PARAMAGNETIC RELAXATION IN SANDSTONES: DISTINGUISHING T1 AND T2 DEPENDENCE ON SURFACE RELAXATION, INTERNAL GRADIENTS AND DEPENDENCE ON ECHO SPACING

摘要

Sandstones have T1/T2 ratio of 1.6, on the average. Clean silica has a T1/T2 ratio of about 1.3. If T2 changes with echo spacing in a homogeneous applied magnetic field, the change is interpreted to be due to diffusion in internal gradients. We demonstrate that when the paramagnetic materia material is iron, the increase in the l T1/T2 ratio above that of clean silica is due to diffusion in internal gradient. Furthermore, when the paramagnetic sites are small enough, no dependence on echo spacing is observed with conventional low low-field NMR spectromet spectrometers. Echo spacing dependence is observed ers. When the paramagnetic materials become large enough or form a ‘shell’ around each silica grain such that the length scale of the region of induced magnetic gradients is large compared to the diffusion length durin during the g time of the echo spacing. The basis for these assertions is a series of experiments and calculations described below. Silica sand coated with 0.12 nm Fe Fe3+ 3+ ion has a T1/T2 ratio close to that of clean sand sand. No echo spacing dependence of T2 relaxatio relaxation is observed. Larger ( n 25 nm and 110 nm nm) magnetite particles coated on silica sand have no echo spacing dependence for lower concentration of particles but show echo spacing dependence for larger concentrations. Largest (2, 2,400 nm) magnetite particles 00 mixe mixed with fine sand have echo spacing dependence d and T1/T2 greater than 2.These experimental observations are being interpreted by theoretical calculations. The magnetite particles are modeled as paramagnetic spheres. Paramagnetic particles on the surfa surface of silica grains ce are modeled as individual particles at low concentrations. At high surface concentrations, they are modeled as a thin, spherical shell of paramagnetic material. Calculations show that the system transitions from the motionally averagin averaging to g the localization regime at high surface concentration of paramagnetic particles.