In this study, an integrated and practical framework has been developed to quantify the static and dynamic characteristics of microscopic pore-throat structure in a tight oil-bearing sandstone formation. Experimentally, thin sections were prepared out of core samples collected from a tight formation, and corresponding analyses were performed by using the scanning electron microscopy and x-ray diffraction techniques. In addition to pore types and clay minerals, the size, fraction, and distribution of grains were identified and classified. Pore and throat size distributions were then determined by using pressure-controlled mercury injection and rate-controlled mercury injection, respectively, to quantify the changes in microscopic pore-throat structure after brine-flow tests. Subsequently, displacement experiments of water flooding and surfactant flooding were performed to determine the upper and lower cutoff value of the movable throat radius. Rock grains primarily containing fine sand and silt were found to possess complex compositions, and both intergranular pores and intragranular dissolution pores were dominant. Nanothroats may have developed in a tight formation because of small radii of pores and throats. Compared with the pore size distribution, throat radius was found to be reduced by 13.9% after brine-flow tests, and the cutoff value of movable throat can be decreased to 0.019 from 0.033 µm by using a surfactant.

In this paper, the gas relative permeability considering slippage effect has been experimentally examined under various experimental conditions (i.e., ambient, high confining pressure, and high temperature). Experimentally, Klinkenberg permeabilities of 12 core samples have been measured by using steady-state flow experiment. It has been found that the Klinkenberg permeability is independent of the experimental temperature and dramatically decreases as confining pressure is increasing. Furthermore, linear correlations have been newly developed between the Klinkenberg permeability and the gas-measured permeability under various conditions. Subsequently, the developed correlations are correspondingly applied to calibrate the gas relative permeability. It has been found that the gas relative permeability can be overestimated without consideration of the slippage effect, i.e., Klinkenberg effect. In addition, the newly developed correlations have been applied to analyze the sensitivity of gas–water relative permeability to gas-measured permeability, confining pressure, and temperature. It is demonstrated that mobile water greatly alleviates the gas relative permeability in comparison to irreducible water. Although an increased confining pressure simultaneously reduces the effective water phase and gas phase permeability, the gas relative permeability increases and the water relative permeability decreases as the confining pressure increases. It is attributed to the fact that the effective water phase permeability is more sensitive to the confining pressure. Given an elevated experimental temperature, the gas relative permeability is reduced while the water relative permeability is enhanced, implying the significance of temperature effect on gas–water relative permeability measurements.

This paper has developed a pragmatic technique to efficiently and accurately determine the Klinkenberg permeability for tight formations with different pore-throat structures. Firstly, the authors use steady-state experiments to measure the Klinkenberg permeability of 56 tight core samples under different mean pore pressures and confining pressures. Secondly, pressure-controlled mercury injection (PMI) experiments and thin-section analyses are conducted to differentiate pore-throat structures. After considering capillary pressure curve, pore types, throat size, particle composition, and grain size, the pore-throat structure in the target tight formation was classified into three types: a good sorting and micro-fine throat (GSMFT) type, a moderate sorting and micro-fine throat (MSMFT) type, and a bad sorting and micro throat (BSMT) type. This study found that a linear relationship exists between the Klinkenberg permeability and measured gas permeability for all three types of pore-throat structures. Subsequently, three empirical equations are proposed, based on 50 core samples of data, to estimate the Klinkenberg permeability by using the measured gas permeability and mean pore pressure for each type of pore-throat structure. In addition, the proposed empirical equations can generate accurate estimates of the Klinkenberg permeability with a relative error of less than 5% in comparison to its measured value. The application of the proposed empirical equations to the remaining six core samples has demonstrated that it is necessary to use an appropriate equation to determine the Klinkenberg permeability of a specific type of pore-throat structure. Consequently, the newly developed technique is proven to be qualified for accurately determining the Klinkenberg permeability of tight formations in a timely manner.

A robust and pragmatical technique was developed to classify flow pathways during long-term waterflooding operations in a hydrocarbon reservoir. More specifically, pore structure analysis, wettability tests, relative permeability tests, and long-term waterflooding experiments were conducted and integrated. Then, effects of pore-throat structures, displacement rates, crude oil viscosities, and wettability on the oil displacement efficiency across different flow pathways were systematically investigated, allowing us to classify flow pathways into the primary and secondary ones. For the former, pore-throat structure significantly affects the efficiency of displacement: for mouth-bar microfacies, cores with larger pore-throat radii and lower fractal dimensions exhibit superior displacement performance, whereas, for point-bar microfacies, it exhibits greater sensitivity to variations in injection parameters. Increasing the injection rate from 0.2 mL/min to 0.5 mL/min can lead to a 7.31% improvement in oil recovery. Also, high-viscosity crude oil leads to an overall decline in displacement efficiency, with a more pronounced reduction observed in the point-bar microfacies, suggesting that complex pore-throat structures are more sensitive to viscous resistance. For the latter, wettability shows its dominant impact with an increase in oil recovery to 7.12% if the wettability index is increased from 0.17 to 0.21 in the point-bar microfacies.