Jinlong Cui,Kun Li,Liwei Cheng,Qingping Li,Zhenfeng Sun,Peng Xiao,Xingxun Li,Guangjin Chen,Changyu Sun
To improve the exploitation efficiency of future commercial marine natural gas hydrate , it is of critical significance to have a solid understanding of the spatial differences of hydrate dissociation. However, few studies have investigated this topic. In this study, a novel L-shape Hydrate Simulator (LHS) with a vertical vessel and a horizontal vessel was established and used to investigate the spatial differences of hydrate dissociation in water-saturated methane hydrate samples by depressurization. It was amazingly found that the hydrate dissociation was abnormally much faster in the location far from the mining well with the highest pressure than in the location near the mining well with the lowest pressure in the vertical vessel. This abnormal phenomenon was caused by the earliest uneven vacating of pore spaces in the vertical vessel. This phenomenon was not obvious in the horizontal vessel as there was little difference in the pore spaces vacating rates in different locations. The pressure reduction leaded to faster hydrate dissociation near the mining well in the horizontal vessel. The experiments on sediments with different permeability indicated that the sediments with higher permeability showed more pronounced spatial difference of hydrate dissociation, and the presence of clay (montmorillonite) in the sediments had significant impacts on the spatial difference of hydrate dissociation. The results in this study demonstrate that vacating of pore spaces of sediments is of determining significance for increasing hydrate dissociation rate in water-saturated sediments and provide important implications for developing methods to enhance the gas production from marine hydrate deposits.
As a new kind of clean and high-energy resource, natural gas hydrate possesses promising prospects in exploitation and utilization. The characteristic of gas production in clay-rich sediments need to be further clarified for developing hydrates more safely and efficiently. To study the dissociation behaviors of methane hydrates in clayey sediments, a novel pressure retention system was applied to create both confined and unconfined environments using a one-dimensional visual hydrate simulator. Two types of methane hydrate deposits were simulated, of which were characterized by excess gas and excess water, respectively. Direct depressurization results showed that in confined environment, the increase of clay content in sediments will result in the decrease of gas phase permeability and gas production rate, but have negligible effect on the final gas recovery ratio. In unconfined environment, the infiltration of overlying seawater will result in the migration of fine-grained particles and extremely low gas recovery ratio. Moreover, the build-up of vertical pressure gradient was observed due to the occurrence of sediment compaction during depressurization process in clayey sediments. It is of significance to improve the effective permeability of the reservior and suppress water seepage for exploiting hydrate safely and efficiently.
Hydrate reservoirs with underlying gas show significant potential for commercial exploitation. An experimental apparatus capable of applying confining pressure and axial pressure was built to simulate the accumulation and dissociation of naturally occurring marine hydrate and a novel method for synthesizing hydrate reservoirs with underlying gas was proposed. With the formed methane hydrate-bearing coarse sediment (HBCS) with underlying gas, hydrate dissociation experiments via depressurization under constrained conditions were carried out at different HBCS proportions. The characteristics of heat transfer and dual-gas co-production during the exploitation of hydrate reservoir at various HBCS proportions were presented. Due to the influence of hydrate decomposition, the diverging temperature variations of the underlying layer and upper HBCS layer were observed at HBCS proportions of 0.21, 0.48, and 0.77. The negative effect of the outside pressure on gas production was not observed in our experimental conditions owing to the adopted coarse quartz sand deposits. The final produced gas volume increases with the HBCS proportion increasing. The produced gas percentage is negative or positive related to the HBCS proportions respectively with the extended production time. In addition, the different production behavior of the underlying gas and hydrate decomposed gas may induce the unexpected exploitation accidents such as blowouts resulting from unstable gas production rate. These findings guide the design of exploitation scheme for reservoirs with different HBCS distribution. And an exploitation method by setting the production well in the underlying gas layer/underlying sediment layer with free gas is then proposed, which can inhibit hydrate reformation and ice generation and improve the production well stability during the hydrate exploitation process.
Jinlong Cui,Xiaohui Wang,Yalong Li,Zhenxi Wang,Guangjin Chen,Liwei Cheng,Sheng Xu,Meng Cao,Liwei Sun
Since the thickness of hydrate-bearing sediments is usually tens to hundreds of meters, the influence of gravity on hydrate dissociation characteristics can no longer be ignored. In this study, experimental simulations were designed to investigate the influences of gravity on hydrate dissociation characteristics by depressurization . The experimental results showed that for sealed water-rich marine hydrate reservoir, gravity could cause more water to migrate downward. The free pore space in sediments was thus rapidly vacated, resulting in faster hydrate dissociation and gas production. The average gas production rate increased from 0.018 mol/min to 0.038 mol/min. In addition, the free pore space at the top of the sediments was vacated first. Hydrate dissociation there was thus faster. For unsealed water-rich marine hydrate reservoir, gravity could not only cause more and faster migration of water, but also more easily cause the formation of dominant channels for water migration, leading to an effective reduction in reservoir pressure . The cumulative water yield increased from 1380g to 8950g. Hydrate thus dissociated. Hydrate dissociated faster at positions close to the mining well where the pressure was lower. These findings can provide deep insights into the spatial evolution of multiphase flows during the exploitation of natural gas hydrates .
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