Focusing mechanism and anisotropy correction of array induction logging responses for shale reservoirs in horizontal wells

We have developed a novel method for correcting resistivity anisotropy in array induction logging (AIL) for shale reservoirs drilled with horizontal wells. This method is featured by two aspects: simplifying software focusing (SF) algorithm from two-dimensional (2D) to one-dimensional (1D) and approximately decoupling the resistivity anisotropy effect. Initially, the typical raw and focused AIL responses are simulated to determine the applicability of the 2D SF algorithm in deviated and horizontal wells. The non-necessity of SF algorithm along vertical direction is also proved for the pure horizontal wells. Subsequently, a 1D SF algorithm that focuses only along the radial direction in horizontal wells is developed to replace the original 2D SF algorithm. This new method eliminates the opacity of traditional SF algorithms and can be easily applied to different AIL tools. The sensitivity of AIL to resistivity anisotropy in homogeneous and layered formations is then discussed, showing that the anisotropy effect can be separated from other effects. Finally, an efficient anisotropy correction scheme using the Levenberg-Marquardt algorithm is established to quickly and accurately correct AIL data in horizontal wells. Results from simulated and measured data demonstrate that this new method effectively removes the anisotropy effect and provides accurate horizontal resistivity for shale beds drilled with horizontal wells. Compared with existing methods, it is significantly faster and compatible with AIL tools from various companies, laying solid foundation for the interpretation and evaluation of shale reservoirs in horizontal wells.