| The near-bit electromagnetic wave antenna system can detect the formation near the bit,identify the interface and fault in time,and adjust the wellbore trajectory,which is an important part of geosteering.Tilted transmitting coil is a new electromagnetic wave antenna system which contains both azimuth and forward probing information,and can measure lateral and forward resistivity at the same time.In this paper,the characteristics of this coil system are studied in detail,the parameters are optimized,and the numerical calculation model of antenna system measurement based on COMSOL software is constructed,which provides a theoretical basis for the research,development,design and application analysis of near-bit electromagnetic wave resistivity logging tool.The following are the main research contents and achievements.In the first part,the numerical calculation method of electromagnetic wave resistivity logging near bit is studied.Based on the analysis of the working principle of the near-bit electromagnetic wave antenna system,the formula for calculating the receiving voltage of the receiving coil is derived.The numerical model of near-bit electromagnetic wave resistivity logging response based on COMSOL software platform is constructed,and the mesh generation method is studied.By comparing with the analytical solution,the rationality of the calculation model and mesh generation and the accuracy of the numerical solution are verified.The second part is to optimize the system parameters of electromagnetic wave antenna near bit.Firstly,the amplitude and phase response characteristics of nine coil arrangements are calculated and analyzed,and the inclined transmitting coil and vertical receiving coil are determined as the best coil arrangements.Secondly,the influence of parameters such as tilt angle,source distance and frequency of transmitting coils on logging response is calculated and studied,and the optimal tilt angle of 45 is selected,and the main receiving source distance and the source distance of two receiving distance coils are reasonably determined.Finally,the conductivity of drill collar,the dielectric constant of formation and the influence characteristics of different mechanical structures of antenna are calculated and analyzed.The results show that when the conductivity of drill collar is greater than 106S/m,it has little effect on the response.The influence of dielectric constant can be ignored;The influence of magnetic core,groove and radome can be ignored.The third part is the research on the influence of environmental parameters and detection characteristics of electromagnetic wave antenna system near bit.Firstly,the influence of formation environment parameters such as borehole radius,mud conductivity,surrounding rock,layer thickness,invasion and anisotropy on the response is studied.The results show that when the emission frequency is 400 k Hz,the phase difference resolution is 1.4 m and the amplitude ratio resolution is 2.1 m.When the transmission frequency is 2MHz,the amplitude ratio resolution and phase difference resolution are both 1.4 m.The detection depth is shallow under low invasion and high invasion.With the increase of dip angle,the amplitude ratio and phase difference are affected by anisotropy,and the higher the emission frequency,the greater the response is affected by anisotropy.Secondly,the detection characteristics such as azimuth angle,forward detection distance and side detection distance are studied.The results show that the amplitude ratio is the largest when the azimuth angles are 60 and 300;When the azimuth angles are 90 and 270,the amplitude ratio is the smallest.When the emission frequency is400 k Hz,2MHz and 10 MHz,the side detection distance is 0.7 m,1m and 1.2 m respectively;The forward probing distances are 0.25 m,0.5 m and 0.8 m,respectively.With the change of stratum contrast,the detection distance also changes.The fourth part studies the response characteristics of the near-bit electromagnetic wave antenna system.In the two strata,the target strata are conductive strata and resistive strata,change the formation tilt angle,and analyze the change of response with bit position under different formation tilt angles.In the third and fifth layers,change the formation inclination angle and observe the response characteristics of instruments passing through the formation.The results show that the larger the dip angle of the formation,the earlier the response changes when the instrument approaches the interface,and the wider the range of response changes with the instrument position.When the dip angle is 90 °,the amplitude ratio and phase difference do not change with the change of bit position. |
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