Abstract:

Production capacity evaluation is a critical task during the early development stages of gas reservoirs. Clarifying the impact of reservoir petrophysical properties, water saturation, and CO₂ volume fraction in fluid composition on gas well production capacity is of great significance for accurately evaluating the production capacity of low-permeability gas reservoirs. This study focused on typical ultra-high temperature, high pressure, and low-permeability gas reservoirs in the Yinggehai Basin. Using the gas-water steady-state flow experiments, the influence of water saturation and CO₂ volume fraction on gas-phase permeability of cores with varying petrophysical characteristics was clarified. Based on the principle of flow velocity approximation, a quantitative multi-factor production capacity chart was established. The results showed that: (1) As water saturation in the rock increased, gas-phase permeability decreased. When water saturation in the target reservoir cores was below the irreducible water saturation (40%), every 10% increase in water saturation corresponded to an average production capacity loss of approximately 12%. When movable water existed, two-phase gas-water flow led to a sharp decline in gas-phase permeability. Specifically, as water saturation rose from 40% to 50%, gas production capacity decreased by about 70%. (2) In ultra-high temperature and high-pressure formations, the presence of CO₂ in the gas composition significantly reduced the gas well production capacity. When the CO₂ content was 28%, the production capacity loss rate was about 12%. As the CO₂ volume fraction increased, its impact on production capacity gradually intensified. When the CO2 volume fraction reached 70%, the production capacity loss rate was around 60%. A multi-factor production capacity evaluation chart was established, incorporating the reservoir petrophysical properties, water saturation, CO₂ volume fraction, and production pressure differentials. This chart provides essential theoretical support and practical guidance for the development of similar gas reservoirs.

Key words: ultra-high temperature and high pressure, low permeability gas reservoir, water saturation, CO₂ volume fraction, production capacity evaluation