Instantaneous Logging Based on a Dynamic Theory of Drilling
Authors: Lutz J. , Raynaud M. , Gstalder S. , Quichaud C. , Raynal J. , Muckleroy J.A. 
Published: December 2007

Vibrations from the tricone bit are measured at the kelly and interpreted at the derrick floor to give an instantaneous log of the formation while it is being drilled. An experimental version of the system has been used in the field for several years. Such instantaneous knowledge is valuable in adjusting drilling variables, in correlating formations, and in warning that a zone of abnormal pressure is being approached.

Introduction

The Societe Nationale des Petroles D'Aquitaine (SNPA) has developed a dynamic theory of drilling based on the creation by the drilling bit of longitudinal vibrations and the transmission of these vibrations through the drillstem. The tricone bit is considered as a generator of mechanical waves in a complex system comprising the drillstem and derrick on one hand and the rock on the other. A theoretical interpretation of the dynamic behavior of the tricone bit permits us to define the vibratory characteristics of the excitation imparted to the rock and the drillstem. Only the tricone bit is considered here.

A dynamic model of drilling operation can be derived from this theory. In this closed-loop model, the characteristics of the simulated rock must be continuously adjusted in order to obtain from the model a signal that agrees with field measurements. In this way, on the basis of the vibratory state measured at the top of the drillstem, it is possible to obtain data on the rock being drilled.

Simple recorders that are accurate and dependable have also been developed to measure the vibrations and the principal parameters. These recorders have been used in the field for about 5 years.

Using the basic ideas derived from the dynamic theory, a simplified means of treating the measurements has given an instantaneous log whose use has a promising future in developing knowledge of the mechanical characteristics of the rock at the time of drilling. This knowledge can lead to better optimization of drilling variables and more effective automation.

The importance of vibratory phenomena in drilling rigs is well known to all drillers. Certain rotational speeds must be avoided because they lead to resonance phenomena in the drill string and the derrick phenomena in the drill string and the derrick phenomena that very quickly take on a spectacular aspect and that can also lead rapidly to severe damage if they are not quickly stopped.

It has been verified many times in the field in particular in turbodrilling with a tricone bit, in which particular in turbodrilling with a tricone bit, in which the rotation of the drillstem can be stopped that the vibratory regimes observed at the kelly sometimes vary quite markedly; the explanation that seems most plausible to us is to be found in a modification of the plausible to us is to be found in a modification of the bit-rock coupling.

We have also been able to observe in the course of numerous tests that the general behavior of tricone bits on our test bench is not always comparable with that in the field. The amount of rock destruction, in particular at high values of drilling parameters, seemed much better in the bench tests, a possible cause being a bit-rock coupling that was more effective because of a more rigid structure.

On the other hand, the remarkable bottom-hole data recorded by Esso Production Research Co. have shown that the bit undergoes parameter variations that are not detected in conventional surface measurements.

JPT

P. 750