Experimental investigation on T2 cutoffs of tight sandstones: Comparisons between outcrop and reservoir cores

T-2 cutoff is a key representative parameter of rock pore structure, which can reflect the degree of mobile fluid as a function of variable water saturation controlled by the extent of centrifuging. However, few studies of T-2 cutoff have been done on tight sandstones. In this study, residual fluid of samples after a series of centrifuges were scanned by NMR to determine T-2 cutoffs (the relaxation time separating mobile from immobile water) and the extent of mobile fluid as a function of variable water saturation controlled by the extent of centrifuging. Results indicate that all reservoir cores exhibit bimodal NMR T-2 distributions, while outcrop cores mainly show unimodal NMR T-2 distributions. Tight sandstones showlittle or no mobile fluid controlled by pore throats with radii >= 1 mu m, including outcrops and reservoir samples. In conclusion, T-2 cutoffs of cores with bimodal NMR T-2 distributions and the ones with unimodal NMR T-2 distributions were obtained,respectively,at 418 psi and 208 psi, while the former cores exhibit larger T-2 cutoffs than the later ones. The critical mobile pore radius for outcrop samples exhibiting a unimodal spectral NMR T-2 distribution is 100 nm, whereas the critical pore radius of 50 nm is identified for reservoir samples which display a bimodal T-2 distribution. T-2gm (The geometric means of T-2 fluid distribution at 100% saturation) may propagate error when applying the Schlumberger Doll Research (SDR) model to tight sandstones, whereas T-2wa (the weighted average NMR T-2 values for the mobile fluid distribution) shows promise as a parameter to predict pore connectivity.