Detection performance of azimuthal electromagnetic logging while drilling tool in anisotropic media

Abstract

Azimuthal electromagnetic (EM) logging while drilling (LWD) has been extensively used in high-angle and horizontal (HA/HZ) wells. However, due to the effects of formation anisotropy, accurate geosteering decision and formation evaluations have become increasingly difficult. To quantitatively analyze the effect of anisotropy on tool responses and data processing, this paper investigates the sensitivity of EM LWD measurements to electric anisotropy and inversion accuracy via forward modeling and inversion. First, a sensitivity factor is defined to quantitatively analyze the sensitivity of the magnetic field components and synthetic signals to electric anisotropy. Then, azimuthal EM LWD responses in anisotropic layered formations are simulated, and the sensitivities to formation parameters for compensated and uncompensated tool configurations are comparatively analyzed. Finally, we discuss the effects of the inversion model on bed boundary inversion in anisotropic formations. Numerical simulation and inversion results show that azimuthal EM LWD can be significantly affected by electric anisotropy. Fortunately, by using a symmetrical compensation configuration, the sensitivity of the geosignals to electric anisotropy can be suppressed, and the boundary detection capability can be further enhanced. Anisotropy normally gives rise to separated resistivity curves and abnormal “horns”; moreover, complicated nonlinear distortion can also arise in geosignals as the tool approaches a bed boundary. If anisotropy effects are ignored in the inversion process, the estimated bed boundary and formation resistivity are usually unreliable, which may mislead geosteering decisions.