Digital assessment of phase changes and heat transfer for hydrates in microstructures using X-ray CT imaging

Abstract

Natural gas-hydrates are plural clean energy resource with rich deposits, and hydrate behavior significantly affects the exploration, production and utilization of these energy resources. However, technical difficulties for effective production and utilization of these energy sources still exist. Hydrate formation using xenon and NaBr brine and X-ray computed tomography (CT) imaging experiments are conducted to investigate microstructural morphology phase behaviors of hydrates, and a novel multi-phase color difference separation method to segment the pore, hydrate, water and grain phases is proposed. Effects of underground temperature gradient on thermal properties during the phase changes of hydrates are analyzed, and their effects on flow properties are studied. Results indicate that the temperature fraction, hydrate phase fraction, latent-heat thermal energy (LTE) and total enthalpy decrease with increasing temperature gradient. Hydrates rapidly form approximately at time = 250s for temperature gradient of 50 K and time = 500s for temperature gradients of 100 K, 150 K, 200 K and 300 K, and the hydrate phase equals to each other approximately at time = 95s. The stable LTE and total enthalpy approaches to a maximum when the temperature gradient is 150 K. Moreover, the temperature fraction and total enthalpy decrease on average by 4.73%, 4.69% and 76.13%, 75.87%, and the hydrate phase fraction increases by 7.89% and 7.88% with increasing microporosity and micropore size from 34.66% to 41.93% and from 37.53 μm to 43.22 μm, respectively. The temperature fraction and total enthalpy increase on average by 5.29% and 76.67% with increasing pore-scale hydrate saturation from 0.73 to 1.00. The LTE remains constant with an increase in hydrate pore-scale variables of microporosity, micropore size and hydrate saturation from 34.66% to 41.93%, from 37.53 μm to 43.22 μm and from 0.73 to 1.00, respectively. In addition, numerical permeability computed by the proposed relation of microporosity and pore-scale hydrate saturation agrees well with experimental results, with the average errors all less than 5%. The temperature and total enthalpy in phase change heat transfer of hydrates improve the permeability. The LTE in phase change heat transfer of hydrates hardly changes the permeability, and hydrate phase decreases the permeability. This work provides the quantitative and qualitative evaluations of phase change and heat transfer behaviors of gas-hydrates, which are helpful and provide excellent tools to develop the techniques for the exploration, production and utilization of these energy resources to decrease the energy crisis.