【作者】 王宁；

【导师】 杨庆；

【作者基本信息】 大连理工大学 ， 土木工程（专业学位）， 2023， 硕士

【摘要】 天然气水合物是一种高效清洁且储量丰富的能源,其商业化开采能够有效的解决我国能源安全和环境问题。海洋中的天然气水合物主要分布在大陆边缘海底沉积物中,水合物的盲目开采可能会引起海底地层承载力下降,继而引起海底滑坡、生产平台塌陷倾覆等问题;另外,甲烷是一种强温室气体,如果开采不当则会对环境产生难以估量的影响。因此,在商业化开采前,有必要对水合物沉积物力学特性进行详细的研究。本文以南海神狐海域沉积物级配人工制备天然气水合物沉积物样品,采用低温高压三轴仪探究水深等因素对其力学特性的影响,分析其影响机理,并依据试验结果提出强度预测公式。主要研究内容和结论如下:(1)采用气饱和制样方法,基于南海神狐海域沉积物级配进行水合物沉积物样品的室内人工制备,同时,对制样方法进行改进并提高水合物饱和度计算准确性:快速通入甲烷气体并关闭阀门,监测气压和温度变化,应用理想气体方程计算甲烷气体的消耗量,推算水合物饱和度。并且进行了沉积物的三轴试验,南海沉积物试样最终超孔压接近有效围压;而剔除了粒径小于0.063mm组分的试样最终超孔压远小于有效围压。故认为细粒成分填充了大颗粒间的孔隙,引起孔压的持续上涨并接近有效围压。(2)对天然气水合物沉积物试样开展三轴剪切试验,探究了不同孔压和不同有效围压对其力学特性的影响。结果表明,孔压和有效围压的提高均能够提升天然气水合物沉积物的强度和剪切模量。对于有效围压为1MPa的试样,当孔压从8MPa提高到16MPa,试样强度提高了14.76%。孔压对粘聚力和内摩擦角均有提高作用。孔压对天然气水合物沉积物强度提高的主要原因包括:(1)增强沉积物颗粒间的接触力;(2)增加水合物内部的应力,引起水合物自身强度提高;(3)增强水合物与沉积物之间的胶结。(3)以不同的配置饱和度制备天然气水合物沉积物试样,进行力学特性试验。结果显示,水合物饱和度的增加对于强度的提高很明显。水合物的自身强度较高,在一定程度上水合物充当沉积物的骨架。提高了饱和度,相当于增加了试样的密实度,使沉积物由松散状态向密实状态转变。随着水合物饱和度的增加,试验现象逐渐由应变硬化向应变软化转变;超孔压曲线表现出由剪缩变为了剪胀。进一步,结合孔压、有效围压和饱和度对水合物沉积物力学特性的影响,提出强度预测理论公式,拟合结果与试验值误差多数小于2%。(4)以不同的降压速率对制备好的水合物样品进行降压分解,并对分解结束后的试样进行剪切。结果表明:降压速率的提高显著降低了分解后沉积物的强度。降压速率增大,甲烷气和水以较高速度流动,可能会带动小颗粒发生迁移,使孔隙中的小颗粒被带走,从而使沉积物强度降低。在剪切过程中,大颗粒间的孔隙不断被粉砂填充,试样发生完全液化。细粒成分对水合物分解后的沉积物强度影响非常大。本文所做工作,一方面能够为天然气水合物储层的强度评价提供一定参考和指导,另一方面有助于更加深入地揭示水深对天然气水合物沉积物影响的内在机制。更多还原

【Abstract】 Methane hydrate is an efficient,clean,and abundant energy source,and its commercial exploitation can effectively solve energy security and environmental issues in China.Methane hydrates are mainly distributed in seafloor sediments at the continental margin.Blind exploitation of hydrates may lead to a decrease in the bearing capacity of the seafloor strata,which in turn can cause problems such as seafloor landslides and collapse of production platforms.Moreover,methane is a strong greenhouse gas,and the environmental impact of improper extraction is difficult to estimate.Therefore,it is necessary to conduct detailed research on the mechanical properties of hydrate sediment before commercial mining.This article uses the sediment grading of the Shenhu Sea area in the South China Sea to artificially prepare sediment,and applies the low-temperature high-pressure triaxial to explore the influence of water depth and other factors on methane hydrate sediment.The mechanism of the influence is analyzed,and a strength prediction formula is proposed based on the experimental results.The main research content and conclusions are as follows:(1)Based on the gas saturation method,the sample preparation method and the hydrate saturation calculation method were improved by manually preparing the sediment from the sediment grading of Shenhu sea area in the South China Sea.And a triaxial test was conducted on the sediment,and the final overpressure of sample 8-1 in the South China Sea sediment was close to the effective confining pressure;The sample with a particle size less than 0.063 mm was removed,and the final pore pressure of the 8-1-nosilt was much lower than the effective confining pressure.Analysis suggests that fine-grained components fill the pores between large particles,causing a continuous increase in pore pressure and approaching effective confining pressure.(2)Methane hydrate sediment samples were prepared using gas saturation method and subjected to triaxial shear tests with different pore pressures and effective confining pressures.The results indicate that pore pressure and effective confining pressure increase the strength and shear modulus of methane hydrate deposits.For the sample with an effective confining pressure of 1MPa,the increase in pore pressure from 8MPa to 16 MPa resulted in a 14.76%increase in sample strength.The pore pressure increases both the cohesion and internal friction angle.There are three reasons why pore pressure increases the strength of methane hydrate sediment: firstly,an increase in pore pressure enhances the contact force between sediment particles;Secondly,the increase in pore pressure increases the stress inside the hydrate,causing an increase in the strength of the hydrate itself;Thirdly,the pore pressure enhances the bonding between hydrates and sediments.(3)Prepare methane hydrate sediment samples with different configure saturation for mechanical properties testing.The results show that saturation has a significant effect on the improvement of intensity.The inherent strength of hydrates is relatively high,and to some extent,hydrates serve as sediment skeletons.Increasing saturation is equivalent to increasing the compactness of the sample,causing the sediment to transition from a loose state to a dense state.As the saturation increases,the stress-strain curve shows a weakening of strain hardening phenomenon;The overpressure curve shows a transition from shear shrinkage to shear expansion.Propose strength formulas for pore pressure,effective confining pressure,and saturation,with the majority of fitting results and experimental values having an error of less than 2%,resulting in good results.(4)After the preparation of hydrate sediment,depressurization decomposition and shearing are carried out at different depressurization rates.The results indicate that the increase in depressurization rate significantly reduces the strength of the decomposed sediment.As the rate of depressurization increases,methane gas and water flow at a higher speed,which may drive small particles in the pores to move,causing them to be carried away,thereby reducing the strength of the sediment.During the shear process,the pores between large particles are constantly filled with silt,and the sample undergoes complete liquefaction.The fine-grained components have a significant impact on the strength of sediment after hydrate decomposition.The work done in this article can provide some reference and guidance for the strength evaluation of methane hydrate reservoirs at the engineering level,and on the other hand,it can help to more deeply reveal the internal mechanism of the influence of water depth on methane hydrate sediment at the theoretical level.更多还原