Stage analysis and production evaluation for class III gas hydrate deposit by depressurization

Natural gas hydrate is of wide distribution and great potential as clean energy. To improve the production performance, the production characteristics of class III gas hydrate are studied by numerical simulation method when initial gas saturation is below the irreducible gas saturation. Based on the gas production behavior, a quantitative method is developed using both the production data and deposit properties to analyze the production process. A new index is introduced to evaluate the energy utilization efficiency of production stages. Then the influencing factors are analyzed. The results indicate that production can be divided into four stages, including slow changing stage, rapid increasing stage, rapid decreasing stage and stable decreasing stage. The boundaries between stages are clearly defined. Compared with other production stages, the first stage has lower energy utilization efficiency. The ratio drop of energy consumed by this stage can enhance the accumulative gas production. The gas flow ability and drawdown pressure impact the production stage and production performance. Optimization of related factors can improve the production performance. Hot fluid injection and fracturing should be considered when reservoir energy is low or gas flow ability is weak.