Experimental Study on CO2 Sequestration in Marine Environments During Hydrate Recovery by Depressurization Combined with Replacement

To protect the environment, sequestering CO2 during the extraction of natural gas hydrates is a highly promising carbon-neutral technology. We investigated methane extraction and CO2 storage by a combined depressurization and CO2-injection method. In the seabed environment, the hydrate-forming period is ~300 min. When injecting liquid carbon dioxide into the reservoir, a replacement rate of ~11.1% is insufficient to meet the requirements for extracting methane and sealing carbon dioxide. Increasing the depressurization pressure can increase gas production. However, an excessively large depressurization pressure will cause the permeability damage rate (PDR) to reach 76.40%, which is not conducive to the continuous progress of the project. Increasing the depressurization pressure (7 MPa) and extending the depressurization pressure time (3 h) can effectively increase the final hydrate saturation (HS) and protect the reservoir permeability. This means higher reservoir stability and more sealed carbon dioxide. A new method has been proposed, namely, the coupling of depressurization and CO2 injection. This method actually extends the depressurization time, resulting in an average reservoir permeability of 1.72 millidarcies, thereby achieving a carbon dioxide storage rate of 27.7%. At the same time, it avoids the repeated implementation of pressure reduction and CO2 injection, reducing the complexity of the project.