2.3.2. Injection of CO2 Liquid

Liquid CO2 was introduced into the cell from the bottom using the gas booster. The injection pressure was 5.6 MPa, and the backpressure at the top of the cell was 5.0 MPa. Thirty-four hours later, the injection pressure and backpressure increased to 6.2 and 5.6 MPa, respectively. The CO2 liquid was injected for more than 17 days while the temperature, pressure, P-wave velocity, and electrical resistance were measured. The formation of CO2 hydrates during the CO2 injection process was expected (refer to Section 3).

#### 2.3.3. Depressurization

The cell was depressurized stepwise using a back-pressure regulator while the inlet valve was closed. Each depressurization step was 0.5 MPa for more than 16 hours. Temperature, pressure, P-wave velocity, and electrical resistance during the depressurization process were measured in the same manner as during CO2 injection process.

#### **3. Experimental Results**
