Gas–Water–Sand Production Dynamics of Offshore Interlayer-Buried Shallow Gas Reservoirs in the South China Sea

Offshore shallow gas resources, once considered potential geological hazards, are now viewed as valuable unconventional oil and gas sources. Despite growing interest, there is a notable lack of comprehensive literature on production characteristics, leaving many aspects of shallow gas production unclear. This study addresses these gaps using test production data and well log analysis from a shallow gas well in the South China Sea to simulate the long-term gas–water–sand production dynamics of an interlayer-buried shallow gas reservoir (IBGR) with thick sequences of thin gas layers interspersed with water-filled interlayers. The simulation showed that after 3 days, the gas production rate was around 64,000 m³/day, close to the field rate of 55,000 m³/day. Over 2 years, the rate dropped to about 23,000 m³/day, indicating a rapid decline. Initially, the pressure drop affected the gas layer, but later, water invasion from the interlayer shifted the pressure drop toward the middle of the IBGR, reducing gas production. Key sensitivity factors affecting gas production were analyzed. Higher gas layer permeability increased both the gas and water output. High residual water saturation in the gas layer reduced water production but boosted gas output, whereas higher interlayer permeability led to more water invasion, decreasing gas production. The risk of sand production was also predicted. During the 2 years of depressurization, sand production ranged from 0.18 to 0.9 m³, posing minimal risk. The contributions of different gas layers to production and the impact of the gas layer distribution on production were evaluated. Different gas layers contributed variably to production, with Gas 5’s contribution rising from 27.3% to 44% over 2 years. Centrally placing productive layers resulted in higher long-term production rates than placing them at the edges due to reduced water invasion. This research provides critical insights into optimizing production strategies for offshore shallow gas reservoirs.