Optimization Design Of Probe Magnet For Nuclear Magnetic Resonance Logging Tool

Nuclear magnetic resonance (NMR) logging is one of the most advanced methods ofunderground oil and gas exploration at the present time. The principle of NMR logging is asfollows: First, the static magnetic field is generated by the permanent magnet located insidethe probe of NMR logging tool and is used to make the hydrogen nucleus in layer magnetized.Then, the radio frequency (RF) coil located inside the probe is employed to generate RF signalto set up resonance in hydrogen nucleus. At last, resonance signal is collected and received togenerate image. NMR logging technology is relatively well-developed abroad, but thetechnology is kept in secret. The domestic research on NMR logging technology mainly ismainly concentrated on the aspect of computer science and geology, such as, establishment oflogging interpretation model, NMR analysis of the core geological features, etc. In this paper,the design problem about an improved probe magnet system for NMR logging tool wasstudied. The main works are as follows:First of all, the static magnetic field distribution of two kinds of aboard typical NMRlogging tool magnets was analyzed by MAGNET software, and two main specifications, i.e.the detection region and detection depth, were discussed. Then, an improved magnet design ofthe probe was proposed, in which the logging method in the middle position way was used todetect the gradient magnetic field. The magnet configuration included main magnet generatingstatic magnetic field and the adjustable magnet adjusting static magnetic field slightly.Secondly, the optimization design of probe magnet structure was completed bycombining response surface model (RSM) and genetic algorithm (GA). The geometryparameters of main magnet and adjustable magnet were selected as optimization variables,respectively, and optimization objective was to increase detection depth out of borehole. RSMof fitting objective function was established by combining Latin hypercube sampling strategyand multiple quadratic radial basis function (RBF), and GA was adopted to find the optimalsolution of established RSM. Finally, the optimum design of probe magnet was finished.Furthermore, based on MAGNET software, the static magnetic field generated byoptimized probe magnet was simulated and analyzed, and the possible detection regionsaround the borehole, detection depth and the magnetic field strength were discussed. The improved effect of proposed magnet structure on detection depth was analyzed.Finally, on the basis of optimized magnet structure, according to the requirement oflogging, the design of the RF coil in accordance with the magnet was completed. Theorthogonality of the RF magnetic field and static magnetic field was studied. The location andvolume of the detection region outside the borehole for the improved probe magnet weredetermined.