Numerical Simulation And Design Of An LWD Acoustic Transducer

LWD Acoustic logging is used to obtain velocity of compressional wave and shear wave while drilling.The technology has been successfully applied to predict formation pore pressure,do real time calibration with seismic data,and measure formation stress and anisotropy.However,manufacturing of the LWD tool still faces many technical challenges.The LWD acoustic transducer design is one of the major challenges.To address the transducer design issue,this thesis uses an FE(finite element)method to investigate LWD acoustic transmitter characteristics and controlling parameters,and suggests the transmitter design parameters on a 5 inch collar tool for the LWD multipole and azimuthal acoustic measurements.During the LWD acoustic measurement,the transducer is placed in the large drill collar.which made performance research complicated.Therefore,it is necessary to investigate the factors that affect transmitter's performance.In this thesis,we first model the conductance curve for a tile transducer element;the modeling result is in good agreement with the laboratory measurement result,suggesting that the transmitter simulation result is reliable.After validating the modeling approach,we establish the FE models for the LWD acoustic multipole and azimuthal transmitters,and investigate the effect of the collar and transducer structure on the transducer resonant frequency.Our analysis results show that the presence of the drill collar increases the transmitter resonant frequency,and suppresses the acoustic wave traveling to collar.In addition,the height,thickness,and angle span of the transmitter elements do not affect the directivity of LWD dipole and quadrupole sources.However,with increasing transmitter height and angle span and decreasing thickness,the resonant frequency of LWD monopole transmitter decreases.Moreover,when transmitter's angle span gets wider,the azimuth resolution of LWD azimuth source significantly decreases.Based on the simulation results,we design LWD multipole and azimuthal transmitters for the 5 inch collar tool.First,for the10-16 kHz operation frequency band of a monopole tool,a design for a 13-kHz center frequency transmitter is considered.For the 5-inch collar tool,the optimal dimension of the LWD multipole source element is obtained,which has 100 mm inner diameter,120 mm outer diameter,126 mm height,and 90 degree angle span.The LWD azimuthal source has 100 mm inner diameter,120 mm outer diameter,130 mm height,and 70-degree angle span.For the given dimensions,we investigate the performance of the LWD transmitter sources using numerical simulation.The results show that the LWD multipole transmitter has a resonant frequency at 13 kHz,having a good multipole radiation directivity and effective collar wave supression,For the LWD azimuthal source,the resonant frequency is 13 kHz with a reasonable azimuth resolution.