Improving Surface WOB Accuracy

摘要

Weight on bit (WOB) is the axial trust force applied to the bit during drilling. It is one of the most important controllable variables of the drilling process. Still, it is quite rare and very difficult to measure it directly, and we are often left with estimating it indirectly through measurements of the tension at the top of the string, or even at the dead line anchor. This indirect WOB measurement, commonly called surface WOB, is most often calculated as measured off-bottom hook loadminus the actual hookload. This method assumes that the reference string weight is constant and does not change during drilling. This paper challenges this assumption by discussing a variety of physical effects that can cause the reference hook load to change, during the drilling of a new stand and after a traditional WOB zeroing procedure is carried out. The paper discusses the following effects that can be used for correcting the reference hook load: 1)well bore friction, 2)added weight in air, 3)well inclination, 4)flow induced lift, 5) nozzle jet lift, 6)cuttings in suspension, and 7) back-pressure in managed pressure drilling operations.