Hydrate dissociation conditioned to depressurization below the quadruple point and salinity addition

Abstract

Techniques have been developed to evaluate the hydrate dissociation behaviour in a brine system by reducing pressure below the quadruple point. Experimentally, hydrate with low initial pressure and temperature is dissociated by a deep depressurization method (DDM) in a one-dimensional (1D) model to evaluate the gas and water production. Theoretically, simulation techniques have been developed to determine the two-phase relative permeability, permeability variation, intrinsic hydrate reaction constant by piecewisely fitting the experimental measurements with consideration of the temperature depression induced by NaCl. Subsequently, the existing semi-empirical permeability reduction models are compared with the simulated average permeabilites to identify the occurrence of hydrate in pores. It is shown from experiments that salinity enhances the potential of gas recovery by enhancing the dissociation driving force (i.e., increasing equilibrium pressure and decreasing equilibrium temperature). Temperature decreases almost in a constant rate at sensible heat dominant stages, while, after reaching the freezing point, it is decreased lowly and even recovered slightly due to heat conduction from boundary and ice fusion. By approximating the energy contributions, it is found that sensible heat is supplied for hydrate dissociation at the initial stage and that heat conduction from boundary and ice fusion accelerates hydrate dissociation at the final stage. It is concluded that the salinity lowers the freezing point from −1.7 to −1.3 °C in the experiment. For numerical simulation, the newly developed dissociation model is found to be able to not only identify fluid flow mechanisms and kinetic hydrate dissociation, but also reproduce ice fusion. With hydrate dissociation and water production, salinity continuously decreases and finally becomes less than its initial value. The Kozeny grain center-filling model is found to be able to capture the permeability variation, implying that hydrate tends to be pore filling dominant in the experiments. The fluid flow and hydrate dissociation kinetics are found to play a dominant role above the freezing point, while, below the freezing point, the significance of heat conduction from boundary and ice fusion is enhanced during the experiments.