Internal Field Gradients In Porous Media

Abstract

The effect of internal field gradients on nuclear magnetic resonance (NMR) spin-spin relaxation (T 2 ) needs to be evaluated for certain cases in NMR well logging. In this paper, systematic methods are developed to calculate the induced magnetic fields and gradients for three types of porous media: spheres, cylinders, and rectangular flakes. Strong internal field gradients were observed on North Burbank (N. B.) sandstones and chlorite/fluid slurries. The experimental observations are compared with calculations. For pores lined with clay flakes, field gradients are concentrated around the sharp comers of the clay flakes regardless of their orientations. The radius of curvature of an object determines the maximum value of the field gradient. Pores lined with clay flakes have the dimensional gradient scaled to the width of the clay flake, whereas for systems of cylinders or spheres the dimensional gradient is scaled to the cylinder or sphere radius. Consequently, thin chlorite clay flakes will have much stronger gradients than larger spherical siderite particles. Both N. B. sandstone and chlorite slurry are simulated as a square macropore lined with rectangular chlorite clay flakes between which are micropores whose total fraction matches that of real systems. The field gradients in the micropores ofN. B. sandstone and chlorite slurry are similar. The mean gradient value of the macropore in the chlorite slurry is much higher than in the N. B. sandstone. Both N. B. sandstone and chlorite slurry have much higher gradients than the field gradients generated by the permanent magnet of logging tools. Measurements of spin-lattice relaxation (T 1 ) and T 2 at different echo spacings (2τ) were performed on N. B. sandstones at various saturation conditions. Gradient values for the whole pore, micropore, and macropore are determined from the slope of the first several data points on the plot of 1/T 2 versus τ 2 . Gradient values from simulations using a 0.2-μm clay width were found to be close to the experimental results for the whole pore and micropore. For macropores, the simulation results match the mean value of the experiments while individual experiments have a larger variation. For chlorite/fluid slurries, the simulation results with a 0.2-μm clay width match well with the mean gradient value of the experiments.