Determination of gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation

Abstract

In this paper, a pragmatic and integrated technique has been developed to experimentally determine the gas-water seepage characteristics with consideration of dynamic pore-throat structure in a tight sandstone gas formation due to a decrease in pore pressure. Conventional experiments were conducted to obtain porosity and permeability of core samples collected from a tight sandstone gas reservoir in the Ordos basin. Then, gas and water flow experiments integrated with the nuclear magnetic resonance (NMR) measurement were employed to obtain the dynamic characteristics of relative permeability curves due to changes in pore-throat structure, while gas permeabilities under different conditions are corrected with consideration of the slippage effect and dynamic pore-throat characteristics. Subsequently, irreducible water saturation and water blocking coefficient were obtained by combining the measured NMR T2 spectrum and gas-water volume, which were used to examine the effects of dynamic evolution of a pore-throat structure on the gas-water micro-distribution and percolation capacity. In the case that the decreased pore pressure inevitably leads to the contraction of pores and throats, although the absolute value of permeability decrease is larger for samples with higher permeability, the relative proportion of permeability decrease is larger for samples with lower permeability, indicating a stronger stress sensitivity. With such a decrease of pore pressure, gas and water relative permeability curves are shifted downwards to the right-hand side, during which the flowing ability of the gas-water system is weakened. The increase in irreducible water saturation is found to be greater in the larger throats, while the relative permeability characteristics and irreducible water saturation of a core sample with a higher permeability have larger changes due to the influence of the dynamic pore-throat structure under the same stress variation. The change in the pore-throat structure due to a decrease in pore pressure imposes the greatest impact on the seepage characteristics at the initial stage.