Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea

Gas hydrate is one kind of potential energy resources that is buried under deep seafloor or frozen areas. The first trial offshore production from the silty reservoir was conducted in the South China Sea by the China Geological Survey (CGS). During this test, there were many unique characteristics different from the sand reservoir, which was believed to be related to the clayed silt physical properties. In this paper, simulation experiments, facilities analysis, and theoretical calculation were used to confirm the hydrate structure, reservoir thermo-physical property, and bond water movement rule. And the behavior of how they affected production efficiency was analyzed. The results showed that: It was reasonable to use the structure I rather than structure II methane hydrate phase equilibrium data to make the production plan; the dissociation heat absorbed by hydrate was large enough to cause hydrate self-protection or reformation depend on the reservoir thermal transfer and gas supply; clayed silt got better thermal conductivity compared to coarse grain, but poor thermal convection especially with hydrate; clayed silt sediment was easy to bond water, but the irreducible water can be exchanged to free water under high production pressure, and the most obvious pressure range of water increment was 1.9–4.9 MPa.     

Seismic fine imaging and its significance for natural gas hydrate exploration in the Shenhu Test Area,South China Sea

Shenhu area in South China Sea includes extensive collapse and diapir structures, forming high-angle faults and vertical fracture system, which functions as a fluid migration channel for gas hydrate formation. In order to improve the imaging precision of natural gas hydrate in this area, especially for fault and fracture structures, the present work propose a velocity stitching technique that accelerates effectively the convergence of the shallow seafloor, indicating seafloor horizon interpretation and the initial interval velocity for model building. In the depth domain, pre-stack depth migration and residual curvature are built into the model based on high-precision grid-tomography velocity inversion, after several rounds of tomographic iterations, as the residual velocity field converges gradually. Test results of the Shenhu area show that the imaging precision of the fault zone is obviously improved, the fracture structures appear more clearly, the wave group characteristics significantly change for the better and the signal-to-noise ratio and resolution are improved. These improvements provide the necessary basis for the new reservoir model and field drilling risk tips, help optimize the favorable drilling target, and are crucial for the natural gas resource potential evaluation.     

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.     

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.     

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.     

加拿大Mallik陆域永冻带天然气水合物成功试采回顾

1998年由加拿大地质调查局和日本国家石油公司领导,在加拿大麦肯齐三角洲永冻带实施的Mallik 2L-38水合物研究井的项目是成功的。但是,由于气候变暖和机械故障,水合物试采目标未能完成。加拿大地质调查局和日本国家石油公司与其它8个国际合作机构于2002年回到Mallik,钻进了3L-38和4L-38两口观测井,以及5L-38生产试验井。在5L-38井中采取了水合物岩心（采取率90%）并进行物探测井等研究,实施了2次短时生产试验——运用MDT的降压法与热注入法的试验。运用热冲洗液循环的热注入法获得了470 m3的气体产量。此后, 2007—2008年又重返于1998年钻成的2L-38井,对其进行加深、扩孔、下套管等,用于实施水合物试采。2007年,在此试验井内储层射孔井段安置潛水泵,抽水降低水位以减小储层压力,在125 h的降压试验期间产生830 m3的气体。试验证明,即使在如此短的时间内,降压法也是有效的。2008年在2L-38井68 d(139 h)的长时间试采过程中,持续产气2000～4000 m3/d,总共产量达13000 m3。此次试采数据表明,降压法对于水合物生产是一种正确可取的方法。2008年加拿大Mallik天然气水物试采项目的成功,是世界水合物研究和开发史上的一座里程碑。     

天然气水合物与油气成藏的相关性研究

对天然气水合物的形成机理与分布特征、现今和地质历史时期天然气水合物的识别标志及其与油气成藏的相关性等方面的主要研究进展与实例的分析显示,天然气水合物在主动陆缘俯冲带增生楔和被动陆缘的陆隆台地断褶区非常发育,以似海底反射、特殊自生碳酸盐岩矿物、非搬运成因的局部沉积破坏或杂乱堆积、大型圆形凹陷和巨大规模的海底滑坡为识别标志,与油气成藏具有共生组合关系.深部气藏渗漏或油藏裂解形成的大量天然气为浅层外源成藏天然气水合物提供气源;古天然气水合物的形成有利于页岩气的保存和成藏;古天然气水合物分解可为浅层气藏提供气源.     

不同注热水温度下水合物开采实验研究

天然气水合物(NGH)是目前备受关注的新型洁净优质能源,注热法是其有效开采方式之一。为了经济有效地进行注热开采,采用自制的一维天然气水合物开采模拟实验装置,在填砂模型中生成相同初始条件的天然气水合物藏, 然后注入热盐水进行开采模拟实验,分析不同注热水温度对开采动态的影响。结果表明：注热水温度越高,水合物开始分解的越早,注热阶段的平均产气速率也越大;不同注热水温度下,热前缘的移动接近于线性,注热温度从40 ℃到100 ℃时,平均热前缘移动速度从015 cm/min增加到02 cm/min,说明注热水温度对热前缘的移动影响不大。注热前期,注热水温度越高,能量效率越高,但注热后期,注热水温度越高,能量效率反而越低。在本组实验条件下,注热水温度从40 ℃增加到100 ℃时,注热开采的能量效率由67减小到24,而平均热前缘移动速度并没有明显降低。因此,为保证注热开采的经济有效,建议在注热前期注高温水,在中后期采用低温水驱替。     

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.     

基于BDS的可燃冰开采环境监测与评估系统设计

天然气水合物(NGH)开发技术已成为当今世界科技创新的前沿.NGH资源需要安全开发和环境保护并重,为加快海底NGH商业化生产步伐,亟需加强海底开采环境长期监测工作,为此设计了一套基于北斗卫星导航系统(BDS)的NGH开采环境监测与评估系统.通过展现大气、海水、海床、井下"四位一体"的3D可视化海洋立体空间场景,实现NG...