Experimental Evaluation of Real-Time Mechanical Formation Classification using Drill String Vibration Measurements

A drill bit creates a hole in subsurface formations by applying weight on bit and rotary speed. The bit is the first segment of the bottom hole assembly (BHA) and the drill string which contacts formation. Today it is possible to monitor drilling parameters such as weight on bit, rotary speed of drill string, azimuthal and inclination angles of the well using downhole sensors. An additional set of parameters recorded and monitored downhole near the bit is the vibration of the drill string as a consequence of the interaction between bit and different formations drilled. Dependent on formation characteristics such as the uniaxial compressive strength of rock, axial vibration is expected to change when the bit contacts formation with changing mechanical properties. Therefore, in case of using drill bits with similar characteristics, recognizing different layers of formation in real-time is expected to be possible if the axial vibration is monitored.

Experiments, using a fully automated laboratory scale drilling rig, the CDC miniRig, were performed. A vibration sensor sub is attached to the drill string above the bit. Concrete cubes with uniform strength as well as layered bonded cubes with different strength were drilled using different ranges of weight on bit and rotational speed. Higher order frequency moments were calculated and used for evaluation of the drilled concrete cubes. The cubes were built with different compressive strength and quantified by uniaxial compressive strength measurements prior to the test. The results of the experiments showed good correspondence with the expected behavior and allowed to differentiate the individual layers of concrete of different strength. The means to identify layers of rock based on vibration analysis are described in this paper. Necessary next steps to translate these results to more complex rocks are presented.