Effects of Gas Dissolution on Gas Migration during Gas Invasion in Drilling

Abstract

Sour gas reservoirs (including CO2 and H2S) are vulnerable to gas invasion when drilling into reservoir sections. The high solubility of the invaded gas in drilling fluid makes the gas invasion monitoring “hidden” and “sudden” for later expansion, and the blowout risk increases. Accurate prediction of gas dissolution is highly significant for monitoring gas invasion. In this study, the gas–liquid flow control equations considering gas dissolution were established. Focusing on the gas dissolution effect, a solubility experiment for CO2 and CH4 in an aqueous solution was performed using a phase equilibrium device. The experimental and simulation results revealed that the addition of CO2 can significantly increase gas dissolution, and the presence of salts decreases it. For solubility prediction of pure CH4 and CO2, the fugacity–activity solubility model, calculated using the Peng–Robinson equation of state, was more accurate than the Soave–Redlich–Kwong equation of state. The Soave–Redlich–Kwong equation of state has higher accuracy for the CO2 and CH4 gas mixture. If the gas dissolution effect is considered for wellbore gas–liquid flow, the time required for the mud pit gain to reach the early warning value increases. When the contents of CO2 and H2S in intrusive gases are higher, the time for mud pit gain change monitored on the ground increases, the concealment increases, and the risk of blowout increases.