Probing the internal field gradients of porous media

Abstract

We devise a modified Carr-Purcell-Meiboom-Gill pulse sequence that allows us to probe the apparent internal field gradient distribution of a fluid-saturated porous medium as a function of the pore size. This distribution is displayed as a two-dimensional map with one axis being the field gradient, another axis being the T2 relaxation time reflecting different pore sizes, and the vertical amplitudes being proportional to the proton population. Such a scheme of two-dimensional representation for fluid-saturated porous media can also be used for the identification of pore fluids using the contrast of their diffusion coefficients.

Exploring the separate NMR responses from crude oil and water in rock cores

“…6b) such an improvement is unnecessary. Even though it has been possible to quantify a bulk region that retums information on the wettability of the system, the impact of the intemal magnetic field gradients during the PFG part of the experiment has not been accounted for [17]. At an operating field of 0.05 T one may argue that the strength of this intemal gradient is usually small.…”

The dependence on magnetic field strength of correlated internal gradient relaxation time distributions in heterogeneous materials

“…An even more effective separation results if the gradient can be correlated with another parameters also dependent on pore size and a 2D inversion performed. One approach followed by Sun and Dunn [26] is to use T 2 relaxation to decouple the internal gradients from pore size. Instead, we choose to use the T 1 times of a system, which are related to pore size as:…”

“…Hence, the overall echo train attenuation may be calculated by averaging over the distribution of gradients in the ensemble of spins. The technique for measurement of internal gradients is a modified CPMG sequence that capitalises [26] upon the changing echo amplitude as a function of echo spacing. We take a constant interval of time, t 0 , and vary the number of 180°pulses, which refocus the magnetisation.…”

Observation of molecular migration in porous media using 2D exchange spectroscopy in the inhomogeneous magnetic field

“…Being an exchange method, the technique senses changes in T 2 values as evidenced by the growth of off-diagonal intensity following a mixing time during which molecules are allowed to diffuse. This method relies on the use of two-dimensional Laplace inversion [5] and is one of a number of 2D separation, correlation and exchange methods [6,3,[7][8][9][10][11] based on this type of analysis. However, there are difficulties associated with the inverse Laplace Transform because of pearling effects [12].…”