Institute of Geophysics, ETH Zürich, Switzerland.
J Magn Reson. 2012 Jul;220:62-9. doi: 10.1016/j.jmr.2012.04.009. Epub 2012 May 2.
Understanding multi-phase fluid flow and transport processes under various pressure, temperature, and salinity conditions is a key feature in many remote monitoring applications, such as long-term storage of carbon dioxide (CO(2)) or nuclear waste in geological formations. We propose a low-field NMR tomographic method to non-invasively image the water-content distribution in electrically conductive formations in relatively large-scale experiments (∼1 m(3) sample volumes). Operating in the weak magnetic field of Earth entails low Larmor frequencies at which electromagnetic fields can penetrate electrically conductive material. The low signal strengths associated with NMR in Earth's field are enhanced by pre-polarization before signal recording. To localize the origin of the NMR signal in the sample region we do not employ magnetic field gradients, as is done in conventional NMR imaging, because they can be difficult to control in the large sample volumes that we are concerned with, and may be biased by magnetic materials in the sample. Instead, we utilize the spatially dependent inhomogeneity of fields generated by surface coils that are installed around the sample volume. This relatively simple setup makes the instrument inexpensive and mobile (it can be potentially installed in remote locations outside of a laboratory), while allowing spatial resolution of the order of 10 cm. We demonstrate the general feasibility of our approach in a simulated CO(2) injection experiment, where we locate and quantify the drop in water content following gas injection into a water-saturated cylindrical sample of 0.45 m radius and 0.9 m height. Our setup comprises four surface coils and an array consisting of three volume coils surrounding the sample. The proposed tomographic NMR methodology provides a more direct estimate of fluid content and properties than can be achieved with acoustic or electromagnetic methods alone. Therefore, we expect that our proposed method is relevant for geophysical applications, such as for monitoring CO(2) injections in saline aquifers or detecting water leakage into nuclear waste deposit sites installed in electrically conductive formations.
理解多相流体在各种压力、温度和盐度条件下的流动和传输过程是许多远程监测应用的关键特征，例如在地质构造中长期储存二氧化碳（CO(2)）或核废料。我们提出了一种低场 NMR 层析成像方法，用于非侵入式地成像相对较大规模实验（∼1 m(3)样品体积）中导电地层中的含水量分布。在地球的弱磁场中操作意味着在电磁场所能穿透导电材料的拉莫尔频率较低。在信号记录之前进行预极化，可以增强与地球磁场中 NMR 相关的低信号强度。为了在样品区域中定位 NMR 信号的起源，我们不采用像常规 NMR 成像那样的磁场梯度，因为在我们所关注的大样品体积中很难控制它们，并且可能会受到样品中磁性材料的影响。相反，我们利用安装在样品体积周围的表面线圈产生的磁场的空间依赖性不均匀性。这种相对简单的设置使仪器价格低廉且便携（它可以潜在地安装在实验室之外的远程位置），同时允许大约 10 cm 的空间分辨率。我们在模拟 CO(2)注入实验中证明了我们方法的一般可行性，其中我们定位并量化了在将气体注入充满水的 0.45 m 半径和 0.9 m 高的圆柱形样品后含水量的下降。我们的设置包括四个表面线圈和一个由三个体积线圈组成的阵列，环绕样品。所提出的层析 NMR 方法比单独使用声学或电磁方法提供了对流体含量和性质的更直接估计。因此，我们预计我们提出的方法与地球物理应用相关，例如监测盐含水层中的 CO(2)注入或检测安装在导电地层中的核废料储存场所中的水泄漏。
