新型核磁共振孔隙介质分析仪的研制

A novel low field nuclear magnetic resonance analyzer for porous media

研制一种新型低场核磁共振孔隙介质分析仪,通过对不同直径岩样的实际测量,可以快速提供含流体孔隙介质的T2分布,T1分布,T1-T2和D-T2二维分布等重要信息,证实了仪器的合理性,有效性,及便捷性。采用Hal-bach磁体和可变尺寸开槽法拉第屏蔽螺线管天线,方便了样品制备和探测效率。低成本线性功率放大器的输出功率可达250 W,能够满足线圈激励要求,并降低成本。有源场效应管组成的隔离电路工作频率较宽,具有通用性。射频三极管组成的负反馈放大电路作为前置放大电路的第一级保证接收回路的低噪声设计,三极管集电极电流由运放为核心的虚拟电流源,使电路设计简化。DSP和FPGA作为主控电路的运算和逻辑控制单元,使回波信号提取和脉冲序列时序生成更灵活。

A novel low field nuclear magnetic resonance （NMR） analyzer for porous media was presented. The analyzer per- mits analysis on water-saturated full cylindrical cores either in the laboratory or at the oil-well field with one-dimensional or two-dimensional measurements, and it can provide T2 distribution, T1 distribution and 2D T1 -T2 maps. The rock test results show that porous media including rock can be measured accurately by one dimensional and two dimensional methods via the analyzer. The sensor consists of an improved Halbach magnet and exchangeable solenoid coils with slotted Faraday shielding. The low cost, linear power amplifier built in IRFS10 and IRF640 produces power up to 250 W with gain of about 50 dB. The active de-coupler uses MOSFET controlled by digital circuit to protect the receiver channel. The gain of wideband preamplifi- er is 56 dB ; the first stage utilizes a low noise wideband discrete RF transistor so that the noise introduced by the preamplifier is negligible. The digital signal processor （DSP） and field programmable gate array （FPGA） are employed to obtain the flex- ible logic control and powerful data processing, to generate all timing and control signal based on one-dimensional or two-di- mensional pulse sequence and obtain the echo train data with digital phase sensitive detection （DPSD）.