Gas-water two-phase percolation mechanism in tight gas reservoirs based on NMR visualization experiment

Key words:  tight gas reservoir  aperture ratio  SE pulse sequence  gas-water relative permeability  gas-water move

Abstract:

In this study, high pressure mercury injection combined with nuclear magnetic T2 spectra was used to investigate pore size distribution of rock samples from the Tai-2 and Shan-2 subdivisions in X well block, Daniudi gas field. In the nuclear magnetic resonance (NMR) visualization experiments, nuclear magnetic image with new spin-echo single point imaging(SE-SPI) sequence complementary was used to study the gas-water two-phase flow, in order to reveal the displacement mechanisms under different conditions in dense gas reservoirs and to understand the two phase migration mechanism in core pores during the process of gas driving water. The results show that, for tight sandstone gas reservoirs, the gas-water two-phase co-permeability area widens with the increase of the displacement pressure difference. Compared with Tai-2, the signal quantity in the core of Shan-2 changes faster, and the gas-water transport speed is faster. The water utilization of the first type reservoir is higher than that of the second type reservoir. With the progress of gas displacement, the signal volume in the small core pores of the Shan-2 increases significantly, indicating that the self-absorption in the small pores can increase the residual water saturation, and meanwhile the gas phase seepage channel can be further expanded, reducing the possibility of water locking.