AbstractUnexpected well deviations can bring the drilling projects lots of financial and technical damages. As a result, investigation of the Bottom Hole Assembly (BHA) tendency and prediction of the probable mechanical behavior of the drilling string, especially when a new configuration is running in the hole, is critical to prevent unexpected failures. In this paper, a drilling project in an Iranian gas field which ended in a catastrophic well trajectory is going to be studied in more detail. Here, we try to answer three main questions. The reasons for the unusual well trajectory and the possibility of predicting this behavior is the first issue. The second question is about the signs of this failure during the operation and the ways that we could detect it earlier. In the third question, the alternative plans that can prevent this problem are examined by studying different BHA configuration and drilling parameters. The major sources of our information are the daily drilling reports, well log data, related published articles, and numerical simulations in WELLPLANTM software of Landmark package. According to the simulation results, BHA design is one of the most effective factors in this case study and its effect could be predicted using BHA tendency analysis before starting the drilling operation. During drilling of this well, some anomalies have been observed in drill string mechanical parameters such as hook load, rotation torque and up and down drags. Simulation of torque and drag charts for some probable well trajectories shows completely different trends for the expected well trajectory and the actual one. The observed data during the drilling operation are similar to the ones simulated for a highly deviated well and are completely unlike the trend of the near-vertical well path. Hence, it was possible to detect the wrong situation if we had simulated the mechanical behavior of the drill string and compare it with the actual observations during the operation. Finally, examination of various BHAs reveals that using an in-gauge stabilizer 10 meters above the bit instead of the one that is 20 meters above the bit could provide better well path control. It is completely obvious from the different build and walk rates which resulted in about 19 different BHA configurations. Moreover, the suggested BHAs demonstrate a good tolerance in case of changing WOB in the desired range. In the end, besides from being careful and alert during the drilling operation, the application of credible drilling simulators is strongly recommended in order to prevent unforeseen situations and also to be prepared if some happen.
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