Abstract: The deep gas reservoirs in Yinggehai Basin, South China Sea is featured by typical characteristics such as high temperature, high pressure, high water content and high CO₂ content, resulting in a very special seepage mechanism. In order to disclose the seepage characteristics of these gas reservoirs and analyze the influencing factors of gas productivity, a experimental system of ultra-high-temperature and high-pressure long core displacement was established to simulate the temperature and pressure conditions of the actual reservoir, and gas seepage experiments were conducted with different water cuts and different CO₂ contents. It was found in the study that the seepage characteristics of the gas reservoir could be divided into four stages: initiation pressure, low-velocity non-Darcy seepage, Darcy seepage and deviation from Darcy seepage; bound water would cause low-velocity non-Darcy seepage, moving water might result in initiation pressure, and bound water would turn into moving water under high pressure difference, making gas seepage deviate from Darcy seepage, causing or aggravating gas-water flowing and reducing gas seepage at high pressure difference; CO₂ in the gas not only prolonged the low-velocity non-Darcy seepage, but also promoted the conversion of bound water into moving water at high content, forming initiation pressure and bringing forward the deviation from Darcy seepage at high pressure difference in advance. Therefore, the increase in water saturation and CO₂ content in these reservoirs will inhibit gas production, and production pressure differential should be strictly controlled to avoid low-velocity non-Darcy seepage and deviation from Darcy seepage at high pressure difference. The results of the study may provide a theoretical basis for the efficient development of these gas reservoirs.

Key words: deep gas reservoir, seepage mechanism, water saturation, CO₂ content, gas production capacity, non-Darcy seepage