Multistep Depressurization-Assisted CH ₄ –CO ₂ Exchange Process for CO ₂ Sequestration and Methane Yield from Hydrate Reservoirs

Natural gas hydrates (NGH) are crystalline materials and are widely distributed across the world. Conventional methods of exploiting resources from NGH reservoirs have limitations in terms of formation stability, sluggish methane production, and adverse environmental impacts. A thermodynamically favorable CO2–CH4 exchange process can be incorporated for natural gas production to ensure seafloor and formation integrity. However, this process has a limitation of slow replacement rates, which makes this method currently economically and technically challenging. In this study, slow multistep depressurization-assisted CH4–CO2 exchange is used for recovering methane and simultaneous CO2 sequestration in lab-simulated reservoir conditions. A one-dimensional reactor cell is used to investigate the kinetics of hydrate formation, dissociation characteristics, and CH4–CO2 exchange process. The parameters studied are hydrate saturation, methane injection pressure, free water content, hydrate saturation distribution, and differential pressure. Produced water and produced gas compositions were also analyzed to study sediment stability and recovery efficiency, respectively. The experiments were planned in three phases, viz., methane hydrate formation, CO2 injection and soaking, and production phase. During the soaking period, CO2 interacts with the existing methane hydrate structure and promotes CO2 hydrate and mixed hydrate formation. The production stage involves the creation of a pressure differential by regulation of the back pressure regulator, which induces the dissociation of the mixed hydrate and creates a driving force to produce the gases. Sand was used as porous media, which promoted multiple hydrate nucleation cycles. Methane injection pressure is found to play a crucial role in determining the hydrate distribution pattern and connectivity among sand grains. During the production cycle, the methane yield efficiency and CO2 storage efficiency were determined. A new hydrate phase equilibrium was seen to exist for mixed CH4 and CO2 hydrates in each production experiment conducted. Gas yield and flow were influenced by pore connectivity and pressure gradients, while total dissolved solids/watercut values in produced water indicated sediment stability. The outcome from this study can provide a database and further guidelines to support any pilot production test, which is/may be planned for CO2 sequestration and methane production from hydrate using the exchange process.