核磁共振测井探测岩石内部磁场梯度的方法

岩石内部磁场梯度对核磁共振测井横向弛豫T₂分布有较大的影响.本文提出了一种利用自旋回波脉冲序列探测岩石内部磁场梯度的二维核磁共振方法，发展了相应的反演方法，得到了含顺磁性物质的饱和水砂岩和泥质砂岩的内部磁场梯度分布规律.结果表明，随顺磁物质含量的增加，岩石内部磁场梯度增大.含顺磁物质的饱和水砂岩颗粒均匀，不含粘土矿物，内部磁场梯度呈单峰分布特征.含绿泥石粘土矿物的饱和水泥质砂岩，T₂分布呈双峰特征，束缚水峰明显，小孔短T₂对应的梯度值大于大孔长T₂对应的梯度值.当绿泥石含量小于15%时，随绿泥石含量的增加，对应束缚水峰的短T₂组分明显增多，T₂分布展宽；当绿泥石含量大于15%以后，较大的岩石内部磁场梯度使短弛豫信息衰减迅速，表现出随绿泥石含量的增加，短T₂组分减少，T₂分布变窄.

NMR logging probing the internal magnetic field gradients of rocks

The internal field gradients in rocks has a big effect on nuclear magnetic resonance (NMR) transverse relaxation T₂ distribution. A two-dimensional NMR method of probing the internal magnetic field gradients of rocks and its inversion algorithms are developed using CPMG pulse sequence. Internal field gradient distributions of water-saturated sandstones and shaly sandstones with paramagnetic impurities are obtained. The results show that internal field gradients of rocks increase with the increasing of content of paramagnetic impurities. The internal field gradients of sandstones with uniform grain show unimodal distributions. The T₂ distributions of water-saturated shaly sandstones with chlorite show bimodal feature with obvious irreducible water peak. The small pores with short T₂ have higher internal field gradients than that of the big pore with long T₂. When the content of chlorite is less than 15%, the short T₂ components corresponding to irreducible water peak increase with increasing the content of chlorite, T₂ distribution becomes wider. However, when the content of chlorite is more than 15%, the higher internal field gradients result in short relaxation component decaying quickly, the short T₂ components decrease, T₂ distribution becomes narrower with increasing the content of chlorite.