Hydrate phase transition and seepage mechanism during natural gas hydrates production tests in the South China Sea: A review and prospect

Natural gas hydrates (NGHs) are globally recognized as an important type of strategic alternative energy due to their high combustion efficiency, cleanness, and large amounts of resources. The NGHs reservoirs in the South China Sea (SCS) mainly consist of clayey silts. NGHs reservoirs of this type boast the largest distribution range and the highest percentage of resources among NGHs reservoirs in the world. However, they are more difficult to exploit than sandy reservoirs. The China Geological Survey successfully carried out two NGHs production tests in the Shenhu Area in the northern SCS in 2017 and 2020, setting multiple world records, such as the longest gas production time, the highest total gas production, and the highest average daily gas production, as well as achieving a series of innovative theoretical results. As suggested by the in-depth research on the two production tests, key factors that restrict the gas production efficiency of hydrate dissociation include reservoir structure characterization, hydrate phase transition, multiphase seepage and permeability enhancement, and the simulation and regulation of production capacity, among which the hydrate phase transition and seepage mechanism are crucial. Study results reveal that the hydrate phase transition in the SCS is characterized by low dissociation temperature, is prone to produce secondary hydrates in the reservoirs, and is a complex process under the combined effects of the seepage, stress, temperature, and chemical fields. The multiphase seepage is controlled by multiple factors such as the physical properties of unconsolidated reservoirs, the hydrate phase transition, and exploitation methods and is characterized by strong methane adsorption, abrupt changes in absolute permeability, and the weak flow capacity of gas. To ensure the long-term, stable, and efficient NGHs exploitation in the SCS, it is necessary to further enhance the reservoir seepage capacity and increase gas production through secondary reservoir stimulation based on initial reservoir stimulation. With the constant progress in the NGHs industrialization, great efforts should be made to tackle the difficulties, such as determining the micro-change in temperature and pressure, the response mechanisms of material-energy exchange, the methods for efficient NGHs dissociation, and the boundary conditions for the formation of secondary hydrates in the large-scale, long-term gas production.©2022 China Geology Editorial Office.     

Migration and accumulation characteristics of natural gas hydrates in the uplifts and their slope zones in the Qiongdongnan Basin,China

Various factors controlling the accumulation of natural gas hydrates (NGHs) form various enrichment and accumulation modes through organic combination. This study mainly analyzes the geological and geophysical characteristics of the NGHs occurrence in the uplifts and their slope zones within the deep-water area in the Qiongdongnan (QDN) Basin (also referred to as the study area). Furthermore, it investigates the dominant governing factors and models of NGHs migration and accumulation in the study area. The results are as follows. (1) The uplifts and their slope zones in the study area lie in the dominant pressure-relief direction of fluids in central hydrocarbon-rich sags in the area, which provide sufficient gas sources for the NGHs accumulation and enrichment through pathways such as gas chimneys and faults. (2) The top and flanks of gas chimneys below the bottom simulating reflectors (BSRs) show high-amplitude seismic reflections and pronounced transverse charging of free gas, indicating the occurrence of a large amount of gas accumulation at the heights of the uplifts. (3) Chimneys, faults, and high-porosity and high-permeability strata, which connect the gas hydrate temperature-pressure stability zones (GHSZs) with thermogenic gas and biogenic gas, form the main hydrate migration system. (4) The reservoir system in the study area comprises sedimentary interlayers consisting of mass transport deposits (MTDs) and turbidites. In addition, the reservoir system has developed fissure- and pore-filling types of hydrates in the pathways. The above well-matched controlling factors of hydrate accumulation enable the uplifts and their slope zones in the study area to become the favorable targets of NGHs exploration.©2022 China Geology Editorial Office.     

叠前同时反演技术在珠江口盆地西部海域天然气水合物储层预测中的应用

珠江口盆地西部海域发育弱BSR或无BSR的天然气水合物储层,常规叠后反演所获得的参数单一,难以精确预测其天然气水合物分布特征。本文在地震道集优化处理、精细速度分析、岩石物理分析及低频模型精确建立的基础上,针对性地采用叠前同时反演技术,对珠江口盆地西部海域天然气水合物储层进行预测,并利用岩相流体概率分析技术对其进行综合识别,实现了对天然气水合物储层地精细刻画。反演预测结果表明,研究区天然气水合物较为发育,预测结果与钻探结果吻合程度较高,应用效果良好。     

基于叠前深度偏移的AVO反演在神狐海域试采区的应用

为了更好地研究神狐海域试采区天然气水合物的展布形态和赋存状况,基于高精度层析速度反演后的叠前深度偏移地震资料进行AVO反演技术研究。为在天然气水合物富集区有效地进行AVO研究,AVO处理前运用叠前深度偏移技术,通过高精度层析反演不断迭代速度模型;然后通过偏移来改善资料的质量,结合实测钻井资料进行AVO正演分析、反演可行性分析、属性交会分析;最后将具有高横向分辨能力的地震资料和高垂向分辨能力的测井资料与地质相结合进行叠前同时反演。该方法可反演出纵、横波阻抗等属性,直接进行岩性和流体识别,结果表明,在AVO反演中引入叠前深度偏移技术,为反演提供了较高质量的叠前共反射点道集和精细的层速度模型,改善了AVO技术在试采区的应用效果,提高了反演的可靠性。反演出的各种属性为AVO分析提供了丰富的信息,能够比较准确地反映地下水合物矿体的展布情况,可用于后续水合物储层的分析及综合研究,对未来的水合物勘探开采具有重要的现实意义。     

Coexistence of natural gas hydrate,free gas and water in the gas hydrate system in the Shenhu Area,South China Sea

Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin, which is on the northern continental slope of the South China Sea. Gas hydrates in this area have been intensively investigated, achieving a wide coverage of the three-dimensional seismic survey, a large number of boreholes, and detailed data of the seismic survey, logging, and core analysis. In the beginning of 2020, China has successfully conducted the second offshore production test of gas hydrates in this area. In this paper, studies were made on the structure of the hydrate system for the production test, based on detailed logging data and core analysis of this area. As to the results of nuclear magnetic resonance (NMR) logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition, the hydrate system on which the production well located can be divided into three layers: (1) 207.8–253.4 mbsf, 45.6 m thick, gas hydrate layer, with gas hydrate saturation of 0–54.5% (31% av.); (2) 253.4–278 mbsf, 24.6 m thick, mixing layer consisting of gas hydrates, free gas, and water, with gas hydrate saturation of 0–22% (10% av.) and free gas saturation of 0–32% (13% av.); (3) 278–297 mbsf, 19 m thick, with free gas saturation of less than 7%. Moreover, the pore water freshening identified in the sediment cores, taken from the depth below the theoretically calculated base of methane hydrate stability zone, indicates the occurrence of gas hydrate. All these data reveal that gas hydrates, free gas, and water coexist in the mixing layer from different aspects.     

松科二井邻域沙河子组含油气地层结构特征——基于测井和地震数据的分析

松辽盆地大陆深部科学钻探工程"松科二井"获得了最完整的沙河子组地层岩心,初步识别出深部页岩气气测异常43层,累计厚度102 m,显示出沙河子组地层具有良好的深部能源前景。为进一步分析松辽盆地沙河子组地层油气资源潜力,本文在前人研究的基础上,结合松科二井测井、岩心数据和地震数据,分析松科二井邻域沙河子组地层的层序特征和空间展布,同时以随钻录井记录到的气测异常为出发点,分析沙河子组地层的测井和地震响应。结果显示:松科二井邻域沙河子组地层具有厚度大、横向延续小的地层展布特点,根据测井、岩心及地震数据,将沙河子组地层分为4个三级层序:SQ1、SQ2、SQ3和SQ4,其中顶部SQ4层序具有较好的油气前景,其岩性以泥岩为主,并具有相对较高孔隙度、低波阻抗的测井曲线特征,在地震剖面上表现为连续强振幅,阻抗反演剖面上存在局部低波阻抗特征,揭示了该区沙河子组深层天然气资源的赋存条件。     

The Sensitive Properties of Hydrate Reservoirs Based on Seismic Stereoscopic Detection Technology

Higher-precision determinations of hydrate reservoirs, hydrate saturation levels and storage estimations are important for guaranteeing the ability to continuously research, develop and utilize natural gas hydrate resources in China. With seismic stereoscopic detection technology, which fully combines the advantages of different seismic detection models, hydrate formation layers can be observed with multiangle, wide-azimuth, wide-band data with a high precision. This technique provides more reliable data for analyzing the distribution characteristics of gas hydrate reservoirs, establishing velocity models, and studying the hydrate-sensitive properties of petrophysical parameters;these data are of great significance for the exploration and development of natural gas hydrate resources. Based on a velocity model obtained from the analysis of horizontal streamer velocity data in the hydrate-bearing area of the Shenhu Sea, this paper uses three VCs(longitudinal spacing of 25 m) and four OBSs(transverse spacing of 200 m) to jointly detect seismic datasets consisting of wave points based on an inversion of traveltime imaging sections. Accordingly, by comparing the differences between the seismic phases in the original data and the forward-modeled seismic phases, multiangle coverage constraint corrections are applied to the initial velocity model, and the initial model is further optimized, thereby improving the imaging quality of the streamer data. Petrophysical elastic parameters are the physical parameters that are most directly and closely related to rock formations and reservoir physical properties. Based on the optimized velocity model, the rock elastic hydrate-sensitive parameters of the hydrate reservoirs in the study area are inverted, and the sensitivities of the petrophysical parameters to natural gas hydrates are investigated. According to an analysis of the inversion results obtained from these sensitive parameters, λρ, Vp and λμ are simultaneously controlled by the bulk modulus and shear modulus, while Vs and μρ are controlled only by the shear modulus, and the latter two parameters are less sensitive to hydrate-bearing layers. The bulk modulus is speculated to be more sensitive than the shear modulus to hydrates. In other words, estimating the specific gravity of the shear modulus among the combined parameters can affect the results from the combined elastic parameters regarding hydrate reservoirs.     

基于水合物指示因子的地震识别方法

速度是水合物勘查的重要参数,但是速度的影响因素众多,给高精度识别水合物带来一定困难.水合物储层岩石物理是研究多种因素对速度影响的有效手段之一.考虑到水合物储层未固结等特征,本文采用SCA-DEM模型,重点对比了孔隙度、饱和度等对纵横波速度以及AVO特征的影响,并构建了一种新的水合物指示因子,消除了由于孔隙变化带来的假异...     

中国南海天然气水合物开采储层水合物相变与渗流机理:综述与展望

【研究目的】中国地质调查局先后于2017年、2020年在南海北部神狐海域成功实施两轮水合物试采,创造了产气时间最长、产气总量最大、日均产气量最高等多项世界纪录,了解和掌握南海天然气水合物开采储层相变与渗流机理,有助于进一步揭示该类型水合物分解机理、产出规律、增产机制等,可为中国海域水合物资源规模高效开采提供理论基础。【研究方法】基于两轮试采实践,笔者通过深入研究发现,储层结构表征、水合物相变、多相渗流与增渗、产能模拟与调控是制约水合物分解产气效率的重要因素。【研究结果】研究表明,南海水合物相变具有分解温度低,易在储层内形成二次水合物等特点,是由渗流场-应力场-温度场-化学场共同作用的复杂系统;多相渗流作用主要受控于未固结储层的物性特征、水合物相变、开采方式等多元因素影响,具有较强的甲烷吸附性、绝对渗透率易突变、气相流动能力弱等特点;围绕南海水合物长期、稳定、高效开采目标,需要在初始储层改造基础上,通过实施储层二次改造,进一步优化提高储层渗流能力,实现增渗扩产目的。【结论】随着天然气水合物产业化进程不断向前推进,还需要着力解决大规模长时间产气过程中温度压力微观变化及物质能源交换响应机制以及水合物高效分解、二次生成边界条件等难题。创新点：南海水合物相变是由渗流场-应力场-温度场-化学场共同作用的复杂系统;南海泥质粉砂储层具有较强的甲烷吸附性、绝对渗透率易突变、气相流动能力弱等特点,多相渗流机理复杂。     

琼东南盆地南部低凸起及其周缘区天然气水合物富集影响因素和成藏模式

为了厘清琼东南盆地南部低凸起及其周缘区天然气水合物富集影响因素及成藏模式,利用天然气水合物钻探获取的钻井、测井及2D/3D地震资料,分析了研究区天然气水合物赋存的地质、地球物理特征,探讨了水合物富集控制成藏的影响因素,建立了水合物成藏模式.结果表明:琼东南盆地南部低凸起及其周缘区位于中央坳陷带南坡的深部流体输导优势方向上.多个站位水合物钻探显示,水合物具有分层、多类型储集层的特征.测井上含水合物层段总体具有高电阻率、低声波时差特征.地震剖面分析显示气烟囱顶部气体横向充注现象明显,气体垂向运移受限.研究区水合物的气源兼具微生物成因和热解气成因.断层、气烟囱以及孔?缝渗漏体系为深层热解气的运移提供了良好的输导条件.浅层块体搬运沉积的快速堆积使得其内部孔隙流体难以迅速排出,从而其孔隙流体压力相比上覆和下伏地层要高,使得下伏流体的垂向输导受阻,形成封盖作用.超压封盖层是研究区多类型储集层水合物主要的控制因素.根据封盖能力的差异性及其对水合物富集程度的影响提出了封闭系统和开放系统两种类型的水合物成藏模式.      

琼东南盆地天然气水合物与浅层气共生体系成藏特征

继我国在神狐海域两次天然气水合物试采成功之后,近几年来在琼东南盆地的勘探证实了天然气水合物的存在,而且钻探表明其与浅层气具有复杂的共生关系.为揭示琼东南盆地深水区天然气水合物与浅层气共生体系成藏特征,结合岩心、测井及三维地震数据,阐明了天然气水合物与浅层气的空间分布特征,研究结果表明,天然气水合物主要赋存在海底以下200 m范围内的沙质沉积物中,且其形成过程与浅层气的垂向运移有关.对天然气水合物与浅层气共生体系成藏特征的深入分析表明,深部热成因气和浅部生物成因气是其重要的气体来源,第四系未固结沙层是良好的储层,且天然气水合物和浅层气共生体系的分布主要受深部气烟囱和断层的控制.浅层气藏为天然气水合物提供稳定的气源条件;第四系块体流沉积与含天然气水合物地层能有效地封堵浅层气的纵向运移,进一步促进浅层气的成藏.因此,天然气水合物的形成与浅层气的发育具有正反馈的相互作用关系,有利于形成更大规模的天然气水合物矿体和浅层气藏,具有良好的商业开发潜力.      

深度域波形指示反演在薄储层预测中的应用:以苏里格气田陕18区块为例

由于地震分辨率的限制,传统的时间域反演方法很难精确预测薄储层的三维空间分布,极大制约了该类储层发育区高效水平井组的部署.要实现这类薄储层的精确预测,关键在于如何井震结合、拓宽频带,获得合理的高频成分.通过对比分析当前主流的反演方法,笔者提出深度域地震波形指示反演方法,该方法能够精确识别陆相薄层砂岩储层.以苏东南地区陕1...     

东濮凹陷薄砂岩储层有效识别技术

东濮凹陷砂岩储层埋藏深、厚度薄、横向变化快,常规地震资料已不能满足储层精细描述的要求。针对该区薄砂岩储层特征,利用谱反演和分频解释技术将时间域的地震信号变换到频率域,获得较原始地震数据更加丰富的数据信息,在此基础上进行模拟退火波阻抗反演和广义多属性拟合综合预测。预测结果表明:谱反演和分频解释技术有效提高了薄储层的分辨率;模拟退火反演精确预测出薄砂岩储层的厚度;而广义多属性拟合技术精确刻画出薄砂岩储层的平面展布范围。多种方法综合最终有效预测出薄砂岩储层的空间分布规律,经钻井证实可靠。      

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.©2022 China Geology Editorial Office.     

Application of wellhead suction anchor technology in the second production test of natural gas hydrates in the South China Sea

Traditional suction anchor technology is mainly used in the fields of subsea structure bearing foundations, single-point mooring systems and offshore wind power. It is characterized by providing sufficient lateral and vertical bearing capacities and lateral bending moment. The anchor structure of a traditional suction anchor structure is improved with wellhead suction anchor technology, where a central pipe is added as a channel for drilling and completion operations. To solve the technical problems of a low wellhead bearing capacity, shallow built-up depth, and limited application of conductor jetting in the second production test of natural gas hydrates (NGHs) in the South China Sea (SCS), the China Geological Survey (CGS) took the lead in independently designing and manufacturing a wellhead suction anchor, which fulfilled the requirements of the production test. This novel anchor was successfully implemented in the second production test for the first time, providing a stable wellhead foundation for the success of the second production test of NGHs in the SCS.©2022 China Geology Editorial Office.     

Saturation evaluation for fine-grained sediments

Accurate quantification of the gas hydrate content in the deep sea is useful for assessing the resource potential and understanding the role of gas hydrates in the global carbon cycle. Resistivity logging data combined with Archie's equation are often used to calculate gas hydrate saturation, but the reliability is dependent on the rationality of the empirical parameter cementation factor and saturation index. At present, an increasing number of fine-grained hydrate-rich sediment regions have been discovered worldwide through drilling efforts, and the reservoir types and hydrate distribution are diverse, which differs greatly from that of coarse-grained reservoirs of hydrate-bearing sediment. This results in vertical variations in m and n through stratigraphy. At present, the saturation evaluation effect of these reservoirs cannot be improved. In this work, a theory for the determination of the cementation factor and saturation index was first proposed to obtain reliable and variable values of the empirical parameters. Then, a hydrate saturation evaluation technique with variables m and n was formed based on the well logging data. This technique was used to evaluate complex fine-grained hydrate-bearing reservoirs in several regions worldwide. It was found that the highest n could be 16, and the log calculation results were more consistent with the core hydrate saturation. Additionally, the cause of the excessively high n values was explained from physical principles, and the result was verified with actually well log data. In future evaluations of the amount of hydrate resources in fine-grained sediment reservoirs worldwide, new saturation estimation methods should be taken into account to advance hydrate research.     

天然气水合物发育的构造背景分析

大量的钻孔资料和地震剖面显示主动大陆边缘的增生楔和被动大陆边缘的俯冲-增生楔、断裂-褶皱系、底辟构造或泥火山、滑塌构造、海底扇、"麻坑"构造和陆地多年冻土区等多种地质构造背景是形成天然气水合物的有利场所,可形成构造圈闭型天然气水合物矿藏。这些地质构造背景一方面大多是深部热成因气、生物成因气或混合成因气体或流体向上运移到海底的通道,形成天然气水合物矿藏;另一方面也可能造成天然气水合物的温压环境改变,致使天然气水合物分解。海底滑塌亦可能是天然气水合物分解所致,是潜在的地质灾害。     

基于波阻抗反演的天然气水合物地震检测技术

天然气水合物作为特殊的地质体,可以有效地粘结碎屑颗粒,降低沉积物孔隙度,它的存在改变了地层沉积物的物理性质,造成天然气水合物与围岩速度反差较大,从而与围岩之间存在明显的波阻抗差。为了对地层中是否有天然气水合物赋存进行地震检测,对南海北部神狐海域的天然气水合物赋存区域的地震资料进行波阻抗反演分析,结果显示波阻抗反演方法能够作为探测天然气水合物的一种技术手段,研究区天然气水合物矿体的波阻抗呈高值分布特征,波阻抗值约为3 850~3 960 g/cm3.m/s。综合分析认为,波阻抗反演方法能够用于天然气水合物的地震探测、储层分析和综合研究工作中,反演结果可以为天然气水合物储量计算提供比较准确的矿体面积和厚度参数。     

青海木里地区天然气水合物三维地震探测

首次在中国祁连山冻土区青海木里地区实施了针对天然气水合物的三维地震探测。根据工区内冻土层厚度、地层岩性、水合物储集空间类型和赋存深度等特 点,设计了三维地震数据采集方案,制定了地震数据处理流程,获得了良好的三维地震探测结果。结果表明,工区内天然气水合物赋存层段地震响应特征与国外冻土区的天然气水合物具有明 显差异;冻土层厚度、岩性、水合物储集空间类型等因素,对该区地震资料振幅、频率、速度等影响显著;水合物层段的地震资料存在低频、中等杂乱振幅等明显的特点,这些特征为该区天然气水合物探测和识别提供了依据。     

南海北部天然气水合物富集特征及定量评价

为寻找有资源前景的高富集天然气水合物及水合物储层的精细刻画方法,利用南海6次钻探发现的高饱和度水合物层的测井、岩心和三维地震数据,分析水合物富集层测井与地震异常特征.发现:（1）不同饱和度的孔隙与裂隙充填型水合物层的测井和地震异常不同,裂隙充填型水合物层具有各向异性;（2）受高通量流体运移的影响,在粉砂沉积物的水合物稳定带底界附近能形成中等饱和度的水合物层,识别标志为稳定带内极性与海底一致的强振幅反射,而非BSR和振幅空白;（3）裂隙充填型中等饱和度水合物层在地震剖面上表现为地层上拱和弱-中等强度振幅反射.储层-疏导-气源的耦合控制着水合物的富集特征和分布,断层与流体运移控制着细粒粉砂质沉积物中水合物的富集与厚度.基于饱和度岩相的统计学反演,能识别3 m非水合物和低饱和度水合物层及空间分布.