The Fundamental Research Of Acoustoelectric Remote Detection Logging While Drilling

Geosteering technology is of great significance for safe drilling,which can improve the accuracy of drilling operations and enhancing the efficiency of drilling.The acoustic remote dection technology,with the advantage of imaging the geologic structures around borehole from 1m to 10 m,has been widely used in wireline logging and has a considerable potential for geosteering.However,for the technology,the inner and outer borehole energy distribute unevenly and as a result of the amplitude of reflection wave is much lower than the one of the direct wave in borehole.This problem would be more serious under strong noise interference generated in the logging while drilling(LWD)environment,the signal to noise ratio of the reflection wave would further decrease in this scenario.The problem confines the application of acoustic method utilized in geosteering.In addition,the acoustic remote detection technology is not sensitive to an electric impedance interface.Therefore,a new LWD remote dection method is urgently required to both can enhance the signal to noise ratio of reflection wave and enable detecting interfaces with difference in elastic or electric impedance.In this dissertation,acoustoelectric coupling effect existing in porous rock is employed to solve these two problems.The major work of this dissertation consists three parts as follows:In the first part,the characteristics of the electrical responses under the scenario of LWD with an interface outside the wellbore are studied.Based on the Pride equations which is a classical acoustoelectric coupling theory,a physical model of the LWD with an interface outside the wellbore is constructed.The solution of the elastic and electrical filed for inside and outside borehole media are derived under cylindrical coordinates and the Real Axis Integration method is used to calculate the electrical responses received by electrodes in the well.The results illustrate that there are two kinds of electrical signal related to the interface: reflected coseismic electric field and interface radiated electromagnetic(EM)wave.Both these two kinds of signals carry the interface information so that they can be used for remote detection logging.However,similar to the conventional reflected acoustic method,the reflected coseismic electric signal is also interfered by the direct coseismic electric signal inside the wellbore and can only detect elastic impedance interface.The interface radiating EM wave generated at the interface can arrive at the electrodes before the direct coseismic electric signal inside the wellbore when the source-receivers distance is large enough.Therefore,the interface radiating EM wave can avoid the interference of the direct coseismic electric signal inside the wellbore and can greatly improve the signal to noise ratio.Moreover,interface radiating EM waves can not only detect the elastic impedance interfaces but also the electrical impedance interfaces which always accompany with low elastic impedance.In the second part,the acoustoelectric conversion efficiency of interface radiating EM waves on the interface is quantitatively analyzed.The reflection,transmission coefficients and Poynting energy flux induced by plane fast compressional wave,SV wave,and SH wave incident on elastic and electrical impedance interfaces are derived.The coefficients are then verified by the conservation law of energy.Then,the influence of the incident angle and frequency on the reflection and transmission coefficient of the interface radiating EM waves were systematically calculated and analyzed.Three conclusions can be inferred from above calculation.The reflection coefficients of interface radiating EM wave excited by SH wave is greater than the one excited by fast compressional wave or SV wave at a majority of angles,the advantages of SH wave are more dominant when incident angle is small,i.e.,SH wave incident at a small angle is an optimized choice to interface radiating electromagnetic wave for remote detection.There is an optimal excitation frequency for the reflection and transmission coefficients of the interface radiating electromagnetic waves.It is proved that the interface radiating EM wave is better than the reflection elastic wave for detecting an electrical impedance interface.Using the calculated reflection coefficient,a fast forward modeling method is proposed for calculating interface radiating EM wave and is verified by an analytical solution.In the third part,an experimental study of the interface radiating EM wave was conducted.The interface radiating EM waves generated by a finite-width pulse acoustic wave at the interface of fluid-porous rock were measured,and their velocity,phase and amplitude variation characteristics were analyzed.The experiment results illustrated that the acoustic wave can generate the interface radiating electromagnetic wave propagated at the speed of light at the fluid-porous sandstone interface;for the case of non-zero offset,its amplitude is the smallest rather than the largest at the position of interface;its phase gradually changes along with the amplitude change and reverses on the interface.In order to explain the experiment phenomenon,a theoretical modeling of the fluid-porous interface model was operated and the results are in good agreement with the experiment results.Both the experimental and theoretical results can been reasonably explained by electric dipole model.