Study on gas-water two-phase seepage law and flow characteristics of the matrix-fracture system in shale gas reservoirs

As a clean energy source, shale gas plays a vital role in supporting China's strategic objectives of carbon peaking and carbon neutrality through its efficient development. Due to the low porosity, low permeability, complex pore structure, and strong heterogeneity of deep shale gas reservoirs, the gas-water two-phase seepage law and flow characteristics remain unclear. This study focuses on deep shale gas in the Western Chongqing block as the research subject. Through the gas-water displacement experiments combined with NMR techniques, the gas-water two-phase seepage laws and flow characteristics of the shale gas matrix-fracture system were revealed. The results show that: (1) In the water displacement gas experiment, formation water cannot break through the matrix-type sample, with its displacement velocity decreasing over time and eventually approaching zero. Under the same time conditions, the higher the displacement pressure, the faster the displacement velocity, and the higher the NMR signal. The matrix-fracture type sample breaks through in a short time, and the time when the NMR signal is stable is much less than that of the matrix-type sample. The displacement pressure is negatively correlated with the NMR signal. (2) In the reverse gas displacement experiment, the gas cannot break through the matrix-type sample. For the matrix-fracture type sample, as the displacement pressure increases, the water phase relative permeability increases, the irreducible water saturation decreases, the gas phase relative permeability decreases, the co-permeability zone slightly expands, and the equal permeability point shifts to the lower left. The findings provide a theoretical basis for the efficient development of shale gas in China.