Characterization of Overall Pore Size Distribution Using N₂GA, MICP, NMR: A Case From Chang 7 Tight Sandstone in Ordos Basin, China

Abstract

Tight sandstone shows a wide pore distribution with pore size continuously ranging from several nanometer to hundred micron. There are many methods used for pore size distribution (PSD) characterization, but no single method can completely cover the broad range of PSD of tight sandstone. N₂ gas adsorption (N₂GA) which is one of the most important methods to evaluate pore structure of porous material can accurately obtain a narrow range of meso-pore (2-50nm) distribution. Mercury injection capillary pressure (MICP) has been widely used to evaluate pore structure of reservoir rocks, with the range of measurement optimally from 30nm to 200mm. Low field nuclear magnetic resonance (NMR) provides a fast, convenient and nondestructive tool for characterizing complex porous media, particularly the petroleum reservoir rocks, obtaining the PSD of all kinds of water-filled pores. And T₂ spectrum distribution can be calibrated for PSD with the gas-water centrifugal experiment, according to certain linear relationship between transverse relaxation time T₂ values and radius. Therefore, N₂GA, MICP and NMR are comprehensively applied to investigate the overall pore size distribution of Chang 7 tight sandstone.

N₂GA exhibits excellent measurement ability of meso-pores, with pore volume(< 30nm) of N₂GA occupied 10.07% of total pore volume. BJH curves exhibit a low amplitude bimodal or unimodal meso-pore distribution, while pore distribution of MICP and NMR is more closed to the x axis within 30nm.The PSD of MICP and NMR is a unimodal and bimodal curves respectively, both composed of partial meso-pores and macro-pores(>50nm). In addition, pores volume(>30nm) of total pore volume of MICP and NMR is about 75.13% and 93.02% respectively. So the overall PSD of Chang 7 tight sandstone mainly ranges from 1.7nm to 35μm, with the 1.7 to 30nm obtained by N₂GA and the 30nm to 35μm obtained by MICP or NMR. What's more, comparison of PSD and its parameters such as porosity, median pore radius and peak value obtained from MICP and NMR reflects obvious differences and indicates pore-throat distribution always moving in the direction of smaller pores,as large of pores are controlled by narrow throats in the Chang 7 tight sandstone.

The combination of pore size distribution obtained from N₂GA, MICP and NMR can not only fully cover the range of pore size from tight sandstone, but also reflect different aspects of tight sandstone pore structure.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017