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

Abstract

T2 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 T2 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 T2 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 T2 distributions, while outcrop cores mainly show unimodal NMR T2 distributions. Tight sandstones showlittle or no mobile fluid controlled by pore throats with radii ≥1 μm, including outcrops and reservoir samples. In conclusion, T2 cutoffs of cores with bimodal NMR T2 distributions and the ones with unimodal NMR T2 distributions were obtained,respectively,at 418 psi and 208 psi, while the former cores exhibit larger T2 cutoffs than the later ones. The critical mobile pore radius for outcrop samples exhibiting a unimodal spectral NMR T2 distribution is 100 nm, whereas the critical pore radius of 50 nm is identified for reservoir samples which display a bimodal T2 distribution. T2gm (The geometric means of T2 fluid distribution at 100% saturation) may propagate error when applying the Schlumberger Doll Research (SDR) model to tight sandstones, whereas T2wa (the weighted average NMR T2 values for the mobile fluid distribution) shows promise as a parameter to predict pore connectivity.