IMECH-IR  > 流固耦合系统力学重点实验室
含水合物沉积物的动静力学特性及水平井 开采的井口土层稳定性研究
Alternative TitleDynamic and Static Characteristics of Hydrate Sediment and Wellhead Soil Layer Stability Analysis of Horizontal Well Mining
孙芳芳
Thesis Advisor张旭辉
2019-05-28
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline工程力学
Keyword天然气水合物,动三轴,力学性质,剪切带,水平井开采
Abstract

天然气水合物(简称水合物)作为非常规油气资源中非常有潜力的能源资源之一,对其勘探和开发的研究工作已列入我国的战略规划。但是,水合物开采过程中水合物的分解相变可能引起储层土体的软化与喷发,进而导致井口土层与附近结构物的安全性问题;大规模的水合物分解甚至会引起海底滑坡、甲烷泄露等灾害。

在此背景下,本文针对南海水合物试采,以室内实验为主,研究含水合物沉积物的动、静态宏微观力学特性;基于理论分析和数值模拟,分析水平井开采时井口附近土层的稳定性问题,以期对工程应用提供参考。

首先,利用含水合物沉积物合成、分解与动静态力学特性测量的实验装置,分别以粉细砂、黏土为骨架,开展了含水合物沉积物的动载特性实验,获得了在不同动载幅值、频率、饱和度条件下的动力学响应参数以及残余静强度的变化规律。结果表明:含水合物粉细砂质沉积物和黏土质沉积物的动载后相对残余静强度(残余静强度与直接静载的参考强度之比)随动载幅值的变化皆存在临界相对动载幅值点(动载幅值与有效围压之比),即当相对动载幅值超过临界点时,沉积物响应由振动致密向振动破坏转变,一般来说相对残余静强度先增后减,且临界点分别是0.550.4,而分解后的含水合物黏土质沉积物在动载作用后几乎失去承载力。根据实验数据,初步提出了评估含水合物沉积物动载作用后的残余静强度的幂指数模型,为储层在地震等动荷载作用下的稳定性评估提供基本参数。

其次,利用含水合物沉积物的三轴-CT微观观测一体化实验装置,开展了不同饱和度和围压下的甲烷水合物细砂质沉积物宏微观力学实验,获得了宏观应力应变曲线,并且观测到剪切带的发育。结果表明:高水合物饱和度的沉积物,应力应变曲线呈现弹脆性破坏的特点;有效围压越小,饱和度越高,应变软化与剪胀现象越明显;高饱和度时,水合物分布相对均匀,而低饱和度时,水合物多呈分散的块状、团状的集合体分布,且剪切破坏多开始于水合物分布具有明显交界面的区域;随着饱和度的增大交叉型剪切带出现,且剪切带的宽度逐渐减小,倾角逐渐增大。

最后,针对水平井降压开采中水合物分解引起的地层变形问题的多时间尺度特点,进行了解耦分析计算。先提出一维水合物降压开采分解渗流的理论模型,再利用岩土计算软件FLAC3D模拟了不同水平井井长、分解半径以及储层倾角条件下,水合物开采过程中井口土层以及上覆层的变形。结果表明:随着分解半径的增大,井口上方以及上覆层顶部的位移呈近似线性趋势递增;随着储层倾角的增大,水平井井口以及上覆层的变形量显著,呈近似抛物线型增加;当分解半径小于水合物层厚度的0.1倍,且储层倾角在附近时,水平井的井长对储层井口以及上覆层的变形无明显影响。该结果可为我国南海第二次水合物试采井口土层稳定性评估提供参考依据。

Other Abstract

Natural gas hydrate is a kind of non-conventional resources with high potential. Study on the exploration and exploitation has been taken as the strategy plan of our country. However, the natural gas hydrate may dissociate and accordingly causes the softening and outburst of the soil layer, which can lead the deformation of the well, and even geological disasters such as marine landslips.

Aiming at the trial production of natural gas hydrate in the South China Sea, a series of experiments are carried out to study the dynamic and static mechanical properties of hydrate-bearing sediments(HBS); Numerical simulation is processed by using the business software FLAC3D to study the stratum deformation and instability around the horizontal well.

Firstly, the triaxle tests were conducted to obtain the dynamic and static mechanical properties of HBS with silt sand and clay as the skeleton by using the apparatus of HBS synthesis and dissociation. The effects of dynamic load amplitude, load frequency and hydrate saturations on HBS were studied. The residual strength after dynamic loading was obtained. The results show that there is a critical relative dynamic loading amplitude (the ratio of loading amplitude to effective confined pressure) for the relative static strength (ratio of residual static strength to initial static strength) of HBS with the skeleton of either silt or clay. That means, once the relative dynamic loading amplitude exceeds the critical value, the curve of residual static strength versus relative dynamic loading amplitude will develop rapidly, and the sediments will change from vibration-induced compaction to vibration-induced failure. The critical values are 0.55 and 0.4 for HBS with the skeleton of silt and clay respectively. Generally, the relative residual strength increases first and then decrease rapidly with the rise of the relative loading amplitude. The relation between the residual static strength and the relative dynamic load amplitude was presented based on the experimental data. It is hoped to provide a reference for the evaluation of safety due to dynamic load, e.g. earthquake during HBS exploitation.

Secondly, by using the integrated experimental platform of triaxial and CT for geotechnique, the macro-mechanical properties of HBS were measured, and the micro characteristics, such as the distribution of hydrates in the pores, the occurrence of the shear band were observed. The effects of saturation of natural gas hydrate and confined pressure were studied. The results show that the stress-strain curve presents the stain softening under high-hydrate saturation. The smaller the effective confining pressure, and the higher the hydrate saturation, the more obvious the strain softening phenomenon and dilation. The distribution of natural gas hydrate in sediments is relative uniform at high saturation while it was in the form of dispersed mass at low saturation. The shear failure initiates from the interface between the gas hydrate and the soils. A single shear band occurs and develops at low saturation, while at high saturation, the cross-type shear bands prevail, the width of shear band becomes small and the angle becomes large.

Finally, the decoupling method is presented to capture the hydrate dissociation, heat conduction and seepage. The initial and boundary conditions are given at different dissociation times. Analysis of the formation deformation caused by natural gas hydrate decomposition by depressurization was carried out. The business software FLAC3D is used to simulate the deformation of the soil layer around a horizontal well, considering the effects of the well length, dissociation length and the slope angle. The results show that with the increase of the dissociation length, the displacement of the soil layer above the well and at the surface of the overlying layer increases linearly. With the increase of the slope angle, the deformation increases in an approximate parabolic type. When the dissociation length is less than 0.1 times the thickness of the natural gas hydrate layer and the slop angle is about 3°, the effects of the well length on the deformation are weak. These results can be taken as the references for the trial exploitation of HBS in the South China Sea.

 

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79105
Collection流固耦合系统力学重点实验室
中国科学院力学研究所
Recommended Citation
GB/T 7714
孙芳芳. 含水合物沉积物的动静力学特性及水平井 开采的井口土层稳定性研究[D]. 北京. 中国科学院大学,2019.
Files in This Item:
File Name/Size DocType Version Access License
完整_3_终极1.docx(31006KB)学位论文 开放获取CC BY-NC-SAApplication Full Text
Related Services
Recommend this item
Bookmark
Usage statistics
Export to Endnote
Google Scholar
Similar articles in Google Scholar
[孙芳芳]'s Articles
Baidu academic
Similar articles in Baidu academic
[孙芳芳]'s Articles
Bing Scholar
Similar articles in Bing Scholar
[孙芳芳]'s Articles
Terms of Use
No data!
Social Bookmark/Share
All comments (0)
No comment.
 

Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.