Inspecting internal magnetic field gradients in volcanic rocks

Nuclear magnetic resonance (NMR) is being used since 1990 in the petroleum industry. NMR is a powerful tool for petrophysical properties estimation (porosity, permeability, pore size distribution, and irreducible saturation). Despite its large success in the conventional carbonate and sandstone reservoirs, some tight sandstones, volcanic and metamorphic rocks, contain a high amount of paramagnetic and clay minerals, which can complicate the interpretation of NMR results. These complications are due to the inhomogeneities of the internal magnetic field generated by the magnetic susceptibility contrast between the pore-fluid and the matrix. The magnitude of the internal gradients depends on the strength of the background magnetic field, magnetic susceptibility contrast, and pore size.

Many studies are focused on the investigation of the effect of clay and paramagnetic minerals on the internal gradient and their implications on the NMR-derived petrophysical properties mainly of the high magnetic susceptibility sandstones. The primary goal of this analysis is to investigate the magnitude of the internal magnetic gradient of volcanic rocks with different alteration grad and its relationship with the rock properties (magnetic susceptibility, iron, and manganese content, pore type, and pore size).

The data were collected using the Minispec q10®, with Larmor frequency of 10 MHz, on the water-saturated samples with magnetic susceptibility between 26.8 10^-3 and -0.4 10^-3 SI. The average effective internal gradient was calculated from the slope of the mean log relaxation rate (T2gm^-1) versus the squared echo time (TE²). The preliminary results show that samples presented a multi-distribution of T2 peaks corresponding to the different pore types observed for these samples (micro, meso, and macropores). The average effective internal magnetic field gradient calculated from the slope of T2gm^-1 vs TE2 ranges from 0 to 43.16 T.m^-1. The average effective internal gradient increases with the increase of magnetic susceptibility and decreases as the T2gm increase, suggesting that the pore size also impact internal gradient magnitudes. However, No clear relation exists between iron content and average effective internal gradient.