Acoustic Advance Ranging Method in Gas Drilling and Its Numerical Simulation

The safety of gas drilling is greatly influenced by local geological conditions of reservoirs. It is difficult to identify formation reflection signals in gas drilling due to the influence of noise generated from gas. Therefore, a near-bit impact source sub while drilling was designed, and a method of acoustic advance ranging for gas drilling was proposed on the basis of the self-excitation and self-reception principle of seismic waves. In this way, measurement accuracy could be raised, and the uncertainty of lithological interfaces ahead of the bit could be reduced. Specifically, the attenuation effect of polytetrafluoroethylene (PTFE) on vibration coda waves was analyzed through an impact test; the feasibility of the acoustic advance ranging method in gas drilling was verified through numerical simulations, and the influence of detection distance on the amplitude of reflected waves was analyzed. The impact test revealed that the coda waves of vibration waves on the hollow cylinder were significantly attenuated upon the use of PTFE. Numerical simulations indicated that the boundary conditions of a perfectly matched layer (PML) can effectively eliminate the interference of boundary reflection of the model, and the ranging error calculated from the arrival time of reflected P-waves was 1 m. In addition, the relative intensity of reflected P-waves shows a gradual increase, and the amplitude of reflected waves decreases greatly as the increase in detection distance. The research results verify the feasibility of detecting lithological interfaces ahead of the bit by near-bit impact sources and provide a new idea for the development of acoustic advance detection technology while drilling in gas drilling.