BHA sag correction

This article provides a brief overview of the MWD STD approach to correcting BHA sag, but for more detailed information, please refer to this material.

Did you know that accurate geosteering requires the BHA sag correction in real-time? The TVD error caused by BHA sag can reach 1 meter per 100 meters when drilling a horizontal section. BHA sag is the misalignment between the direction-and-inclination module and the borehole centreline due to deflection of the MWD drill collar due to gravity and wellbore curvature. The BHA sag correction is implemented in the following our products:

MWD STD platforms encapsulate a novel BHA modeling algorithm based on energy minimization. The algorithm determines contact points BHA with the borehole walls in real time and provides enhanced accuracy performance with respect to the conventional sag correction. MWD STD sag correction was verified using laser probe and MWD measurements of a deviated borehole drilled in a laboratory: the average deviation between the measured and calculated data was approximately 0.01 deg:

The novel BHA deflection modeling validation vs real data

Also, the algorithm was verified by RSS vs MWD measurement comparison. The average deviation between the calculated and measured sensor readings was approximately 0.03 deg or less then 1 sigma of the accelerometer uncertainty.

The new algorithm calculates the points of contact between the BHA and the walls of the borehole automatically, eliminating the need for prior constraints such as the requirement of having a stabilizer in the BHA or the assumption of fixed contact points. This reduction in assumptions leads to a higher degree of accuracy and stability in the solution, especially in a deviated wellbore:

Stability of the commercial and MWD STD FE methods