The second natural gas hydrate production test in the South China Sea

Clayey silt reservoirs bearing natural gas hydrates (NGH) are considered to be the hydrate-bearing reservoirs that boast the highest reserves but tend to be the most difficult to exploit. They are proved to be exploitable by the first NGH production test conducted in the South China Sea in 2017. Based on the understanding of the first production test, the China Geological Survey determined the optimal target NGH reservoirs for production test and conducted a detailed assessment, numerical and experimental simulation, and onshore testing of the reservoirs. After that, it conducted the second offshore NGH production test in 1225 m deep Shenhu Area, South China Sea (also referred to as the second production test) from October 2019 to April 2020. During the second production test, a series of technical challenges of drilling horizontal wells in shallow soft strata in deep sea were met, including wellhead stability, directional drilling of a horizontal well, reservoir stimulation and sand control, and accurate depressurization. As a result, 30 days of continuous gas production was achieved, with a cumulative gas production of 86.14 ×10⁴ m³. Thus, the average daily gas production is 2.87 ×10⁴ m³, which is 5.57 times as much as that obtained in the first production test. Therefore, both the cumulative gas production and the daily gas production were highly improved compared to the first production test. As indicated by the monitoring results of the second production test, there was no anomaly in methane content in the seafloor, seawater, and atmosphere throughout the whole production test. This successful production test further indicates that safe and effective NGH exploitation is feasible in clayey silt NGH reservoirs. The industrialization of hydrates consists of five stages in general, namely theoretical research and simulation experiments, exploratory production test, experimental production test, productive production test, and commercial production. The second production test serves as an important step from the exploratory production test to experimental production test.     

The first offshore natural gas hydrate production test in South China Sea

Natural gas hydrates (NGH) is one of key future clean energy resources. Its industrialized development will help remit the huge demand of global natural gas, relieve the increasing pressure of the environment, and play a vital role in the green sustainable growth of human societies. Based on nearly two decades’ studying on the reservoir characteristics in the South China Sea (SCS) and the knowledge of reservoir system, the China Geological Survey (CGS) conducted the first production test on an optimal target selected in Shenhu area SCS in 2017. Guided by the “three-phase control” exploitation theory which focused on formation stabilization, technologies such as formation fluid extraction, well drilling and completing, reservoir stimulating, sand controlling, environmental monitoring, monitoring and preventing of secondary formation of hydrates were applied. The test lasted for 60 days from May 10^th when starting to pump, drop pressure and ignite to well killing on July 9^th, with gas production of 3.09×10⁵ m³ in total, which is a world record with the longest continuous duration of gas production and maximal gas yield. This successful test brings a significant breakthrough on safety control of NGH production.     

Economic Critical Resources for the Industrial Exploitation of Natural Gas Hydrate

Since the implementation of several pilot production tests were in natural gas hydrate (NGH) reservoirs in terrestrial and marine settings, the study of NGH has entered a new stage of technological development for industrial exploitation. Prior to the industrial exploitation of any given NGH reservoir, the economic feasibility should be examined. The first step of economic evaluation of a NGH reservoir is to know whether its resource amount meets the requirement for industrial exploitation. Unfortunately, few relevant studies have been conducted in this regard. In this study, the net present value (NPV) method is employed to estimate the economic critical resources required for the industrial exploitation of NGHs under different production scenarios. Sensitivity analysis is also performed in order to specify the effects of key factors, such as the number of production wells, gas price, technological improvement and tax incentive, on the economic critical resources. The results indicate that China requires the lowest economic critical resource for a NGH reservoir to be industrially exploited, ranging from 3.62 to 24.02 billion m3 methane. Changes in gas price and tax incentives also play significant roles in affecting the threshold and timeline for the industrial exploitation of NGH.     

RMR技术在海域天然气水合物钻探中的适应性分析

深水表层天然气水合物沉积体埋藏浅、地层软、未固结,泥浆密度窗口窄,使用常规隔水管泥浆循环钻进钻遇水合物分解会引起井周孔隙压力及储层水合物饱和度发生变化,导致水合物储层泥浆密度窗口更窄,引发井壁以及井口失稳等问题,并且时间长、成本高。文章介绍了无隔水管泥浆回收(RMR)技术原理,对该技术在深水表层海域天然气水合物钻探中的适应性进行了分析。结果表明,RMR技术利用双梯度原理,能有效解决目前深水表层钻井窄密度窗口难题;实时监控系统提高遇险机动性,严防硫化氢等气体发生泄漏;能够简化井身结构,缩短建井周期,降低建井成本;同时对海洋零排放,安全环保。     

低温固体氧化物燃料电池法开采天然气水合物

热开采法、抑制剂刺激法等传统天然气水合物开采方法将破坏水合物地层,不能有效地解决海底地层破坏和开采使用化石燃料燃烧后所存在的碳储存和碳捕捉问题.介绍了低温固体氧化物燃料电池法开采天然气水合物的方法.该方法利用CO2置换天然气水合物地层中的甲烷,运送到低温固体氧化物燃料电池的阳极,在阴极通入空气,发生电化学反应,产生电能,并将产出的CO2通入到天然气水合物地层,置换出甲烷.该方法简单、实用,克服了传统开采方法所存在的问题,发电效率较传统发电方法大幅提高.     

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。     

Geomechanical issues in the exploitation of natural gas hydrate

As a clean fossil fuel with great reserves, natural gas hydrate (NGH) is widely regarded as an important future alternative energy source. NGH is widely distributed in onshore tundra and shallow sedimentary layers in the deep sea. These sedimentary layers typically exit shallow burial depth, poor diagenesis and low strength characteristics; moreover, the decomposition of NGH can also greatly reduce reservoir strength. Therefore, NGH development can easily causes many geomechanical problems, including reservoir instability, sand production and seabed landslides, etc., which may further trigger a series of environmental disasters such as tsunamis, natural gas leakage and the acceleration of global warming. This study mainly reviews the research progress regarding geomechanical issues in NGH development, including mechanical properties of NGH-bearing sediments, borehole stability, hydraulic fracturing, sand production, reservoir settlement and seabed landslides. In addition to previous research achievements regarding geomechanical problems in NGH exploitation, the limitations and challenges are also discussed, and several questions and insightful suggestions are put forward for future research from our point of view.     

深海甲烷电化学原位长期监测技术及其在海洋环境调查和天然气水合物勘探中的意义

深海溶解甲烷浓度数据连续获取的方法技术,对于海洋环境和天然气水合物开发过程中甲烷扩散作用及通量的动态监测,具有重要的科学意义和实际应用价值。本文较详细地介绍了依据"海水脱气、气体样品定量输入、电化学高精度检测"技术思路,采用"增压排液整机系统控制的海水循环、减压稳流、气液分离、烃类组分高精度检测技术改进"方法,研发"深海甲烷电化学原位长期监测技术"的关键环节和技术方法。结合原位传感器在胶州湾港口为期94天底水长期监测实验获取的数据成果,对原位传感器的技术性能、数据质量、地质效果进行了研究评价。结果表明:(1)原位传感器量程甲烷指标达到0.01～10 000 nmol/L,灵敏度达到0.01 nmol/L,对烃类组分检测具有较好的稳定性和选择性;(2)监测水域溶解甲烷数值范围19.01～106.87 nmol/L,正常甲烷背景32.41 nmol/L,局部异常甲烷背景80.60 nmol/L,资料显示异常与污水排放过程对海水环境污染有关;(3)实测甲烷数据成果地球化学特征与胶州湾海域海水环境以往调查研究成果符合,证明了实测数据的客观性和科学性;(4)海试监测试验成果证明,原位传感器测试性能可靠、结构设计合理、设计思路科学,基本具备了海洋科学调查中对海水甲烷浓度数据获取的能力,在未来海洋天然气水合物开发过程中对甲烷扩散作用的动态监测及深海甲烷浓度通量的长期监测中,具有实际应用价值和科学意义。     

Natural gas hydrates in the Qinghai-Tibet Plateau: Characteristics,formation, and evolution

The Qinghai-Tibet Plateau (also referred to as the Plateau) is the largest area bearing alpine permafrost region in the world and thus is endowed with great formation conditions and prospecting potential of natural gas hydrates (NGH). Up to now, one NGH accumulation, two inferred NGH accumulations, and a series of NGH-related anomalous indicators have been discovered in the Plateau, with NGH resources predicted to be up to 8.88×10¹² m³. The NGH in the Qinghai-Tibet Plateau have complex gas components and are dominated by deep thermogenic gas. They occur in the Permian-Jurassic strata and are subject to thin permafrost and sensitive to environment. Furthermore, they are distinctly different from the NGH in the high-latitude permafrost in the arctic regions and are more different from marine NGH. The formation of the NGH in the Plateau obviously couples with the uplift and permafrost evolution of the Plateau in spatial-temporal terms. The permafrost and NGH in the Qilian Mountains and the main body of the Qinghai-Tibet Plateau possibly formed during 2.0–1.28 Ma BP and about 0.8 Ma BP, respectively. Under the context of global warming, the permafrost in the Qinghai-Tibet Plateau is continually degrading, which will lead to the changes in the stability of NGH. Therefore, The NGH of the Qinghai-Tibet Plateau can not be ignored in the study of the global climate change and ecological environment.©2021 China Geology Editorial Office.     

煤矿井下钻孔内瓦斯浓度监测传感器研制

当前,煤矿井下钻孔作业时,瓦斯监测系统只能反映钻孔孔口处瓦斯抽采量,无法获得钻孔内某个区段的瓦斯抽采效果, 随着煤矿井下瓦斯抽采钻孔孔深增加,沿钻孔长度方向瓦斯抽采效果出现明显分区,不同孔深处有效抽采半径出现较大差异,导致煤矿井下瓦斯抽采钻孔布置难度较大,不确定性增加。针对此问题,设计一种煤矿井下钻孔内瓦斯浓度监测传感器,该传感器基于可调谐半导体激光吸收光谱(Tunable Diode Laser Absorption Spectroscopy,TDLAS)原理,可实现钻孔内多点同时在线监测,保证了孔内无源,实现了本质安全。首先,分析TDLAS瓦斯测量基本原理,从气体分子吸收光谱原理出发,介绍了激光光源的选择,并根据比尔?朗伯定律推算瓦斯气体浓度解算公式。然后,在此基础上进行瓦斯浓度监测传感器设计,包括光程设计、结构设计、保护工艺设计和孔中操作流程4方面。最后,从性能和可靠性2方面出发,进行相对误差测试、稳定性测试、响应时间测试、与非色散红外传感器性能对比和防水防尘测试。设计的瓦斯浓度监测传感器直径40 mm,长度80 mm,传感器本质安全,结构上能够很好地适用于煤矿井下钻孔内应用。性能测试中,传感器全量程最大相对误差2.8%,小于孔内瓦斯浓度±6%的监测标准;稳定性测试中,传感器数据的波动范围在0.015%,稳定性为0.28%,满足稳定性小于1%的要求;传感器的响应时间约为8 s,满足响应时间小于10 s的要求;与非色散红外传感器对比测试中,设计的TDLAS瓦斯浓度监测传感器的相对误差和响应时间都明显优于非色散红外传感器。可靠性测试中,传感器长时间处于高湿度环境中,其测量精度并未受到影响,保护工艺可有效防水。性能测试和可靠性测试结果表明,瓦斯浓度监测传感器能够很好地满足孔内瓦斯浓度监测需求,在煤矿井下孔中监测方面具有很好的应用前景。      

中国南海天然气水合物第二次试采主要进展

泥质粉砂型天然气水合物被认为是储量最大开采难度亦最大的水合物储层,2017年南海天然气水合物试采,初步验证了此类水合物储层具备可开采性。在总结前次试采认识的基础上,对试采矿体进行优选、精细评价、数值与试验模拟和陆地试验,中国地质调查局于2019年10月—2020年4月在南海水深1225 m神狐海域进行了第二次天然气水合物试采。本次试采攻克了钻井井口稳定性、水平井定向钻进、储层增产改造与防砂、精准降压等一系列深水浅软地层水平井技术难题,实现连续产气30 d,总产气量86.14×104m3,日均产气2.87×104m3,是首次试采日产气量的5.57倍,大大提高了日产气量和产气总量。试采监测结果表明,整个试采过程海底、海水及大气甲烷含量无异常。本次成功试采进一步表明,泥质粉砂储层天然气水合物具备可安全高效开采的可行性。     

城市垃圾填埋场甲烷资源量与利用前景

垃圾填埋气的主要成分为CH4、CO2等气体,可严重污染大气、地下水和生态环境,并对全球气候变暖产生一定的影响;同时填埋气也是一种清洁可再生能源和资源,回收和利用垃圾填埋气可实现环境、安全、能源、资源、经济多重效益。目前,垃圾填埋气的利用主要为甲烷利用。本文介绍了填埋气中甲烷资源量的计算方法,采用一阶动力模型对国内城市垃圾填埋气中的甲烷排放量进行了计算和预测,获得了城市生活垃圾填埋气中甲烷的资源量的范围,并分析了国内垃圾填埋气排放的特点和趋势以及国内外对填埋气利用的途径、方法及效果。结合清洁发展机制（CDM）项目和国情分析了垃圾填埋气的利用前景,并提出了填埋气回收利用的主要问题和建议,为国内城市生活垃圾填埋气的利用提供了有益参考。     

Sensitivity Analysis of Multi-phase Seepage Parameters Affecting the Clayey Silt Hydrate Reservoir Productivity in the Shenhu Area, South China Sea

Natural gas hydrate (NGH) is an important future resource for the 21st century and a strategic resource with potential for commercial development in the third energy transition. It is of great significance to accurately predict the productivity of hydrate-bearing sediments (HBS). The multi-phase seepage parameters of HBS include permeability, porosity, which is closely related to permeability, and hydrate saturation, which has a direct impact on hydrate content. Existing research has shown that these multi-phase seepage parameters have a great impact on HBS productivity. Permeability directly affects the transmission of pressure-drop and discharge of methane gas, porosity and initial hydrate saturation affect the amount of hydrate decomposition and transmission process of pressure-drop, and also indirectly affect temperature variation of the reservoir. Considering the spatial heterogeneity of multi-phase seepage parameters, a depressurization production model with layered heterogeneity is established based on the clayey silt hydrate reservoir at W11 station in the Shenhu Sea area of the South China Sea. Tough + Hydrate software was used to calculate the production model; the process of gas production and seepage parameter evolution under different multi-phase seepage conditions were obtained. A sensitivity analysis of the parameters affecting the reservoir productivity was conducted so that: (a) a HBS model with layered heterogeneity can better describe the transmission process of pressure and thermal compensation mechanism of hydrate reservoir; (b) considering the multi-phase seepage parameter heterogeneity, the influence degrees of the parameters on HBS productivity were permeability, porosity and initial hydrate saturation, in order from large to small, and the influence of permeability was significantly greater than that of other parameters; (c) the production potential of the clayey silt reservoir should not only be determined by hydrate content or seepage capacity, but also by the comprehensive effect of the two; and (d) time scales need to be considered when studying the effects of changes in multi-phase seepage parameters on HBS productivity.     

天然气水合物开采过程甲烷气泄漏海床基高密度电阻率法监测效果模拟与分析

为实现天然气水合物开采过程沉积物内甲烷气泄漏过程有效监测,设计了一种海床基原位电学监测方法。为界定该方法对不同模式泄露甲烷气的探测能力,以南海神狐海域天然气水合物试采区为研究区,构建相应地质及电阻率模型,模拟利用设计电学系统对其进行监测,计算得到不同采集参数的电阻率剖面,并对其进行对比分析。研究结果表明：海水层会在探测剖面上某深度区间内形成层状低阻异常带,对该深度区间有效电信号形成压制。将该异常带顶界深度定义为系统有效探测深度后,发现该深度受电极距直接影响,10 m极距偶极装置有效探测深度约为50 m。电学探测剖面对有效探测深度内分布的层状和团状甲烷气聚集区、慢速甲烷气泄露区、沿断层泄露的甲烷气区具有良好反映能力,数据处理得到的相对电阻率剖面与电学探测剖面相比能更好地反映甲烷气聚集区边界。该监测方法能够实时监测含气区空间变化。     

Geochemical insights into contribution of petroleum hydrocarbons to the formation of hydrates in the Taixinan Basin,the South China Sea

Methane hydrate in the South China Sea(SCS)has extensively been considered to be biogenic on the basis of itsδ13C and δD values.Although previous efforts have greatly been made,the contribution of thermogenic oil/gas has still been underestimated.In this study,biomarkers and porewater geochemical parameters in hydrate-free and hydrate-bearing sediments in the Taixinan Basin,the SCS have been measured for evaluating the contribu-tion of petroleum hydrocarbons to the formation of hydrate deposits via a comparative study of their source inputs of organic matters,environmental conditions,and microbial activities.The results reveal the occurrence of C14-C16 branched saturated fatty acids(bSFAs)with relatively high concentrations from sulfate-reducing bacteria(SRBs)in hydrate-bearing sediments in comparison with hydrate-free sediments,which is in accord with the positive δ13C values of dissolved inorganic carbon(DIC),increasing methane concentrations,decreasing alka-linity,and concentration fluctuation of ions(Cl-,Br,SO2-,Ca2+,and Mg2+).These data indicate the relatively active microbial activities in hydrate-bearing sediments and coincident variations of environmental conditions.Carbon isotope compositions of bSFAs(-34.0％o to-21.2％o),n-alkanes(-34.5％o to-29.3％o),and methane(-70.7％o to-69.9％o)jointly demonstrate that SRBs might thrive on a different type of organic carbon rather than methane.Combining with numerous gas/oil reservoirs and hydrocarbon migration channels in the SCS,the occurrence of unresolved complex mixtures(UCMs),odd-even predominance(OEP)values(about 1.0),and biomarker patterns suggest that petroleum hydrocarbons from deep oil/gas reservoirs are the most probable carbon source.Our new results provide significant evidence that the deep oil/gas reservoirs may make a contribution to the formation of methane hydrate deposits in the SCS.     

天然气水合物勘探-开发-储运-环境响应研究新进展--第五届天然气水合物国际会议简介

主要根据第五届天然气水合物国际会议(ICGH5)的资料,介绍了天然气水合物勘探、开发、储运和环境响应研究的新进展.在勘探、开发方面,主要使用多维地震和核磁共振等方法来研究天然气水合物的分布情况,并根据天然气水合物声阻抗与沉积物孔隙度之间的关系来估算天然气水合物的资源量;在储运方面,与传统的液化天然气方法相比,以天然气水合物丸粒形式储运天然气所需的费用和所消耗的总能量以及所释放出的CO2相对较少;在环境响应方面,对深海储存CO2以及CH4流体对海洋和大气影响的研究,为解决天然气水合物可能引起的环境问题提供了新的思路.最后,介绍了各国天然气水合物的研究情况,并对天然气水合物研究中尚需解决的问题进行了展望.     

页岩气开发的地下水环境背景值、监测指标及污染示踪方法研究——以焦石坝区块为例

页岩气开采过程中水力压裂及废水回注可能带来的环境问题受到越来越多关注。由于水力压裂开采技术涉及的污染物种类众多,因此建立敏感性监测指标对于识别潜在的地下水污染具有重要意义。本文以中国涪陵页岩气田焦石坝区块为例,首先确定了地下水环境背景值（包括水化学、同位素及溶解气）;通过页岩水与浅层地下水的端元对比,确定了地下水环境敏感性监测指标与污染示踪方法。结果显示浅层地下水TDS在146~402 mg·L-1之间,属于HCO₃-Ca·Mg型,地下水属于年轻地下水（含3 H、14C接近100 pmC）,地下水甲烷含量均小于0.01 mg·L-1,水中溶解气的甲烷（CH₄）体积比低于0.006 4%,δ13C-CH₄总体小于-50‰,为生物成因甲烷（显著区别于热成因页岩气）。涪陵页岩气田焦石坝区块页岩水TDS约为海水两倍,为大气降水成因,显著区别于北美的（蒸发）海水起源。根据页岩水和浅层地下水水文地球化学特征的差异,建立了确定地下水敏感性监测指标的框架,识别出焦石坝区块地下水敏感性指标（7项）。页岩气开发对地下水潜在的污染主要包括气体污染和溶解固体组分污染。对于气体,建立了甲烷含量及同位素组成的端元,并且应用惰性气体进行了两个端元识别;对于溶解固体组分污染,基于浅层地下水背景值数据,可利用本文给出的敏感性指标进行判别。本项研究对于中国页岩气开发的地下水环境保护具有重要的意义,有助于完善页岩气开发过程中地下水环境监测和潜在污染示踪。     

青海木里三露天井田煤系天然气水合物成藏模式与勘查开发建议

天然气水合物因其能量密度巨大、使用方便、燃烧值高、清洁无污染等特点,被誉为21世纪最有希望的战略资源,是常规油气资源的潜力巨大的一种替代能源.青海木里地区三露天井田天然气水合物的发育是该区含煤岩系的充足气源和高寒山区适宜的水合物稳定带耦合作用的结果.将赋存在煤系中的天然气水合物定义为煤系天然气水合物,并将其纳入煤系非常...     

青藏高原天然气水合物遥感探测研究

介绍了天然气水合物的遥感探测方法,即:首先选择适当的ETM+、ASTER以及Quickbird等遥感图像,分析可见光、短波红外、热红外波段的影像特征,反演地表温度;然后结合野外调查和前人资料,寻找天然气水合物的直接和间接遥感解译标志,进行遥感解译。研究结果表明,青藏高原天然气远景区出现的几种遥感解译标志包括:地表上空高温异常(尤其是水体上空)、泥火山、沼泽地水体中大量气泡和棕黄色沉淀物,以及碳酸盐化、粘土化等指示信息。本研究为进一步开展大规模天然气水合物遥感调查和圈定靶区提供了基础方法。      

Characteristics of major and trace elements in surface sediments of the Makran Accretionary Prism,Pakistan and their implications for natural gas hydrates

To accurately identify the natural gas hydrates (NGH) in the sea area of the Makran Accretionary Prism, Pakistan, this paper presents the testing and analysis of major and trace elements in sediment samples taken from two stations (S₂ and S₃) in the area by the China Geological Survey. As shown by testing results, all major elements are slightly different in content between the two stations except SiO₂ and CaO. This also applies to the trace elements that include Sr and Ba primarily and Cr, Ni and Zn secondarily. It can be concluded in this study that the tectonic setting of the Makran Accretionary Prism is dominated by oceanic island arc and that provenance of the Makran Accretionary Prism is dominated by felsic igneous provenance, which is at the initial weathering stage and mainly consists of granodiorite. Besides terrigenous detritus, there are sediments possibly originating from Makran-Bela Ophiolite from the northwestern part and Murray Ridge igneous rocks from the southeastern part. The V/Cr, Ni/Co, and V/(V+Ni) ratios indicate that sediments of the two stations are in an oxidation-suboxidation environment. However, the authors infer that the sedimentary environment of the sediments 3.0 m below the seafloor tends to be gradually transformed into a reduction environment by comparison with the Qiongdongnan Basin in the South China Sea where NGH has been discovered. The sediments in the Makran Accretionary Prism are rich in organic matter, with total organic carbon (TOC) content greater than 1%. According to comprehensive research, the organic matter in the sediments mainly originates from marine algae and has high TOC content, which is favorable for the formation of NGH.