Experimental And Numerical Study Of The Factors On The Gas Production From Hydrates In Porous Media

Natural gas hydrate is the focus of a special attention as a potential energy resource due to its characteristics of wide distribution, a vast amount of reserve, high energy capacity, etc. In this study, an attempt was made to address the hydrate dissociation behavior, the sensitivity of factors on gas production and the evaluation of different exploitation methods with an investigation of experimental and numerical simulation.In the first part of the work, experiments were conducted to understand the formation process of hydrates in porous media, and to determine the permeability of the hydrate-bearing porous media. The results showed that the characteristic of hydrate formation can be affected by the initial water saturation. By comparing the experimentally determined permeability with those calculated using the empirical permeability correlations, the relationship of hydrate saturation and hydrate accumulation habits in porous media was found, and then a novel permeability model based on the weighted combination of pore-filling and grain coating model was proposed. Secondly, considering the hydrate accumulation habits in porous media, the expressions of special surface area for hydrate dissociation were determined with the comparison of the experimental data and numerical results.One-dimensional (1D) based on IMPES and two-dimensional (2D) cylindrical, r-z, based on fully implicit (FI) solution method were developed for dissociation of hydrate in porous media. Sensitivity analysis was done with the parameters, including hydrate accumulation habits in porous media, simulation scale, initial water saturation, etc. The performance of gas production from hydrates induced by different depressurizing approaches or depressurization with wall-heating method was evaluated.The results indicated that the hydrates dissociate along the radial and longitudinal direction, and the dissociation in the radial direction is earlier than that of the longitudinal direction of the laboratory-scale hydrate sample. The effects of the hydrate distribution in porous media were examined by adapting the conceptual models of hydrate accumulation habits into simulations to govern the evolution of permeability with hydrate decomposition. The simulations suggested that the performance of gas production with the hydrate coating models is superior to that of hydrate filling models. There existed a "switch" value (the "switch" absolute permeability) for laboratory-scale hydrate dissociation in porous media. The absolute permeability had almost no influence on the gas production behavior when the permeability exceeded the "switch" value. In this study, the "switch" value of the absolute permeability was estimated to be50md. On the other hand, it can be found that the cumulative gas production is not affected by the assumption of stationary water phase with the condition of simulation scale length vs. diameter L/d<50; while there would be some opposite results presented in the gas production performance under a larger simulation scale L/d>50. Moreover, for the case of Sw,e (initial water saturation)<Swr,e (irreducible water saturation), or Sw,e>Swr,e, there were different control mechanisms dominating the process of hydrate dissociation and gas production. The flow ability dominated the gas production behavior under the condition of Sw,e>Swr,e.Finally, the gas production performance from hydrates induced by different depressurizing approaches or depressurization with wall-heating method indicated that the depressurization process with depressurizing range has significant influence on the final gas production. On the contrary, the depressurizing rate only affects the production lifetime. More amount of cumulative gas can be produced with a larger depressurization range or lowering the depressurizing rate for a certain depressurizing range. The gas production from hydrate by means of depressurization with wall-heating method is higher than that of depressurization or wall-heating method, and the wall-heating is shown to affect only a limited region. For the depressurization with wall-heating method, a higher initial hydrate saturation can resulted in a lower gas production rate, and the effect of initial temperature and initial pressure on the gas production can be neglected, which were very different from that by using depressurization.