Numerical Simulation Of Look-Ahead Acoustic Logging-While-Drilling Based On Phased Array Technology

With the increase of drilling operations in high-angle and horizontal wells,the exploration of geological abnormal bodies ahead of the drill bit has been an urgent requirement of the petroleum industry.However,there are few acoustic logging solutions for this problem so far.Therefore,a novel look-ahead acoustic logging-while-drilling(LWD)scheme is developed based on the phased array technology for the detection of the anomalies ahead of the drill bit in this study.The responses of phased array acoustic LWD tools under different formation conditions are investigated using numerical simulation algorithms,and the feasibility of these tools in look-ahead acoustic LWD is analyzed.This study is helpful to solve the fundamental technical problems for look-ahead acoustic LWD and provides the theoretical basis for the development of industrial prototypes.The two-dimensional(2D)finite-difference time-domain(FDTD)algorithm in a cylindrical coordinate system and the three-dimensional(3D)FDTD algorithm in a rectangular coordinate system are derived respectively for the numerical simulations of borehole acoustic fields.Based on a high-performance computer cluster,a new parallel FDTD algorithm is proposed for elastic wave equations with the MPI and Open MP hybrid programming technology.The hybrid parallel algorithm distributes the total task into multiple processes based on MPI,and assigns the subtask of each process into multiple threads based on Open MP at the same time.Each process has independent memory space,and each thread of the process shares the memory space.The results show that,compared with the parallel FDTD algorithm based on Open MP,the hybrid parallel FDTD algorithm based on MPI and Open MP can use multiple nodes of the computer cluster to complete the computing tasks in parallel.Therefore,the hybrid parallel algorithm can greatly improve the computing speed and effectively reduces the memory consumption of a single node.The directional-radiation principle of acoustic energy is introduced based on phased array acoustic transmitters.The acoustic fields inside and outside the borehole generated by the linear phased array(LPA)acoustic transmitters are numerically simulated using the 2D FDTD algorithm,and the application effects of the LPA acoustic transmitters are analyzed in controlling the amplitudes of borehole guided waves and the energy distribution of formation P waves.The results show that,compared with the monopole acoustic transmitter,the amplitudes of refracted P and S waves can be enhanced,and the angular width and the deflection angle of the main radiation beam of formation P waves can be controlled,by adjusting the parameters of the LPA acoustic transmitter(such as,the element number,the delay time between the excitation signals applied to adjacent elements,and the amplitude weighting of the excitation signals applied to all elements).Compared with the scanning-radiation scheme,the directional-radiation scheme with lower implementation difficulty has a better application prospect in practical measurements.The scanning-reception principle of acoustic energy is introduced based on phased array acoustic receiver stations.The responses of phased array acoustic LWD tools with one or two formation interfaces away from the borehole are numerically simulated using the 3D FDTD algorithm,and the application effects of the arcuate phased array(APA)and cylindrical phased array(CPA)acoustic receiver stations are analyzed for acquiring the azimuth angles of geological abnormal bodies outside the borehole.The results show that,the amplitude distribution of echo P-P waves received by different azimuthal receiver elements is helpful for determining the azimuth angle of a single formation interface outside the borehole.Compared with the individual-reception mode,the azimuth measurement accuracy of the APA acoustic receiver station is significantly improved when working in the scanning-reception mode.Compared with the APA scanning-reception scheme,the CPA scanning-reception scheme performs better in simultaneously measuring the azimuth angles of two anomalies outside the borehole.The responses of monopole and phased array acoustic LWD tools in a situation involving a formation interface ahead of the drill bit are numerically simulated using the3 D FDTD algorithm.The application effects of the phased array acoustic transmitters and receiver stations in look-ahead acoustic LWD are analyzed.The results show that,the acoustic fields of look-ahead acoustic LWD are more complex than those of conventional reflection acoustic logging,because of the wave scattering at the well bottom.Compared with the monopole acoustic transmitter,the amplitudes of echo P-P waves can be considerably increased by directionally enhancing the energy radiated to the formation ahead of the drill bit,based on the LPA acoustic transmitter.Compared with the monopole acoustic receiver,the azimuth angle of the formation interface ahead of the drill bit can be obtained by analyzing the amplitude distribution of echo P-P waves in the scanning-reception waveforms,based on the APA or CPA acoustic receiver station.The data processing methods of conventional reflection acoustic logging can be applied to look-ahead acoustic LWD for the similar wave propagation characteristics.