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.

The exploitation of natural gas hydrates by the pressure reduction method is affected by the decomposition heat absorption effect, and the range of the formation temperature reduction area is expanding. At the same time, the temperature reduction phenomenon is more significant around the production wells under the influence of gas throttling and expansion effects, and hydrate formation will occur under certain temperature and pressure conditions, leading to blockage of effective seepage channels in the reservoir in the region and elevation of seepage resistance, which may affect the output of hydrate decomposition gas. A numerical simulation model is constructed for the purpose of studying the secondary hydrate generation pattern around the well, analyzing the impact of secondary hydrates around wells on the production capacity, and assessing the effectiveness of prevention methods to inform the actual production of hydrates. The results demonstrate that secondary hydrate is typically formed in the near-well area of the upper part of the production well, and the secondary hydrate around the upper part of the production well is the first to be formed, exhibiting the highest saturation peak and the latest decomposition. The formation of the secondary hydrate can be predicted based on the observed change in temperature and pressure, and the rate of secondary hydrate formation is markedly rapid, whereas the decomposition rate, approximately 0.285 mole/d, is relatively slow. Additionally, the impact of secondary hydrates on cumulative gas production is insignificant, and the effect of secondary hydrates on capacity can be ignored. Hot water injection, wellbore heating, and reservoir reconstruction can effectively eliminate secondary hydrates around the well. Reservoir reconstruction represents a superior approach to the elimination of secondary hydrates, which can effectively enhance production capacity while preventing the generation of secondary hydrates.

In recent years, a new type of natural gas hydrate reservoir (designated as Class 1S reservoir) has been discovered in the Qiongdongnan Basin. Within this hydrate reservoir, free gas and hydrate coexist within the same stratum. The Class 1S reservoir is comprised of three distinct zones: the gas accumulation zone, the three-phase zone, and the hydrate-bearing zone. It exhibits significant commercial development potential. This paper analyzes the formation mechanism and geological context of Class 1S hydrates. A geological model was established and numerical simulation methods were employed to evaluate its production capacity, elucidating the evolutionary patterns of hydrate saturation distribution at different well locations. The simulation results indicate that production wells should be prioritised in gas accumulation zones in order to achieve the highest cumulative gas production. Additional production wells may be considered in later stages to enhance recovery rates. Secondary hydrate formation significantly impacts production in Hydrate-bearing zone and three-phase zone. Measures such as wellbore heating can be employed to minimize secondary hydrate formation around the wellbore.

As ice-like crystals and non-stoichiometric compounds comprising gas and water, natural gas hydrates have drawn significant attention as a potential alternative energy source. This work focuses on holistically reviewing theoretical modeling and numerical studies conducted on the production of gas from natural gas hydrates. Firstly, fundamental models for the dissociation of a hydrate in a porous sediment are summarized in terms of the phase equilibrium and dissociation kinetics. The main features of different models and improvements for them are identified by clarifying crucial driving mechanisms and kinetic parameters. Subsequently, various numerical works addressing the dissociation of a hydrate in a porous sediment and the flow characteristics in a wellbore are reviewed, including aspects such as the theoretical background, computational scheme, and the physics involved. In general, profiting from a significant capacity to solve nonlinear differential equations, numerical simulations have contributed to great progress in fundamentally understanding the mechanism driving gas production and in developing effective exploitation methods. Owing to the substantial fundamental physics involved in the exploitation of natural gas hydrates, existing challenges, alternative strategies, and future directions are provided correspondingly from a practical application perspective.

Cold seeps are natural “outcrop” evidence of marine gas hydrates and represent one of the most direct indicators of gas hydrate occurrence. This study used high-resolution 3D seismic data covering approximately 400 km² in the deep-water southern Qiongdongnan Basin, northern South China Sea. Sub-bottom profiles and multibeam bathymetric data were also incorporated. Several seismic features indicative of fluid migration and accumulation were recognized, including gas chimneys, bottom-simulating reflectors (BSRs), seafloor pockmarks, seismic pull-up events, and bright spots. The results confirm that gas chimneys act as the main pathways for the upward migration of deep high-temperature, high-pressure fluids. The analysis shows that the sustained activity of gas chimneys not only supplies hydrocarbon gases for hydrate accumulation but also, through associated thermal fluids, can cause the dissociation of pre-existing hydrates, resulting in high-flux methane seepage and consequently nurturing cold seep systems. Through in situ observations and sampling by the remotely operated vehicle (ROV) Haima, a “Haima Cold Seep Area” associated with gas hydrates was discovered in the Qiongdongnan Basin. The study indicates that the identification of fluid seepage conduits on 3D seismic profiles is the most critical seismic characteristic for cold seep recognition. As direct outcrop evidence of hydrate occurrence, cold seeps can effectively compensate for the ambiguity of indirect indicators such as BSR, thus providing a reliable basis for fine-scale assessment of marine gas hydrate resources and investigation of accumulation mechanisms.