Parameters optimization of storage capacity of hole-bottom freezing sampling technique for natural gas hydrates

Abstract

The coolant must be pre-stored in the sampler before the freezing procedure for natural gas hydrate sampling is applied. The coolant's storage capacity throughout the samplerlowering procedure is crucial to ensure successful sampling. In this study, the key factors influencing storage capacity were coolant density, dry ice specific surface area, ambient pressure, and temperature difference. An orthogonal method was used to analyze each factor's level of influence and potential action processes. The results indicated that ambient pressure, specific surface area, coolant density, and temperature difference all had significant impact. Ambient pressure affects the phase-change path of dry ice, and high pressure increases the likelihood of dry ice melting, greatly reducing latent heat. The larger specific surface area could help to generate a compact dry ice layer to protect the interior, but it may cause cold energy loss during the freezing process. Dry ice, with a smaller specific surface area, may be a better option. Low-temperature alcohol can separate the dry ice layer from the surrounding environment, allowing for heat exchange. However, a low coolant density may promote heat exchange between the alcohol layer and surrounding environment, resulting in the loss of dry ice. The appropriate coolant formulation comprised of a mixture of 2.5 kg of granular dry ice and 1 L of alcohol, temperature difference maintained at 105 K, and the working pressure of 0.1 MPa.