油水体系水合物浆液流动保障研究进展

海洋油气田的高效、安全开采是践行国家深海战略、探索未来战略能源的必要手段。在深海低温、高压环境下运行的多相混输管道的堵塞已成为能源行业安全生产和流动保障的棘手问题,而天然气水合物的快速形成是造成这一问题的主要诱因。明晰油水体系水合物浆液的堵塞机理,开发设计绿色、环保的低剂量水合物抑制剂,掌握浆液的流动特征,将为深水油田多相混输管道的流动保障提供理论基础和技术指导。为此,本文从水合物堵塞过程、化学法防治以及水合物浆液的流动特性等方面总结概括了油水体系水合物浆液的流动保障研究进展。油水体系水合物堵塞主要由水合物的生长、聚集、管壁黏附和着床沉积造成,文章提出:应量化水合物生长、聚集、管壁黏附和着床沉积等因素对流动阻力和管道堵塞的影响,建立科学的工业管道水合物堵塞风险评估理论体系;借助微观尺度实验和分子动力学模拟,厘清不同类型的动力学抑制剂(KHIs)抑制机理和防聚剂(AAs)防聚机理;将实验探索、理论分析和数值模拟相结合,解耦颗粒微观行为与流场宏观物理参数间的关系,定量表征浆液流变性和流动阻力。

聚乙烯己内酰胺链端改性及其对甲烷水合物形成的抑制作用研究

注入动力学抑制剂是一种有效缓解天然气水合物管道堵塞的方法。本文以动力学抑制剂聚乙烯基己内酰胺(PVCap)结构为基础,将氧乙基和酯基引入PVCap的分子链端,合成了新抑制剂PVCap-XA1,在高压定容反应釜内评价了PVCap-XA1对甲烷水合物形成的抑制作用,并采用粉末X射线衍射、低温激光拉曼光谱和冷冻扫描电子显微镜研究了抑制剂对甲烷水合物结构和形态的影响。实验结果表明,相同实验条件下PVCap-XA1比PVCap具有更好的抑制作用;微观测试表明PVCap-XA1的加入没有改变甲烷水合物的晶体结构,但会使甲烷水合物晶面扭曲变形,可以降低水合物大小笼占有比(I_(L)/I_(S)),使得甲烷分子更难进入水合物大笼,同时PVCap-XA1的加入使甲烷水合物的微观形貌由多孔有序变得更致密而不利于气体通过。

南海神狐海域水合物对岩心电阻率的影响

研究南海神狐海域天然气水合物的电学特性,对资源量的估算具有重要意义。选取南海神狐海域SH7B站位岩心样品,采用高过冷度和冰点以下温度震荡等方式,合成含天然气水合物的岩心样品,实验研究含天然气水合物海底沉积物的电学特性。甲烷在3.6℃形成天然气水合物时,岩心样品电阻率从1.220Ω·m(水饱和状态)增加至2.150Ω·m(水合物饱和度S_h为37.7%);当S_h≤18.0%时,样品电阻率变化明显,水合物开始胶结于沉积物颗粒;当18.0%<S_h≤25.0%时,电阻率变化较平缓,水合物可能以接触模式存在于沉积物孔隙;当S_h>25.0%时,大量的水合物可能胶结于沉积物孔隙,以致电阻率增加较快。最终甲烷气体难以扩散至被水合物封闭的孔隙流体中,电阻率趋于稳定。

基于分形孔隙模型的含天然气水合物沉积物电阻率数值模拟

电阻率法是确定天然气水合物(以下简称水合物)饱和度的重要方法,通过数值模拟可以有效研究含水合物沉积物的电阻率特性,但过去构建的孔隙模型由于约束条件较少,而与实际的孔隙结构存在着较大的差异。为此,基于自然界沉积物具有自相似特征,选定地毯总边长为3和颗粒边长为1的谢尔宾斯基地毯作为沉积物的分形孔隙模型,根据等效电阻网络模式建立了含水合物沉积物的电导模型,并利用上述模型分析了孔隙度、孔隙水电导率、沉积物骨架电导率等因素对含水合物沉积物电阻率与水合物饱和度关系的影响。研究结果表明:(1)含水合物沉积物的电阻率可以表示为孔隙度、面积比、微观结构尺寸、孔隙水电导度、沉积物骨架电导率及经验参数的函数;(2)孔隙水电导率和孔隙度的减小都会导致沉积物电阻率的增大;(3)含水合物沉积物的电阻率随水合物饱和度的增大而增大;(4)在高水合物饱和度范围内,含水合物沉积物的电阻率随沉积物颗粒骨架电导率的增大而明显减小。结论认为:在一定的水合物饱和度范围内,该分形孔隙模型计算结果与实验数据和测井数据都能较好地吻合,准确度较高。

祁连山冻土区天然气水合物储层岩石热物性实验研究

采用扫描电镜测试了祁连山冻土区天然气水合物储层泥岩和砂岩微观结构。在显微镜下观察到,泥岩由微小均匀的片状多边形块体构成,这种微观结构使得泥岩中水合物以浸染状赋存;而砂岩内部存在分散状孔隙,砂岩中水合物为孔隙赋存。采用瞬变平面热源法测试了含甲烷水合物储层岩石的导热系数和热扩散率。在温度为-9.41~9.41℃时,干泥岩导热系数为0.577~0.853 W·m-1·K-1,含甲烷水合物储层泥岩导热系数为0.704~1.050 W·m-1·K-1。在温度为-8.11~9.28℃时,干砂岩导热系数为0.828~1.271 W·m-1·K-1,含甲烷水合物储层砂岩导热系数为3.850~4.555 W·m-1·K-1。在温度为-9.41~9.41℃时,干泥岩热扩散率为0.712~0.894 mm2·s-1,含甲烷水合物储层泥岩热扩散率为0.792~1.006 mm2·s-1,干砂岩热扩散率为1.198~1.674 mm2·s-1,含甲烷水合物储层砂岩热扩散率为1.403~1.769 mm2·s-1。测试数据表明,对于孔隙型水合物,测定导热系数是一种较好的辨识水合物存在的辅助手段。

非成岩水合物储层降压开采过程中出砂和沉降实验研究

天然气水合物多赋存在非成岩地层中,在开采过程中易出现出砂和沉降情况,制约了天然气水合物的安全高效长期开采。为研究水合物开采过程中的温压、产气、产水、出砂和沉降情况,在自主研发水合物出砂及防砂模拟装置上进行了不同条件下的开采模拟实验。研究表明,在前两个生产阶段,产水含砂率和出砂粒径随着水合物开采而逐渐增大;水合物细砂储层产气速率增加会增大携液能力,导致携砂能力增强而增大出砂风险,同时高产气速率促进井筒温度降低导致冰相生成,存在冰堵的风险;开采过程中的储层沉降与储层水合物含量相关性较大,而产气速率和降压速率对储层沉降的影响与产气模式有关。水合物开采中后期进行增产作业会增加储层出砂风险和沉降速率,进一步探讨了该实验对日本2013年第一次海域水合物试采出砂情况的推测,提出水合物开采分阶段分级防砂的概念。

Effect of microwave on formation/decomposition of natural gas hydrate

Natural gas hydrate (NGH) reservoirs have been considered as a substantial future clean energy re-source and how to recover gas from these reservoirs feasibly and economically is very important. Mi-crowave heating will be taken as a promising method for gas production from gas hydrates for its ad-vantages of fast heat transfer and flexible application. In this work, we investigate the formation /decomposition behavior of natural gas hydrate with different power of microwave (2450MHZ), pre-liminarily analyze the impact of microwave on phase equilibrium of gas hydrate,and make calculation based on van der Waals-Platteeuw model. It is found that microwave of a certain amount of power can reduce the induction time and sub-cooling degree of NGH formation, e.g., 20W microwave power can lead to a decrease of about 3℃ in sub-cooling degree and the shortening of induction time from 4.5 hours to 1.3 hours. Microwave can make rapid NGH decomposition, and water from NGH decomposi-tion accelerates the decomposition of NGH with the decomposition of NGH. Under the same pressure, microwave can increase NGH phase equilibrium temperature. Different dielectric properties of each composition of NGH may cause a distinct difference in temperature in the process of NGH decomposi-tion. Therefore, NGH decomposition by microwave can be affected by many factors.

Gas hydrate formation in fine sand

Gas hydrate formation from two types of dissolved gas (methane and mixed gas) was studied under varying thermodynamic conditions in a novel apparatus containing two different natural media from the South China Sea. The testing media consisted of silica sand particles with diameters of 150-250 μm and 250-380 μm. Hydrate was formed (as in nature) in salt water that occupies the interstitial space of the partially water-saturated silica sand bed. The experiments demonstrate that the rate of hydrate formation is a function of particle diameter, gas source, water salinity, and thermodynamic conditions. The initiation time of hydrate formation was very short and pressure decreased rapidly in the initial stage. The process of mixed gas hydrate formation can be divided into three stages for each type of sediment. Sand particle diameter and water salinity also can influence the formation process of hydrate. The conversion rate of water to hydrate was different under varying thermodynamic conditions, although the formation processes were similar. The conversion rate of methane hydrate in the 250-380 μm sediment was greater than that in the 150-250μm sediment. However, the sediment grain size has no significant influence on the conversion rate of mixed gas hydrate.

Influence of 3A molecular sieve on tetrahydrofuran (THF) hydrate formation

Visual observation of the THF hydrate formation process in the presence of a 3A molecular sieve has been made at normal atmosphere and below a temperature of zero by microscopy. The results indicate that a 3A molecular sieve can induce the nucleation of the THF hydrate and promote the THF hydrate growth. With the existence of a 3A molecular sieve, the growth rate of THF hydrate is between 0.01 and 0.05 μm/s. In comparison with the system without any 3A molecular sieve, the growth rate increases about 4 nm/s. After the THF hydrate grows into megacryst, the crystals will recombine and partially change under the same condition.

Molecular dynamics simulations of the mechanisms of thermal conduction in methane hydrates

The thermal conductivity of methane hydrate is an important physical parameter affecting the processes of methane hydrate exploration,mining,gas hydrate storage and transportation as well as other applications.Equilibrium molecular dynamics simulations and the Green-Kubo method have been employed for systems from fully occupied to vacant occupied sI methane hydrate in order to estimate their thermal conductivity.The estimations were carried out at temperatures from 203.15 to 263.15 K and at pressures from 3 to 100 MPa.Potential models selected for water were TIP4P,TIP4P-Ew,TIP4P/2005,TIP4P-FQ and TIP4P/Ice.The effects of varying the ratio of the host and guest molecules and the external thermobaric conditions on the thermal conductivity of methane hydrate were studied.The results indicated that the thermal conductivity of methane hydrate is essentially determined by the cage framework which constitutes the hydrate lattice and the cage framework has only slightly higher thermal conductivity in the presence of the guest molecules.Inclusion of more guest molecules in the cage improves the thermal conductivity of methane hydrate.It is also revealed that the thermal conductivity of the sI hydrate shows a similar variation with temperature.Pressure also has an effect on the thermal conductivity,particularly at higher pressures.As the pressure increases,slightly higher thermal conductivities result.Changes in density have little impact on the thermal conductivity of methane hydrate.