Experimental Investigation On The Spatial Differences In Dissociation Strength During Natural Gas Hydrate Exploitation Via Depressurization

Natural gas hydrates(NGHs)are important green alternative energy source,with the characteristics of large resource reserves,high energy density,wide distribution and clean combustion.Depressurization method is considered as one of the most effective and economic strategies for the development of NGHs.It is of great significance to clarify the characteristics of dissociation and production during the depressurization of NGHs.However,the spatial difference of hydrate dissociation and its influencing factors during the depressurization of NGHs are still unclear.In this paper,a systematic study on the spatial differences of hydrate dissociation by depressurization is carried out.A novel Lshape hydrate simulator is established and used to investigate the spatial differences of hydrate dissociation by depressurization in different systems.The influence mechanism of the gravity and the use of dual horizontal wells are also investigated.The main contents of this study are as follows:(1)A novel design method of interconnected multi-reactor NGHs simulator was proposed.Through this design method,the required spatial differences of hydrate dissociation during the depressurization of NGHs can be obtained.According to this design method,a preliminary L-shape hydrate simulator was design and established.Gassaturated and water-saturated methane hydrate reservoirs were prepared,and the homogeneity of the distribution of the prepared hydrates was evaluated.It is found that for gas-saturated hydrate reservoirs,the instantaneous temperature at each measuring point shows the same trend and change range,and both increase rapidly to 284.05 K and then decrease slowly to 278.15 K.It indicates that the distribution of the prepared gassaturated methane hydrate is homogeneous.In addition,for water-saturated hydrate reservoirs,the regeneration of hydrate only occurs at the gas-water interface in the process of brine injection.The rise range,change trend and change duration of the instantaneous temperature at each measuring point are basically the same,indicating that the final prepared water-saturated hydrate is homogeneously distributed.(2)A systematic experimental study was carried out on the spatial differences of methane hydrate dissociation by depressurization in confined gas-saturated environment.The effects of mining pressure,particle size of quartz sand and montmorillonite were investigated respectively.The restriction mechanism of hydrate dissociation was analyzed and the dissociation kinetics model was established.It is found that hydrate dissociates faster at the locations far from the mining well than near the mining well.The dissociation of hydrate is completed earlier at the location far from the mining well.These phenomena are affected by the migration of water in the sediments.In other words,the lower the water saturation in the sediments,the faster the dissociation of hydrate.The experimental results also show that the spatial difference of hydrate decomposition is more pronounced in sediments with better permeability and at lower mining pressure.The established hydrate dissociation kinetics model is in good agreement with the experimental data in quartz sand sediments.(3)The study on the spatial differences of hydrate dissociation by depressurization in the vertical and horizontal directions in confined water-saturated environment were carried out.The controlling factors of hydrate dissociation were analyzed and the effects of particle size of quartz sand and montmorillonite were investigated.It is found that in confined water-saturated environment,hydrate dissociation is controlled by the release of free pore space in the sediments.In the vertical direction,the uneven release of free pore space caused by the migration of water in the sediments leads to the spatial difference of hydrate dissociation.Hydrate dissociates faster at the locations far from the mining well with higher instantaneous pressure.In the horizontal direction,the pressure gradient in the sediments leads to the spatial difference of hydrate dissociation.In addition,the spatial difference is more pronounced in sediments with higher permeability,and montmorillonite in the sediments has a significant influence.(4)In this section,the strategy of placing the reactor horizontally and vertically was used to investigate the influence of gravity on the spatial difference of hydrate dissociation and production characteristics.Experiments were carried out on the mining process of hydrates by depressurization in three different environments.The results show that in confined gas-saturated environment,gravity has no obvious influence on the final gas recovery ratio and cannot cause changes in the trend of spatial difference.However,gravity can cause more pronounced spatial difference.In confined water-saturated environment,gravity doses not influence the final gas recovery ratio.However,gravity can the spatial difference of hydrate dissociation in the vertical direction to be different from that in the horizontal direction.In unconfined water-saturated environment,gravity can not only cause more and faster water migration,but also more easily lead to the formation of dominant channels for water migration in the sediments.This leads to a relatively faster rate of hydrate dissociation near the mining well.(5)The effects of dual-horizontal wells on the improvement of hydrate production characteristics and on the spatial differences of hydrate dissociation were studied.It is found that in confined gas-saturated environment,the use of dual-horizontal wells only improves the gas production efficiency by less than 50% and the final gas recovery ratio remains unchanged.The dual-horizontal wells can cause the characteristic migration of water in the sediments,resulting in relatively slower dissociation of hydrate at the two horizontal wells.In confined water-saturated environment,the use of dual-horizontal wells can improve the gas production efficiency by more than 80%.The use of dualhorizontal wells can cause the migration of water in the sediments,which leads to relatively faster hydrate dissociation far from the mining well.In unconfined watersaturated environment,the use of dual-horizontal wells can significantly improve the gas recovery ratio,but it will result in greater water production.The pressure gradient in the sediments leads to the spatial differences of hydrate dissociation.Hydrate dissociates faster at the bottom mining well where the pressure is lower.