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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.
Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea. However, there remains a lot of key questions yet to be resolved, particularly about the mechanisms and the controls of gas hydrates enrichment. Numerical simulaution would play signficant role in addressing these questions. This study focused on the gas hydrate exploration in the Shenhu Area, Northern South China Sea. Based on the newly obtained borehole and multichannel reflection seismic data, the authors conducted an integrated 3D basin modeling study on gas hydrate. The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation, such as temperature, pressure, hydrocarbon source, and tectonic setting. Gas hydrates are most concentrated in the Late Miocene strata, particularly in the structual highs between the Baiyun Sag and the Liwan Sag, and area to the south of it . It also proved the existence of overpressure in the main sag of source rocks, which was subject to compaction disequilibrium and hydrocarbon generation. It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage. The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.
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.
Drilling results suggest that the thickness of natural gas hydrates (NGHs) in the Shenhu Area, South China Sea (SCS) are spatially heterogenous, making it difficult to accurately assess the NGHs resources in this area. In the case that free gas exists beneath hydrate deposits, the frequency of the hydrate deposits will be noticeably attenuated, with the attenuation degree mainly affected by pore development and free gas content. Therefore, the frequency can be used as an important attribute to identify hydrate reservoirs. Based on the time-frequency characteristics of deposits, this study predicted the spatial distribution of hydrates in this area using the frequency division inversion method as follows. Firstly, the support vector machine (SVM) method was employed to study the amplitude versus frequency (AVF) response based on seismic and well logging data. Afterward, the AVF response was introduced as independent information to establish the nonlinear relationship between logging data and seismic waveform. Then, the full frequency band information of the seismic data was fully utilized to obtain the results of frequency division inversion. The inversion results can effectively broaden the frequency band, reflect the NGHs distribution, and reveal the NGHs reservoirs of two types, namely the fluid migration pathway type and the in situ self-generation self-storage diffusion type. Moreover, the inversion results well coincide with the drilling results. Therefore, it is feasible to use the frequency division inversion to predict the spatial distribution of heterogeneous NGHs reservoirs, which facilitates the optimization of favorable drilling targets and is crucial to the resource potential assessment of NGHs.
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.
Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate. In this study, low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples. Results show that the signal intensity of T2 distribution isn’t affected by sediment type and pore pressure, but is affected by temperature. The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T2 distribution. The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media. The water conversion rate fluctuates during the hydrate formation. The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media. For the fine sand sample, the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate, but causes a large water-sand contact area resulting in a low water conversion ratio (Cw=96.2%). The clay can reduce the water conversion rate and ratio, especially montmorillonite (Cw=95.8%). The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.
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 enrichment incubations were conducted in this study. The results show that the methane oxidation rates in the studied sediments were 2.03‒2.36 μmol/gdw/d, which were higher than those obtained by sediment incubations from other areas in marine ecosystems. Thus the authors suspect that the methane oxidation potential of methanotrophs was relatively higher in sediments from the Shenhu Area. After the incubations family Methylococcaea (type I methanotrophs) mainly consisted of genus Methylobacter and Methylococcaea_Other were predominant with an increased proportion of 70.3%, whereas Methylocaldum decreased simultaneously in the incubated sediments. Collectively, this study may help to gain a better understanding of the methane biotransformation in the Shenhu Area.
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.
Natural gas hydrates (NGHs) are a new type of clean energy with great development potential. However, it is urgent to achieve safe and economical NGHs development and utilization. This study established a physical model of the study area using the FLAC3D software based on the key parameters of the NGHs production test area in the South China Sea, including the depressurization method, and mechanical parameters of strata, NGHs occurrence characteristics, and the technological characteristics of horizontal wells. Moreover, this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well. The results are as follows. With the dissociation of NGHs, the overburden strata of the NGHs dissociation zone subsided and formed funnel-shaped zones and then gradually stabilized. However, the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress. Specifically, distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well, which was thus prone to suffering shear failure. Moreover, apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure. Therefore, it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.
Blockage in water-dominated flow pipelines due to hydrate reformation has been suggested as a potential safety issue during the hydrate production. In this work, flow velocity-dependent hydrate formation features are investigated in a fluid circulation system with a total length of 39 m. A 9-m section pipe is transparent consisted of two complete rectangular loops. By means of pressurization with gas-saturated water, the system can gradually reach the equilibrium conditions. The result shows that the hydrates are delayed to appear as floccules or thin films covering the methane bubbles. When the circulation velocity is below 750 rpm, hydrate is finally deposited as a “hydrate bed” at upmost of inner wall, narrowing the flow channel of the pipeline. Nevertheless, no plugging is observed during all the experimental runs. The five stages of hydrate deposition are proposed based on the experimental results. It is also revealed that a higher driving pressure is needed at a lower flow rate. The driving force of hydrate formation from gas and water obtained by melting hydrate is higher than that from fresh water with no previous hydrate history. The authors hope that this work will be beneficial for the flow assurance of the following oceanic field hydrate recovery trials.
The distributed acoustic sensor (DAS) uses a single optical cable as the sensing unit, which can capture the acoustic and vibration signals along the optical cable in real-time. So it is suitable for monitoring downhole production activities in the process of oil and gas development. The authors applied the DAS system in a gas production well in the South China Sea for in situ monitoring of the whole wellbore for the first time and obtained the distributed acoustic signals along the whole wellbore. These signals can clearly distinguish the vertical section, curve section, and horizontal production section. The collected acoustic signal with the frequency of approximately 50 Hz caused by the electric submersible pump exhibit a signal-to-noise ratio higher than 27 dB. By analyzing the acoustic signals in the production section, it can be located the layers with high gas production rates. Once an accurate physical model is built in the future, the gas production profile will be obtained. In addition, the DAS system can track the trajectory of downhole tools in the wellbore to guide the operation. Through the velocity analysis of the typical signals, the type of fluids in the wellbore can be distinguished. The successful application of the system provides a promising whole wellbore acoustic monitoring tool for the production of marine gas hydrate, with a good application prospect.
How natural gas hydrates nucleate and grow is a crucial scientific question. The research on it will help solve practical problems encountered in hydrate accumulation, development, and utilization of hydrate related technology. Due to its limitations on both spatial and temporal dimensions, experiment cannot fully explain this issue on a micro-scale. With the development of computer technology, molecular simulation has been widely used in the study of hydrate formation because it can observe the nucleation and growth process of hydrates at the molecular level. This review will assess the recent progresses in molecular dynamics simulation of hydrate nucleation and growth, as well as the enlightening significance of these developments in hydrate applications. At the same time, combined with the problems encountered in recent hydrate trial mining and applications, some potential directions for molecular simulation in the research of hydrate nucleation and growth are proposed, and the future of molecular simulation research on hydrate nucleation and growth is prospected.
Grain-displacing hydrate deposits exist at many marine sites, which constitute an important part of methane hydrate resources worldwide. Attributed to the difficulties in acquiring field data and synthesizing experimental samples, the formation and property characterization of grain-displacing hydrate remains less understood and characterized than the pore-filling hydrate in current literature. This study reviews the formation mechanisms of grain-displacing hydrate from the perspective of geological accumulation and microscale sedimentary property. The experimental methods of synthesizing grain-displacing hydrate in the laboratory and the current knowledge on the property of grain-displacing hydrate sediment are also introduced. Shortcomings in current theories and suggestions for future study are proposed. The work is hoped to provide valuable insights for the research into the hydrate accumulation, geophysics, and hydrate exploitation targeted at the grain-displacing hydrate in the marine sediments.