First-Hand:Sometimes Well Weighers are Just Lucky
Submitted by Sam Gibbs
Applying math and science increases your chances of being correct when making a diagnosis about an oil well. Being lucky also helps. We happened upon a certain well with a rod part in the presence of a new customer. He was intrigued by our company system and was following us from well to well. Rod parts are easy to find, even without high-tech tools. The downhole pump and part of the rod string had fallen off and were resting at the bottom of the tubing. The unit was over-balanced. The prime mover was racing on the upstroke and struggling to lift the counterweights on the downstroke. Furthermore, fluid was not being produced to the surface. Nothing was coming out of the bleeder. Probably the polished rod, usually shiny, had turned black because the stuffing-box packing was not being lubricated by fluid at the surface.
When downhole pump cards were calculated with the computer, a rod part revealed itself with an absurd and un-interpretable pump card shape. The reason was that an incorrect rod design had been supplied to the computer (part of the rod string of record broke off). In our customer’s presence, we made a stab at where the rod part occurred. The computer told us the theoretical buoyant weight of the complete rod string. We made a standing-valve test which gave the buoyant weight of the rod string without pump load. The difference in theoretical weight and measured weight allowed us to determine where the rod part was.
We estimated that the 201st rod had broken (in the 0.75 in. section). Our customer called a pulling unit contractor to fish the parted rods and place the well back on production. We went on to the next well. Two days later, our customer called us to say that the break was found in the 201st rod. He was impressed.
In reality, we were just lucky. Our load cell could not reliably measure any closer than about 250 pounds, which is the weight of six rods. The standing-valve test was obscured by the downhole friction, which decreased our ability to measure the exact buoyant weight of the rods. Yet, we benefited immeasurably by our good fortune. Our customer was an influential engineer in a major oil company. Because of his advocacy of us, we worked for the major oil company throughout the United States, wherever they had rod-pumped wells to analyze.