companies, are as technical. But all well analysis expertise does not come from high science. Much of it comes from just living and spending time around pumping wells. There are also situations in which we learn just by accident. We happen to be in the right place at the right time to observe something that we would never see otherwise, even by applying technology to our jobs. One such instance occurred when we were hired to determine why polished rods kept breaking on a certain well.
When we parked the analysis truck at the well, we looked for the usual things that are known to cause polished-rod parts. One is a tilted carrier bar which causes a large bending stress just under the clamp. The carrier bar on the subject well was not tilted. We also examined the unit to make sure it was level and placed properly over the well so as not to flex the polished rod when it stroked up and down.
Slack was thrown and the load cell was installed between the carrier bar and the clamp. I was in back, operating the unit, and my young colleague was in front, handling the dynamometer equipment. He was relatively new on the job. When we stroked the unit upward the first time, he did not confirm that the reins were tracking properly on the horse head. They were not. When pulling the rods to replace the pump, the horse head must be set aside to make room for the service unit. Afterward, the service unit crew apparently failed to replace the bolts that securely fasten the horse head to the beam. When we threw slack and removed the rod load, the horse head shifted sideways and did not shift back into place when the load was reapplied. Thus, one rein fell off the horse head when we stroked the unit upward the first time. The unit was not equipped with bridle bails that would normally prevent the reins from falling off the horse head.
We were in big trouble. The polished rod was bent severely to one side and the carrier bar was anything but level. Somehow, we had to get the unit back to bottom and shove the rein back onto the horse head. Apprehensively, we gently stroked the unit downward and threw slack in the bridle. We shoved the horse head back into the proper position so the reins would track correctly and laboriously pushed the rein back on the horse head. We complete the well analysis without further incident.
The computer analysis did not explain why polished rods were breaking on the well, but the wreck we witnessed did. It is common for pumps to stick on the downstroke (and on the upstroke, too). When the pump sticks on the downstroke, slack is thrown in the bridle and the rod load is briefly removed from the unit. This, evidently, was happening to the well, probably during the night when no one was around to observe the event. When slack was thrown, the horse head would shift as we observed, and a rein would fall off. The unit would keep running and the polished rod would be flexed one way and the other for hundreds of strokes until it broke. This is equivalent to bending a malleable wire repeatedly until it breaks.
The lease operator would later drive by the well and discover the wreckage. The lifting sub, clamp, and polished-rod stub would be lying nearby on the ground. He would not know what happened except that the unit had suffered another polished-rod break.
The practical conclusion to draw from this story is to make sure that service unit crews re-bolt the horse head securely in place after each pulling job. Also, make sure that all units are equipped with bridle bails to prevent the reins from jumping off the horse head should slack be thrown in the bridle. Our customer followed these recommendations and later said that the well had not suffered another polished-rod part.
The main philosophical message of this story is that we do not learn everything about well analysis from science. Living long enough to gain practical experience is the best teacher of all.