Selection criteria and feasibility of the inversion model for azimuthal electromagnetic logging while drilling (LWD)

Azimuthal electromagnetic logging while drilling (LWD), which is capable of providing accurate position information approaching bed boundary, has been widely applied in real-time geosteering. For the inversion speed and precision of azimuthal electromagnetic LWD data, the key lies in the selection of proper inversion model and corresponding optimization algorithm. In this study, we first simplified the complex three-dimensional (3D) inversion of data into a series of one-dimensional (1D) inversion problems by using the dimensionality reduction scheme. Then, the feasibility and inversion performance of various 1D inversion models and different optimization methods were investigated, and the best combination between the inversion model and inversion algorithm was also obtained. Numerical simulation results show that the selection of 1D inversion model is dominated by the thickness of targeted beds, whereas the determination of inversion algorithm relies on the total layers amount of the inversion model. For the formation with thickness larger than 4.0 m, the single boundary inversion model and gradient optimization method are recommended. When the bed thickness is between 1.0 m and 4.0 m, the two-boundary inversion model instead of the single-boundary one is needed to estimate upper and lower boundaries around the borehole. For the inversion of azimuthal electromagnetic LWD data of thin layers, the multiple-boundary inversion model and the Bayesian algorithm should be employed. Key words: logging-while-driling, azimuthal electromagnetic logging, electromagnetic logging, inversion model, inversion algorithm, geosteering, bed boundar