A new method of estimating tortuosity and pore size in unconventional formations using nmr restricted diffusion measurements

Tortuosity and pore size are two petrophysical properties useful for estimating permeability. Given the small pore systems in unconventional rocks, the free diffusion of the pore fluids is restricted by the pore walls. NMR restricted diffusion measurements of fluids in porous media can provide both tortuosity and pore-size information. In this study, we focus on a series of low permeability organic-rich chalks with connate water present. The core samples are pressure-saturated with two hydrocarbons, high-pressure methane (C1) and decane (C10). NMR measurements are conducted under pressure to obtain the restricted diffusivity of the hydrocarbon-bearing pore space. In planning the NMR restricted diffusivity measurements, an optimum series of diffusion-encoding times are chosen for the pulse-field gradient (PFG) pulse sequence to obtain the correlation between the restricted diffusivity (D) and free diffusion length (D). The Padé fit on D versus D! is then used to estimate the tortuosity and pore-size of the hydrocarbon-filled pore space. The restricted diffusivity of C10 lies in the short diffusionlength regime, which determines the surface-To-volume ratio and pore-body size. The restricted diffusivity of high-pressure C1 lies in the long diffusion-length regime, which determines the tortuosity. We also report on the accuracy of the tortuosity and poresize using propagation of errors in the Padé fit.