Real-Time Downhole Shock Measurements Increase Drilling Efficiency and Improve MWD Reliability

Abstract

Abstract Many BHA and MWD failures are the direct result of high shock and vibration produced by the drilling process. Shocks resulting from high lateral vibrations have been shown to contain the most energy and do the most damage to downhole electrical components. High lateral and torsional vibrations can result in tool joint fatigue and twist-offs, and can be shown to reduce roller cone bit bearing life and damage PDC bit cutters. Many published attempts have been made to model downhole vibrations from surface measurements so that rotary speeds can be selected to minimize vibration. However, downhole data shows that currently these models have limited application in practice. Contact between the drillpipe and wellbore damps outs the transmission of vibrations in directional wells. In vertical wells, most models are highly sensitive to boundary conditions (e.g. BHA/wellbore contact points, hole size, bit/rock interaction) which result in limited predictive value. A simple, practical and cost-effective solution to the problem has been found. An accelerometer has been installed in MWD systems to count lateral shocks above a programmable "normal" value. A "normal" value of 25 g's has been found to give good field results in offshore environments. The number of shocks is transmitted to the surface every one or two minutes. This enables the driller to modify the rotary speed, and weight-on-bit, to eliminate or significantly reduce the downhole vibration. Eliminating lateral vibrations is expected to minimize axial and some torsional vibrations because there appears to be significant mode coupling. Examples from the field show that: – changes in rotary speed can have an enormous effect on the downhole shock environment. – surface measurements of vibration are a poor indicator of the downhole environment. – downhole shocks often correlate well with lithology as indicated by the MWD gamma ray. – PDC bit damage can be avoided by using the shock measurement. – MWD reliability has been improved. As a result of the improvements, the shock measurement has been made a standard component of all new MWD tools.