Calorimetry to Study Metastability of Natural Gas Hydrate at Atmospheric Pressure and Temperatures below 0°C

Abstract

Abstract: Natural gas hydrate is metastable when stored at atmospheric pressure and temperatures below 0°C. The hydrate is regarded as metastable since, under these pressure‐temperature conditions, the rate of dissociation is very low. Isothermal calorimetry was used to study the rate of dissociation, which is a measure of the metastability. Low temperatures and large samples improve the stability. The isothermal method is being developed to quantify metastability. Scanning calorimetry was used to determine the hydrate number and amount of free water. In the scanning method, the natural gas hydrate sample was pressurized with methane. The calorimeter was operated below the methane hydrate equilibrium line and above the natural gas hydrate equilibrium line. Prior to analysis, the sample was conditioned in a separate heating‐cooling cycle. This was necessary to eliminate undesirable thermal responses due to desorption as the ice melted. Desorption occurred because the sample was refrigerated (−20°C) under a high natural gas pressure, but analyzed under a relatively low methane pressure. The scanning method is being developed to analyze natural gas hydrate that contains large amounts of free water.