Parametric Stability Analysis of Marine Risers with Multiphase Internal Flows Considering Hydrate Phase Transitions

This study investigates the effects of multiphase internal flows that consider hydrate phase transitions on the parametric stability of marine risers.A numerical model of the multiphase internal flow that considers a hydrate phase transition is established.The model first solves the flow parameters and subsequently obtains the natural frequencies of risers with different gas intake ratios.The stability charts of marine risers with different gas intake ratios are plotted by applying Floquet theory,and the effects of the gas intake ratio on the instability and vibration response of the risers are identified.The natural frequency increases with an increase in the gas intake ratio;thus,instability zones move to higher frequency ranges in the stability charts.As the increasing gas intake ratio reduces the damping effect of the Coriolis force,the critical amplitude of the heave in the unstable region decreases,especially when hydrodynamic damping is not considered.As a result,higher-order unstable regions are excited.When in an unstable region,the vibration response curve of a riser with a high gas intake ratio excited by parametric resonance diverges quickly due to parametric resonance.

Hydrate phase transition and seepage mechanism during natural gas hydrates production tests in the South China Sea:A review and prospect

Natural gas hydrates(NGHs)are globally recognized as an important type of strategic alternative energy due to their high combustion efficiency,cleanness,and large amounts of resources.The NGHs reservoirs in the South China Sea(SCS)mainly consist of clayey silts.NGHs reservoirs of this type boast the largest distribution range and the highest percentage of resources among NGHs reservoirs in the world.However,they are more difficult to exploit than sandy reservoirs.The China Geological Survey successfully carried out two NGHs production tests in the Shenhu Area in the northern SCS in 2017 and 2020,setting multiple world records,such as the longest gas production time,the highest total gas production,and the highest average daily gas production,as well as achieving a series of innovative theoretical results.As suggested by the in-depth research on the two production tests,key factors that restrict the gas production efficiency of hydrate dissociation include reservoir structure characterization,hydrate phase transition,multiphase seepage and permeability enhancement,and the simulation and regulation of production capacity,among which the hydrate phase transition and seepage mechanism are crucial.Study results reveal that the hydrate phase transition in the SCS is characterized by low dissociation temperature,is prone to produce secondary hydrates in the reservoirs,and is a complex process under the combined effects of the seepage,stress,temperature,and chemical fields.The multiphase seepage is controlled by multiple factors such as the physical properties of unconsolidated reservoirs,the hydrate phase transition,and exploitation methods and is characterized by strong methane adsorption,abrupt changes in absolute permeability,and the weak flow capacity of gas.To ensure the long-term,stable,and efficient NGHs exploitation in the SCS,it is necessary to further enhance the reservoir seepage capacity and increase gas production through secondary reservoir stimulation based on initial reservoir stimulation.With the constant progress in the NGHs industrialization,great efforts should be made to tackle the difficulties,such as determining the micro-change in temperature and pressure,the response mechanisms of material-energy exchange,the methods for efficient NGHs dissociation,and the boundary conditions for the formation of secondary hydrates in the large-scale,long-term gas production.

Electronic factor in electron self-exchange reaction of hydrated redox ion pairs from thermodynamic data

An accurate scheme for determining the electronic factor of the electron self-exchange reaction in solution is presented in this paper. The used various activation parameters and slopes of potential energy surfaces are obtained in terms of an improved activation model and the accurate potential function determined from the vibrational spectroscopic and thermodynamic data. The coupling matrix elements are determined using numerical integral method over the perturbed double-zeta Slater type state functions. Theoretical results of electronic factor in this work are found in close agreement with those extracted from experimental rate constant data and to be less than unity. Results indicate that outer-sphere electron transfer reactions in solution involving hydrated transition metal ions are nonadiabatic in nature.

An In-Situ MRI Method for Quantifying Temperature Changes during Crystal Hydrate Growths in Porous Medium

Given the complexity of the thermo-hydro-chemically coupled phase transition process of hydrates,real-time in-situ observations are required.Thermometry maps are particularly essential in analyzing the heat transfer process during the growth and dissociation of crystal hydrates.In this study,we present the temporally and spatially resolved thermometry of the formation of tetrahydrofuran hydrates based on the temperature dependence of the chemical shift of the water proton.Images of temperature changes were synchronously obtained using a 9.4 T^(1)H magnetic resonance imaging(MRI)system to predict the saturation level of the aqueous solution,phases of the solid hydrates,and the positive temperature anomaly of the exothermic reaction.It was observed that variations in the MRI signal decreased while the temperature rise differed significantly in space and time.The results predicted in this study could have significant implications in optimizing the phase transition process of gas hydrates.