A method and system for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates

Abstract

This invention belongs to the field of research on the landward distribution boundaries of natural gas hydrates. It discloses a method and system for studying the fate of methane at the landward distribution boundaries of marine natural gas hydrates, including: seismic interpretation and attribute analysis of present-day BSRs; seismic interpretation and attribute extraction of ancient BSRs to calculate their development depths; calculation of the hydrostatic pressure at the time of the ancient BSRs; simulation and prediction of the ancient BSR positions to determine the location of the landward distribution boundaries of the ancient hydrates; demarcation of the main areas of the study area; identification of whether submarine seepage features are present near the landward distribution boundaries of the hydrates, and interpretation of possible vertical and lateral fluid migration pathways and free gas accumulation; determination of the fate of methane at the boundaries and analysis of the main controlling factors; and calculation of the hydrate decomposition zone volume at the landward distribution boundaries of the hydrates based on the positions of the ancient and present BSRs. This invention identifies multiple types of methane fates at the landward distribution boundaries of marine natural gas hydrates, enabling a correct understanding of the environmental impacts of different methane fates and their effects on hydrate reservoirs.

Description

Technical Field

[0001] The present invention belongs to but is not limited to the technical field of studying the landward distribution boundary of natural gas hydrates, and in particular relates to a method and system for studying the fate of methane at the landward distribution boundary of natural gas hydrates in marine areas. Background Art

[0002] Natural gas hydrates are widely distributed globally and are considered a clean alternative energy source of the future. Over 90% of these hydrates are found in marine environments. Experts estimate that the methane resources found in natural gas hydrates in sediments within 3,000 meters of water depth worldwide are 2.1 trillion cubic meters, enough to meet human energy needs for 1,000 years. Ruppel and Kessler (2017) estimated the carbon reserves in hydrates to be 1,800 GtC. The "China Mineral Resources Report 2018," based on the types and occurrence of natural gas hydrates and geological conditions, preliminarily estimates that the amount of natural gas hydrate resources in my country's waters is approximately 80 billion tons of oil equivalent, approaching the amount of conventional oil resources in my country and approximately double the amount of conventional natural gas resources. At current consumption levels, this amount could meet my country's energy needs for nearly 200 years. In addition to being able to provide a large amount of clean energy, natural gas hydrates are believed to play an important role in marine geological disasters and ecological threats (such as submarine landslides, ocean acidification, etc.) and global warming due to their shallow burial depth, poor stability, and the fact that the methane they contain has a stronger greenhouse effect than carbon dioxide.

[0003] In deep-sea environments, the gas hydrate stability zone is a wedge-shaped body that is defined by the interface between the seabed and the bottom-simulating reflection layer (BSR), and thins toward the land. In seismic data, the intersection of the BSR and the seabed is the landward limit of the GHSZ—Gas Hydrate Stability Zone (LLGHSZ). Figure 1 Due to its shallow burial depth and lack of overlying thick sedimentary strata, the landward distribution boundary of hydrates is more susceptible to the influence of ocean conditions such as climate and submarine currents, resulting in frequent hydrate formation and decomposition. Although the hydrate content at the landward hydrate boundary accounts for only ~3.5% of the natural gas hydrate system, previous studies have shown that when the hydrate stability domain moves upward, the methane released from the hydrate decomposition zone between the ancient and modern BSRs can migrate laterally for tens of kilometers to the boundary, and can transport the methane decomposed in the main area of ​​the hydrate stability domain to the boundary, thereby effectively supplementing the source of methane at the landward distribution boundary of hydrates. Currently, the landward distribution boundary of natural gas hydrates has become a hot topic in hydrate research. One of the important issues is the fate of the free methane gas at the landward distribution boundary of hydrates.

[0004] Under the backdrop of global warming, the hydrate stability zone retreats seaward, with hydrate decomposition predominating at the continental boundary. Numerous large-scale seepage features, such as pockmarks and gas plumes, have been discovered near the LLGHSZ, the continental boundary of hydrates in various regions. For example, 570 previously undiscovered gas plumes have been discovered at depths of 50 to 1700 mbsl along the Atlantic margin of the United States, approximately 440 of which are located near the present-day LLGHSZ. This may be related to the vertical dynamic migration of the gas hydrate stability zone as seafloor water temperatures fluctuate. Berndt et al. (2014) suggested that Arctic warming is driving hydrate decomposition, leading to large-scale methane seepage near the LLGHSZ and accelerating global warming. Westbrook et al. (2009) also calculated the rising velocity of gas plumes in the West Spitsbergen margin (near the Arctic). A rough estimate suggests that the methane released by gas hydrate decomposition near the LLGHSZ could reach 20 teragrams per year, a significant contribution to the annual global carbon cycle. Methane seeps at the boundary represent the migration of methane from the lithosphere to the hydrosphere, which may affect the environment and climate and also destroy hydrate reservoirs.

[0005] At present, the research on the landward distribution boundary of natural gas hydrates mainly focuses on the large-scale leakage characteristics such as a large number of pits and gas plumes distributed along the boundary. However, not all hydrate boundaries have obvious methane leakage characteristics. In addition to leakage, it is necessary to explore whether there are other types of methane migration and accumulation patterns near the landward boundary of hydrates. In the Mauritanian waters of West Africa, there are high-amplitude anomalies representing free gas accumulation in the shallow non-hydrate stable domain at the landward distribution boundary of natural gas hydrates, and the updip direction is blocked by carbonate rocks with chaotic reflection characteristics. This phenomenon represents another destination of the hydrate distribution boundary that is different from large-scale leakage: lateral migration and accumulation of methane, that is, methane decomposed from hydrates migrates laterally and accumulates in the stratigraphic trap of the shallow non-hydrate stable domain ( Figure 2 Ancient pits below the seafloor are visible above some shallow free gas reservoirs, likely representing methane leakage caused by later overpressure. This pattern suggests that methane at the boundaries of the hydrate stability zone can migrate further laterally to shallower depths, slowing direct leakage of methane to the seafloor or even seawater at these boundaries. However, further details are urgently needed, including the process, mechanism, and controlling factors of lateral methane migration, as well as environmental impacts distinct from methane leakage.

[0006] In view of the above analysis, the technical problems that need to be solved urgently in the existing technology are:

[0007] While the continental boundaries of hydrates have become a hot topic in hydrate research, most studies have focused primarily on large-scale seepage patterns, such as the numerous pockmarks and gas plumes distributed along these boundaries. Currently, known methane fates at hydrate boundaries include regeneration of natural gas hydrates in shallow sediments; seepage into shallow seafloor sediments or seawater, where anoxic or oxygen-consuming oxidation produces carbon dioxide; formation of chemoautotrophic communities and carbonate precipitation on the seafloor; and emission into the atmosphere, where it contributes to climate change as a greenhouse gas. However, not all hydrate boundaries exhibit distinct methane seepage patterns. Beyond seepage and regeneration, other types of methane migration and accumulation patterns near continental hydrate boundaries should be explored. This information can be used to further analyze whether these other methane fates are safer or more severe than vertical seepage at the boundaries, both for environmental and climatic impacts. Summary of the Invention

[0008] In response to the problems existing in the prior art, the present invention provides a method and system for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates.

[0009] The present invention is achieved by providing a method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, the method specifically comprising:

[0010] S1: Select the LLGHSZ, a landward distribution boundary of natural gas hydrates visible in seismic data, and perform seismic interpretation and attribute analysis on the present-day BSR;

[0011] S2: If there is a paleo-BSR below the present-day BSR, perform seismic interpretation and attribute extraction on the paleo-BSR to understand its development location;

[0012] S3: If the seismic characteristics of the paleo-BSR are not obvious, the paleo-BSR can be predicted by numerical simulation and the development depth of the paleo-BSR can be calculated by selecting a hydrate phase equilibrium formula suitable for the study area;

[0013] S4: Research the data and literature of the study area to obtain the sea level change data of the work area. Combined with the current seabed depth, calculate the hydrostatic pressure value at the ancient BSR at that time to obtain the sea level change and bottom water temperature data of the work area;

[0014] S5: By adjusting parameters such as thermal conductivity and geothermal gradient, the location of the paleo-BSR can be simulated and predicted, and the location of the landward distribution boundary of the paleo-hydrate can be determined;

[0015] S6: Combining the seismic interpretation and numerical simulation prediction results of ancient and modern BSR, the study area is divided into three main areas: the main area of ​​the hydrate stability zone, the landward distribution boundary of the hydrate, and the shallow non-hydrate stability zone at the boundary;

[0016] S7: Conduct seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are submarine seepage features near the landward boundary of the hydrate. By analyzing the reflection anomalies of the strata between the ancient and modern BSRs, interpret possible vertical and lateral fluid migration pathways and free gas accumulation.

[0017] S8: Use coherence volumes and dip-type attributes to characterize reflection breaks and discontinuities in seismic data;

[0018] S9: Determine the vertical / lateral migration characteristics and pathways of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain beyond the boundary, as well as the accumulation and destruction of free gas along the migration pathways. Specifically evaluate the fate of methane after hydrate decomposition in the main area of ​​the stability domain.

[0019] S10: Determine the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the study area, determine the fate of methane at the boundary and analyze its main controlling factors;

[0020] S11: Based on the ancient and present BSR positions, the volume of the hydrate decomposition zone in the landward distribution boundary of hydrates can be calculated;

[0021] S12: Summarize the main factors controlling the different methane fates at the landward boundary of hydrates.

[0022] Furthermore, in S1, the work area with the intersection characteristics of BSR and seabed is selected as the study area, and seismic interpretation and attribute analysis are performed on the present BSR. Its seismic characteristics are strong amplitude, negative polarity, and oblique cutting of isochronous strata, so as to grasp the position of the present BSR.

[0023] Furthermore, the S3, natural gas hydrate phase equilibrium formula can be selected according to parameters such as gas composition, pore water salinity, sediment pore size, etc. of a specific work area.

[0024] Furthermore, S4 investigates the data and literature of the study area to obtain the sea level change data of the work area, and calculates the hydrostatic pressure value at the ancient BSR at that time in combination with the current seabed depth:

[0025] P=ρsw*g*(Dold-sb+Dbsr) (3)

[0026] Where Dbsr is the depth from the ancient seabed to the ancient BSR, ρsw is the seawater density, ρsw = 1028 kg / m3, and g is the acceleration of gravity, g = 9.81 m / s2. The temperature at the bottom boundary of the gas hydrate stability domain can be calculated by different methods. One method is to use the bottom water temperature and the equilibrium, stable one-dimensional thermal diffusion coefficient (such as 10-6 m2 s-1 in the Mauritanian waters) and the geothermal gradient to calculate the temperature of the sedimentary stratum. Alternatively, it can be calculated using the two-dimensional heat diffusion conduction model (4):

[0027]

[0028] Among them, ρb, Cb, Kx and Kz are the sediment density, specific heat capacity and conductivity, respectively. By determining the temperature distribution data in the sediment, the hydrate stability bottom boundary in a certain geological history period, that is, the development position of the ancient BSR, can be predicted based on the hydrate phase equilibrium formula.

[0029] Furthermore, the S7, by analyzing properties such as the dip angle of the seabed, can characterize the characteristics of methane seepage, such as pits and carbonate reefs developed at the hydrate boundary. The plane map of the RMS amplitude properties of the strata between the ancient and modern BSR can reveal the distribution of fluid migration paths in the hydrate decomposition zone and the accumulation of free gas.

[0030] Furthermore, the S8 utilizes coherence volume and dip attributes to characterize reflection interruptions and discontinuous structures in seismic data, such as faults, gas chimneys, and carbonate units.

[0031] Furthermore, the S10 determines the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the sea area of ​​the study area: the dynamic migration of the hydrate stability domain provides decomposition and release of methane, faults and gas chimneys and other channels provide vertical migration channels, the lithologic and stratigraphic spatial configuration provides long-distance lateral migration paths, and the shallow stratigraphic and lithologic traps at the landward boundary of the hydrate provide methane accumulation conditions; combined with the spatial matching relationship of various elements such as the hydrate decomposition zone, methane migration path, free gas accumulation and leakage, the methane migration, accumulation or leakage pattern at the landward distribution boundary of the hydrate in the study area is established, the fate of the methane at the boundary is determined, and its main controlling factors are analyzed.

[0032] Furthermore, the S11, combined with the previous research results on parameters such as the porosity and hydrate saturation of the hydrate reservoir in the study area, preliminarily calculated the amount of methane decomposed within the fluid migration path of the area and estimated the amount of methane leaked or accumulated in different destinations. The above results are used to guide the quantitative evaluation of the climate-hydrate interaction relationship at the hydrate distribution boundary and the destructive effect on the hydrate reservoir there.

[0033] Furthermore, the S12 compares the methane migration and accumulation pattern at the landward boundary of the Mauritanian Sea in West Africa with the large-scale submarine methane leakage pattern at other landward boundaries of hydrates (such as the northern Atlantic continental margin of the United States and the Svalbard Sea in the polar region), analyzes the main controlling factors of different methane fates at the landward boundary of hydrates, and conducts a summary analysis from the perspectives of gas source, migration momentum, migration pathways, and based on factors such as the differences in hydrate saturation in different regions and different layers, and the differences in stratigraphic lithology (such as whether water channel sand bodies with good permeability are developed laterally), to clarify the different methane fate types at the landward distribution boundary of hydrates and their main controlling factors.

[0034] Another object of the present invention is to provide a system for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, the system specifically comprising:

[0035] Input module, used to obtain 2D and 3D seismic data of the landward distribution boundary of hydrate in the developed sea area;

[0036] Numerical simulation module, used for numerical simulation of the lower bound of hydrate stability region;

[0037] Attribute analysis module, used for interpretation and attribute analysis of current BSR earthquakes;

[0038] Ancient BSR position prediction module, used to predict the ancient BSR position;

[0039] The regional division module is used to combine the seismic interpretation of ancient and modern BSR and the numerical simulation prediction results to divide the study area into three main areas: the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary;

[0040] The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone long-distance lateral migration and to establish migration, accumulation, and leakage patterns in different regions;

[0041] The methane amount calculation module is used to preliminarily calculate the amount of methane from hydrate decomposition at the landward boundary and the amount of methane that has migrated laterally to the boundary;

[0042] Output module, used to determine the fate of methane at the boundary and analyze its main controlling factors.

[0043] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0044] First, this invention provides a method for studying the fate of methane at marine natural gas hydrate distribution boundaries. This cutting-edge research describes the fate of methane at hydrate distribution boundaries, in addition to seepage, when geological processes cause the hydrate stability bottom boundary to migrate upward. This method is of great significance for comprehensively and objectively understanding the environmental and climatic impacts of hydrate systems. Following the research plan of "determining the dynamic migration process of the hydrate stability zone - characterizing and analyzing the migration, accumulation, and seepage patterns of methane in the hydrate decomposition zone - summarizing the carbon migration characteristics of the hydrate distribution boundary - and analyzing the key factors controlling the different methane fates at the hydrate distribution boundary," this method reconstructs the dynamic migration process of the hydrate stability zone in a specific study area since the last glacial period and under the background of global warming, as well as the vertical and lateral migration, accumulation, and seepage patterns of decomposed methane. This method identifies the key factors and mechanisms controlling the different methane fates at the hydrate distribution boundary, guiding accurate assessment of the environmental impacts of different methane fates and their destructive effects on hydrate reservoirs.

[0045] Second, the technical solution of the present invention fills the technical gap in the industry at home and abroad:

[0046] While the continental boundaries of hydrates have become a hot topic in hydrate research, most studies have focused primarily on large-scale seepage patterns, such as the numerous pits and gas plumes distributed along these boundaries. Currently, known methane fates at hydrate boundaries include regeneration of natural gas hydrates in shallow sediments; seepage into shallow seafloor sediments or seawater, where anoxic or oxygen-consuming oxidation produces carbon dioxide; formation of chemoautotrophic communities and carbonate precipitation on the seafloor; and emission into the atmosphere, where it contributes to climate change as a greenhouse gas. However, not all hydrate boundaries exhibit clear methane seepage patterns, and the question of whether other methane fates exist besides vertical seepage has yet to be fully explored. To address this scientific question, the technical solutions provided by the present invention provide additional insights into the potential for methane migration and accumulation near the continental boundaries of hydrates, in addition to seepage and regeneration. The technical solution provided by the present invention can guide further analysis of whether other types of methane fate are safer or have more serious consequences for the environment and climate compared with vertical methane leakage at the boundary. It has important guiding significance for a comprehensive and objective understanding of the environmental and climate impacts of hydrate systems.

[0047] The technical solution of the present invention solves the technical problems that people have been eager to solve but have never been able to solve successfully:

[0048] Not all hydrate boundaries show clear signs of methane seepage, leading to a lack of in-depth research into whether other methane fates exist besides vertical seepage. The technical solution of this invention addresses this question to some extent, and the research approach can also be applied to the study of fate types in other marine hydrate development zones.

[0049] Third, the present invention addresses the distribution of natural gas hydrates in marine areas and the fate of their methane, resolving the technical difficulties of the prior art in accurately delineating the boundaries of hydrate stability zones and predicting the fate of methane migration and accumulation. Prior art methods suffer from insufficient precision in identifying the landward distribution boundaries of hydrates, particularly in detecting methane migration mechanisms and accumulation phenomena, where clear seismic signals and model support are difficult to obtain. This impacts the scientific development of hydrate resources and environmental impact assessments. The present invention significantly improves the accuracy and depth of hydrate distribution boundary identification by improving seismic interpretation methods and combining numerical simulation with multi-attribute analysis.

[0050] The present invention also represents a technological advancement by accurately delineating the main body and landward boundary of the hydrate stability domain through a comprehensive analysis of the present and ancient positions of bottom boundary reflections (BSRs). This addresses the traditional inability to dynamically predict changes in the hydrate stability boundary with factors such as sea level and temperature. Through numerical simulation and historical data analysis, the present invention accurately reconstructs hydrate stability zones during different geological periods, including the Last Glacial Maximum (LGM). This provides a scientific basis for the dynamic assessment of hydrate resources and helps predict future hydrate resource distribution trends under changing environmental conditions.

[0051] Furthermore, this invention significantly enhances our understanding of methane migration pathways and accumulation mechanisms. Addressing the existing problem of unclear identification of vertical and lateral fluid migration pathways, this invention employs methods such as coherence volume and dip attribute analysis to characterize seismic reflection interruptions and discontinuities, enabling accurate identification of the vertical and lateral migration pathways of methane following hydrate decomposition. This technological advancement provides an important basis for assessing the environmental risk of methane entering the water column following hydrate decomposition, contributing to research on the marine methane cycle and marine environmental protection.

[0052] Finally, the present invention provides a systematic methane fate analysis process for industrial applications, which can effectively evaluate the release, migration, and environmental impact of methane in different hydrate distribution areas. Traditional methods have limitations in methane fate assessment, making it difficult to clearly determine the final destination of methane and its impact on marine ecosystems. Through comprehensive data analysis and model prediction, the present invention systematically reveals the various fate pathways of methane after decomposition in the stable domain and their main controlling factors, providing important technical support and guidance for the safe development and environmental assessment of natural gas hydrate resources, and has significant industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 (A) is a schematic diagram of the marine natural gas hydrate stability zone and distribution boundary provided by an embodiment of the present invention;

[0054] Figure 1 (B) is the intersection of the natural gas hydrate phase equilibrium curve and the geothermal gradient curve provided by the embodiment of the present invention, which is the BSR development location. The BSR depth becomes shallower as the seabed depth decreases until it intersects with the seabed at point A.

[0055] Figure 1 (C) is a seismic profile provided by an embodiment of the present invention showing the BSR distribution characteristics. The BSR depth becomes shallower as the seabed depth decreases until it intersects with the seabed at point A.

[0056] Figure 2 (AB) is a plane map of the RMS amplitude attributes of the R1 and R2 layers in the Mauritanian waters of West Africa provided by an embodiment of the present invention, showing features such as hydrate distribution boundaries, high amplitude anomalies (HAAs), and gas migration paths;

[0057] Figure 2 (C) is a seismic profile provided by an embodiment of the present invention showing that the hydrate distribution boundary is a shallow high-amplitude free gas accumulation, and its updip direction is blocked by carbonate rocks with chaotic reflection characteristics;

[0058] Figure 2 (D) is a three-dimensional combination of the RMS amplitude attribute map, Chaos attribute plane map, and seismic profile of the R1 layer provided by an embodiment of the present invention, showing the spatial matching relationship between the hydrate distribution boundary, free gas accumulation, and carbonate rock;

[0059] Figure 3 This is a flow chart of a method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates provided by an embodiment of the present invention;

[0060] Figure 4 This is a technical roadmap for a method of studying the fate of methane at the landward distribution boundary of marine natural gas hydrates provided by an embodiment of the present invention;

[0061] Figure 5 is a schematic diagram of a natural gas system at a continental margin provided by an embodiment of the present invention, showing the main area of ​​the hydrate stability domain, the landward boundary area of ​​the hydrate, and the non-hydrate stability domain;

[0062] Figure 6 This is a system module diagram for studying the fate of methane at the landward distribution boundary of natural gas hydrates in the sea area provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0063] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0064] like Figure 3 、 Figure 4 As shown, an embodiment of the present invention provides a method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, the method specifically comprising:

[0065] S1: Select the LLGHSZ, a landward distribution boundary of natural gas hydrates visible in seismic data, and perform seismic interpretation and attribute analysis on the present-day BSR;

[0066] S2: If there is a paleo-BSR (also known as relict BSR) below the present-day BSR, perform seismic interpretation and attribute extraction on the paleo-BSR to understand its development location;

[0067] S3: If the seismic characteristics of the paleo-BSR are not obvious, the paleo-BSR (such as the position of the bottom boundary of the hydrate stability zone during the last glacial maximum (LGM)) can be predicted through numerical simulation. By selecting a hydrate phase equilibrium formula suitable for the study area, the development depth of the paleo-BSR can be calculated.

[0068] S4: Research the data and literature of the study area to obtain the sea level change data of the work area. Combined with the current seabed depth, calculate the hydrostatic pressure value at the ancient BSR at that time to obtain the sea level change and bottom water temperature data of the work area;

[0069] S5: By adjusting parameters such as thermal conductivity and geothermal gradient, the location of the paleo-BSR can be simulated and predicted, and the location of the paleo-hydrate landward distribution boundary (i.e., the paleo-LLGHSZ) can be determined.

[0070] S6: Combining the seismic interpretation and numerical simulation prediction results of ancient and modern BSR, the study area is divided into three main areas: the main area of ​​the hydrate stability zone, the landward distribution boundary of the hydrate, and the shallow non-hydrate stability zone at the boundary;

[0071] S7: Conduct seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are submarine seepage features near the landward boundary of the hydrate. By analyzing the reflection anomalies of the strata between the ancient and modern BSRs, interpret possible vertical and lateral fluid migration pathways and free gas accumulation.

[0072] S8: Use coherence volumes and dip-type attributes to characterize reflection breaks and discontinuities in seismic data;

[0073] S9: Determine the vertical / lateral migration characteristics and pathways of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain beyond the boundary, as well as the accumulation and destruction of free gas along the migration pathways. Specifically evaluate the fate of methane after hydrate decomposition in the main area of ​​the stability domain.

[0074] S10: Determine the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the study area, determine the fate of methane at the boundary and analyze its main controlling factors;

[0075] S11: Based on the ancient and present BSR positions, the volume of the hydrate decomposition zone in the landward distribution boundary of hydrates can be calculated;

[0076] S12: Summarize the main factors controlling the different methane fates at the landward boundary of hydrates.

[0077] An embodiment of the present invention relates to a method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, which mainly reveals the migration process of methane in the hydrate stability zone by interpreting and analyzing current and ancient BSR (bottom boundary reflection interface) and related seismic data.

[0078] First, this method identifies the landward distribution boundary (LLGHSZ) of the natural gas hydrate zone in the study area. Seismic interpretation and attribute analysis of the present-day BSR are performed to determine the BSR's specific location and related parameters. If a residual BSR exists beneath the present-day BSR, further seismic attribute extraction is performed to understand the ancient BSR's location in the area. If the paleo-BSR is not clearly characterized, numerical simulation tools can be used to predict the depth of the paleo-BSR during the last glacial period (LGM). Combined with the hydrate phase equilibrium formula, the bottom boundary of the stability zone can be calculated to determine the paleo-BSR's depth.

[0079] Secondly, by researching historical data and literature in the study area, we obtained data on sea level changes and seafloor depths in the work area, calculated the hydrostatic pressure during the ancient BSR, and inferred the impact of sea level changes and bottom water temperature during this period. Next, based on parameters such as geothermal gradient and thermal conductivity, we further predicted the location of the ancient BSR through numerical simulations. This allowed us to delineate the main hydrate stability zone, the landward distribution boundary, and the unstable zone shallower than the boundary, providing reference areas for the fate of methane.

[0080] On this basis, seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the intervening reflectors were conducted to identify submarine seepage characteristics near the landward boundary of hydrates, analyze reflection anomalies between ancient and modern BSRs, and explore potential vertical and lateral fluid migration pathways and free gas accumulation. Coherence volumes and dip-based seismic attributes were then used to delineate reflection discontinuities and structures, providing support for further understanding of fluid migration pathways.

[0081] Finally, by comprehensively analyzing the above data, the fluid migration paths in different regions of the study area and the free gas accumulation phenomenon on the migration paths are determined, and the fate of methane after hydrate decomposition is specifically evaluated. The volume of the hydrate decomposition zone is calculated by combining the ancient and modern BSR positions, and the main controlling factors of methane fate in different regions are summarized, so as to completely reveal the methane migration and accumulation mechanism and fate characteristics at the landward boundary during the dynamic migration of marine gas hydrates.

[0082] For S1, select the work area with the intersection characteristics of BSR and the seabed as the study area, conduct seismic interpretation and attribute analysis on the current BSR. Its seismic characteristics are strong amplitude, negative polarity, and oblique cutting of isochronous strata, and master the position of the current BSR.

[0083] For S3, the gas hydrate phase equilibrium formula can be selected according to parameters such as gas composition, pore water salinity, and sediment pore size in the specific work area. Lu & Sultan (2008) established a series of hydrate phase equilibrium formulas for different systems based on the gas hydrate thermodynamic model. For example, for the pure methane and brine system, the hydrate phase equilibrium formula is shown in (1);

[0084] P(T,S) = exp((Cs·S + Ds)·T)·exp(Es·S)·Fs (1)

[0085] where, P is the pressure (kPa), T is the temperature (K), S is the pore water salinity (0 < S ≤ 0.050 mol NaCl / mol H2O), the Cs coefficient is 0.1711726397, the Ds coefficient is 0.1711726397, the Es coefficient is -34.14836102, and the Fs coefficient is 1.010769956×10-9.

[0086] For the pure methane, brine and sediment pore size system, the hydrate phase equilibrium formula is shown in (2).

[0087]

[0088] where, P is the pressure (kPa), T is the temperature (K), rp is the pore radius (6×10-9—1×10-6), the Ra coefficient is 3.053808015×10-11, the Rb coefficient is 0.1065001844, the Rc coefficient is -2.490775613×10-18, and the Rd coefficient is 0.6834013617×10-9.

[0089] For S4, investigate the data and literature of the study area, obtain the sea level change data of the work area, and combine the current seabed depth to calculate the hydrostatic pressure value at the ancient BSR at that time:

[0090] P=ρsw*g*(Dold-sb+Dbsr) (3)

[0091] Where Dbsr is the depth from the ancient seabed to the ancient BSR, ρsw is the seawater density, ρsw = 1028 kg / m3, and g is the acceleration of gravity, g = 9.81 m / s2;

[0092] Research the data and literature in the study area to obtain data on sea level changes and bottom water temperature in the work area. The temperature at the bottom boundary of the natural gas hydrate stability zone can be calculated by different methods. One method is to use the bottom water temperature and the equilibrium, stable one-dimensional thermal diffusion coefficient (such as 10-6m2 s-1 in the Mauritanian waters) and the geothermal gradient to calculate the temperature of the sedimentary strata. Alternatively, it can be calculated using the two-dimensional heat diffusion conduction model (4):

[0093]

[0094] Where ρb, Cb, Kx, and Kz represent the sediment density, specific heat capacity, and conductivity, respectively. Once the temperature distribution data within the sediment is determined, the hydrate stability base, or the location of the paleo-BSR, can be predicted based on the hydrate phase equilibrium equation at a specific geological period.

[0095] The S6 is divided into three main areas: the main area of ​​the hydrate stability domain, the landward distribution boundary area of ​​hydrate, and the shallow non-hydrate stability area at the boundary, as shown in Figure 2. Figure 5 shown.

[0096] The S7, by analyzing properties such as seafloor inclination, can characterize the characteristics of methane seepage, such as pits and carbonate reefs developed at the hydrate boundary. The plane map of the RMS amplitude properties of the strata between ancient and modern BSRs can reveal the distribution of fluid migration paths in the hydrate decomposition zone and the accumulation of free gas.

[0097] The S8 uses coherence volume and dip attributes to characterize reflection interruptions and discontinuous structures in seismic data, such as faults, gas chimneys, and carbonate units.

[0098] The S10 is to determine the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the sea area of ​​the study area: the dynamic migration of the hydrate stability domain provides decomposition and release of methane, faults and gas chimneys and other channels provide vertical migration channels, the lithologic and stratigraphic spatial configuration provides long-distance lateral migration paths, and the shallow stratigraphic and lithologic traps at the landward boundary of the hydrate provide methane accumulation conditions; combined with the spatial matching relationship of various elements such as the hydrate decomposition zone, methane migration path, free gas accumulation and leakage, the methane migration, accumulation or leakage pattern at the landward distribution boundary of the hydrate in the study area is established, the fate of methane at the boundary is determined, and its main controlling factors are analyzed.

[0099] The S11 study, combined with previous research on hydrate reservoir porosity, hydrate saturation, and other parameters in the study area, initially calculated the amount of methane decomposed within the fluid migration pathways in the study area and estimated the amount of methane leaking or accumulating in different fates. These results were used to guide the quantitative evaluation of climate-hydrate interactions at hydrate distribution boundaries and their destructive effects on hydrate reservoirs in these locations.

[0100] The S12 described above compares the methane migration and accumulation pattern at the landward boundary of the Mauritanian Sea in West Africa with the large-scale submarine methane leakage pattern at other landward boundaries of hydrates (such as the northern Atlantic continental margin of the United States and the Svalbard Sea in the polar region), analyzes the main controlling factors of different methane fates at the landward boundary of hydrates, and conducts a summary analysis from the perspectives of gas source, migration dynamics, migration pathways, etc., according to factors such as the differences in hydrate saturation in different regions and different layers, and the differences in stratigraphic lithology (such as whether there are lateral water channel sand bodies with good permeability), to clarify the different methane fate types at the landward distribution boundary of hydrates and their main controlling factors.

[0101] like Figure 6 As shown, an embodiment of the present invention provides a system for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, specifically comprising:

[0102] Input module, used to obtain 2D and 3D seismic data of the landward distribution boundary of hydrate in the developed sea area;

[0103] Numerical simulation module, used for numerical simulation of the lower bound of hydrate stability region;

[0104] Attribute analysis module, used for interpretation and attribute analysis of current BSR earthquakes;

[0105] Ancient BSR position prediction module, used to predict the ancient BSR position;

[0106] The regional division module is used to combine the seismic interpretation of ancient and modern BSR and the numerical simulation prediction results to divide the study area into three main areas: the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary;

[0107] The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone long-distance lateral migration and to establish migration, accumulation, and leakage patterns in different regions;

[0108] The methane amount calculation module is used to preliminarily calculate the amount of methane from hydrate decomposition at the landward boundary and the amount of methane that has migrated laterally to the boundary;

[0109] Output module, used to determine the fate of methane at the boundary and analyze its main controlling factors.

[0110] The present invention is implemented in the waters of Mauritania, West Africa, such as Figure 2 As shown, no obvious methane seepage is observed at the gas hydrate distribution boundary in the study area. Instead, high-amplitude anomalies, representing free gas accumulation, are observed in the non-hydrate stability zone shallower than the landward boundary of the gas hydrate distribution. These anomalies are blocked updip by carbonate rocks with chaotic reflectivity. This phenomenon represents an alternative fate at the hydrate distribution boundary, distinct from large-scale seepage: lateral methane migration. This occurs when methane from hydrate decomposition migrates laterally and updip along permeable sandstone layers (which serve as fluid pathways) until it encounters a stratigraphic trap blocked by carbonate rocks. There, the methane accumulates in the shallow stratigraphic trap of the non-hydrate stability zone. Ancient pits buried below the seafloor are visible above some of the shallow free gas reservoirs, likely representing methane leakage caused by later overpressure. This pattern indicates that methane at the hydrate stability zone boundary can further migrate laterally to shallower depths, delaying direct seepage to the seafloor or even seawater at the boundary and migrating the methane to shallower depths. Previous studies have shown that methane is more likely to escape into the atmosphere when it seeps in water depths less than 150 m. At depths greater than 150 m, methane is consumed in the seawater and cannot enter the atmosphere to impact climate (McGinnis et al., 2006). Therefore, this methane fate may be more likely to cause methane to enter the atmosphere than methane seeps at the boundaries of natural gas hydrate distribution, potentially leading to more severe climate impacts.

[0111] Mcginnis DF, Greinert J, Artemov Y, Beaubien SE, Wüest A. 2006, Fateofrising methane bubbles in stratified waters: How much methane reaches the atmosphere? Journal of Geophysical Research:Oceans,111(C9).

[0112] It should be noted that the embodiments of the present invention can be implemented by hardware, software, or a combination of software and hardware. The hardware portion can be implemented using dedicated logic; the software portion can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. Those skilled in the art will appreciate that the above-mentioned devices and methods can be implemented using computer-executable instructions and / or contained in processor control code, for example, such as a carrier medium such as a disk, CD or DVD-ROM, a programmable memory such as a read-only memory (firmware), or a data carrier such as an optical or electronic signal carrier. The devices and modules of the present invention can be implemented by hardware circuits such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., can also be implemented by software executed by various types of processors, or can be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

[0113] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with this technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates, characterized in that: The method specifically includes: S1: Select the LLGHSZ, a landward distribution boundary of natural gas hydrates visible in seismic data, and perform seismic interpretation and attribute analysis on the present-day BSR; S2: If there is a paleo-BSR below the present-day BSR, perform seismic interpretation and attribute extraction on the paleo-BSR to understand its development location; S3: If the seismic characteristics of the paleo-BSR are not obvious, the paleo-BSR can be predicted by numerical simulation and the development depth of the paleo-BSR can be calculated by selecting a hydrate phase equilibrium formula suitable for the study area; S4: Obtain sea level change data for the work area, combine it with the current seabed depth, calculate the hydrostatic pressure value at the ancient BSR at that time, and obtain sea level change and bottom water temperature data for the work area; S5: By adjusting the thermal conductivity and geothermal gradient parameters, the paleo-BSR location is simulated and predicted, thereby determining the location of the landward distribution boundary of the paleohydrate. S6: Combining the seismic interpretation and numerical simulation prediction results of ancient and modern BSR, the study area is divided into three main areas: the main area of ​​the hydrate stability zone, the landward distribution boundary of the hydrate, and the shallow non-hydrate stability zone at the boundary; S7: Conduct seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are submarine seepage features near the landward boundary of the hydrate. By analyzing the reflection anomalies of the strata between the ancient and modern BSRs, interpret possible vertical and lateral fluid migration pathways and free gas accumulation. S8: Use coherence volumes and dip-type attributes to characterize reflection breaks and discontinuities in seismic data; S9: Determine the vertical / lateral migration characteristics and pathways of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain beyond the boundary in the study area, as well as the accumulation and destruction of free gas along the migration pathways, and specifically evaluate the fate of methane after hydrate decomposition in the main area of ​​the stability domain; S10: Determine the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the study area, determine the fate of methane at the boundary and analyze its main controlling factors; S11: Calculate the volume of the hydrate decomposition zone at the landward distribution boundary of hydrates based on the ancient and present BSR positions; S12: Summarize the main factors controlling the different methane fates at the landward boundary of hydrates.

2. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: In S1, the work area with the intersection characteristics of BSR and seabed is selected as the study area, and seismic interpretation and attribute analysis are carried out on the present BSR. Its seismic characteristics are strong amplitude, negative polarity, and oblique cutting of isochronous strata, so as to grasp the position of the present BSR.

3. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S3, natural gas hydrate phase equilibrium formula is selected based on the gas composition, pore water salinity, and sediment pore size parameters of the specific work area.

4. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: S4, obtains the sea level change data of the work area, combines it with the current seabed depth, and calculates the hydrostatic pressure value at the ancient BSR at that time: P = ρsw *g *(Dold-sb+Dbsr) (3) Where Dbsr is the depth from the ancient seabed to the ancient BSR, ρsw is the seawater density, ρsw = 1028 kg / m 3 , g is the acceleration due to gravity, g=9.81 m / s 2 The temperature at the bottom boundary of the gas hydrate stability domain is obtained by the following two methods: one is to calculate the temperature of the sedimentary formation using the bottom water temperature and the equilibrium, stable one-dimensional thermal diffusion coefficient and geothermal gradient; the other is to calculate using the two-dimensional heat diffusion conduction model (4): （4） in, b, Cb, Kx and Kz are the sediment density, specific heat capacity and conductivity, respectively. The temperature distribution data in the sediment was determined, and the hydrate stability bottom boundary, that is, the development position of the ancient BSR, in a certain geological history period was predicted based on the hydrate phase equilibrium formula.

5. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S7, through the dip attribute analysis of the seabed, depicts the characteristics of methane seepage reflected by the pits and carbonate reefs developed at the hydrate boundary. The RMS amplitude attribute plane map of the strata between the ancient and modern BSR reveals the distribution of fluid migration paths in the hydrate decomposition zone and the accumulation of free gas.

6. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S8 uses coherence volume and dip attributes to characterize reflection interruptions and discontinuities in seismic data, including faults, gas chimneys, and carbonate units.

7. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S10 is to determine the methane migration and accumulation process and mechanism at the landward boundary during the dynamic migration of natural gas hydrates in the sea area of ​​the study area: the dynamic migration of the hydrate stability domain provides methane released by decomposition, faults and gas chimney channels provide vertical migration channels, the lithologic and stratigraphic spatial configuration provides long-distance lateral migration paths, and the shallow stratigraphic and lithologic traps at the landward boundary of the hydrate provide methane accumulation conditions; combined with the spatial matching relationship between the hydrate decomposition zone, methane migration path, free gas accumulation and leakage elements, the methane migration, accumulation or leakage pattern at the landward distribution boundary of the hydrate in the study area is established, the fate of the methane at the boundary is determined, and its main controlling factors are analyzed.

8. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S11 preliminary calculation study is to study the amount of methane decomposed within the fluid migration path of the region and estimate the amount of methane leaking or accumulating in different destinations. The above results are used to guide the quantitative evaluation of the climate-hydrate interaction relationship at the hydrate distribution boundary and the destructive effect on the hydrate reservoir there.

9. The method for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates according to claim 1, characterized in that: The S12 compares the methane migration and accumulation patterns at the landward boundary of the Mauritanian waters in West Africa with the large-scale submarine methane leakage patterns at other landward boundaries of hydrates, analyzes the main controlling factors of different methane fates at the landward boundary of hydrates, and conducts a summary analysis from the perspectives of gas source, migration momentum, and migration pathways, based on the differences in hydrate saturation and stratigraphic lithology in different regions and layers, to clarify the different methane fate types at the landward distribution boundary of hydrates and their main controlling factors.

10. A system for studying the fate of methane at the landward distribution boundary of marine natural gas hydrates based on claims 1-9, characterized in that: The system specifically includes: Input module, used to obtain 2D and 3D seismic data of the landward distribution boundary of hydrate in the developed sea area; Numerical simulation module, used for numerical simulation of the lower bound of hydrate stability region; Attribute analysis module, used for interpretation and attribute analysis of current BSR earthquakes; Ancient BSR position prediction module, used to predict the ancient BSR position; The regional division module is used to combine the seismic interpretation of ancient and modern BSR and the numerical simulation prediction results to divide the study area into three main areas: the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary; The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone long-distance lateral migration and to establish migration, accumulation, and leakage patterns in different regions; The methane amount calculation module is used to preliminarily calculate the amount of methane from hydrate decomposition at the landward boundary and the amount of methane that has migrated laterally to the boundary; Output module, used to determine the fate of methane at the boundary and analyze its main controlling factors.

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