Monitoring a Drilling Trajectory by Using the Drill-Bit Signal as a Source

Abstract

Summary A deviation from the desired drilling trajectory creates problems and costs. Monitoring the drill-bit trajectory can help to take action in advance to adjust the drilling process to the desired trajectory. In this study we suggest a possible approach to detect the deviation of the drill-bit using seismic while drilling (SWD) methods. SWD allows seismic measurements without stopping the drilling process and will therefore enable near real-time corrections for the drilling trajectory. We use the drill-bit as a seismic source and an array of seismic receivers on the surface to record the drilling signals. Because the drill bit will create a noisy source signature we apply cross-coherence to retrieve the seismic signal as it would be emitted by a standard seismic borehole source. A cross-correlation between the records at a reference depth and recordings due to deeper source positions reveals a specific change in travel times, when the drill-bit source is located along a deviating drilling trajectory. By using 2D finite-difference modelling of wave propagation for a typical hard rock environment, we show that this method can deliver sufficient information about the deviation that a correction of the drilling process is possible.