The nuclear magnetic resonance logging while drilling (NMR LWD) technique has advantages of providing real-time petrophysical information of in-situ oil & gas reservoirs during drilling. However, rigorous and harsh downhole conditions during NMR measurement pose great challenges for NMR LWD sensor design and implementation, including restricted space, motion states and limited downhole power supply. In this paper, fundamental engineering and scientific issues of NMR-LWD sensors are discussed based on special drilling downhole conditions and special features of NMR measurements. Key suggestions for NMR LWD sensor design are proposed. These are mandrel centered-type measurement, axis-symmetric low gradient fields, single frequency, moderate depth of investigation (DOI) and region of interest (ROI) height, a simple and practical assembly structure suitable for drill collars. A novel NMR LWD probe design is proposed based on the above understanding, where DOI and ROI are developed by using an optimized magnet configuration and permanent magnetic materials. The main magnet assembly is based on a classical inside-out configuration with a size constraint of a standard 6.75 inch non-magnetic drill collar, which is then improved by utilizing a circular focusing magnet between them. Static magnetic field distributions of the designed NMR sensor are given with 2D and 3D FEM methods. Characteristics were investigated including Larmor frequencies, static magnetic field gradient and sensitive volumes. The static magnetic field B0z produced by this magnet assembly is rotationally symmetrical with high resonant frequency, signal intensity and a compromised magnetic gradient, which has the advantage of alleviating axial and radial movement effects on measurement. The TR coil is a solenoid made of several turns of copper belts, resulting in an axisymmetric RF B1 field, which has a natural perpendicular relationship with B0z. Tuning and matching procedures are also discussed and calculated, several reference charts of capacities combination are given for rapid resonance circuit adjustment. The prototype antenna is tuned to have an impedance of about 500 Ω consistent with the spectrometer’s to improve power transmission efficiency. A full-size prototype NMR LWD sensor was built and tested jointly with the electronic circuits on a testing platform under laboratory conditions. In-house electronics and acquisition systems are used to validate the sensor design. Multi TW and TE CPMG experiments are carried out while the sensor is in a static state. Echo trains are successfully obtained and T2 distribution is demonstrated using 1D inverse Laplace transforms. The echo trains have adequate SNR and all T2 distribution have a single peak in accordance with fluid samples prepared as expected. The results show the NMR LWD design’s feasibility and potential.
Key words：
nuclear magnetic resonance logging while drilling; focusing magnet; probe; gradient magnetic field
LI Xin, LUO Sihui, XIAO Lizhi, SUN Zhe, WANG Zhengduo. Construction method study of NMR probe for logging while drilling. Petroleum Science Bulletin, 2020, 02: 172-181.
