Design of an Innovative Downhole NMR Scanning Probe

Abstract

Currently, commercial downhole nuclear magnetic resonance (NMR) tools can measure depth (along with borehole axis) and radial (perpendicular with borehole axis) profile information simultaneously. However, in many cases, the lack of information due to the strong heterogeneity, partial invasion of the drilling mud around the borehole or borehole collapse will have a serious impact on applications. In order to ensure the validity of each measurement, acquisition of azimuthal information is as important as the depth and radial profile information. In this paper, we described the performance and response simulation of an innovative centralized downhole NMR scanning probe (SMRT). This probe uses hollow cylinder magnets as main magnets to produce azimuthally symmetrical polarized (related to the $z$ -axis) static magnetic field ${B} _{0}$ . Circular focusing magnets and high permeability materials are added between the main magnets to simultaneously adjust the field homogeneity along with $z$ -axis, increase the height of sensitive region and enhance the strength of ${B} _{0}$ field in sensitive region. Coil array is composed of eight independent coil units with the same performance but different azimuthal selection function used to scan the formation around the probe so that eight azimuthally distinguishable sensitive regions can be investigated. Magnets and coil array configuration are optimized with the finite-element method, and ${B} _{0}$ field and ${B} _{1}$ field is calculated to obtain the spin dynamic response. Numerical simulation results show that NMR signals from different azimuthal sections had no overlapped feature (sensitive volume of each azimuthal direction is thin shell of arc length approximate 45°) and high azimuthal resolution is feasible.