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.     

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.     

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。     

南海神狐海域天然气水合物地球物理测井评价

我国南海北部神狐海域的天然气水合物钻探过程中采用电缆测井来识别水合物储层,使用了自然伽马、电阻率、密度、声波全波列、井温—井方位、井径及中子等7种测井仪器,测量的参数主要包括地层的自然放射性、深(浅)探测电阻率、密度、纵波速度、温度、井径、长(短)源距中子计数率及井眼方位,这些参数对于确定天然气水合物的赋存位置起到非常重要的作用。详细介绍了神狐海域天然气水合物测井的工作方法和基本步骤,参照国外相关分析,针对其中某站位钻孔ZK1的地层孔隙度及天然气水合物饱和度进行初步评价。结果表明密度测井和电阻率测井两种方法求出的地层孔隙度的一致性较好,而计算的天然气水合物饱和度值则高于孔隙水淡化分析得到的值,因此尚需结合研究区岩心分析数据来提高解释精度。研究结果对未来我国的天然气水合物测井评价具有指导意义。     

南海北部神狐海域天然气水合物成藏特征

天然气水合物作为一种新型、洁净、潜在的新能源,越来越引起世界各国科学家的重视,对天然气水合物研究也进一步深入,但天然气水合物作为一种能源矿产,对其成藏机制的研究相对较少。针对我国天然气水合物调查研究相对较为详细的神狐海域,从其物质来源、气体运移通道、成藏条件等角度探讨神狐海域天然气水合物的成藏特征,指出白云凹陷古近纪埋藏的巨厚烃源岩是其成藏的主要物质基础;底辟构造发育区是形成水合物流体向上运移的主要通道;新近纪晚期大面积发育的滑塌体是水合物的主要赋存区。神狐海域具备天然气水合物成藏的优越条件,是进一步勘探水合物的远景区。     

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.     

Large borehole with multi-lateral branches: A novel solution for exploitation of clayey silt hydrate

ontact area are two main ways to raise the productivity of hydrate. An exploitation technique based on large borehole with multi-lateral branches (LB & MB) was proposed in this paper. This technique is mainly intended for the clayey silt hydrate reservoir in the South China Sea, and its main purpose is to alleviate the sand output from formation for maintaining the stability of the reservoir and to greatly increase the gas productivity of the reservoir. In this paper, the following aspects were mainly expounded: definition of the basic geometric parameters for layout of multi-lateral branches in clayey silt hydrate reservoir, simulation of the stimulation effect of a typical well profile with two branches, and prediction and simulation of the reservoir failure risk in a well profile with eight branches. The results show that the LB & MB effectively improves the flow field in the formation, raises the productivity of the reservoir and may also help to decrease the produced water-gas ratio (WGR). When the lateral branches spacing is too small, the failure zones around adjacent lateral branches overlap each other, possibly causing reservoir failure in a larger range. Therefore, the geometric parameters of multi-lateral branches depend on the dual control of the productivity and geotechnical risk factor of reservoir. Further study is being carried out, so as to obtain the optimal combination of parameters of multi-lateral branches.     

Preliminary results of environmental monitoring of the natural gas hydrate production test in the South China Sea

Natural gas hydrate (NGH) is considered as one of the new clean energy sources of the 21st century with the highest potential. The environmental issues of NGH production have attracted the close attention of scientists in various countries. From May 10 to July 9, 2017, the first offshore NGH production test in the South China Sea (SCS) was conducted by the China Geological Survey. In addition, environmental security has also been effectively guaranteed via a comprehensive environmental monitoring system built during the NGH production test. The monitoring system considered sea-surface atmosphere methane and carbon dioxide concentrations, dissolved methane in the sea water column, and the seafloor physical oceanography and marine chemistry environment. The whole process was monitored via multiple means, in multiple layers, in all domains, and in real time. After the production test, an environmental investigation was promptly conducted to evaluate the environmental impact of the NGH production test. The monitoring results showed that the dissolved methane concentration in seawater and the near-seabed environment characteristics after the test were consistent with the background values, indicating that the NGH production test did not cause environmental problems such as methane leakage.     

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.     

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.     

波阻抗反演在南海北部神狐海域天然气水合物勘探中的应用

针对神狐海域的地质构造和天然气水合物的赋存特征,以重点测线三维地震数据为基础,分析讨论了基于宽带约束的模拟退火波阻抗反演方法、流程和关键技术问题,定量获得了含天然气水合物沉积物的波阻抗特征。结果表明:基于宽带约束的模拟退火波阻抗反演数据具有较高的有效垂向分辨率和较好的横向连续性;神狐海域高波阻抗异常反映了含天然气水合物沉积层,而不连续异常低波阻抗层是水合物层之下游离气的表现,这与钻探结果吻合。由此可见,基于宽带约束的模拟退火波阻抗反演可为天然气水合物层识别和预测、勘探目标圈定、钻探井位选择提供重要依据。     

Distribution of gas hydrate reservoir in the first production test region of the Shenhu area,South China Sea

In May and July of 2017, China Geological Survey (CGS), and Guangzhou Marine Geological Survey (GMGS) carried out a production test of gas hydrate in the Shenhu area of the South China Sea and acquired a breakthrough of two months continuous gas production and nearly 3.1 × 10⁵ m³ of production. The gas hydrate reservoir in the Shenhu area of China, is mainly composed of fine-grained clay silt with low permeability, and very difficult for exploitation, which is very different from those discovered in the USA, and Canada (both are conglomerate), Japan (generally coarse sand) and India (fracture-filled gas hydrate). Based on 3D seismic data preserved-amplitude processing and fine imaging, combined with logging-while-drilling (LWD) and core analysis data, this paper discusses the identification and reservoir characterization of gas hydrate orebodies in the Shenhu production test area. We also describe the distribution characteristics of the gas hydrate deposits and provided reliable data support for the optimization of the production well location. Through BSR feature recognition, seismic attribute analysis, model based seismic inversion and gas hydrate reservoir characterization, this paper describes two relatively independent gas hydrate orebodies in the Shenhu area, which are distributed in the north-south strip and tend to be thicker in the middle and thinner at the edge. The effective thickness of one orebody is bigger but the distribution area is relatively small. The model calculation results show that the distribution area of the gas hydrate orebody controlled by W18/W19 is about 11.24 km², with an average thickness of 19 m and a maximum thickness of 39 m, and the distribution area of the gas hydrate orebody controlled by W11/W17 is about 6.42 km², with an average thickness of 26 m and a maximum thickness of 90 m.     

Velocity-porosity relationships in hydrate-bearing sediments measured from pressure cores,Shenhu Area,South China Sea

Evaluating velocity-porosity relationships of hydrate-bearing marine sediments is essential for characterizing natural gas hydrates below seafloor as either a potential energy resource or geohazards risks. Four sites had cored using pressure and non-pressure methods during the gas hydrates drilling project (GMGS4) expedition at Shenhu Area, north slope of the South China Sea. Sediments were cored above, below, and through the gas-hydrate-bearing zone guided with logging-while-drilling analysis results. Gamma density and P-wave velocity were measured in each pressure core before subsampling. Methane hydrates volumes in total 62 samples were calculated from the moles of excess methane collected during depressurization experiments. The concentration of methane hydrates ranged from 0.3% to 32.3%. The concentrations of pore fluid (25.44% to 68.82%) and sediments (23.63% to 54.28%) were calculated from the gamma density. The regression models of P-wave velocity were derived and compared with a global empirical equation derived from shallow, unconsolidated sediments data. The results were close to the global trend when the fluid concentration is larger than the critical porosity. It is concluded that the dominant factor of P-wave velocity in hydrate-bearing marine sediments is the presence of the hydrate. Methane hydrates can reduce the fluid concentration by discharging the pore fluid and occupying the original pore space of sediments after its formation.©2022 China Geology Editorial Office.     

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。     

南海神狐海域MIS12期以来的碳酸盐旋回与水合物分解

对南海神狐海域水合物钻探区SH1B和SH5C等2个钻孔顶部0~39.41 m和0~23.85 m的连续沉积物开展高分辨率碳酸盐旋回研究,结合浮游有孔虫氧同位素地层学、AMS¹⁴C年代学和生物地层学进行地层划分。结果表明：SH1B孔MIS12期以来CaCO₃曲线总体上具有明显的冰期和间冰期旋回,表现为较典型的冰期低、间冰期高的“大西洋型”旋回特征,具有良好的地层意义;SH5C孔除MIS1-MIS2期外,MIS3-MIS8期的CaCO₃含量较SH1B孔的要低,冰期和间冰期旋回性较差,MIS3-MIS 4和MIS7期出现明显的低钙事件;同时,SH5C孔MIS3-MIS5期浮游有孔虫G.ruber的δ¹³C值明显负偏,平均值为-0.11‰;两孔MIS1期的沉积速率是MIS2期的近2倍,与前人的研究结果相反,为MIS2期部分地层缺失所致。SH1B孔受水合物分解影响不大,而SH5C孔受水合物分解影响明显。SH5C孔MIS2期的地层缺失事件和MIS3-MIS4、MIS7期的低钙事件以及MIS3-MIS5期δ¹³C值负偏事件主要发生在末次冰期,推测与下伏水合物分解产生的海底滑塌、海水酸化等环境效应有关。     

Stability analysis of submarine slopes in the area of the test production of gas hydrate in the South China Sea

In this paper, the mechanical properties of gas hydrate-bearing sediments (GHBS) were summarized and the instability mechanism of submarine hydrate-bearing slope (SHBS) was analyzed under the background of the test production of gas hydrate in the northern part of the South China Sea. The strength reduction finite element method (SRFEM) was introduced to the stability analysis of submarine slopes for the safety of the test production. Two schemes were designed to determine the physical and mechanical parameters of four target wells. Through the division of the hydrate dissociation region and the design of four working conditions, the range and degree of hydrate dissociation at different stages during the test production were simulated. Based on the software ABAQUS, 37 FEM models of SHBS were set up to analyze and assess the stability of the submarine slopes in the area of the test production. Necessary information such as safety factors, deformation, and displacement were obtained at different stages and under different working conditions. According to the calculation results, the submarine slope area is stable before the test production, and the safety factors almost remains the same during and after the test production. All these indicate that the test production has no obvious influence on the area of the test production and the submarine slopes in the area are stable during and after the test production.     

南海神狐海域天然气水合物声波测井速度与饱和度关系分析

利用南海北部神狐海域A站位的地震和测井资料综合分析神狐海域含天然气水合物沉积层的声波测井速度及水合物饱和度的分布特征和变化规律,并对水合物饱和度的理论计算值和实测值进行对比分析,同时对水合物稳定带的纵波速度特征与饱和度的关系进行了综合研究。结果表明：神狐海域A站位的水合物层厚度约20 m,纵波速度在1 873～2 226m/s之间,水合物饱和度在15.0%～47.3%之间变化,水合物饱和度值相对较高;受海底复杂地质因素的影响,根据岩心孔隙水的氯离子淡化程度实测的水合物饱和度随声波速度的变化并不是单一的正比例关系,而是随声波速度的升高而上下波动,波动幅度在10%～20%之间,总体趋势上随声波速度的升高而升高,并集中分布在理论曲线附近;利用热弹性理论速度模型计算并校正后的水合物饱和度随声波速度的增加而有规律地增加,水合物饱和度的理论计算值与实测数据比较吻合,说明所建立的岩石物理模型正确,模型参数选取合理。根据声波速度计算水合物饱和度这一方法可扩展到整个研究区域,并为研究区的水合物资源量评价提供基础数据。     

Identification of functionally active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area,South China Sea

Large amounts of gas hydrate are distributed in the northern slope of the South China Sea, which is a potential threat of methane leakage. Aerobic methane oxidation by methanotrophs, significant methane biotransformation that occurs in sediment surface and water column, can effectively reduce atmospheric emission of hydrate-decomposed methane. To identify active aerobic methanotrophs and their methane oxidation potential in sediments from the Shenhu Area in the South China Sea, multi-day enrichm...     

南海北部陆坡区神狐海域构造特征及对水合物的控制

通过对南海北部陆坡区神狐海域高精度2D和3D地震资料的精细解释,在研究区共识别出4种构造类型,分别为气烟囱(流体底辟)、区域大尺度断层、深水扇中的正断层和滑移体中的滑脱断层。气烟囱具有直立的通道形态,其内部结构可划分为杂乱反射带、模糊反射带和顶部强振幅区域。大尺度断层位于水合物钻探区的西北部和东北部,断层规模大,对深部地层表现出明显的控制作用。深水扇中的正断层广泛发育于上新世的深水扇中,特别是在水合物钻探区西部进积特征明显的深水扇中,正断层的数量更多。滑移体中的滑脱断层在神狐海域的第四纪地层中非常常见,在剖面上呈雁列式分布。研究结果表明,大尺度断层由于和水合物钻探区的距离较远,对于水合物的成藏可能不起控制作用。气烟囱和规模小数量多的断裂体系为含气流体的运移提供了垂向和侧向的输送通道,构成了水合物的流体运移体系。当富含甲烷气体的流体通过这些垂向-侧向的运移通道时,在合适的温压条件下,被适于水合物聚集的沉积体所捕获,就有可能形成水合物。水合物钻探区内东西部构造特征的差异,使得研究区内形成了不同的流体运移体系,这可能是控制钻探区水合物不均匀性分布的一个关键因素。