天然气组分对水合物形成温度的影响研究

天然气的水合物形成温度不仅和压力有关,而且不同组分的天然气在等压下形成水合物的温度也不同,因此有必要对天然气中各主要组分与水合物形成温度之间的关系进行研究,有效地确定水合物抑制剂的用量,有利于水合物的防治工作。文章研究了乙烷、丙烷、丁烷和戊烷与天然气水合物形成温度的关系,得到了丙烷和丁烷对水合物的影响最大,而乙烷和戊烷对水合物的影响较小的结论,从而为根据天然气不同的气质组分进行水合物的防治提供了科学的依据。     

天然气水合物抑制过程中甲醇用量的影响

采用可视化高压流体测试装置,考察了甲醇含量对陕北某气田天然气水合物生成条件的影响。实验结果表明,气体组成对其水合物的生成条件有较大的影响,大分子气体或液态烃的存在可显著降低水合物的生成压力;甲醇含量对水合物生成的温度降有较大影响,甲醇含量越高,水合物生成温度降越大;水合物生成压力的对数(lgp)与温度(t_e)呈线性关系,不同甲醇含量时水合物生成条件的lgp～t_e曲线相互平行;甲醇质量分数小于30%时,Hammerschmidt方程和Nielsen-Bucklin方程对水合物生成温度降的预测偏差较小,但甲醇质量分数大于30%时,预测偏差较大;采用Nielsen-Bucklin修正式,预测甲醇用量的偏差小于2%。     

环状化合物-甲烷水合物稳定性的分子模拟

分子动力学模拟(MD)可为天然气以水合物形式固化提供微观分析。本文利用计算机模拟,恒温等压(NPT)系综下,分别研究了温度为273K、283K、293K,压力为2MPa条件下,环状化合物环戊烷、四氢呋喃、四氢吡喃3种促进剂对Ⅱ型甲烷水合物体系稳定性的影响。结果表明:添加促进剂可使天然气水合物在温和条件下稳定存在;当温度升高,体系稳定性下降;相同条件下,促进作用排序为环戊烷四氢呋喃四氢吡喃;客体分子直径与晶穴直径之比接近1时体系较稳定。     

水合物抑制方法及编程计算

在给定组分条件下，天然气水合物的生成与否主要取决于压力和温度，正确地预测天然气节流后的温度及计算抑制剂用量，可为防止水合物堵塞措施提供技术依据。针对上述问题，选用较合理的方法对水合物生成条件进行预测，通过对水化物生成条件的分析和相关图版的拟合，建立天然气水合物预测模型及合理计算抑制剂注入量模型，对抑制水化物工艺参数编程进行仿真计算，提出合理的工艺参数。     

酸性天然气生成水合物条件实验测定与应用

针对酸性天然气藏开发易生成水合物,堵塞井筒及生产管线的问题,运用定温搜索压力法,对中国西部地区2个高含CO₂的天然气藏气样进行水合物生成条件实验测定。研究表明:天然气中高CO₂含量提高了地层水中HCO^-₃的离子含量,从而增加了地层水的矿化度;与纯CH₄气体相比,相同温度条件下,CO₂的存在显著降低了天然气水合物生成所需的地层压力。对于目标天然气流体,在低矿化度地层水中,生成水合物条件与在纯水中没有明显差别;在较高矿化度(16 970.7 mg/L)地层水中生成天然气水合物压力显著高于纯水体系。进一步基于Clausius-Clapeyron方程计算所合成水合物的分解焓为62 kJ/mol,生成的天然气水合物为Ⅰ型结构。最后结合实验数据和生产动态数据,对目标气藏开发过程水合物生成特征进行分析,预测井筒内是否生成水合物以及出现水合物的井筒位置,该研究为现场生产预防水合物生成具有借鉴意义。     

天然气水合物相平衡的实验研究

天然气水合物相平衡研究是天然气水合物勘探开发和海洋环境保护研究的基础,也是最重要的前缘问题。为此,对取自中国西部油田某2口井的分离器气气样1和气样2在纯水、地层水和配置水等3种水样,以及3 MPa、6 MPa、9 MPa、12 MPa等4个压力体系下对生成水合物的温度进行了实验研究。研究结果表明：随着实验压力的增加,同种气样生成天然气水合物的温度越高;随着水样矿化度的增加,同种气样生成天然气水合物的温度越低;在同一水样中,组分、组成不同的天然气,甲烷含量越高,其形成天然气水合物的温度越低。     

天然气管线水合物生成影响因素研究

介绍了天然气管线中水合物生成条件、以及水合物的生成对管线正常输送和安全运行的影响;提出了天然气管线水合物生成影响因素比较框图,对不同输送工况下管道中水合物的生成进行了分析,得出天然气管线中水合物生成影响因素有输量、起点压力、起点温度和管径,其中输量影响最大,起点压力影响最小,适当增大输量、提高起点温度、降低起点压力和减小管径,可以缩小水合物生成范围甚至避免水合物生成。     

气井井筒水合物预测研究

天然气中的各种组分分子在一定温度和压力下,与游离水结合,形成结晶笼状固体,在生产的过程中会堵塞油管,严重的时候甚至造成停产.考虑井筒中天然气温度和压力的耦合,从井底逐点迭代计算出井筒中天然气温度、压力的分布,再用判别水合物的热力学模型逐点判别是否形成水合物和找出水合物生成的具体位置,为防止水合物的生成提供指导作用.可以通过降低井筒压力和提高井筒温度或降低天然气体系的形成温度2个方面着手预防水合物的生成,防止水合物生成可作为开发前期选择油管考虑的一个因素.     

气井井下节流降压工艺方法探讨

结合气井生产特点，对汪家屯气田水合物的生成机理及产生规律进行了研究，阐述了水合物形成机理，即天然气水合物是天然气中的水和气体在低温高压下的产物，其形成与天然气组分和地层水的矿化度、温度和压力有关。为探索新的水合物的预防技术，在易形成水合物气井上，开展了井下节流防治水合物工艺试验，其原理是将地面气嘴移到井下产层上部油管内，使天然气的节流降压膨胀过程发生在井内。通过井下油嘴节流、降温后的天然气仍可吸收地层温度，降低井筒内天然气压力，提高采出天然气的井口温度，破坏水合物的生成条件，达到防止水合物生成的目的。     

水下采油树节流阀中天然气水合物生成分析

采用数值模拟的方法,结合天然气水合物生成的温度-压力曲线,对水下采油树系统中套筒式节流阀内的天然气水合物生成情况进行分析。将节流阀内实际压力与天然气水合物生成的临界压力的最大差值定义为最大等效压力,在此基础上,研究了进口温度、出口压力及相对开度对最大等效压力的影响,并提出了一种新的天然气水合物预测方法——最大等效压力法。根据此方法,得到了在特定进口温度和出口压力条件下的水合物生成的临界相对开度区间。相对开度在此区间内,节流阀内不会生成水合物;在此区间外则有水合物生成。研究结果对天然气生产及输送过程中的流动保障具有重要意义。     

Dissociation Rate of THF-methane Hydrates

Abstract

A number of papers and research projects suggest that stranded natural gas can be transported in a solid hydrate state at higher temperatures or lower pressures compared to conventional transportation systems (LNG and CNG). The self-preservation effect of methane hydrate can probably be improved by the use of a third component besides CH₄ and water. Tetrahydrofuran (THF) is a promoter that greatly reduces the required formation pressures. In the present work the influence of THF on the decomposition kinetics of mixed THF-CH₄ hydrates was studied to evaluate the THF stabilization effect. The experimental work, carried out with the help of a reaction calorimeter, has revealed that the dissociation rate of mixed THF hydrates is lower (on average by one order of magnitude) than that of simple methane hydrates. Mixed hydrates can also be stored for short periods at temperatures over 0°C. However, the best preservation conditions (among the experimented ones) are realized at ?1°C and 3 MPa. (about 66 days required for complete dissociation).     

多孔介质中天然气水合物生成的主要影响因素

目前有关天然气水合物(以下简称水合物)的研究主要集中在物理化学性质考察和开采(分解)方法探索方面。在进行后者的研究过程中,地层渗流过程的物理模拟至关重要,但目前借助于石油开采研究中广泛应用的填砂管等多孔介质对水合物进行动态过程的研究却鲜有报道。为此,利用河砂填砂管在岩心驱替装置上进行了甲烷水合物生成过程的物理模拟,考察了地层温度、甲烷压力及地层模型性质参数等对水合物生成过程的影响。结果表明:(1)利用冰融水作为地层模型的束缚水可显著提升甲烷水合物的生成速率;(2)多孔介质条件下过程驱动力(即实验压力或温度偏离水合物相平衡对应值的程度)对甲烷水合物的生成起着决定性作用;(3)当甲烷压力高于水合物相平衡压力1.4倍以上,或者实验温度低于相平衡温度3℃以下时,甲烷水合物生成诱导期几乎不随温压条件的变化而变化;(4)渗透率、含水饱和度、润湿性等参数对实验中甲烷水合物的生成率不构成明显影响。     

块状甲烷水合物分解动力学特征及其影响因素

与冷泉相关的块状甲烷水合物是非常规天然气资源开发的重点目标之一。为了了解其分解动力学特征以便于制订合理的开发方案，利用高压差示扫描量热仪实验测试了块状甲烷水合物的生成与分解过程，将分解的吸热效应与分解速度相关联，分析不同环境下块状甲烷水合物分解瞬时速度和平均速度的变化特征，然后，基于实验结果采用经典的甲烷水合物分解动力学模型计算得到不同压力下甲烷水合物分解活化能，进而评价分解表面积、温度、压力和矿化度等因素对甲烷水合物分解速度的影响。研究结果表明：①随着压力升高，甲烷水合物分解活化能逐渐增大，在此次实验测试条件下其数值介于27.5～28.5 kJ/mol；②在去离子水溶液中，甲烷水合物的分解瞬时速度呈现先增加后减小的趋势，在分解早中期其累计分解物质的量随时间的变化关系呈指数函数形式增长，后期则呈缓慢线性增长；③在孔隙水溶液中，甲烷水合物的分解瞬时速度也呈现先增加后减小的变化趋势，但较之于去离子水溶液，孔隙水溶液中甲烷水合物的分解瞬时速度峰值出现的时间较晚，孔隙水溶液矿化度对水合物分解速度的促进作用弱于温度的影响；④对影响去离子水溶液中块状甲烷水合物分解速度的因素按照影响程度由大到小排序，结果依次为分解表面积、温度、压力。结论认为，在储层改造的基础上，热激法是块状甲烷水合物开采的合理方式。     

OCEANIC METHANE HYDRATES: A "FRONTIER" GAS RESOURCE

Methane hydrates are ice-like compounds consisting of natural gas (mainly methane) and water, whose crystal structure effectively compresses the methane: each cubic metre of hydrate can yield over 150 cu.m of methane. Hydrates "cement" sediments and impart considerable mechanical strength; they fill porosity and restrict permeability. Both biogenic and thermogenic methane have been recovered from hydrates.
Hydrates occur in permafrost regions (including continental shelves), and are stable in ocean-floor sediments below water depths of about 400 m in the "Hydrate Stability Zone" (HSZ). This is a surface-parallel zone of thermodynamic equilibrium that extends down from the sediment surface to a depth determined by temperature, pressure and local heat flow. Methane and water are stable below the HSZ.
Although the economic recovery of hydrates has taken place in Arctic regions, oceanic hydrates offer far greater potential as an energy resource. A variety of traps for methane gas can be formed by oceanic hydrates. In addition to the gas within the hydrates themselves, simple gas traps in closures beneath the HSZ in the vicinity of bathymetric highs, and complex traps involving both hydrate and structural/stratigraphic components, have been observed.
It has been estimated that at least twice as much combustible carbon occurs associated with methane hydrates as in all other fossil fuels on Earth. The evaluation of methane in, and associated with, oceanic hydrates therefore constitutes a major energy exploration frontier.     

OCEANIC METHANE HYDRATE: THE CHARACTER OF THE BLAKE RIDGE HYDRATE STABILITY ZONE, AND THE POTENTIAL FOR METHANE EXTRACTION

Oceanic methane hydrates are mineral deposits formed from a crystalline "ice" of methane and water in sea-floor sediments (buried to less than about 1 km) in water depths greater than about 500 m; economic hydrate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by "freezing" the sediment and by filling the pore spaces in a manner similar to water-ice in permafrost. Concentrated hydrate deposits may be underlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seismic reflection records indicate that trapped gas may blow-out naturally, causing large-scale seafloor collapse.
In this paper, we consider both the physical properties and the structural integrity of the hydrate stability zone and the associated free gas deposits, with special reference to the Blake Ridge area, SE US offshore, in order to help establish a suitable framework for the safe, efficient, and economic recovery of methane from oceanic gas hydrates. We also consider the potential effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess the ambient pressure effect on the production of methane by hydrate dissociation, and attempt to predict the likelihood of spontaneous gas flow in a production situation.     

An intelligent optimization–based prediction model for natural gas hydrate formation in a deepwater pipeline

Temperature, pressure, and composition of gas mixtures in deepwater pipelines promote rapid formation of gas hydrates. To avert this dilemma, it is more significant to find out the temperature and pressure limits in gas hydrates formation of the deepwater pipeline. The objective of this research is to develop an optimization method that finds the optimal temperature and pressure profile for natural gas hydrate formation conditions and an error calculation method to find the realistic approach of the hydrate formation prediction model. A newly developed correlation model is computing the hydrate formation pressure and temperature for a single component of methane (CH₄) gas. The proposed developed prediction model is based on the 2 and 15 constant coefficients and holds a wide range of temperature and pressure data about 2.64 to 46°C and 0.051 to 400 MPa for pure water and methane, respectively. The reducing error discrepancies are 1.2871, 0.35012, and 1.9052, which is assessed by GA, PSO, and GWO algorithms, respectively. The results show the newly developed optimization algorithms are in admirable compliance with the experimental data and standards of empirical models. These correlations are providing the capability to predict gas hydrate forming conditions for a wide range of hydrate formation data.     

Numerical solution for natural gas production from methane hydrate dissociation

This paper describes a one-dimensional model for natural gas production from the dissociation of methane hydrate in a confined reservoir by a depressurizing well. The approach accounts for the heat released by hydrate dissociation and convection–conduction heat transfer in the gas and hydrate zone. The system of governing equations is solved using a finite-difference scheme. For different well pressures and reservoir temperatures, the gas flow, the pressure and temperatures conditions in the reservoir are simulated. Distributions of temperature and pressure in the hydrate and gas regions and time evolutions of natural gas output also are evaluated. It is shown that the gas production rate is a sensitive function of well pressure. In addition, both heat conduction and convection in the hydrate zone is important. The simulation results are compared with the linearization approach and the shortcomings of the earlier approach are discussed.     

电阻率在天然气水合物三维生成及开采过程中的变化特性模拟实验

为获取更多的清洁高效能源,全球范围内都正在开展与天然气水合物（以下简称水合物）开采相关的研究,其中电阻率作为表征水合物变化的一个重要参数也被纳入了重点研究范畴,但其在水合物三维生成及开采过程中的变化特性尚未见报道。为了得到三维系统下水合物的电阻率变化数据,利用三维水合物反应釜,实验室模拟研究了水合物在多孔介质中生成及利用双水平井注热开采实验过程中电阻率的变化特性。结果发现：①电阻率总体上随着水合物的生成而升高,随着其分解而下降;②电阻率与水合物饱和度并不呈完全的线性关系,当水合物饱和度升高到一定程度时,电阻率变化减缓;③在水合物生成过程中发现水合物生成存在“爬壁效应”--水合物在多孔介质中的生成并不同步,水合物在边界区域明显多于中心区域;④在水合物开采过程中发现电阻率不仅随水合物分解的变化而变化,而且还与开采过程中的流体流动有着较大关系,所以利用电阻率作为水合物开采的特征指标时需要先排除流体流动的干扰。     

Characterization of Gas Hydrates by Modulated Differential Scanning Calorimetry

Abstract

Modulated Differential Scanning Calorimetry (MDSC) has been applied to the study of methane, ethane, and propane hydrates at different hydrate and ice concentrations. The reversing thermodynamical component of the MDSC curves, makes it possible to characterize such hydrates.

Methane and ethane hydrates show the melting-decomposition peak at a temperatures higher than the ice contained in the sample, while propane hydrate melts and decomposes at a lower temperature than the ice present in the sample. The hydrate peaks tend to disappear if the hydrate is stored at atmospheric pressure. Guest size and cavity occupation fix the heat of dissociation and stability of the hydrates, as confirmed by parallel tests on tetrahydrofurane hydrates.