Numerical simulation for subsidence control in CO2 storage and methane hydrate extraction

Gas hydrates are increasingly viewed as a promising alternative to traditional fossil fuels. However, their extraction process poses risks to structural integrity, potentially causing significant subsidence. In this study, we developed a Thermo-Hydro-Mechanical-Chemical (THMC) model to analyze the impact of gas hydrate extraction on seabed subsidence. Our investigation focused on the influence of bottom hole flowing pressure, initial hydrate concentration, gas saturation, permeability, porosity, and rock thermal conductivity on subsidence during gas hydrate extraction via depressurization.

The results show that seabed subsidence is affected by various factors such as bottom hole flowing pressure, initial hydrate concentration, gas saturation, permeability, porosity, and rock thermal conductivity. It was noted that significant subsidence is associated with low initial hydrate concentration, high permeability, porosity, low gas saturation, low rock thermal conductivity, and a notable pressure drop of 79.31%.

To address this issue, we propose a seabed subsidence mitigation strategy involving CO₂ injection. This approach not only safeguards offshore infrastructure and coastal communities but also helps reduce CO₂ emissions, aligning with global climate change mitigation efforts. In our model, CO₂ injection occurs in the subsurface reservoir at the interface between the free water zone and hydrate-bearing formations. The CO₂ hydrates formation process releases heat, which dissociates methane hydrates, allowing the methane to be replaced by CO₂ molecules and move towards the production well.

Our analysis reveals that increasing injection temperature and rate significantly reduces subsidence. Additionally, the range of investigated injection pressures, which included pressures equal to and more than double the initial reservoir pressure, showed inconsequential impacts on seabed subsidence.

The effectiveness of subsidence reduction is significantly enhanced by injecting a CO₂/N₂ mixture compared to pure CO₂ injection. The most substantial reduction in subsidence occurred when a mixture of CO₂ and N₂ in a 50/50 vol/vol ratio was injected at a high rate.

These findings offer crucial insights for optimizing the efficiency and control of gas hydrate extraction methods. They emphasize the importance of employing balanced injection strategies to minimize environmental risks and ensure sustainable energy extraction.