Numerical modeling and analysis of responses of eccentric electromagnetics logging while drilling tool

Key words:  electromagnetic logging-while-drilling  tool eccentricity  finite-difference time-domain  anisotropic absorbing boundary

Abstract:

In order to model the electromagnetic logging responses while drilling with tool eccentricity and to provide theoretic basis for the eccentricity correction, the responses with tool eccentricity were simulated and analyzed based on the finite-difference time-domain (FDTD) framework. Non-uniform gridding technique in cylindrical coordinates and staircase approximation technique were utilized to reach fine simulation of borehole, coils, tool eccentricity. In addition, the UPML absorbing boundary condition was implemented to absorb reflection field, which could reduce the model size and improve simulation speed. By comparing the simulation results with the one-dimension analytical solution and the semi-analytic results with tool eccentricity, the algorithm was verified favorably. In addition, the responses of the Halliburton EWR-Phase4Electromagnetic LWD tool under different eccentricity distances, mud resistivity, borehole sizes, coil-spacing and frequencies were simulated. The results show that the phase difference resistivity is affected more by tool eccentricity than the attenuation resistivity. With larger coil-spacing and borehole size and higher frequency, the responses are more sensitive to tool eccentricity. In conventional sand-shale reservoirs, for 2MHz frequency, eccentricity correction needs to be applied to the responses of middle coil-spacing and long coil-spacing when the mud resistivity is less than 0.1Ω·m.