Three-dimensional Finite Difference Simulation Of Acoustic Logging While Drilling And Study On Acoustic Insulation Performance Of Grooved Drill Collar

Acoustic logging-while-drilling(LWD)detects the formation around the borehole and provides information for geological guidance.This technology has real-time measurement characteristics,which expected to reduce the cost of measurement.The existing logging data indicates that the collar waves,with early arrival time and large amplitude,in the monopole acoustic LWD cover up the acoustic signals from the formation,which makes it impossible to extract the accurate velocity of the formation compressional wave.The predecessors used the groove on the drill collar to eliminate the drill collar wave,but the acoustic insulation mechanism of grooving is still unclear.It is necessary to study the propagation characteristics of the collar wave in the acoustic LWD and the acoustic insulation mechanism of the grooved structure.In this paper,the three-dimensional parallel finite difference time domain(FDTD)algorithm is used to study the propagation characteristics of the collar wave in the acoustic LWD.The acoustic field of acoustic LWD under the periodic grooved drill collar is simulated.Progress has been made mainly in the following areas:The FDTD algorithm of three-dimensional rectangular coordinate system in the fluid-solid composite structure with high acoustic impedance difference is programmed,and the acoustic field response of the acoustic LWD is simulated by this algorithm.In order to verify the correctness of FDTD algorithm,the relationship between the volume strain produced by the annular force source inside the borehole and the displacement generated by the monopolar source outside the borehole is established based on the principle of reciprocity of elastodynamics.In the absence of analytic solution comparison when the drill collar grooved and eccentric,the correctness of the FDTD algorithm in simulating acoustic LWD field response is verified by the reciprocity principle.The FDTD algorithm is used to simulate the full-wave of the acoustic LWD and the drill collar placed in an infinite fluid.It is found that the amplitude of the reflected collar wave,which propagates from drill collar to the borehole and reflects back to the outside surface of the drill collar after reaching the borehole wall,accounts for an important proportion of the total amplitude of the collar wave.In order to analyze the influence of drill collar structure on the propagation of the collar wave,the acoustic field of the drill collar truncated in the middle and the drill collar truncated at the end is simulated.The results show that the collar wave disappears at the truncation point when the drill collar is truncated.The collar wave is converted into Stoneley wave after truncation.In order to illustrate the difference between stress response and displacement response in acoustic LWD,the wave fields of stress response and displacement response are simulated by FDTD algorithm,and the characteristics of stress and displacement waveforms are analyzed.The relative amplitude of Stoneley wave is the largest in the full wave of the radial stress and the collar wave is the second in the full wave.The relative amplitude of the collar wave is the largest in the full wave of the radial displacement.It is noteworthy that the formation compressional wave is significanttly observed in the full wave of the axial displacement at the borehole.The distribution law of the collar wave amplitude along the radial direction is studied.It is found that the maximum amplitude of the collar wave is located at the outer surface of the drill collar in the radial displacement response.However,the maximum amplitude of drill collar wave is located inside the drill collar in the radial stress response,and the amplitude of the collar wave at the borehole wall is larger than that at the outer surface of drill collar.It is also found that the radial displacement response is more sensitive to the change of formation parameters.The larger the acoustic impedance of formation,the larger the amplitude of the collar wave in the radial displacement response at the outer surface of drill collar,but the smaller the amplitude of the collar wave at the borehole wall.FDTD algorithm is used to simulate the acoustic field response of different grooved collar structures aim at the collar models of three periodic grooving structures(interior groove,exterior groove and staggered groove).It is found that grooving can weaken the amplitude of the collar wave,but grooving can cause some of the collar wave to be converted into Stoneley wave.The variation of the amplitude of the collar wave and the converted Stoneley wave with frequency under different grooving parameters is studied.It is found that the interior groove causes the low point of the collar wave and the converted Stoneley wave to move to low frequency,while the exterior groove causes the low point of the collar wave and the converted Stoneley wave to move to high frequency.The interior and exterior groove causes both low point of the collar wave and the converted Stoneley wave to move to low frequency and high frequency.It is recognized that the frequency range of the velocity of the compressional wave extracted from the acoustic LWD field under the interior and exterior staggered grooved collar structure is larger than that of the other two grooved structures.FDTD algorithm is used to calculate the full wave of the dipole and quadrupole acoustic LWD field under grooved collar structure.It is found that there are significant differences on multipole acoustic LWD field between the effect of exterior groove and interior groove.In the case of dipole acoustic LWD field,the velocity of fluid-solid interface wave increases and the velocity of collar flexural wave decreases under exterior grooved drill collar,while the velocity of fluid-solid interface wave remains unchanged and the velocity of collar flexural wave decreases under interior grooved drill collar.In the case of quadrupole acoustic LWD field,the velocity of the formation screw wave increases under exterior grooved drill collar,but it does not change under interior grooved drill collar.The frequency range of dipole and quadrupole acoustic LWD in grooved collar case is proposed based on the understanding of the effect of grooving on multipole acoustic LWD field.The velocity of fluid-solid interface can be used to invert the velocity of the formation shear wave when the frequency of acoustic source is between 4.5 k Hz and 6.5 k Hz in dipole acoustic LWD.The velocity of the formation screw wave can be used to invert the velocity of the formation shear wave when the frequency of acoustic source is between 2.0 k Hz and 6.0 k Hz in quadrupole acoustic LWD.In order to analyze the influence of the drill collar eccentricity on multipole acoustic LWD field,FDTD algorithm is used to simulate the acoustic field responses of monopolar,dipole and quadrupole acoustic LWD when drill collar eccentricity occurs.It is found that the influence on the acoustic LWD field of the drill collar eccentricity under different excitation modes is different.The amplitude of the collar wave and the Stoneley wave are larger in near eccentric position and smaller in far eccentric position,while the velocity of the Stoneley wave is smaller in near eccentric position and larger in far eccentric position.The velocity of the formation compressional wave can be extracted from the full wave of monopolar acoustic LWD under the grooved drill collar structure,and the eccentric distance can be identified according to velocity variation law of the Stoneley wave.The velocity of the fluid-solid interface at the near eccentric position is small while it is large at the far eccentric position in dipole acoustic LWD.However,when the collar is eccentric,the dispersion degree of formation spiral wave of the stress full wave is different under different eccentric positions in quadrupole acoustic LWD.There is a significant error in inversion of the velocity of the formation shear wave by using the velocity of the formation spiral wave.In this paper,the analysis of the propagation characteristics of the collar wave and the study of the acoustic insulation performance of grooved drill collar have guiding significance for the development of acoustic LWD tool and the signal processing.