Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance

Abstract

Based on the experiments of nitrogen gas adsorption (N2GA) and nuclear magnetic resonance (NMR), the multifractal characteristics of pore structures in shale and tight sandstone from the Chang 7 member of Triassic Yanchang Formation in Ordos Basin, NW China, are investigated. The multifractal spectra obtained from N2GA and NMR are analyzed with pore throat structure parameters. The results show that the pore size distributions obtained from N2GA and NMR are different, and the obtained multifractal characteristics vary from each other. The specific surface and total pore volume obtained by N2GA experiment have correlations with multifractal characteristics. For the core samples with the similar specific surface, the value of the deviation of multifractal spectra Rd increases with the increase in the proportion of large pores. When the proportion of macropores is small, the Rd value will increase with the increase in specific surface. The multifractal characteristics of pore structures are influenced by specific surface area, average pore size and adsorption volume measured from N2GA experiment. The multifractal characteristic parameters of tight sandstone measured from NMR spectra are larger than those of shale, which may be caused by the differences in pore size distribution and porosity of shale and tight sandstone.

A Fractal Permeability Model of Tight Oil Reservoirs Considering the Effects of Multiple Factors

“…Thus, an investigation into the flow capability, which is characterized by permeability, of fluid flow in tight reservoirs is urgently needed. Tight reservoirs contain enormous micro/nanopores, in which the fluid flow exhibits micro/nanoscale flow [8][9][10]. The oil flow in micro/nanoscale pores is distinct from that at a macroscopic level [11], and many indoor experiments on fluid flow through micro/nanopores have been conducted.…”

Fractal Analysis of Tight Sandstone Petrophysical Properties in Unconventional Oil Reservoirs with NMR and Rate-Controlled Porosimetry

“…Tight sandstone usually has complex pore structures and wide micro- and nanopore size distributions, which are quite different from the physical properties of conventional sandstone. , Although there are various pore structure analysis techniques, NMR and RCP have been widely used to characterize the pore structure and physical properties of tight sandstones. − NMR is a nondestructive testing method and has been widely applied for characterizing pore size and fluid saturation in core samples. − RCP is an advanced experimental method for studying the pore structures of porous media. ,, It can record the changes in the total mercury intrusion volume with the intrusion pressure to obtain information of the pores and throats. The mercury is injected at a very low quasistatic rate to capture the fluctuation in capillary pressure, and subisons and risons can be used to distinguish pores from throats. , The different pore and throat sizes lead to different mercury intrusion pressures, which can be used to accurately obtain the sizes, quantity, and distributions of pores and throats.…”

Experimental Study and Mathematical Model of Residual Oil Distribution during Gas Flooding in Unconventional Oil Reservoirs with Low-Field NMR

“…Several techniques, such as NMR, high-pressure mercury intrusion (HPMI), scanning electron microscopy, and X-ray computed tomography, are widely used to study the pore throat structure in unconventional oil reservoirs. ,− Among these techniques, HPMI has been effectively used to study pore throat structure characteristics . NMR offers the advantages of a short measurement time without damage to core plugs and therefore has been widely used to analyze pore structures and quantify residual oil distributions. , Wang and Zeng used rate-controlled porosimetry and NMR to characterize the movable fluid distribution in tight sandstone.…”