Search Results (34)

To accommodate the extreme thermodynamic effects and erosion damage in throttling equipment for ultra-high-pressure natural gas wells (175 MPa), a coupled multiphase flow erosion numerical model for nozzles was established. This model incorporates a real gas compressibility factor correction and is based on the renormalized k-ε RNG (Renormalization Group k-epsilon model, a turbulence model that simulates the effects of vortices and rotation in the mean flow by modifying turbulent viscosity) turbulence model and the Discrete Phase Model (DPM, a multiphase flow model based on the Eulerian–Lagrangian framework). The study revealed that the nozzle flow characteristics follow an equal-percentage nonlinear regulation pattern. Choked flow occurs at the throttling orifice throat due to supersonic velocity (Ma ≈ 3.5), resulting in a mass flow rate governed solely by the upstream total pressure. The Joule–Thomson effect induces a drastic temperature drop of 273 K. The outlet temperature drops below the critical temperature for methane hydrate phase transition, thereby presenting a substantial risk of hydrate formation and ice blockage in the downstream outlet segment. Erosion analysis indicates that particles accumulate in the 180° backside region of the cage sleeve under the influence of secondary flow. At a 30% opening, micro-jet impact causes the maximum erosion rate to surge to 3.47 kg/(m²·s), while a minimum erosion rate is observed at a 50% opening. Across all opening levels, the maximum erosion rate consistently concentrates on the oblique section of the plunger front. Results demonstrate that removing the front chamfer of the plunger effectively improves the internal erosion profile. These findings provide a theoretical basis for the reliability design and risk prevention of surface equipment in deep ultra-high-pressure gas wells. Full article

The methane flux in the Dongsha area in the northern South China Sea is relatively high. The results indicate the presence of both shallow and deep gas hydrate reservoirs at the Site DS-W08. The gas hydrate reservoir in this area is mainly composed of fine-grained sediments, and high-saturation gas hydrates are present. The shallow-GHR (8–24 mbsf) exhibits a maximum hydrate saturation of 14% (pore volume). The deep-GHR (below 65 mbsf) shows a maximum hydrate saturation of 33% The suspended sedimentation process on the banks of turbidity currents and the deep-water traction current sedimentation process play potentially important roles in the enrichment of gas hydrates. To investigate the influence of sedimentary processes on gas hydrate accumulation, this study analyzed gas hydrate saturation, sediment grain size, grain compositions, biological components, and geochemical characteristics of hydrate-bearing and adjacent layers at Site DS-W08. Sediment grain size analysis suggests that the studied layer was formed through the interaction of turbidity current-induced overbank suspended deposition and traction current deposition. By comprehensively analyzing the comparison of sediment Sr/Ba ratios and the data of foraminifera and calcareous nannofossils, it is found that the bank deposits and traction current deposits triggered by turbidity currents correspond to glacial periods and interglacial periods, respectively. Analysis of biological components shows that layers with high foraminifera content and traction current-modified sediments are more favorable for gas hydrate accumulation. Hydrate reservoirs are all composed of traction current deposits, and the cap rock rich in foraminifera fossils at the top promotes hydrate formation; while the fine-grained turbidites formed during the turbidite deposition process inhibit hydrate accumulation. This study aims to deepen the understanding of the enrichment mechanism of natural gas hydrates and support the commercial development of fine-grained sediments in the northern South China Sea. Full article

This study investigated methane hydrate formation and dissociation within a temperature range of 280 to 290 K and a pressure range of 5.5 to 13 MPa. These conditions are relevant to natural gas systems, where methane is the primary component of natural gas. Either experimental or thermodynamic models were used to predict the conditions of formation of gas hydrates. The Van der Waals–Platteeuw model based on statistical thermodynamics is the basis of the existing thermodynamic models for predicting the conditions of hydrate formation. In this work, the stepwise heating method was applied to determine the thermodynamic equilibrium points of methane gas in a constant volume system. The CPA (Cubic Plus Association) equation of state and the Van der Waals–Platteeuw model were employed to simulate hydrate formation conditions. Experimental equilibrium data for pure methane were compared with results from previous studies (Deaton and Frost, Nakamura, Jhaveri and Robinson, De Roo, and others). The results showed excellent agreement, with an average absolute temperature error of less than 0.1%. This high level of accuracy confirms the reliability of the experimental procedures and thermodynamic modeling approaches used in the study to accurately predict hydrate formation conditions, being critical for designing and operating natural gas systems in order to avoid hydrate accumulation. Full article

The Shenhu area, located on the northern continental slope of the South China Sea, is a confirmed gas hydrate-enriching region, but the sedimentary unit, causative mechanisms, and evolution processes of the strata that contain hydrate remain unclear. Furthermore, the recognition of bottom simulating reflectors (BSRs) rests on qualitative description; there is no quantitative method for the characterization or three-dimensional depiction of BSRs. This review examines the sedimentary features and key factors controlling gas hydrate distribution in the region, based on high-resolution sequence stratigraphy combined with drilling and logging data from hydrate drilling projects in the Shenhu area. The main findings of this study include (1) BSRs are mainly distributed in the ridges of the continental slope and in the slip blocks of the side slope, with hydrates developing along a thin layer (10–40 m) below the hydrate stability zone, as confirmed by drilling results; (2) The distribution of BSRs is strongly influenced by the presence of gas chimneys, the migration of deepwater channels, and the erosion and sedimentation processes of canyons, all of which are directly or indirectly related to the accumulation, distribution, and formation of hydrate reservoirs; (3) The sand factor is generally less than 10%, and BSRs accumulate in areas where the sand factor is higher (4–10%). Hydrate saturation shows a positive correlation with porosity. This research also identifies the early Pleistocene erosion/resedimentation event as a key factor that controls the non-homogeneous distribution of hydrates in the region. This research highlights the role of deepwater canyon erosion and slumping processes in controlling gas hydrate formation, offering new insights into the impact of dynamic geological processes on hydrate accumulation. This study provides valuable knowledge for future hydrate exploration and global resource assessments. Full article

This study investigates the natural gas hydrates within the Qiongdongnan Basin by integrating well-log and seismic data. Through pre-stack inversion and rock physics analysis, key parameters such as P-wave and S-wave impedances were utilized to distinguish hydrate-bearing formations from other geological bodies. A low-frequency model was constructed using the Inverse Distance Weighting (IDW) algorithm to improve the precision of parameter inversion. This study employs a multi-constraint inversion strategy, incorporating hard constraints from multiple wells and soft constraints from geological frameworks, ensuring reliable inversion results. Findings indicate that hydrate reservoirs are characterized by increased wave velocity and density due to hydrate accumulation, providing insights into the spatial distribution and characteristics of hydrates. This research enhances the understanding of hydrate reservoirs and offers valuable data for exploration in the Qiongdongnan Basin. Full article

Natural gas hydrates are a clean and efficient new energy source with the potential to replace conventional energy, holding significant production value. Studying hydrate accumulation systems is fundamental for hydrate resource evaluation and target reservoir selection. The Shenhu area of the South China Sea has abundant hydrate resources, and drilling data show that the hydrate distribution in this area exhibits noticeable heterogeneity. Aiming at this phenomenon, we used a novel thermo-flow/chemical coupled simulator to simulate the hydrate accumulation system based on the actual geological strata and provide a detailed depiction of the evolution of components in the process. The results indicate that favorable migration conditions can accelerate hydrate formation. However, to achieve the rapid formation of thick, high-saturation hydrate layers, the fluid migration conditions must be complemented by effective fluid aggregation conditions. Furthermore, a sensitivity analysis of the fault morphology was conducted, revealing that larger-scale faults are more conducive to rapid hydrate accumulation. In summary, this study provides a quantitative analysis of the hydrate accumulation process and its key influence factors using a novel simulator, offering theoretical support for resource evaluation and an exploration of hydrate distribution. Full article

One of the most studied permafrost-associated gas hydrate accumulations in Arctic Alaska is the Eileen Gas Hydrate Trend. This study provides a detailed re-examination of the Eileen Gas Hydrate Trend with a focus on the gas hydrate accumulation in the western part of the Prudhoe Bay Unit. This integrated analysis of downhole well log data and published geophysical data has provided new insight on structural, stratigraphic, and reservoir controls on the occurrence of gas hydrates in the Eileen Gas Hydrate Trend. This study revealed the relatively complex nature of the gas hydrate occurrences in the Eileen Gas Hydrate Trend, with gas hydrates present in a series of coarsening upward, laterally pervasive, mostly fine-grained sand beds exhibiting high gas hydrate saturations. Most of the gas hydrate-bearing reservoirs in the Eileen Gas Hydrate Trend are laterally segmented into distinct northwest- to southeast-trending fault blocks, occur in a combination of structural–stratigraphic traps, and are only partially hydrate filled with distinct down-dip water contacts. These findings suggest that the traditional parts of a petroleum system (i.e., reservoir, gas source, gas migration, and geologic timing of the system formation) also control the occurrence of gas hydrates in the Eileen Gas Hydrate Trend. Full article

The Qiongdongnan Basin (QDNB), located in the north of the South China Sea, is a Cenozoic rift basin with abundant oil and gas resources. Large flake hydrates have been found in the core fractures of Quaternary formations in the deep-water depression of the QDNB. In order to understand the spatial distribution patterns of these fractures, their geneses in sedimentary basins, and their influences on gas migration and accumulation, such fractures have been observed using high-resolution 3D seismic images and visualization techniques. Four types of fractures and their combinations have been identified, namely bed-bounded fractures/microfaults, unbounded fractures, fracture bunches, and fracture clusters. Bed-bounded fractures/microfaults are mainly short and possess high density; they have developed in mass transport depositions (MTDs) or Meishan and Sanya Formations. The unbounded fractures/microfaults that occur in Miocene–Pliocene formations are mainly long and discrete, and are dominantly caused by strong tectonic movements, the concentration of stress, and sustained intense overpressure. The fracture bunches and fracture clusters that occur in Oligocene–Early Miocene formations have commonly developed with the accumulation of large numbers of fractures and may be related to the release of pressure, diapirs, and basement fault blocks (228.9 ± 1 Ma). In this study, six fluid charging or leakage models are proposed based on distinct fracture types, assuming the uniform conductivity of each fracture. In a 3D space view, a vertical decrease in the fracture scale (number or density) will more likely result in gas supply than dispersion, thus promoting the accumulation of gas in the reservoirs. Nevertheless, the fractures above the Bottom Simulating Reflect (BSR)/seismic anomaly are excessively developed, and bed-bounded fractures within a particular layer, such as MTDs, can easily cause seabed leakage. These results are useful for explaining the vertical migration of gas/fluids in areas and formations with less developed gas chimneys, faults, diapirs, and other structures, particularly in post-rifting basins. Full article

Fine-grained natural gas hydrate (NGH) reservoirs are widely distributed across the world and bear more than 90% of global NGH. However, it is difficult to exploit this kind of NGH reservoir economically and environmentally using conventional methods. Water-jet cutting is an efficient and environmentally friendly technique for mining such hydrate reservoirs, as the production process does not depend on mass and heat transfer within the formations. In this work, a series of physical experiments were conducted to clarify the erosion performance of marine hydrate-bearing sediment (HBS) impacted by water jets. The results show that the accumulation of sediment particles and hydrate particles at the bottom of erosion hole severely inhibits the vertical erosion of HBS by water jet. For a particular jet flow rate, the jet distance has an optimal value, which is between 4 mm and 28 mm. Moreover, the upwelling flow containing solid particles has a significant impact on the erosion of the hole top. In reservoirs with a low hydrate saturation (20–40%) and reservoirs with a high hydrate saturation (60–80%), the erosion holes exhibit a gourd shape and a bamboo shape, respectively. In addition, the volume erosion efficiency and the depth erosion efficiency are more sensitive to the variation in jet flow rate than jet distance and hydrate saturation. This study can provide theoretical and technical support for the application of water-jet cutting in the exploitation of marine HBS. Full article

Gas hydrates have been considered as a new energy that could replace conventional fossil resources in the future because of their high energy density, environmental friendliness, and enormous reserves. To further analyze the potential distribution of gas hydrate stability zones (GHSZ) and the formation of a gas hydrate system in the Shenhu area of the South China Sea (SCS), a 3D petroleum simulation model (PSM) was built from 3D seismic interpretations and all available geological data. Based on the thermal calibration of the 3D model, the evolution of the GHSZ, hydrocarbon generation and migration, and the formation and accumulation of gas hydrates were simulated for the first time in the area. Thermal simulation shows that the methane source of gas hydrate originated from shallow biogenic gas and deep thermogenic gas. Most areas are dominated by shallow biogenic gas, while, only about 3% of the deep thermogenic gas derived from Enping Formation source rock and contributed to the gas hydrate formation within a few areas in the southeast. The thermogenic gas migrated vertically into the GHSZ through connecting faults, mud diapir, and/or gas chimney to form gas hydrate. The source rocks of the Wenchang Formation, a deep thermogenic gas source, began to enter the main hydrocarbon generation window at 28.4 Ma. The Enping source rock began to generate oil from 25 Ma on and gas from 16 Ma on. Since 5.3 Ma, most areas of the source rocks have generated a gas window, and only the shallower parts in the east still in the oil window, which had lasted until now. The shallow biogenic gas source rocks from the Hanjiang, Yuehai, and Wanshan formations generated gas in different periods, respectively. The Qionghai Formation began to generate hydrocarbon from 0.3 Ma and until now. Other results show that the GHSZ developed mainly during the Quaternary and Neogene (Wanshan Formation) and the GHSZ is thicker in the southern area and thinner in the northern part with a positive correlation with water depth. Starting at 11.6 Ma, the GHSZ developed in the Hanjiang Formation in the south of the Shenhu area and gradually expanded to the north to cover most of the study area at 5.3 Ma during the Yuehai Formation. From 1.8 Ma on, the GHSZ covered the entire study area. At the same time, the GHSZ in the Hanjiang Formation disappeared because of the change in temperature and pressure. At present, the GHSZ in the Yuehai Formation has disappeared, while the Quaternary and Wanshan are the two main formations for GHSZ development. The formation and distribution of gas hydrates are fundamentally controlled by the space-time coupling between the hydrocarbon generation and expulsion time and distribution of the GHSZ. The simulation results of gas hydrate accumulation and distribution were verified by drilling results and the matching rate is 84%. This is the first time that 3D simulation was successfully conducted with PSM technology in the Shenhu area and it provides important guidance for gas hydrate study in other areas of the SCS. Full article

2D numerical modeling algorithms of multi-component, multi-phase filtration processes of mass transfer in frost-susceptible rocks using nonlinear partial differential equations are a valuable tool for problems of subsurface hydrodynamics considering the presence of free gas, free water, gas hydrates, ice formation and phase transitions. In this work, a previously developed one-dimensional numerical modeling approach is modified and 2D algorithms are formulated through means of the support-operators method (SOM) and presented for the entire area of the process extension. The SOM is used to generalize the method of finite difference for spatially irregular grids case. The approach is useful for objects where a lithological heterogeneity of rocks has a big influence on formation and accumulation of gas hydrates and therefore it allows to achieve a sufficiently good spatial approximation for numerical modeling of objects related to gas hydrates dissociation in porous media. The modeling approach presented here consistently applies the method of physical process splitting which allows to split the system into dissipative equation and hyperbolic unit. The governing variables were determined in flow areas of the hydrate equilibrium zone by applying the Gibbs phase rule. The problem of interaction of a vertical fault and horizontal formation containing gas hydrates was investigated and test calculations were done for understanding of influence of thermal effect of the fault on the formation fluid dynamic. Full article

Compared with the deeply buried marine gas hydrate deposits, gas hydrates in the shallow subsurface, close to and at the seafloor, have attracted more attention owing to their concentrated distribution, high saturation, and easy access. They accumulate at relatively shallow depths <100–120 m and occur as gas hydrate-bearing mounds (also known as hydrate outcrops, pingoes) at the seafloor derived from the growth of hydrates in the shallow subsurface or as pure hydrate chunks formed by gas leakage. This paper reviews and summarizes such gas hydrate systems globally from the perspective of gas sources, migration pathways, and accumulation processes. Here, we divided them into four categories: fault-chimney-controlled, diapir-fault-controlled, fault-controlled, and submarine mud volcano-controlled deposits. Gas chimneys originate immediately above the restricted regions, mostly affected by faults where high gas concentrations trigger elevated pore fluid pressures. Diapirism derives a dendritic network of growth faults facilitating focused gas discharge and hydrate formation near the seafloor. Furthermore, pre-existing faults or fractures created by overpressured gas from greater depths in accretionary tectonics at convergent margins act as preferential pathways channeling free gas upwards to the seafloor. Gas flux rates decrease from the submarine mud volcano center to its margins, creating a concentric pattern of distributing temperature, gas concentrations, and hydrate contents in shallow sediments around the mud volcano. Hydrate-bound hydrocarbons are commonly of thermogenic origin and correspond to high-background geothermal conditions, whereas microbial gas is dominant in a few cases. The presence of heavier hydrocarbons mitigates the inhibition of hydrate formation by salt or heat. Fluid migration and pathways could be compared to the “blood” and “bones” in an organic system, respectively. The root of a pathway serves as the “heart” that gathers and provides considerable free gas concentrations in a restricted area, thereby triggering pore fluid pressures as one important drive force for focused fluid flow in impermeable sediments (the organic system). Besides the suitable temperature and pressure conditions, a prerequisite for the formation and stability of hydrate deposits in the shallow subsurface and at the seafloor is the sufficient supply of gas-rich fluids through the hydrate stability zone. Thus, the proportion of gas migrating from deep sources is significantly larger than that trapped in hydrates. As such, such marine hydrate deposits seem more like temporary carbon storage rather than the main culprit for climate warming at least in a short period. Full article

Gas hydrates comprise one of the largest reservoirs of organic carbon on Earth. Marine gas hydrate predominantly consists of biogenic (i.e., microbially generated) methane molecules trapped within lattice-like cages of frozen water molecules. Sedimentary organic matter is the feedstock for methanogens producing gas in anaerobic sub-seafloor environments. Therefore, an understanding of the minimum amount of organic material (measured as carbon and hydrogen content) necessary for methanogenesis to result in appreciable volumes of hydrocarbons is central to understanding the requirements for gas hydrate formation. Reactive transport modelling by workers over the past 20 years suggests minimum requirements of ~0.3–0.5. wt. % TOC (total organic carbon) for gas hydrate formation, while earlier workers predicted TOC as low as ~0.1–0.2. wt. % could produce biogenic gas. However, the hydrogen content (recognized as the limiting reagent in hydrocarbon generation for over 50 years) needed for biogenic gas generation and gas hydrate formation is poorly understood. Furthermore, the minimum organic contents needed for gas hydrate formation have not been investigated via basin-scale computational modeling. Here, we construct a synthetic 3-D basin and gas hydrate system model to investigate minimum sediment TOC and hydrogen (HI, hydrogen index) contents needed for gas hydrate formation. Our modelling suggests that, under geologically favorable conditions, TOC as low as 0.1. wt. % (paired with 100 HI) and HI as low as 50 (paired with 0.2. wt. % TOC) may produce biogenic gas hydrate saturations above 1%. Our modelling demonstrates the importance of basin-scale investigation of hydrocarbon systems and demonstrates how the confluence of favorable structural controls (e.g., faults, folds, anticlines) and stratigraphic controls (e.g., carrier beds, reservoirs) can result in gas hydrate accumulations, even in organic-poor settings. Full article

Formation and dissociation of pore gas hydrates in permafrost can change its properties, including fluid flow capacity. Permeability is one of the most significant parameters in the study of hydrate-containing rocks, especially in the case of gas burial or extraction. Gas permeability variations in frozen sand partially saturated with CO₂ or CH₄ hydrates are studied experimentally at a constant negative temperature of −5 °C, as well as during freezing–thawing cycles. The gas permeability behavior is controlled by the formation and dissociation of pore gas hydrates in frozen sand samples. The samples with an initial ice saturation of 40 to 60% become at least half as permeable, as 40% of pore ice converts to hydrate. The dissociation process of accumulated hydrates was modeled by both depressurizing methane or CO₂ to atmospheric pressure and by stepwise injection of gaseous nitrogen up to 3 MPa into a frozen sample. In sand samples, with a decrease in gas pressure and without subsequent injection of nitrogen, a decrease in pore hydrate dissociation due to self-preservation was noted, which is reflected by a deceleration of gas permeability. Nitrogen injection did not lead to a decrease in the rate of dissociation in the frozen hydrate-containing sample, respectively, as there was no decrease in the rate of gas permeability. Full article

The Mackenzie Delta (MD) is a permafrost-bearing region along the coasts of the Canadian Arctic which exhibits high sub-permafrost gas hydrate (GH) reserves. The GH occurring at the Mallik site in the MD is dominated by thermogenic methane (CH₄), which migrated from deep conventional hydrocarbon reservoirs, very likely through the present fault systems. Therefore, it is assumed that fluid flow transports dissolved CH₄ upward and out of the deeper overpressurized reservoirs via the existing polygonal fault system and then forms the GH accumulations in the Kugmallit–Mackenzie Bay Sequences. We investigate the feasibility of this mechanism with a thermo–hydraulic–chemical numerical model, representing a cross section of the Mallik site. We present the first simulations that consider permafrost formation and thawing, as well as the formation of GH accumulations sourced from the upward migrating CH₄-rich formation fluid. The simulation results show that temperature distribution, as well as the thickness and base of the ice-bearing permafrost are consistent with corresponding field observations. The primary driver for the spatial GH distribution is the permeability of the host sediments. Thus, the hypothesis on GH formation by dissolved CH₄ originating from deeper geological reservoirs is successfully validated. Furthermore, our results demonstrate that the permafrost has been substantially heated to 0.8–1.3 °C, triggered by the global temperature increase of about 0.44 °C and further enhanced by the Arctic Amplification effect at the Mallik site from the early 1970s to the mid-2000s. Full article