Combined proton T1N and CPMG T2N studies of water saturated sandstone core plugs

Diffusion dynamics for water in a series of sandstone core plugs with a broad range of permeabilities was studied using both the Carr-Purcell-Meiboom-Gill (CPMG) and inversion recovery experiments. Both the transverse and longitudinal magnetization curves were found to fit well to stretched exponential relaxation kinetics. At short times, the transverse magnetization is well described by an expression for free diffusion averaged over a distribution of pore sizes. The stretch exponents for the transverse and longitudinal magnetization are shown to be simply related to the width of the pore size distribution. A cross over from free to restricted diffusion is evident in the dependence of T2 with increasing diffusion time set by the interpulse spacing tau in the CPMG experiment. The T2(tau) data is fit to a model which interpolates between the limits of free and restricted diffusion. A length derived from this model is shown to provide a simple estimate of the absolute fluid flow permeability.