Effect of pore-throat structure on irreducible water saturation and gas seepage capacity in a multilayer tight sandstone gas reservoir

Abstract

In this study, effects of pore-throat structure on gas seepage capacity in a multilayer tight sandstone gas reservoir and the interlayer interference characteristics during commingled multilayer production have been experimentally investigated. More specifically, representative core samples were selected from a multilayer tight sandstone gas reservoir in the eastern Ordos Basin according to a statistical analysis of various cores with respect to their petrophysical properties. Then, high-pressure mercury intrusion (HPMI) experiments were conducted to obtain capillary pressure curves of core samples collected from each layer, while their corresponding pore-throat structure characteristics were evaluated based on median throat radius, cutoff throat volume ratio, pore-throat skewness, and fractal dimension. Subsequently, combined with the nuclear magnetic resonance (NMR) technique, gas-water seepage experiments with the collected core samples of each layer were performed to obtain the relative permeability curves and T₂ spectrum distribution curves. Considering the effect of pore-throat structure heterogeneity and water saturation on gas slippage, gas relative permeabilities of core samples were corrected. According to irreducible water saturation distribution, gas relative permeability together with water locking damage coefficient, irreducible water saturation and gas seepage capacity of each layer were quantitatively assessed. In addition, depletion experiments from single- and two-layer cores were conducted to examine the impact of pressure differences and pore-throat structure variations on interlayer interference. The heterogeneity of throats is found to be the main factor dominating irreducible water saturation. With the aggravating heterogeneity in the pore-throat structure, there exists an increase in irreducible water saturation and water locking saturation. Irreducible water is principally distributed in small pores/throats controlled by capillary force, leading to a more serious water locking phenomenon. With a decrease in proportion of small throats and a reduction in structure heterogeneity of large throats, irreducible water mainly occupies as a form of membrane in large pores/throats whose proportion and heterogeneity are the key to gas seepage capacity. With an increase in proportion of large throats and a reduction in their structure heterogeneity, the damage coefficient due to water locking becomes smaller, gas relative permeability at the irreducible water saturation increases, and the gas seepage capacity is enhanced. With a deterioration of pore-throat structures, irreducible water saturation increases, water locking phenomenon intensifies, and gas seepage capacity is weakened. The increase in disparity of interlayer pore-throat structure leads to heightened levels of interlayer interference. The interlayer pressure differentials play a crucial role in determining the extent of interlayer interference in a commingled multilayer production process. In a high-pressure layer, its gas flow rate initially remains relatively high, resulting in a rapid decline in the pressure gradient within the core at the outlet region. In a low-pressure layer, however, water locking is exacerbated by the backflow, thereby reducing the gas seepage capacity in the outlet region and ultimately diminishing its gas production rate. Differences in pore-throat structure are a significant factor affecting interlayer interference. The more pronounced the discrepancy in pore-throat structure between layers, the more severe the impact on the interlayer interference, and subsequently the lower the gas production rate.