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Natural gas hydrate is widely distributed, shallow-buried, clean, and pollution-free and has enormous reserves, it is regarded as the alternative clean energy source in the oil and gas field with the most potential. Pressure coring is the only way to drill for gas hydrate core on the surface under in situ conditions, which is of great value for analyzing its occurrence conditions and reserves comprehensively. Based on this, a new wireline pressure coring system (WPCS) with a ball valve seal was designed and developed in this paper; it was applied in the deep sea for the first time in the South China Sea hydrate survey voyage of the Guangzhou Marine Geological Survey (GMGS). A total of 15 runs of deep-sea gas hydrate drilling and coring applications were carried out, and they tested well. The experimental water depth was 1700–1800 m, and the coring depth below the seafloor was about 100–150 m. The formation consisted of sandy hydrate and argillaceous hydrate. The results showed the following. (1) The success rate of ball valve turn-over could reach almost 100% in the argillaceous hydrate reservoir, although there are some isolated cases of pressure relief. Meanwhile, drilling in the sandy hydrate reservoir, the success rate was only 54.55%. (2) When drilling in the argillaceous hydrate reservoir, the core recovery rate could reach 80%, while in the sandy hydrate reservoir, it was almost 0%. In practice, the sandy formation with gas hydrate is stiff to drill compared to the performance in argillaceous formations. After our analysis, it was believed that the ball valve and core tube could be easily plugged by sand debris during the sampling of sandy hydrate formation. Moreover, the sandy core is easily plugged into the core liner because of the high friction of sand grains in clearance. (3) The pressure-holding effect of the core drilling tool was related to the formation of hydrate, the sealing form of the ball valve, and the environmental pressure. Sandy hydrate formations often caused the ball valve to jam, while the muddy hydrate formation did not. The research results of this paper have reference value for the further optimization of the WPCS structure, the optimization of drilling parameters, and the design parameters of the ball valve structure, which could be better used for the pressure coring of gas hydrate and subsequent research work in the future. [ABSTRACT FROM AUTHOR]

In this paper, a one-dimensional model of gas–water two-phase productivity for hydrate depressurization is established, which takes into account permeability variation and gas–water two-phase flow. By solving the coupled algebraic equations of dissociation front position, equilibrium temperature, and pressure in an iterative scheme, the movement law of the hydrate dissociation front and the evolution process of temperature and pressure near the well were obtained, and the effects of bottom hole pressure, reservoir temperature, and hydrate saturation on productivity were analyzed. The results show that the hydrate reservoir is divided into a decomposed zone and an undecomposed zone by the dissociation front, and the temperature and pressure gradients of the former are greater than those of the latter. Reducing bottom hole pressure, increasing reservoir temperature, and increasing hydrate saturation all lead to an increase in temperature and pressure gradient in the decomposed zone. Methane gas production is a sensitive function of bottom hole pressure, reservoir temperature, and hydrate saturation. The lower the bottom hole pressure, the higher the reservoir temperature, the lower the hydrate saturation (within a certain range), and the higher the gas production rate. The trend of the water production curve is the same as that of gas, but the value is 3–4 orders of magnitude smaller, which may be due to the large difference in the viscosity of gas and water, and the gas seepage speed is much larger than that of water. [ABSTRACT FROM AUTHOR]

The China Geological Survey (CGS) has carried out a large number of surveys and core drilling over the deepwater area of Qiongdongnan Basin (QDNB) in the South China Sea and discovered the natural gas hydrate system controlled by the gas chimney with a high geothermal gradient. The complex hydrate system consists of a sandy hydrate reservoir distributed around a lateral transition gas-hydrate mixed zone and a free gas zone in the middle. The hydrate and gas are distributed in the same layer, which is thin but potentially valuable for commercial exploitation. In this paper, a geological model of the target hydrate system in QDNB was established based on the results of several rounds of drilling. The method of numerical simulation was utilized to assess the production capacity of the target hydrate system and clarify the evolution of hydrate and gas saturation distribution with different well positions. The simulation results indicate that the producer well built in the center of the highly-saturated hydrate zone has a limited gas production capacity, with a cumulative production of only 7.25 × 106 m3 in 9 years. The well built at the boundary of the hydrate zone can rapidly link up the gas in the transition zone through a large production pressure differential, but it lacks control over the hydrates and its dissociated gas in the transition zone—the cumulative gas production volume from hydrate accounts for only 12.3%. As for the wells built in the transition zone and gas zone, they can directly invoke the free gas production capacity. Free gas is produced as the formation pressure reduces and hydrate is induced to dissociate, making the gas from the hydrate the subsequent production capacity. The cumulative production can exceed 6 × 108 m3 in 9 years. The stable production duration can extend to 2645 days, and the cumulative proportion of gas at the wellhead from hydrate reaches close to 30%. It is necessary to avoid the free water layer. The bottom water coning would improve the water production by 40% and shorten the stable production duration. In summary, the complex hydrate system of this type in the QDNB has the potential for industrialized exploitation. In the future, the well group can be used for the further improvement of the hydrate utilization rate. [ABSTRACT FROM AUTHOR]

The five-spot well is a promising technique for enhancing the inherent low gas production rate from silty natural gas hydrate (NGH) reservoirs with low permeability in the South China Sea (SCS). However, the production characteristics and the responses to key geological and engineering factors are still unclear and warrants investigation. Herein, we first establish a three-dimensional reservoir model based on the SH2 NGH deposits at SCS with the implementation of a five-spot well. We systematically examine the gas production enhancement effect and the unexpected well-to-well interaction induced by long-term depressurization. A sensitivity analysis on the effects of bottomhole pressure (P btm), permeability (k) and hydrate saturation (S H) is conducted to elucidate the key factors controlling the gas production enhancement. Our results reveal that the five-spot well significantly improves the gas productivity with cumulative gas production increased to 4.12 times that of a single well over 10 years. The well interference on gas production is not significant in the early stage but becomes more significant as depressurization sustains. Due to the relatively slow pressure propagation at the inner well, contribution to gas production from the inner well decreases over time while the outer wells increase. Based on the sensitivity analysis, gas production increases with decreasing P btm , increasing k , and decreasing S H with P btm and k as the key controlling factors. The findings provide valuable design basis for the adoption of a multi-well system in enhancing gas production for targeted NGH reservoirs in the next field production trial at SCS. [Display omitted] • A five-spot well is designed for enhancing gas production from silty natural gas hydrate deposits • A thermo-hydraulic-chemical model is set up to examine the gas production enhancement effect • A 33 sensitivity study of bottomhole pressure, permeability and hydrate saturation is conducted • Gas production increased to 4.12 times that of a single well but with well-to-well interference [ABSTRACT FROM AUTHOR]

Clayey-silty hydrate-bearing sediments are widely distributed worldwide, presenting significant potential for natural gas hydrate (NGH) extraction. However, fundamental thermodynamic and kinetic properties of CH 4 hydrate (MH) within these sediments remain subjects of debate, necessitating further investigation. This study aimed to investigate the kinetics of MH formation and dissociation in clayey-silty sediments suspensions, alongside morphological examinations. Results demonstrated that SH-W02-B sediments (SS) effectively shorted the induction time (t ind), with even a minute mass fraction of 0.1 wt% SS reducing t ind by 14 min compared to pure water (41 min). Lower mass fractions (<5.0 wt%) exhibited slight promotion of MH growth kinetics, whereas higher mass fractions (>10.0 wt%) markedly promoted MH growth kinetics, peaking at 144 v/v (20.0 wt%), correspondingly, and facilitating the conversion of H 2 O to MH to ∼70%. Morphological observation unveiled the formation of a thin clay layer at the gas-liquid interface, retarding MH formation and inducing the development of an anti-seepage hydrate layer, leading to the initial slow growth phase. Subsequent upward water migration facilitated the transport of fine clay particles, while the progressive encasement of clay particles by hydrate disrupted the fine clay layer, triggering the rapid hydrate formation phase and ensuing stratification between fine and coarse sediments particles. During hydrate dissociation, a heterogeneous distribution of fine and coarse sediments particles manifested in the reservoir. Moreover, the results carrying profound implications on understanding NGH occurrence pattern and developing recovery strategies from clayey-silty hydrate-bearing sediments. [Display omitted] • Clayey-silty core sediments were recovered near the China second field production test well. • X-ray diffraction analysis reveals 31 wt% clay with primary illite/smectite mixed layer in core sediments. • The presence of core sediments expedite CH 4 hydrate nucleation and enhance the growth kinetics. • Transport of fine clay particles leads to two-stage growth with stratified hydrate-sediments layers. [ABSTRACT FROM AUTHOR]

Large-scale natural gas hydrate accumulations (generally over one billion cubic meters of natural gas) in coarse-grained marine sediments are the most valuable types for commercial exploration and exploitation. The formation of such a gas hydrate deposit generally requires a sufficient natural gas supply such as deeply buried thermogenic gas source. However, the worldwide discovered thermogenic gas hydrate deposits are rare, leaving relatively a poor understanding of their formation and evolution. Based on the geochemical and microbial analyses of eleven hydrate-related gas and five sediment samples from the thermogenic gas hydrate deposits in the Qiongdongnan Basin in northern South China Sea, we systematically investigated the origin and evolution of thermogenic hydrocarbons within sandy gas hydrate reservoirs. Results show that natural gas within high saturated gas hydrate-filled sandy sediments is predominately of thermogenic (the proportion is 57.1–70.5%), characterized by "dry gas" (99.38–99.7% C 1 , 0.26–0.47% C 2 and <0.5% C 3+), relatively lower C 1 /(C 2 +C 3) ratio (187–376), and heavier δ13C–C 1 values (−55.0 to −31.3‰). In addition, typical biomarkers (including the alkanes, tricyclic terpanes, hopanes and steranes) generated from mature source rocks were detected in the hydrate-filled fine sand reservoir, as well as the biomarker indicators (including the unresolved complex mixtures (UCMs) hump and the 25-norhopane homologous series) for severe hydrocarbon biodegradation. Furthermore, large amounts of hydrocarbon degrading bacteria (50.5% of bacteria) and methanogenic archaea (34.3% of archaea) were detected within hydrate-filled sandy sediment. Our results indicated that the deeply buried thermogenic source can indeed provide sufficient gas supply for the formation of a large-scale gas hydrate deposit, but thermogenic liquid hydrocarbons within gas hydrate reservoirs are very susceptible to be degraded and consumed by hydrocarbon degrading bacteria. In addition, we also found that the clay components of argillaceous gas hydrate reservoirs can adsorb and store large amounts of in situ immature organic matter, which can significantly obscure or cover up the original geochemical signatures of thermogenic hydrocarbons therein. These findings suggest that the relative contribution of deeply buried thermogenic source for gas hydrate accumulation worldwide may be underestimated, because we have previously ignored the fact that thermogenic hydrocarbons within gas hydrate reservoirs would be significantly consumed by hydrocarbon degrading microbes or diluted by in situ immature sedimentary organic matter. Our findings are highly significant for the evaluation and exploration of thermogenic gas hydrate accumulations worldwide. • Typical thermogenic gas hydrates within sandy reservoir in South China Sea were reported. • Biodegraded thermogenic hydrocarbons were extracted from sandy gas hydrate reservoir. • Abundant hydrocarbon degrading bacteria were detected in sandy gas hydrate reservoir. • Clay components of hydrate reservoir significantly obscure the contribution of thermogenic gas source. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reveals the inhibition effect of representative clays from South China Sea on CH 4 hydrate experimentally. • Thermodynamic inhibition is more significant for Na-montmorillonite than kaolinite and illite. • Develops a thermodynamic model accounting for the reduced water activity on clay surface. • Implications provided on the stability and fluid production from South China Sea clayey hydrate-bearing sediments. Clay minerals are widely distributed in hydrate-bearing sediments worldwide, especially in the clayey-silty sediments in the South China Sea. The debate on the influence of clay on the thermodynamics of natural gas hydrate (NGH) remains and impedes the fundamental understanding of hydrate-clay interactions and the design of effective production strategies for energy recovery. Specifically, the phase equilibria of MH in the presence of clay and how different types of water associated with clay particles affect NGH thermodynamics and stability are not fully elucidated and warrant investigation. In this study, the phase equilibria of methane hydrate (MH) were measured in the presence of typical clays from the South China Sea, i.e., montmorillonite, kaolinite, and illite. The shift of MH phase equilibria curve to a more stringent pressure was identified with Na-montmorillonite showing the most significant inhibition. The leftward shift (inhibition) of phase equilibrium temperature (T eq) is up to 3.9 K at a pressure of 10.40 MPa for Na-montmorillonite compared with pure water. Furthermore, we develop a thermodynamic model based on the classical Chen-Guo model first accounting for the change in water activity (a w) due to cation exchange of Na-montmorillonite. The developed model exhibits good predictability with the measured phase equilibira data. The inhibition on the phase equilibria of MH is linked to the reduction of a w due to the clay-bound water for clays with a swelling nature. The findings of this study offer direct evidence and explanations on how clay minerals thermodynamically inhibit CH 4 hydrate. It provides valuable insights into the resource estimation of clayey-silty NGH in nature and facilities the design of suitable production strategies targeting clay-rich hydrate-bearing sediments. [ABSTRACT FROM AUTHOR]

Properties of host sediments are critical to understanding the formation and occurrence of gas hydrates in nature. This study investigates the mineralogy and pore characteristics of marine gas hydrate-bearing sediments (GHBS) within and below two high-saturation zones cored in drilling site W01 in the Qiongdongnan Basin in the South China Sea. The study involves an original application of high-resolution SEM imaging and automated mineralogy to fields-of-view (FOVs) large enough to undertake quantitative analyses at micro- to nano-scales. The samples are mainly fine-grained (<63 μm) and mineralogically dominated by illite, montmorillonite and quartz, significantly higher in clay mineral content than GHBS cored elsewhere in the South China Sea. Porosities determined from thin sections are from 27.6% to 33.0% and the pores are dominated by diameters of 0.1–1 μm. A sample from the upper high-saturation GHBS is rich in clay minerals and clay-sized grains with pores mainly 0.1–1 μm in size, while a sample from the lower zone of high-saturation GHBS has less clay minerals, more sands, and pore diameters of >1 μm are as important as those 0.1–1 μm in size. A third sample from an underlying zone has similar mineral compositions, grain-size and pore-size distributions to those in the uppermost high-saturation hydrate bearing zone. The largest pores are due to microfossil cavities, but their contribution to total porosity is <5%, suggesting their role in fine-grained sediments should not be overestimated. It is inferred that grain size and pore size, affected by variations in mineral content, are important factors influencing differences in hydrate accumulation. The geological source of gas and its migration are also factors determining the distribution of gas hydrate in the study area. Our approach using large FOVs to obtain quantitative micro- and nano-scale data from SEM images facilitates correlation to centimeter- and meter-scale information from well logs and seismic data, and offers opportunities to obtain new insights into the in-situ occurrence and accumulation of gas hydrate and its effects on physical properties of gas hydrate reservoirs. • First application of SEM-based methods to fields of view large enough to allow quantitative analyses of mineralogy and pore characteristics of gas hydrate-bearing sediments (GHBS). • GHBS of the Qiongdongnan Basin are clay-rich and dominated by illite, montmorillonite and quartz, distinct from coarser-grained GHBS elsewhere in the South China Sea. • High-saturation GHBS at different depths show difference in contents of clay minerals, sand grains and pores >1 μm. • Microfossils form the largest cavities but account for <5% of pore area, suggesting their role in improving porosity in fine-grained GHBS should not be overestimated. [ABSTRACT FROM AUTHOR]