|Alternative Title||Simulation on seepage in decompressing process for natural gas hydrate deposits|
|Place of Conferral||北京|
|Keyword||天然气水合物 含水合物土层 力学性质 降压 渗流 试采|
Gas hydrate is the strategic energy resource of the 21st century in China, and exists extensively in South China Sea. In 2017, the trial production of gas hydrate was conducted by the depressurization method. Test results show that the gas production rate reduces due to the lack of thermal supply, and can not meet the requirements of the commercial exploitation. Previous research work focuses on water-gas two phase seepage, and evaluates the production rate of different mining methods (thermal injection, depressurization, CO2 displacement, etc.), while the mechanism of gas hydrate dissociation, heat transfer, and fluid seepage are not well understood. Experiments by depressurization-induced hydrate dissociation are conducted considering different hydrate saturations and depressurization modes to provide a reference for the data explanation of the trial production in South China Sea in this paper.
First, a series of tests on the static and dynamic properties of hydrate-bearing sediments (HBS) were conducted by using silt sand as skeleton. The effects of high saturation hydrate on the failure strength of HBS, and dynamic load amplitudes on the failure strength of unsaturated HBS were obatained. The test results show that when the hydrate saturation exceeds 55%, the failure strength decreases; the failure strength after hydrate dissociation decreases with the increase of hydrate saturation; the critical phenomenon occurs when the relative dynamic load amplitude exceeds 0.5, and the failure and liquification occur. The mechanical properties provide a reference for the experimental set-up of seepage in decompressing process for natural gas hydrate deposits.
Second, the gas hydrate deposits were prepared simulating the physical and mechanical parameters of in-situ gas hydrate stratum, and depressurization method was used to dissociate the hydrate into gas and water in the porous media. Different hydrate saturations were considered, and one-step and multi-step depressurization method were adopted. The temperatures, pressures, produced gas instant and cumulative volume during hydrate dissociation were recorded, and the propagation of the gas seepage length was calculated. The results show that the propagation length of the gas seepage is proportional to the square root of time. The freezing of released water from gas hydrate dissociation and the reformation of gas hydrate occur when the initial temperature is close to the freezing point. Multi-step depressurization can control the dissociation rate of gas hydrate, allow the temperature recovery of the deposits, and high gas production rate can be reached. The simulated results are consistent with the field trial test.
Third, the sand production is found to be a detrimental problem during depressurization gas hydrate production. We regard that the mechanism is seepage-induced fine grain migration in the porous media. Hence, the apparatus simulating the seepage-induced fine grain migration in deposits is developed. Different soils were adopted to prepare the deposits. The effects of grain size distribution, mass percentage of the fine grain, and pressure gradient on the sand production were investigated. The results indicate that under a same seepage pressure gradient, the sand production depends on the percentage of the fine grains and the pore structure of the porous media. Under a same grain size distribution, sand production increases linearly with the pressure difference.
In this paper, the seepage phenomenon during depressurization-induced gas hydrate dissociation in deposits is obtained. Subsequent research on the modeling of the multi-physical effects and the formulation of the seepage with fine grains during gas hydrate dissociation are suggested.
|罗大双. 天然气水合物矿层降压过程中渗流现象的模拟实验[D]. 北京. 中国科学院大学,2018.|
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