NMR SURFACE RELAXIVITY DETERMINATION USING NMR APPARENT DIFFUSION CURVES AND BET MEASUREMENTS

摘要

The surface relaxivity is an important parameter in low field NMR T2 or T1 relaxation experiments.It can be determined from the apparent diffusion curve at short time.We show in this paper the applicability and limitations of this technique in various situations(grain packs,sandstones,carbonates) and compared the results with BET. We show first that the V/S ratio is better determined using a fit considering the entire apparent diffusion curve D(t) from short to long diffusion time typically in the range 10 to 1000 ms.The method is validated using a grain pack made of a reference material used in BET measurements with a known specific surface area. We show various measurements illustrating the proposed procedure.On grain packs made with the same material,the measured V/S ratios are proportional to the grain size.On a suite of Fontainebleau sandstones with different porosity and permeability,we obtained a weak variation of the pore size distribution when scaled by surface relaxivity, as expected for this type of formation.On carbonates,the specific surface is often too high to allow reliable NMR S/V measurements with the actual instrument limitations.In these cases,the BET measurement can be used instead.After calibration,the V/S distributions obtained from T2 measurements are smaller than mercury injection derived throat size distributions,as expected.