Initial estimation of hydrate formation temperature of sweet natural gases based on new empirical correlation

Production,processing and transportation of natural gases can be significantly affected by clathrate hydrates.Knowing the gas analysis is crucial to predict the right conditions for hydrate formation.Nevertheless,Katz gas gravity method can be used for initial estimation of hydrate formation temperature （HFT） under the circumstances of indeterminate gas composition.So far several correlations have been proposed for gas gravity method,in which the most accurate and reliable one has belonged to Bahadori and Vuthaluru.The main objective of this study is to present a simple and yet accurate correlation for fast prediction of sweet natural gases HFT based on the fit to Katz gravity chart.By reviewing the error analysis results,one can discover that the new proposed correlation has the best estimation capability among the widely accepted existing correlations within the investigated range.

Development of a least squares support vector machine model for prediction of natural gas hydrate formation temperature

Hydrates always are considered as a threat to petroleum industry due to the operational problems it can cause.These problems could result in reducing production performance or even production stoppage for a long time.In this paper, we were intended to develop a LSSVM algorithm for prognosticating hydrate formation temperature(HFT) in a wide range of natural gas mixtures. A total number of 279 experimental data points were extracted from open literature to develop the LSSVM. The input parameters were chosen based on the hydrate structure that each gas species form. The modeling resulted in a robust algorithm with the squared correlation coefficients(R^2) of 0.9918. Aside from the excellent statistical parameters of the model, comparing proposed LSSVM with some of conventional correlations showed its supremacy, particularly in the case of sour gases with high H_2S concentrations, where the model surpasses all correlations and existing thermodynamic models. For detection of the probable doubtful experimental data, and applicability of the model, the Leverage statistical approach was performed on the data sets. This algorithm showed that the proposed LSSVM model is statistically valid for HFT prediction and almost all the data points are in the applicability domain of the model.

Passability test and simulation of sand control string with natural gas hydrates completion in large curvature hole

To meet the requirements of marine natural gas hydrate exploitation,it is necessary to improve the penetration of completion sand control string in the large curvature borehole.In this study,large curvature test wells were selected to carry out the running test of sand control string with pre-packed screen.Meanwhile,the running simulation was performed by using the Landmark software.The results show that the sand control packer and screen can be run smoothly in the wellbore with a dogleg angle of more than 20°/30 m and keep the structure stable.Additionally,the comprehensive friction coefficient is 0.4,under which and the simulation shows that the sand control string for hydrate exploitation can be run smoothly.These findings have important guiding significance for running the completion sand control string in natural gas hydrate exploitation.

Research and application of thermal insulation effect of natural gas hydrate freezing corer based on the wireline-coring principle

Natural gas hydrate(NGH)holds great promise as a source of clean energy.It is critical for acquiring the largest possible in situ NGH core for NGH eigen features and resource assessment.However,the existing NGH coring technology has limitations,such as temperature increments,limited coring diameters,low coring rates,and complex coring structures.Therefore,this study designs and proposes an NGH freezing coring(NGHFC)method and verifies the freezing and coring capacities of the NGHFC method in laboratories and experimental wells.Results suggest that NGHFC shows good freezing and heat-retention properties.A freezing core heat transfer model is developed.According to the actual air temperature and operating time,the optimum initial temperature of the cold source can be determined using this model.The average coring rate of NGHFC can reach 77.86%.The research results will provide a new idea of coring gas hydrates.

Wellbore temperature distribution during drilling of natural gas hydrate formation in South China sea

The natural gas hydrate resources in the South China Sea alone are about 85 trillion cubic meters.In the drilling process of marine gas hydrate,the natural gas hydrate will decompose and produce gas,as the rising of temperature and dropping of the pressure in the annulus.This process will have a significant impact on drilling safety.Therefore,it is necessary to study the wellbore temperature distribution during the drilling of marine hydrate layer.In this paper,the wellbore temperature distribution of safe drilling in hydrated formation is taken as the research goal,and the research status of relevant domestic and international wellbore temperatures was investigated.According to the characteristics of the marine environment and reservoir-forming characteristics of hydrate reservoirs in the South China Sea,the wellbore temperature distribution model of offshore drilling wellbore under the condition of hydrate decomposition was established.The temperature distribution curve of drilling straight wellbore in hydrate layer of South China Sea was obtained.When drilling the hydrate reservoir,the distribution regularity of the wellbore temperature is similar to that of the conventional offshore drilling wellbore.However,the temperature of the wellbore annulus near the hydrate decomposition site is lower than the ambient temperature,mainly due to the hydrate decomposition endothermic.In this paper,the sensitivity analysis of several main parameters of the wellbore temperature distribution of drilling straight wellbore in hydrate layer of South China Sea was carried out.Through the conduction of experiment and numerical simulation,we have get some new findings:(1)The hydrate saturation has little effect on the wellbore temperature;(2)As the drilling fluid displacement increases,the annulus temperature of the wellbore above the mudline increases,and the temperature of the wellbore below the mudline decreases continuously;(3)As the density of the drilling fluid increases,the temperature at the wellhead decreases,and the temperature at the bottom of the well increases slightly;(4)The greater the rate of penetration of the well,the temperature at the upper part of the wellbore decreases,and the temperature at the bottom of the wellbore increases;Among them,the penetration rate has the most obvious effect on the annular temperature.The results are expected to be helpful to guide the drilling process of marine gas hydrate and offer some references.

Rapidly Estimating Natural Gas Compressibility Factor

Natural gases containing sour components exhibit different gas compressibility factor （Z） behavior than do sweet gases. Therefore, a new accurate method should be developed to account for these differences. Several methods are available today for calculating the Z-factor from an equation of state. However, these equations are more complex than the foregoing correlations, involving a large number of parameters, which require more complicated and longer computations. The aim of this study is to develop a simplified calculation method for a rapid estimating Z-factor for sour natural gases containing as much as 90% total acid gas. In this article, two new correlations are first presented for calculating the pseudo- critical pressure and temperature of the gas mixture as a function of the gas specific gravity. Then, a simple correlation on the basis of the standard gas compressibility factor chart is introduced for a quick estimation of sweet gases＇ compressibility factor as a function of reduced pressure and temperature. Finally, a new corrective term related to the mole fractions of carbon dioxide and hydrogen sulfide is developed.

Laboratory measurement of longitudinal wave velocity of artificial gas hydrate under different temperatures and pressures

The longitudinal wave velocity and attenuation measurements of artificial gas hy- drate samples at a low temperature are reported. And the temperature and pressure dependence of longitudinal wave velocity is also investigated. In order to under- stand the acoustic properties of gas hydrate, the pure ice, the pure tetrahydrofuran (THF), the pure gas hydrate samples and sand sediment containing gas hydrate are measured at a low temperature between 0℃ and –15℃. For the pure ice, the pure THF and the pure gas hydrate samples, whose density is 898 kg/m3, 895 kg/m3 and 475 kg/m3, the velocity of longitudinal wave is respectively 3574 m/s, 3428 m/s and 2439 m/s. For synthesized and compacted samples, the velocity of synthesized samples is lower than that of compacted samples. The velocities increase when the densities of the samples increase, while the attenuation decreases. Under the con- dition of low temperature, the results show that the velocity is slightly affected by the temperature. The results also show that wave velocities increase with the in- crease of piston pressures. For example, the velocity of one sample increases from 3049 up to 3337 m/s and the other increases from 2315 up to 2995 m/s. But wave velocity decreases from 3800 to 3546 m/s when the temperature increases from –15℃ to 5℃ and changes significantly close to the melting point. Formation con- ditions of the two samples are the same but with different conversion ratios of wa- ter. The results of the experiment are important for exploration of the gas hydrate resources and development of acoustic techniques.

Developing a robust correlation for prediction of sweet and sour gas hydrate formation temperature

There are numerous correlations and thermodynamic models for predicting the natural gas hydrate formation condition but still the lack of a simple and unifying general model that addresses a broad ranges of gas mixture.This study was aimed to develop a user-friendly universal correlation based on hybrid group method of data handling(GMDH)for prediction of hydrate formation temperature of a wide range of natural gas mixtures including sweet and sour gas.To establish the hybrid GMDH,the total experimental data of 343 were obtained from open articles.The selection of input variables was based on the hydrate structure formed by each gas species.The modeling resulted in a strong algorithm since the squared correlation coefficient(R2)and root mean square error(RMSE)were 0.9721 and 1.2152,respectively.In comparison to some conventional correlation,this model represented not only the outstanding statistical parameters but also its absolute superiority over others.In particular,the result was encouraging for sour gases concentrated at H2S to the extent that the model outstrips all available thermodynamic models and correlations.Leverage statistical approach was applied on datasets to the discovery of the defected and doubtful experimental data and suitability of the model.According to this algorithm,approximately all the data points were in the proper range of the model and the proposed hybrid GMDH model was statistically reliable.

Analysis of factors affecting microwave heating of natural gas hydrate combined with numerical simulation method

As a clean and abundant unconventional natural gas resource,natural gas hydrate(NGH)holds the characteristics of safety,high efficiency and sustainable exploitation,which helps to alleviate the energy shortage of China,reduce the foreign-trade dependence of oil and gas,and ensure the national energy security.Microwave heating is a significant method that has been used in natural gas hydrate exploration.By using the microwave heating,the NGH in the reservoir formations would be heated,decomposed and stimulated thanks to taking advantage of microwave heating's unique characters:efficiency,high speed,clean and pollution-free.This paper established the temperature under microwave heating gas hydrate distribution theory model,and by using the finite element method for simulating temperature field of microwave heating gas hydrate,this paper analyzed the natural gas hydrate in the microwave field temperature distribution in the influencing factors.Microwave has a significant heating effect on the hydrate reservoir in the immediate vicinity of wellbore,and it is not affected by the initial conditions of reservoir.The temperature can rise to above 50℃ within 1 h which is higher than the phase equilibrium temperature at the time of hydrate decomposition and is helpful to improve the decomposition rate of hydrate.The frequency is set at 915 MHz,and the feed port has a spiral arrangement with a length of 10 mm,which greatly expands the microwave heating range.

Prediction of natural gas hydrate formation region in wellbore during deepwater gas well testing

Wellbore temperature field equations are established with considerations of the enthalpy changes of the natural gas during the deep-water gas well testing. A prediction method for the natural gas hydrate formation region during the deep-water gas well testing is proposed, which combines the wellbore temperature field equations, the phase equilibrium conditions of the natural gas hydrate formation and the calculation methods for the pressure field. Through the sensitivity analysis of the parameters that affect the hydrate formation region, it can be concluded that during the deep-water gas well testing, with the reduction of the gas production rate and the decrease of the geothermal gradient, along with the increase of the depth of water, the hydrate formation region in the wellbore enlarges, the hydrate formation regions differ with different component contents of natural gases, as compared with the pure methane gas, with the increase of ethane and propane, the hydrate formation region expands, the admixture of inhibitors, the type and the concentrations of which can be optimized through the method proposed in the paper, will reduce the hydrate formation region, the throttling effect will lead to the abrupt changes of temperature and pressure, which results in a variation of the hydrate formation region, if the throttling occurs in the shallow part of the wellbore, the temperature will drop too much, which enlarges the hydrate formation region, otherwise, if the throttling occurs in the deep part of the wellbore, the hydrate formation region will be reduced due to the decrease of the pressure.

气体水合物稳定性相图的研究进展

天然气水合物在我国未来的能源战略中将占有重要地位,相关的基础研究具有重要性和紧迫性。无论是自然界的水合物矿藏还是实验室合成的水合物样品,水合物的形成和稳定存在都需要特定的压强(P)和温度(T)条件,因此,确定气体水合物的相稳定性与相边界,构建不同组分的水-气体混合物的温压(P-T)相图,一直是水合物研究的核心问题,特别是对于水合物材料的合成、水合物矿藏的开采和运输具有重要意义。基于此,从实验和理论方面介绍气体水合物相图的研究进展。

气体水合物形成条件的第一性原理热力学研究

天然气水合物在我国未来的能源战略中将占有重要地位,相关的基础研究具有重要性和紧迫性。水合物的研究涉及石油化工、材料科学、能源科学、凝聚态物理、物理化学等多个学科,是一个典型的交叉学科研究领域。在当前的气体水合物研究中,固相水合物的相稳定性是共通的关键科学问题。拟在第一性原理水平上,通过对分子间相互作用的精确描述,对典型的甲烷水合物建立化学势相图,进而获得水合物稳定存在的温度、压强热力学条件,建立天然气水合物成藏的微观物理图像。

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。

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3MPa、6MPa、9MPa、12MPa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。

影响天然气水合物形成因素的实验研究

本文主要就影响天然气水合物形成的各种因素进行实验研究 ,包括气体组成、温度压力、含水饱和度、离子浓度、气体搅拌、CO2 酸性气体等的影响。通过实验数据和实验结果的分析得出各种因素对天然气水合物形成的正负影响和影响程度 ,从而对天然气水合物有更深入地了解 ,对解决现场生产实际问题具有直接性的指导作用。

深水天然气井水合物预测研究

为了抑制深水天然气井生产测试时生成水合物,针对深水环境海底高压低温特性,研究水合物生成与井筒压力、温度分布和含液量的关系,通过井筒分段方式建立井筒温度分布预测模型,考虑冷凝水和凝析油的影响,建立含液天然气井井筒压力分布模型。实例计算表明,低产气量、低井口压力状态生产测试时和开关井过程中,靠近海床的井筒及以上部分温度下降显著,有利于水合物生成。理论计算和实测值吻合较好,说明建立的模型和理论计算结果能很好地指导实际生产。

地面测试中天然气水合物影响分析及工艺技术对策

油气井地面测试过程中由于天然气经过针阀、弯头、孔板等很容易形成水合物,导致测试管线堵塞,影响测试安全。四川油气井测试公司近几年来对如何抑制油气井测试过程中天然气水合物的生成做了大量工作。文中分析了地面测试过程中天然气水合物的形成条件和控制措施,并结合这些控制措施在川渝气田的实际应用效果,进行了综合分析和总结,对今后地面流程测试工艺中针对水合物方面的设计、施工和地面流程本身做进一步改进提供了一定的依据。

高含硫湿气管道水合物形成影响因素

应用统计热力学方法,研究了不同含量H2S对湿气输送管道形成天然气水合物压力和温度条件的影响,并使用HYSYS软件对高含硫湿气输送管道压力和温度的变化规律进行过程模拟。结果表明H2S含量越高,越容易形成水合物;高含硫湿气输送管道的压力变化和温度变化趋势相似,水相的存在加剧了管道压力和温度的变化,上坡处管道更易形成水合物;在高压条件下,低水相含量即可形成饱和自由水环境,且水相含量相对较高的气流更易形成水合物。

祁连山哈拉湖坳陷地质构造特征及天然气水合物成藏地质条件研究

祁连山哈拉湖坳陷与已发现天然气水合物的木里坳陷同属于南祁连盆地次级坳陷,有相似的水合物成藏条件,但由于该区地质工作程度低,坳陷内第四系覆盖层下的地质构造信息不清,对该区的天然气水合物成藏条件缺乏认识。为查明哈拉湖坳陷的地质构造特征和天然气水合物成藏条件,在哈拉湖坳陷区内开展了物化探综合探测及研究,结果表明:(1)调查区内可划分出4个凹陷区、2个凸起区,调查区内可划分出26条断裂;(2)调查区永冻土层分布特征以新生界沉积区以大面积片状和基岩出露区以岛状分布为主,永冻土层发育与高程、地形、地表覆盖层及表层土壤水分等因素密切相关;(3)调查区发现了两处地球化学异常,以酸解烃重烃、酸解烃干燥系数、顶空气甲烷和荧光光谱为指标组合,显示了两种不同的异常特征类型,异常浓集中心明显,强度较高;(4)哈拉湖坳陷的烃原岩条件相对差,烃源岩的保存完整性和印支-燕山期圈闭构造是水合物成藏的关键,哈拉湖坳陷区北部和地球化学Ⅰ号异常区可为水合物成藏远景区。本文为南祁连盆地下一步天然气水合物资源调查与评价提供了依据。

天然气水合物保真取样钻具的试验研究

由于天然气水合物生存环境及特性特殊,所以了解其物化特性最有效的方法是通过钻探取样。天然气水合物钻探取样的关键是要有满足要求的钻探设备、钻探工具、钻探工艺及特殊的冲洗液。介绍了天然气水合物取样钻具的结构、原理、特点、钻进工艺及试验结果。根据海洋钻井的特点,提出了中国实施天然气水合物钻探经济实用的施工方案。