Hydrate Accumulations Research Articles

Natural Gas Migration Pathways and Their Influence on Gas Hydrate Enrichment in the Qiongdongnan Basin, South China Sea

2D and 3D seismic data and basin simulation were used to investigate the gas hydrate distribution and natural gas migration pathways in the Qiongdongnan Basin (QDNB). Hydrate-related amplitude anomalies and extensive bottom simulating reflectors (BSRs) were mapped within the uppermost part. Based on their seismic reflection characteristics, the three main types of natural gas migration pathways and their distributions in the QDNB were identified through high-resolution seismic data. Basin modeling was carried out to document the migration efficiency of different migration pathways and their effects on hydrate enrichment. The basin modeling results show the following: (1) Diapirs, fault structures, and fractures constitute the three main types of natural gas migration pathways that transport the thermogenic gas from the deep to shallow layers in the QDNB. (2) The three migration pathways impact hydrate enrichment in different ways. Diapirs and faults contribute significantly to hydrate enrichment due to their higher migration efficiency. In comparison, the migration efficiency of the fracture systems is lower, with minimal benefit to hydrate enrichment. (3) The natural gas hydrate in the QDNB is mainly distributed along the diapirs and deep faults and generally scattered around the fracture system. These conclusions indicate that the migration pathways in the QDNB are regionally distributed and are closely related to hydrate accumulation.

The Controlling Factors of the Natural Gas Hydrate Accumulation in the Songnan Low Uplift, Qiongdongnan Basin, China

Hydrocarbon charging stages and natural gas hydrate accumulation models were established in the Songnan Low Uplift, Qiongdongnan Basin (QDNB), China. Detailed geochemical analysis, paleotemperature and paleopressure analyses, seismic interpretation, and hydrocarbon charging characterization were conducted to investigate the controlling factors of natural gas hydrate accumulation. The Yacheng and Lingshui formations in the Lingshui Sag were identified as effective source rocks. The δ13C1 values of the gas hydrates vary from −35.97‰ to −59.50‰, following a direction from the Sag Center to the Low Uplift, indicating that δ13C1 values became relatively lighter during gas migration. Seismic data evidence revealed that large-scale faults, laterally distributed sandstones, and gas chimneys were comprehensive, however efficient hydrocarbon migration pathways. Systematic overpressure was developed in the Yacheng and Huangliu formations, which offer sufficient hydrocarbon migration impetus. Hydrocarbon-bearing aqueous inclusions and their coexisting aqueous inclusions were observed in the Huangliu Formation in the Lingshui Sag, indicating four stages of hydrocarbon charging in the QDNB. Based on the comprehensive analysis in this research, two gas hydrate accumulation models were proposed in the Songnan Low Uplift, QDNB, as follows: 1) a mixture of gas migrated by large-scale faults and a thermogenic-biogenic mixed gas model and 2) gas hydrates from laterally distributed sandstones sealed by MTDs.

Sand-rich gas hydrate and shallow gas systems in the Qiongdongnan Basin, northern South China Sea

In 2019, the sand-rich gas hydrate and shallow gas (GHSG) reservoirs have been drilled in the Qiongdongnan Basin, northern South China Sea. As a conventional energy resource, shallow gas can provide a continuous source of gas for gas hydrate accumulation, which is also the focus of future research on gas hydrates. Using core-log-seismic data, the distributions and sedimentary environments of the sand-rich GHSG reservoirs were identified and characterized. Then, the sand-rich GHSG systems were illustrated, and the interactions of gas hydrate with shallow gas were analyzed in details in the Qiongdongnan Basin. At last, an accumulation model for the GHSG systems was proposed. This model shows that natural gas in the deep parts of the hydrocarbon-rich sag migrated laterally to the study area to form deep natural gas reservoirs, and then is transported vertically through gas chimneys to form GHSG system. The system mainly includes gas sources, gas migration pathways, reservoirs, and seals, of which gas migration pathways are key influencing factors. Moreover, the whole system is a dynamically balanced system in which shallow gas reservoirs can provide adequate gas for gas hydrate formation; meanwhile, gas hydrates can be seals to prevent the leakage of free gas and facilitate a larger-scale shallow gas reservoir formed, so these two items are complementary and indispensable in the system.

Experimental Insights Into the In Situ Formation and Dissociation of Gas Hydrate in Sediments of Shenhu, South China Sea

Natural gas hydrates as sustainable energy resources are inherently affected by mineral surfaces and confined spaces in reservoirs. However, the habits of hydrates in geological sediments are still an open question. In this work, we systemically studied the process of hydrate formation and dissociation in sediments from the Shenhu area of the South China Sea to examine the evolution of hydrate saturation and permeability in sediments and their relationship. Characterization of samples indicates that sediments of the Shenhu area are mainly composed of clay and fine sand grains and provide a large number of nanopores for hydrate accumulation. For in situ observations enabled by low-field nuclear magnetic resonance methods, the formation of hydrates shows a different kinetic behavior with an induction time compared to hydrate dissociation. Estimated by variations of hydrate saturation (%) over time, the rate of hydrate formation is around 12%/min, while the dissociation rate increases to 3%/min with the higher temperature. With the presence of hydrates, pore space and thus permeability of sediments decreased obviously by one and three orders of magnitude when the hydrate saturation is 20 and 45%, respectively. Compared to models with the assumption of grain-coating and pore-filling hydrates, the tendency of permeability evolution from NMR measurements is between fitted lines from models. It highlights that the existing models considering a single pattern of hydrate growth cannot precisely describe the relationship between permeability and hydrate saturation. Hybrid hydrate habits coexist in sediments resulting from heterogeneous pore structures and thus complex gas–water distributions.

Sediment Microstructure in Gas Hydrate Reservoirs and its Association With Gas Hydrate Accumulation: A Case Study From the Northern South China Sea

Exploration and pilot production have confirmed that gas hydrates in the Shenhu area on the northern continental slope of the South China Sea have enormous resource potential. However, a meticulous depiction of gas hydrate reservoirs based on sediments is limited. The distributed low-flux gas hydrates are mainly deposited in the Shenhu area, and the gas hydrate saturation exhibits extreme vertical heterogeneity. In this study, we focused on the sediment microstructure of gas hydrate reservoirs. Based on the variation in gas hydrate saturation, the study interval was divided into non-gas hydrate (non-GH) as well as I-, II-, and III-gas hydrate reservoir layers. We analyzed the relationship between sediment microstructure and gas hydrate reservoirs based on computed tomography scans, specific surface area analysis, and scanning electron microscopy observations. The results showed that the sediment in gas hydrate reservoirs had three types of pores: 1) intergranular pores between coarse grains (CG-intergranular pores), 2) intergranular pores between fine grains (FG-intergranular pores), and 3) biologic grain pores (BG-pores). The CG- and FG-intergranular pores were mainly formed by the framework, which consisted of coarse minerals (such as quartz and feldspar) and clay minerals, respectively. The BG-pores were mainly formed by the coelomes of foraminifera. CG-intergranular pores and BG-pores can provide effective reservoir space and increase the permeability of sediment, which is conducive to gas hydrate accumulation. The FG-intergranular pores reduce permeability and are not conducive to gas hydrate accumulation. Clay minerals and calcareous ultramicrofossils with small grain sizes and complex microstructures fill the effective reservoir space and reduce the permeability of sediment; additionally, they improve the adsorption capacity of sediment to free gas or pore water, which is not conducive to gas hydrate formation and accumulation. The results of our study explicitly suggest that the microstructure of sediment is an important controlling factor for gas hydrate accumulation and reveals its underlying mechanism.

Double bottom simulating reflectors and tentative interpretation with implications for the dynamic accumulation of gas hydrates in the northern slope of the Qiongdongnan Basin, South China Sea

The origin of double bottom simulating reflectors (BSRs) and their relationship with gas hydrate systems are of great significance to the exploration of gas hydrate resources in submarine settings. Based on seismic data, this paper presents the first identified double BSRs in the northern slope of the Qiongdongnan Basin (QDNB), northern South China Sea. The imaged double BSRs are located in submarine ridges associated with a canyon system. These two BSRs (BSR1 and BSR2) are characterized by two subparallel high-amplitude reflections that mimic the seafloor’s distribution but have the opposite polarity. Patches of enhanced reflections (ERs) with high acoustic impedance indicate the occurrence of gas hydrate above BSR1 and between BSR1 and BSR2. The ERs indicate that the free gas terminates against BSR2 and is located beneath BSR2. The velocity spectrum of the strata shows that the interval velocity above the BSR1 and between BSR1 and BSR2 reaches as high as ∼2450 m/s, suggesting the presence of gas hydrates. The numerical modelling results of the gas hydrate stability zone (GHSZ) show that the geothermal gradient of the study area is 31.3–48.9 °C/km. Three potential formation mechanisms for these double BSRs are proposed: (1) variations in the P-T conditions in the GHSZ caused by erosion and redeposition associated with the submarine canyon; (2) precipitation of gas hydrates with different structures due to a supply of both thermogenic gas and biogenic gas; and (3) a relic paleo-BSR formed by the upward migration of a geothermal fluid through possible gas chimneys resulting in gas hydrate dissociation. BSR1 is correlated with the BGHSZ of structure I (SI) gas hydrates, and BSR2 may be correlated with the BGHSZ of structure II (SII) gas hydrates. Therefore, SI and SII gas hydrates may coexist in the study area, and SII gas hydrates and free gas may be present between BSR1 and BSR2. The occurrence of double BSRs suggests the dynamic accumulation of gas hydrates and free gas due to the erosion and redeposition of a submarine canyon, which shifted the BGHSZ and resulted in the differential distribution of the gas hydrates. A model illustrating the migration and accumulation of the hydrocarbons and gas hydrates associated with the occurrence of double BSRs is proposed, providing a reference for future gas hydrate exploration in the deepwater QDNB.

Mineralogy and pore characteristics of marine gas hydrate-bearing sediments in the northern South China Sea

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.

Spatial-Temporal Evolution of the Gas Hydrate Stability Zone and Accumulation Patterns of Double BSRs Formation in the Shenhu Area

The current study examines the methane gas hydrate stability zone (GHSZ) in the Shenhu area in the northern South China Sea (SCS) as an example to calculate the thickness of the GHSZ and reconstruct its evolution since 8.2 Ma. Two mechanisms for typical double BSRs in the Shenhu area are shown, and the relationship between the evolving thickness of the GHSZ and the dynamic accumulation of NGHs at typical stations in the Shenhu area is clarified. The results show that the thickness of the GHSZ varies over time with overall thickening in the Shenhu area. The current thickness of the GHSZ is between 160.98 and 267.94 m. Two mechanisms of double BSRs in the Shenhu area are summarized: the double BSRs pattern based on changes in formation temperature, pressure and other conditions and the double BSRs pattern based on differences in gas source and composition. The formation process and occurrence characteristics of double BSRs and hydrate at site SH-W07-2016 in the Shenhu area are also closely related to the changes in thickness of the GHSZ. In addition, the age when gas source first enters the GHSZ has a considerable influence on the dynamic accumulation process of hydrate. Since the formation of hydrate above the BSR at site SH-W07-2016, the GHSZ has experienced up to two periods of thickening and two periods of thinning at this site. With the changes in the thickness of the GHSZ, up to two stages of hydrate formation and at most two stages of hydrate decomposition have occurred. This paper is of great value for understanding the formation of multiple bottom-simulating reflectors (BSRs) as well as the migration, accumulation and dissipation of natural gas hydrate (NGH) during the dynamic accumulation process.

Source-Reservoir Characteristics and Accumulation of Gas Chimney-Type Gas Hydrates in Qiongdongnan Basin, Northern South China Sea

The Qiongdongnan Basin is an important gas hydrate exploration area in the South China Sea, but the gas hydrate accumulation process is poorly understood. By selecting an Lingshui (LS) target area and using first-hand geochemical data, three-dimensional seismic data, and an independent thermal insulation and pressure maintaining shipborne core analysis system, in this study, comprehensive geological evaluation was carefully conducted around the gas source and reservoir of the gas chimney hydrates, and the accumulation process was investigated. Geochemical data for a total of 47 sets of gas samples revealed that the gas source of the hydrates in the study area was mainly thermogenic gas supplemented by mixed gas. It was predicted that the contribution of the thermogenic gas to the hydrate accumulation was up to 70%. Using the independent shipborne core analysis system, the characteristics of the low-temperature combustible gas hydrates were determined, and the reservoir in the study area was characterized as non-diagenetic to weakly diagenetic, rich in clay and silt (D50 = 15.1–34.1 μm), weakly self-sustaining, and strongly heterogeneous. Based on the differentiated seismic response of the hydrate layer-gas bearing hydrate layer-shallow gas layer at the top of the gas chimney, the accumulation process in the target area was determined to be as follows: remote thermogenic gas transportation, local microbial genetic gas accumulation, episodic dynamic gas chimney reservoir formation, and source-reservoir control. In addition, the exploration ideas of an effective supply from a gas source and a shallow large-scale sand body are emphasized. The results of this study provide an important reference for the trial production of natural gas hydrates in the South China Sea.

Geophysical characteristics of a focused fluid flow system in the western South China Sea

High-resolution multichannel seismic data and multibeam terrain data are used to analyse and identify a focused fluid flow (FFF) system in the central and northern parts of the Zhongjiannan Basin in the western South China Sea. The authors classify and summarise the geophysical characteristics of an FFF system in the study area. It is divided into a fault-related FFF system and a column-related FFF system. In addition, the FFF system in the study area is mainly located within the tectonic activity area, thin slope break belt and land slope area. The rich FFF system in the study area is an important channel for fluid migration, which is conducive to gas hydrate accumulation. The FFF system in the study area is poorly studied, and the distribution research results of the FFF system in this area can be used more widely in hydrocarbon and gas hydrate exploration and must be taken into account when assessing seabed stability.

The Structure of Permafrost in Northern West Siberia: Geophysical Evidence

The permafrost of Arctic West Siberia stores extremely rich resources of hydrocarbon fuels that remain a key energy source and an important element of the global economy. A large amount of natural gas in permafrost is bound in gas hydrates which may become an alternative fuel of the future. Shallow subsurface in the permafrost area of northern West Siberia has been studied by transient electromagnetic (TEM) soundings to estimate the permafrost thickness and to detect faults as channels for fluids and heaving features as possible indicators of gas hydrate accumulations. The shallow transient electromagnetic (sTEM) surveys were conducted in discontinuous and continuous permafrost at two sites in northern West Siberia (the northeastern Yamal Peninsula and the southern Gydan Peninsula), with a focus on the vicinities of lakes and cryogenic landforms (frost mounds). The sTEM method resolves well the heterogeneous permafrost structure, with faults, numerous unfrozen zones (taliks), and frost mounds marked by resistivity and seismic anomalies. Some lakes are located above faults, and their origin may be related to deformation, whereas the frost mounds may mark deep-seated fluid dynamic processes. Local thickening of permafrost may be associated with the presence of gas hydrates. The detected features may trace gas migration pathways and reveal possible sites of gas emission.

Geochemical Characteristics and Gas‐to‐Gas Correlation of Two Leakage‐type Gas Hydrate Accumulations in the Western Qiongdongnan Basin, South China Sea

AbstractIn recent years, a series of highly saturated leakage‐type gas hydrates have been discovered in the western Qiongdongnan Basin (QDNB), South China Sea. Based on the molecular compositional and isotopic characteristics of the gas samples relevant to the gas hydrates collected from the two leakage‐type gas hydrate accumulations in the GMGS5 and GMGS6 drilling zones, a detailed geochemical gas‐to‐gas correlation was conducted in this study, in order to further understand the geochemical characteristics and possible hydrocarbon sources of these gas hydrates. The natural gas relevant to the gas hydrates in the GMGS5 block is characterized by wet gas (67.96% &lt; %C1 &lt; 98.58%, C1/C1+ &lt; 0.9) and significant molecular and carbon isotope fractionation within the depth profile, whereas the gas samples from the GMGS6 block exhibit the characteristics of dry gas (99.25% &lt; %C1 &lt; 99.81%, C1/C1+ &gt; 0.9) and lack molecular and carbon isotope fractionation. Approximately 40%–60% of the methane within the gas hydrate is of microbial origin, while the C2+ gas components are typical coal‐type gas that are derived from thermogenic source rocks or deeply‐buried natural gas fields. In addition, typical in situ primary microbial methane (–80.6‰ &lt; δ13C‐C1 &lt; −67.7‰) was discovered in well Wb, which was applied to estimate the contribution of the microbial gas to the gas hydrates. The gas‐to‐gas correlation results show that the hydrate gases within the two leakage‐type gas hydrate accumulations in the GMGS5 and GMGS6 blocks are geochemically different, suggesting that they may have been derived from different source kitchens. Our results further indicate that the deeply‐buried thermogenic gas significantly contributed to the shallowly‐buried gas hydrates in the western QDNB and multiple effective thermogenic source kitchens provided the hydrocarbon gas in the gas hydrate accumulations.

Numerical Simulations in Support of a Long-Term Test of Gas Production from Hydrate Accumulations on the Alaska North Slope: Reservoir Response to Interruptions of Production (Shut-Ins)

Numerical Simulations in Support of a Long-Term Test of Gas Production from Hydrate Accumulations on the Alaska North Slope: Reservoir Response to Interruptions of Production (Shut-Ins)

Message Passing Interface (MPI) Parallelization of Iteratively Coupled Fluid Flow and Geomechanics Codes for the Simulation of System Behavior in Hydrate-Bearing Geologic Media. Part 2: Parallel Performance and Application

Summary The reservoir geomechanics simulator (RGMS or RGM simulator), a geomechanics simulator based on the finite element method and parallelized using the Message Passing Interface (MPI), is developed in this work to model the stresses and deformations in subsurface systems. RGMS can be used standalone or coupled with flow and transport models. pTOUGH+HYDRATE (pT+H) V1.5, a parallel MPI-based version of the serial TOUGH+HYDRATE (T+H) V1.5 code that describes mass and heat flow in hydrate-bearing porous media, is also developed. Using the fixed-stress split iterative scheme, RGMS is coupled with the pT+H V1.5 to investigate the geomechanical responses associated with gas production from hydrate accumulations. In the second paper of this series, the parallel performance of the codes is tested on the Texas A&amp;M University Ada Linux cluster using up to 512 processes and on a Mac Pro computer with 12 processes. The investigated problems are: Group 1: Geomechanical problems solved by RGMS in 2D Cartesian and cylindrical domains and a 3D problem, involving 4×106 and 3.375×106 elements, respectively; Group 2: Realistic problems of gas production from hydrates using pT+H V1.5 in 2D and 3D systems with 2.45×105 and 3.6×106 elements, respectively; Group 3: The 3D problems in Group 2 solved with the coupled RGMS-pT+H V1.5 simulator, fully accounting for geomechanics. Two domain partitioning options are investigated on the Ada Linux cluster and the Mac Pro, and the code parallel performance is monitored. On the Ada Linux cluster using 512 processes, the simulation speedups (a) of RGMS are 218.89, 188.13, and 284.70 in the Group 1 problems, (b) of pT+H V1.5 are 174.25 and 341.67 in the Group 2 cases, and (c) of the coupled simulators are 134.97 and 331.80 in the Group 3 cases. The results produced in this work show the necessity of using full geomechanics simulators in marine hydrate-related studies because of (a) the associated pronounced geomechanical effects on production and displacements and (b) the effectiveness of the parallel simulators developed in this study, which can be the only realistic option in these complex simulations of large multidimensional domains.

Message Passing Interface (MPI) Parallelization of Iteratively Coupled Fluid Flow and Geomechanics Codes for the Simulation of System Behavior in Hydrate-Bearing Geologic Media. Part 1: Methodology and Validation

Summary The Reservoir GeoMechanics Simulator (RGMS or RGM simulator), a geomechanics simulator based on the finite element method (FEM) and parallelized using the Message Passing Interface (MPI), is developed in this work to model the stresses and deformations in subsurface systems. RGMS can be used standalone or coupled with flow and transport models. pTOUGH+ HYDRATE (pT+H) V1.5, a parallel MPI-based version of the serial TOUGH+HYDRATE (T+H) V1.5 code that describes mass and heat flow in hydrate-bearing porous media, is also developed. Using the fixed-stress split iterative scheme, RGMS is coupled with the pT+H V1.5 codes to investigate the geomechanical responses associated with gas production from hydrate accumulations. In the first paper of this series, we discuss the governing equations underlying physics and their mathematical representation in the modeling of the geomechanics, methane hydrate, and coupled problems as well as the numerical methods and the parallelization processes (involving a domain decomposition method based on the MPI approach) used for the parallel simulators. Two 2D problems (in Cartesian and radial-cylindrical coordinates) and a 3D Cartesian coordinate problem are created to validate the FEM and the parallelization method in RGMS. The displacements and the maximum principal effective stresses obtained from the RGMS solution of these three problems are compared to those from the commercial software Ansys Mechanical and are shown to practically coincide. The parallelization of pT+H V1.5 is validated by comparing its results to those from the serial T+H V1.5 code in a study that involved (a) fluid production from a large-scale 2D cylindrical system describing a real-life oceanic hydrate deposit and (b) a simplified geomechanical model based on hydrate-dependent pore compressibility. The coupling method is validated by comparing the numerical results to the analytical solutions of the Terzaghi and the McNamee-Gibson problems. The parallelization validation of the coupled simulator is achieved by comparing the results obtained for different numbers of processes in the solution of the problems used for the pT+H V1.5 parallelization validation with the full geomechanical model. The results clearly demonstrate the validity and reliability of the parallel codes (a) RGMS, (b) pT+H V1.5, and the (c) coupled pT+H V1.5 and RGM simulators, which can be used to solve the large-scale physics of complex problems.

Modeling the Effects of Sedimentation on Natural Occurrences of CH4 Hydrates in Marine Sediments

This study explores the relationships that sedimentation rate and transport properties have with the formation and evolution of hydrates in fine-grained marine sediments and their corresponding bottom simulating reflector (BSR) responses. Using a series of one-dimensional simulations of multiphase, multicomponent flow and transport of mass and heat through porous media, a slab of sediments through sedimentation is modeled. The boundary conditions are set to emulate the pressure and geothermal gradients and its resulting gas hydrate stability zone (GHSZ). Hydrates are formed by injecting methane gas through the bottom of the grid and letting it migrate and reach the boundary of stability. The resulting hydrate accumulation is subjected to different sedimentation rates and replicated with different intrinsic permeability. With sedimentation, the geothermal gradient is displaced upward and the boundary of stability shoals. Through methane recycling, the distribution of phases changes through cycles of slow melting and rapid reformation. This results in a dynamic flow barrier that relocates the base of the GHSZ over geological time, in response to the variations of both pressure and salinity. The characteristics of a BSR response will be tied to the stage of the melting cycle.

A Numerical Model for Determining Deep Methane Flux Linked to the Free Gas Zone: Application to the Ocean Drilling Program Site 995 and Implications for Regional Deep Methane Flux at the Blake Ridge

The lack of the quantification of deep dissolved methane flux prevents us from accurately understanding hydrate accumulation and distribution at a given geologic setting where vertically upward methane advection dominates the hydrate system. The upward deep methane flux was usually applied as an assumed value in many previous studies. Considering the deep methane flux changes the methane concentration in the pore water and further affects the phase transfer between the gas and aqueous phases depending on the in situ methane concentration, we link gas bubbles distribution to deep dissolved methane flux. Here, we constructed a numerical model to quantify the dissolved methane flux from depth based on the parameters related to gas bubble distribution, including the residual gas saturation in sediments and the free gas zone (FGZ) thickness. We then applied our model to ODP Site 995 at the Blake Ridge where methane was sourced from deep layers. Our model results predict an upward deep methane flux of 0.0231 mol/m2/a and the occurrence of another gas interval in deeper sediments, which are consistent with seismic data. We further explored the influence of upward methane flux on hydrate accumulation and found that the thin hydrate occurrence zone at nearby Site 994 likely resulted from a small deep methane flux. Combined with the previous conclusion of high deep methane flux at Site 997, we showed that along the Blake Ridge drilling transect the estimated deep methane fluxes decrease with increasing distance from the crest of the ridge. This approach for quantifying deep methane flux is complementary to the current hydrate accumulation model and provides new insights into the regional methane flux estimation at the Blake Ridge.

Effects of depressurization on gas production and water performance from excess-gas and excess-water methane hydrate accumulations

• Various water saturations simulate Classes I and II hydrate deposits in nature. • Ambient temperature controls periphery dissociation in excess-gas hydrate deposits. • Sensible heat dominates spatially uniform dissociation in excess-water deposits. • Secondary hydrate formation are prevented by adjusted gas production pressure. • Optimized depressurization is crucial to improve water production and CH 4 recovery. Depressurization is considered as the most promising technique for hydrate exploitation, as it achieves the highest energy profit ratio and is the most technologically feasible. However, the exploitation of excess-water hydrate accumulations generates high water production, leading to increased cost, poor energy efficiency, and problems with sand during operation. Thus, water management is crucial to gas recovery by the depressurization of different classes of hydrate accumulations, yet relevant studies remain limited. In this study, synthetic hydrate samples were prepared to simulate two types of natural methane hydrate sediments: Class 1 accumulations (excess-gas hydrate) and Class 2 accumulations (excess-water hydrate). Hydrate dissociation was conducted using a variety of depressurization approaches, and MRI imaging was employed to characterize water performance and methane recovery. Methane hydrate preferentially dissociated along the peripheries of the excess-gas samples due to more efficient heat dissipation. Methane hydrate dissociated more uniformly in the excess-water samples because the high specific heat capacity of water enabled the supply of extra heat. Furthermore, pressure histories, mean intensity change in MRI images, and water variations were monitored to analyze the characteristics of hydrate dissociation, changes in porosity, intrinsic permeability and reservoir heat, water and gas production rates, and possible secondary hydrate formation. The results of this study suggest that an optimized depressurization approach, such as stepwise depressurization, could improve methane recovery from Class 1 and Class 2 methane hydrate accumulations.

Geological, geophysical, and geochemical characteristics of deep-routed fluid seepage and its indication of gas hydrate occurrence in the Beikang Basin, Southern South China Sea

Deep-routed fluid seepage conduits are widely distributed in the Southern South China Sea (SSCS), and gas hydrates have been discovered inside these conduits through drilling and logging. Due to a lack of integrated geological, geophysical, and geochemical data, deep-routed fluid seepage conduits and their relationship to gas hydrate occurrence are still unclear. In this study, a comprehensive investigation of fluid seepage was conducted through systematic analysis of high-resolution 2-D seismic data, multibeam bathymetry data, headspace gas and carbon isotope components, authigenic carbonates, pore water, biomarkers, and hydrate phase equilibrium simulation. Multiple types of deep-routed conduits—which typically manifested as mud volcanoes, diapirs, fluid-escape pipes, and paleo-uplift associated faults—were identified. These manifestations were generally accompanied by Bottom Simulating Reflectors (BSRs), blanking zones, locally enhanced reflections, "pulled-up" reflections, pockmarks, mounds, and gas plumes, which indicated fluid seepage and possible gas hydrate accumulation. The Sulfate Methane Transition Zone (SMTZ) in the fluid seepage sites occurred at relatively shallow depths of 325–660 cmbsf (centimeter below the seafloor), suggesting a high methane flux and sufficient gas supply. The carbon isotope composition of the methane (−92.6‰ < δ13C < −50.2‰) and the detection of ethane, propane, and n-butane, as well as multiple geological configuration biomarkers, all indicate that deep-sourced hydrocarbons and thermogenic gas have migrated into the shallow layer through deep-routed conduits. Several geological and geochemical anomalies, including gas expansion, gas bubbling in piston core sediments, negative pore water chloride anomalies, and δ18O enrichment (3.8‰ < δ18O < 4.1‰) in the cold seep carbonates, provide evidence of hydrate dissociation during core recovery. The deep-routed fluid seepage and the accompanying hydrocarbon gas supply have a significant influence on the potential thermogenic gas-sourced hydrate accumulation and evolution. Overall, the co-existence of fracture-filling hydrates and pore-filling hydrates formed from the same supply of thermogenic gas and biogenic gas was inferred through comprehensive study and comparison with adjacent hydrate drilling areas. This article is the first systematic study of the characteristics of hydrate accumulation under the fluid seepage background in the Beikang Basin, and it provides a favorable reference for revealing the relationship between deep-sourced fluid seepage and hydrate formation. However, more exploration work needs to be conducted to confirm the above assumptions.

Insight into the plugging mechanism in water-continuous hydrate slurries

Oil-water-gas co-transport often occurs in offshore oil/gas production, in which gas hydrate formation could lead to severe flow assurance issues and subsequent safety and environmental risks. As the offshore oil/gas fields mature, water production increases and the flow conditions may convert from an oil-continuous to a water-continuous system. To understand the hydrate plugging mechanism of water-continuous hydrate systems, the flow characteristics of methane hydrate-water slurries was experimentally investigated by using a high pressure vertical loop. The slurry viscosity of different hydrate fractions was measured using an in-line viscometer. Accumulation, deposition and sloughing of the hydrate particles were investigated by video imaging and viscosity measurement. The plugging mechanism in a water-continuous hydrate system was extensively analyzed and hydrate deposition and sloughing were observed. The density difference and agglomeration of hydrate particles is considered as the major cause of hydrate accumulation and deposition, which subsequently instigates plugging in water-continuous systems.