The True Weight on Bit

Abstract

Drilling optimization is important for drilling companies in order to be cost effective and competitive. Using correct weight on bit (WOB) is a part of drilling optimization to achieve the optimal rate of penetration (ROP) for the given drilling situation. Whereas too low WOB might result in low ROP, too high WOB can result in a damaged drill bit. It is therefore important to be able measure or calculate the actual WOB precisely to avoid the mentioned scenarios. The main goal of this project was to develop two alternative WOB agents that use real time drilling data (RTDD) such as hook load (HKL), measured bit depth (DBTM), measured hole depth (DMEA) and rotation of the drill string (RPM) as input to calculate WOB. The first agent alternative uses the difference in HKL when rotating off-bottom and when rotating on-bottom to calculate WOB. The weight of the drill string is calculated in the second alternative, and iterations with different WOB values are performed until the calculated hook load matches the measured HKL with a small margin. Both agent alternatives performed well when testing them with RTDD from drilling of the 17 ½ inch section of the Gullveig K-4 AH well. The author believes that both the alternatives periodically perform better than the WOB model by Schlumberger when testing on the RTDD. The second agent alternative gives similar WOB results as WELLPAN when not circulating. However, when circulating, the results deviate. This might indicate that the viscous force model used in the second alternative needs adjustments to improve the accuracy. Given normal well conditions, the change in total hook load (THKL) when running in hole (RIH), equals the extra weight of the drill string due to increased length of the drill string. The author believes that by adding this extra weight to the current THKL reading in-between THKL measurements, the first alternative will give the most accurate WOB calculations of the two. In order to validate the accuracy of the two WOB agent alternatives, comparison with measured downhole WOB data is necessary. Unfortunately, this type of data was not available during the work of present thesis.