Studies On The Finite-difference Time-domain Simulation Of Acoustic Logging While Drilling And The Propagation Characteristics Of The Collar Wave

This thesis studies the numerical simulation of acoustic logging while drilling(LWD) by finite-difference time-domain (FDTD) and the propagation characteristics ofthe collar wave. One of the serious problems faced by acoustic LWD technology atpresent is: the amplitude of the collar wave, which generates from the transmitter andpropagates through the drill collar to the receivers, is much larger than the acousticsignal from the formation. It is hard to accurately extract the velocities of thecompressional and the shear waves if not so many grooves are cut in the dill collar inorder to guarantee the mechanical behaviors of drill pipe. The researchers in EarthResources Laboratory of Massachusettes Institute of Technology tried to eliminate theinterference of the collar wave by using the seismoelectric LWD method based onelectrokinetic effects in rocks. The electrokinetic effect is a peculiar phenomenon influid-filled porous media. Therefore, the electromagnetic signals received by theseismoelectric LWD can only be generated in porous formations. If the velocities of thecompressional and the shear waves can be extracted from the electromagnetic signals, itis not required to cut grooves in the drill collar.From the numerical simulation of seismoelectric LWD, the fluctuations andpetrophysics study group in department of mechanics of Harbin Institute of Technologyrecently found that the collar wave group exists in the full waveforms o f theelectromagnetic fields. And then the suspected collar waves were also found from themeasured seismoelectric signal in small-scaled model well in laboratory. Whether thecollar wave exists in the electromagnetic signal of the seismoelectic LWD or not? It hasa great significance of using the seismoelectric LWD to eliminate interference of thecollar wave. The key point to answer this question is to make clear if obvious collarwave propagates to the formation. Therefore, it is required to study the propagationcharacteristics of the acoustic wavefields caused by the drill collar in the LWD model.In this thesis, the FDTD algorithm is implemented of simulating the elastic wavepropagation in an inhomogeneous axisymmetric model. The wavefields in the boreholefluid and in the formation of the monopole acoustic LWD are calculated by the FDTDalgorithm. Special attentions are paid to the propagation characteristics of the collarwave.In Chapter2, the viscoelastic wave equations that origin from the class icalelastodynamics equations are formatted according to the finite-difference theory. TheFortran90is adopted to realize the program of the elastic wave FDTD algorithm in theaxisymmetric model. The velocity-stress staggered finite difference grid and theparameter average method are applied to deal with the inner boundary difference format of different media. The artificial reflex caused by the truncation of the calculativeboundary is eliminated by using the currently recognized most effective perfectlymatched layer absorbing boundary condition. The PML of the unsplit field is alsoadopted in the algorithm.In Chapter3, the time-domain full waveform of the acoustic wireline logging（theaxisymmtric cylindrical layered model for the borehole fluid and the solid formation outof the well）and the acoustic logging while drilling（the complex cylindrical layeredmodel for the fluid in the drill collar, the drill collar, the fluid outside the drill collar andthe solid formation out of the well）are both calculated by using the realized FDTDalgorithm. The calculated results are compared with the results of the semi-analyticalreal axis integration. Moreover, the simulation waveforms which have both thehorizontal layered model and the cylindrical layered model are compared with theprevious calculated results in order to testify the correctness of the algorithm and theprogram in this thesis.Based on the above researches, the propagation characteristics of the drill collarwave are analyzed in Chapter4by calculating the time domain waveform of the fluidoutside the drill collar and the solid formation out of the well, the time-velocity similarcoherence map as well as the gray-scale map of the wave field distribution at differentmoments respectively according to three models, namely, the model of placing the drillcollar in an infinite fluid medium, the model of truncating the drill collar in the middleas well as the model of truncating the drill collar in the end.The studies conclude that the drill collar wave spreads along the drill collar and atthe same time radiates energy towards the outer fluid of the collar and even the solidformation. Similar with the Stoneley wave, the energy of the drill collar wave iscentralized around the drill collar and the wellbore. Compared with the compressionaland the shear wave, the drill collar wave in the formation around the wellbore is stillvery obvious. The absolute amplitude of the drill collar wave in the formation decreasesrapidly with the distance of deviating from the well shaft gradually increasing. Thewave group which propagates with the speed of the velocity of the drill collar wave inthe formation will generate the accompanied electromagnetic field because of theelectrokinetic effect. Therefore, these analyses can explain why the accompaniedelectronic field of the drill collar wave exists in the seismoelectric logging while drillingfull wave generated by numerical simulation and the―suspected‖drill collar signalappeared during the experiment measurement.