Most current diagnostic techniques that stablish downhole conditions from polished rod measurements, are based on simplified analytical solutions and thus have inherent limitations. In the present investigation these limitations are fully explored and a new diagnostic approach, based on the method of characteristics, is introduced in order to overcome said limitations. This new approach is more flexible (since it can be easily extended to include deviated and directional wells), allows the estimation of computational error, yields a more accurate downhole load estimation and compares very favorably with state-of-the-art codes.
A specially designed Downhole Instrumentation Unit (DIU) was used to obtain downhole measurements (at the, pump) in a test performed in a 5625 feet deep rod pumped well. The DIU is a microprocessor based 5 feet long "intelligent sucker rod" that can be screwed in anywhere along the rod string. The results of this test show that the diagnostic technique using the method of characteristics provides a very good estimation of actual loads along the rod string and at the subsurface pump.