Experimental Research Of Gas Hydrates Exploitation In Marine Sediments

Gas hydrates are increasingly considered as a potential energy resource. The energy shortage problem can be solved under the condition that the gas hydrate is exploited availably. Four approaches (depressurization, thermal stimulation, inhibitor injection and CO2 replacement) have been proposed to release methane from gas hydrate reservoir. Among these methods, depressurization and thermal-stimulation are regarded as the most promising and feasible gas production technique. Although natural gas hydrates are known to occur in numerous marine regions, there are still lots of limitation in economy and drilling technique to product gas test in-situ. So the situation decides that the recovery of gas from hydrate reservoirs through laboratory measurement to be a necessary and the best way to obtain the dissociation progress by different production approaches.An experimental apparatus was built to investigate the formation and dissociation process of gas hydrate in"methane gas + sands(0.18-0.25mm) +0.03% SDS solution"and"methane gas + Sediments from South China Sea +0.03% SDS solution"systems respectively by depressurization and thermal-stimulation(including electric heating and hot-water injection) methods. Each run experiment includes the formation and dissociation processes. Moreover, it is the first time to apply the TDR (Time Domain Reflectometry) technique to measure hydrate saturation in real time during gas hydrates formation and decomposition, which can reflect the quantity variety of the hydrate accurately and the behavior of the reaction progress in detail.Some results and improvements have been achieved from the experimental results.Research on the formation of gas hydrates in porous sediments system show the reaction began with the sand and gas interface on the top of the sediments and gradually permeated downwards through the sediments in vertical direction, and in horizontal direction the hydrate formation starts from the outer sediment and then propagated gradually to the inner. Hydrate formation reaction rate equation based on the hydrate saturation is created as:dS_H/dt=k_fxS_H^n* , from which reaction rate constant can be calculated and the overall order of the hydrate formation can be determined.The behavior of gas hydrate dissociation by depressurization is observed through many runs experments. The decomposition process consist three stages: fast dissociation, slow dissociation and dissociation end. Hydrates dissociate earlier on the surface and outer layer of the hydrate-bearing sediments than those in inner. The dissociation regulation is accord with the law of decomposition kinetics.There are three major factors that determine gas production rate by depressurization: degree of depressurization is a significant factor influencing the gas production process which decides the whole dissocioaton trend. Initial ambient temperaturue which affects the dissociation rate obviously after the vessel pressure falls down to the setting dissociation pressure (outlet pressure). Initial hydrate saturation affects the decomposition rate complicated. As a result of the dissociation kinetic behavior is various with different range of hydrate saturation which controled by different dominated reaction elements.Based on the hydrate saturation which can reflect the behavior of the hydrate more accurately, a hydrate dissociation rate equation is obtained as:dS_H/dt=k_fxS_H^n*, from which can calculate the hydrate dissociation rate constant and order of the hydrate reaction. According to the experiment results, the decomposition rates were found to be both first order and zero order depending on the type of sediments. In addition, the empirical equation between the hydrate dissociation rate constant and the effect factors (depressurizing range, initial ambient temperature and initial saturation) is also established.The hydrates decomposition regulation by the electric heating method is acquired. The process of dissociation can be divided into three stages: initial heating and dissociation, heating of the hydrate-bearing sediments, and complete production. The analysis of both energy efficiency and thermal efficiency indicate that the gas hydrates needs to pay more time to dissociate completely, which results in the lower ecnomic and availability.Gas production process by hot-water injection and the factors which affect the rate of hydrate dissociation are investigated. Gas can be recovered from the bottom of the hydrate reservoir to the top gradually by adjusting the valves located in different height of the vessel. Raising the temperature of the hot water can get a higher dissociation rate. The faster speed of injecting hot water, the higher dissociation rate is obtained. In contrast, the speed of the injecting hot water plays a leading role in this method. The higher injecting speed, the higher efficiency in both energy and thermal achieved.It is significant to synthesize the hydrate within the real South China Sea sediments, and then recover the gas by decomposition. The results showed that temperature vibration and increasing the contract surface area between gas and sediments can promote the hydrate formation speed effectively and obtain the gas hydrates with the saturation 27% ultimately. The decomposition is conducted under the conditions of 3℃(initial temperature) and 2MPa (outer pressure). According to variety of the hydrate saturation during the hydrates decomposition, the dissociation constant rate ( K ds=0.71) was calculated and a zero order reaction was observed. It indicted that depressurization method was adequate to produce gas from sedimentds of South China Sea.