Method and system for studying methane homing at land distribution boundary of natural gas hydrate in sea area

Abstract

The invention belongs to the field of natural gas hydrate terrestrial distribution boundary research, and discloses a method and system for researching methane attribution at a sea area natural gas hydrate terrestrial distribution boundary, and the method comprises the steps: carrying out the seismic interpretation and attribute analysis of a current BSR; seismic interpretation and attribute extraction are carried out on the ancient BSR, and the development depth of the ancient BSR is calculated; calculating a hydrostatic pressure value at the ancient BSR at the moment; simulating and predicting the position of the paleo-BSR, and determining the position of a paleo-hydrate landwise distribution boundary; dividing a main area of the research area; identifying whether seabed leakage characteristics are developed near a hydrate land boundary, and explaining possible vertical and lateral fluid migration channels and free gas aggregation; determining boundaries of methane and analyzing main control factors of the boundaries; according to ancient and modern BSR positions, the hydrate decomposition area volume of the hydrate land distribution boundary area can be calculated. According to the method, the multi-type methane homing at the natural gas hydrate distribution boundary of the sea area is mastered, so that people can correctly know the environmental influence of different methane homing and the effect on the hydrate reservoir.

Description

Technical Field

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

[0002] Natural gas hydrates are widely distributed around the world and are considered to be a clean alternative energy source in the future. More than 90% of them exist in the marine environment. According to expert estimates, the methane resources in natural gas hydrates in sediments within a water depth of less than 3,000 meters in the marginal seas, deep-sea troughs and ocean basins around the world are currently 2.1 trillion cubic meters, which can meet human needs for 1,000 years. Ruppel and Kessler (2017) predicted that the carbon reserves in hydrates are estimated to be 1,800 Gt C. The "China Mineral Resources Report 2018" preliminarily predicts that the amount of natural gas hydrate resources in my country's sea areas is about 80 billion tons of oil equivalent, which is close to my country's conventional oil resources and about twice the amount of conventional natural gas resources. At the current consumption level, it can 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 environment, the gas hydrate stability zone is a wedge-shaped body that is defined by the interface between the seafloor and the bottom simulating reflection layer (BSR), and thins toward the land. In seismic data, the intersection of the BSR and the seafloor is the landward limit of the GHSZ—Gas Hydrate Stability Zone (LLGHSZ). Figure 1 ). The landward distribution boundary of hydrates is more susceptible to frequent hydrate formation and decomposition due to its shallow burial depth and lack of overlying thick sedimentary strata, which is affected by ocean conditions such as climate and submarine currents. 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 the methane decomposed in the main area of ​​the hydrate stability domain can be transported to the boundary, thereby effectively supplementing the source of methane at the landward distribution boundary of hydrates. At present, 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 free methane at the landward distribution boundary of hydrates.

[0004] Under the background of global warming, the hydrate stability domain retreats to the sea, and the hydrate decomposition is the main feature at the landward boundary of the hydrate. At present, a large number of large-scale leakage features such as pits and gas plumes have been found near the landward boundary of the hydrate in different regions, such as the LLGHSZ. For example, 570 previously undiscovered gas plumes were found at a depth of 50-1700 mbsl on the Atlantic margin of the United States, of which about 440 are located near the present LLGHSZ, which may be related to the vertical dynamic migration of the gas hydrate stability domain when the seabed water temperature changes. Berndt et al. (2014) believed that the warming of the Arctic caused the decomposition of hydrates, resulting in a large amount of methane leakage near the LLGHSZ, which in turn accelerated global warming. Westbrook et al. (2009) also calculated the rising speed of gas plumes in the West Spitsbergen margin (near the Arctic). According to a rough estimate, the methane released by the decomposition of natural gas hydrates near the LLGHSZ can reach 20 Tg / year, which accounts for a large proportion of the total global carbon cycle each year. 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 hydrate 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 natural gas hydrate. 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 hydrate, and the updip direction is blocked by carbonate rocks with chaotic reflection characteristics. This phenomenon represents another fate 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 traps of the shallow non-hydrate stable domain ( Figure 2 ). Ancient pits below the seafloor can be seen above some shallow free gas reservoirs, which may be methane leakage caused by overpressure in the later period. This pattern indicates that methane at the boundary of the hydrate stability zone can further migrate laterally to the shallow part, delaying the direct leakage of methane to the seafloor or even seawater at the boundary. However, more details need to be analyzed, such as the lateral migration process, mechanism, main controlling factors of methane, and environmental impacts different from methane leakage.

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

[0007] Although the landward distribution boundary of hydrates has become a hot topic in hydrate research, most studies focus on the large-scale leakage characteristics such as the large number of pits and gas plumes distributed along the boundary. The known fate of methane at the hydrate distribution boundary in current research mainly includes the regeneration of natural gas hydrates in shallow sediments; leakage into shallow sediments or seawater on the seabed, and the generation of carbon dioxide through anoxic oxidation or oxygen-consuming oxidation; the formation of chemoautotrophic communities and precipitation of carbonate sediments on the seabed; and emission into the atmosphere as a greenhouse gas affecting climate change. However, not all hydrate boundaries have obvious methane leakage characteristics. In addition to leakage and regeneration of hydrates, whether there are other types of methane migration and accumulation patterns near the landward boundary of hydrates should be explored. On this basis, it is possible to further analyze whether other types of methane fate are safer or more serious than the vertical leakage of methane at the boundary. Summary of the invention

[0008] In view of 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 natural gas hydrate in the sea area.

[0009] The present invention is achieved by 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 gas hydrate visible in seismic data, and conduct seismic interpretation and attribute analysis on the present BSR;

[0011] S2: If there is an ancient BSR below the present BSR, conduct seismic interpretation and attribute extraction on the ancient BSR to understand the development position of the ancient BSR;

[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: Investigate the data and literature of the study area, obtain the data of sea level change in the work area, and calculate the hydrostatic pressure value at the ancient BSR at that time in combination with the current seabed depth, and obtain data such as sea level change and bottom water temperature in the work area;

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

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

[0016] S7: Carry out seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are seafloor leakage characteristics near the landward boundary of hydrates. By analyzing the reflection anomaly characteristics of the strata between ancient and modern BSRs, possible vertical and lateral fluid migration pathways and free gas accumulation can be explained;

[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 paths of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary in the study area, as well as the accumulation and destruction of free gas on the migration path, and 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 controlling factors of 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 the seismic interpretation and attribute analysis of the present BSR are carried out. The seismic characteristics are strong amplitude, negative polarity, oblique cutting and 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. in a specific work area.

[0024] Furthermore, S4 investigated the data and literature in the study area, obtained the sea level change data in the work area, and calculated 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, g is the gravitational acceleration, g = 9.81 m / s2; The temperature at the bottom boundary of the gas hydrate stability domain can be obtained by different methods. One is to use the bottom water temperature and the balanced, 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 strata; it can also be calculated using the two-dimensional thermal diffusion conduction model (4):

[0027]

[0028] Among them, ρb, Cb, Kx and Kz are sediment density, specific heat capacity and conductivity respectively. By determining the temperature distribution data in the sediment, the stable bottom boundary of hydrate 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 the attributes of the seabed such as 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 attributes of the strata between the ancient and modern BSRs can reveal the distribution of fluid migration paths in the hydrate decomposition zone and the accumulation of free gas.

[0030] Furthermore, 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.

[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 lithology and stratigraphic spatial configuration provide long-distance lateral migration paths, and the shallow stratigraphic and lithological closures 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.

[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 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.

[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 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 summary analysis from the perspectives of gas source, migration momentum, migration pathways, etc., according to 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), etc., 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] An input module is used to obtain two-dimensional and three-dimensional 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 limit of hydrate stability domain;

[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 and numerical simulation prediction results of ancient and modern BSR to divide the study area into three main areas: the main area of ​​hydrate stability, the landward distribution boundary area of ​​hydrate, and the shallow non-hydrate stability area at the boundary;

[0040] The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone lateral long-distance 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 migrates 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, the present invention provides a method for studying the fate of methane at the boundary of natural gas hydrate distribution in the sea area. This research content belongs to the frontier field, and it describes the fate of methane at the boundary of hydrate distribution other than leakage when the geological process causes the hydrate stable bottom boundary to migrate upward. It has important guiding significance for comprehensively and objectively understanding the environmental and climate impacts of the hydrate system. According to the research route of "determining the dynamic migration process of the hydrate stability domain - characterizing and analyzing the migration, accumulation and leakage patterns of methane in the hydrate decomposition zone - summarizing the carbon migration characteristics of the hydrate distribution boundary - analyzing the main controlling factors of different methane fates at the hydrate distribution boundary", the dynamic migration process of the hydrate stability domain in the specific research area since the last ice age and under the background of global warming, as well as the vertical and lateral migration, accumulation and leakage patterns of decomposed methane, are reconstructed, the main controlling factors and mechanisms of different methane fates at the hydrate distribution boundary are mastered, and accurate evaluation of the environmental impact of different methane fates and the destructive effects on hydrate reservoirs is guided.

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

[0046] Although the landward distribution boundary of hydrates has become a hot topic in hydrate research, most studies focus on the large-scale leakage characteristics such as a large number of pits and gas plumes distributed along the boundary. The known destinations of methane at the hydrate distribution boundary in current research mainly include the regeneration of natural gas hydrates in shallow sediments; leakage into shallow sediments or seawater on the seabed, and the generation of carbon dioxide through anoxic oxidation or oxygen-consuming oxidation; the formation of chemoautotrophic communities and precipitation of carbonate deposits on the seabed; and emission into the atmosphere as a greenhouse gas affecting climate change. However, not all hydrate boundaries have obvious methane leakage characteristics, so the question of "whether there are other destinations of methane besides vertical leakage" has not been studied in depth. In response to this scientific problem, the technical solution provided by the present invention supplements whether there are other types of methane migration and aggregation patterns near the landward boundary of hydrates besides leakage and regeneration of hydrates. The technical solution provided by the present invention can guide further analysis of whether the fate of other types of methane is safer or will cause more serious consequences on the environment and climate compared with vertical methane leakage at the boundary, and 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 have obvious methane leakage characteristics, so the question of "whether there are other methane destinations besides vertical leakage" has not been studied in depth. The technical solution of the present invention has provided a certain answer to this question, and the research idea can also be used to study the destination types of hydrate development areas in other sea areas.

[0049] Third, the present invention addresses the distribution of natural gas hydrates in the sea and the fate of their methane, and solves the technical difficulties in the prior art of being unable to accurately delineate the boundaries of the hydrate stability domain and predict the migration and accumulation fate of methane. In the prior art, the identification of the landward distribution boundary of hydrates suffers from the defect of insufficient accuracy, especially in the detection of methane migration mechanisms and aggregation phenomena, it is difficult to obtain clear seismic signals and model support, which affects the scientific development of hydrate resources and environmental impact assessment. The present invention significantly improves the accuracy and depth of hydrate distribution boundary identification by improving seismic interpretation methods and combining numerical simulation and multi-attribute analysis.

[0050] The technical progress of the present invention also lies in that, through comprehensive analysis of the current and ancient bottom boundary reflection (BSR) positions, the main area and landward boundary of the hydrate stability domain are accurately delineated, solving the problem that the traditional technology cannot dynamically predict the changes of the hydrate stability boundary with sea level, temperature and other factors. Through numerical simulation and historical data analysis, the present invention accurately reconstructs the hydrate stability area in different geological periods such as the last glacial period (LGM), provides a scientific basis for the dynamic evaluation of hydrate resources, and helps to predict the distribution trend of hydrate resources under future environmental changes.

[0051] In addition, the present invention significantly improves the understanding of methane migration paths and accumulation mechanisms. In view of the problem of unclear identification of vertical and lateral fluid migration paths in the prior art, the present invention uses coherence body, dip attribute analysis and other methods to characterize seismic reflection interruptions and discontinuous structures, so that the vertical and lateral migration paths of methane after hydrate decomposition can be accurately identified. This technological advancement provides an important basis for assessing the environmental risk of methane entering the water column after hydrate decomposition, which is helpful for marine methane cycle research and marine environmental protection.

[0052] Finally, the present invention provides a systematic methane fate analysis process in industrial applications, which can effectively evaluate the methane release, migration and environmental impact in different hydrate distribution areas. Traditional methods have limitations in methane fate assessment, and it is difficult to clarify the final destination of methane and its impact on the marine ecosystem. Through comprehensive data analysis and model prediction, the present invention systematically reveals the various fate paths and main controlling factors of methane after decomposition in the stable domain, 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 stable region and distribution boundary of marine natural gas hydrate 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 in the embodiment of the present invention as the BSR development position, and 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, where 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 diagram of the RMS amplitude attributes of the R1 and R2 layers in the Mauritanian Sea area of ​​West Africa provided by an embodiment of the present invention, showing features such as hydrate distribution boundaries, high amplitude anomalies (HAAs: High amplitude anomalies) 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 spatial combination of the RMS amplitude attribute map, Chaos attribute plane map and seismic section 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 natural gas hydrate in the sea area provided by an embodiment of the present invention;

[0060] Figure 4 It is a technical roadmap of a method for studying the fate of methane at the landward distribution boundary of natural gas hydrate in the sea area 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 a main area of ​​a hydrate stability domain, a landward boundary area of ​​a hydrate, and a non-hydrate stability domain;

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

[0063] In order to make the purpose, technical solution 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 used 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 natural gas hydrate in the sea area, and the method specifically includes:

[0065] S1: Select the LLGHSZ, a landward distribution boundary of gas hydrate visible in seismic data, and conduct seismic interpretation and attribute analysis on the present BSR;

[0066] S2: If there is an ancient BSR (also called residual BSR, Relict BSR) below the current BSR, conduct seismic interpretation and attribute extraction on the ancient BSR to understand the development position of the ancient BSR;

[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 domain during the last glacial maximum (LGM)) 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.

[0068] S4: Investigate the data and literature of the study area, obtain the data of sea level change in the work area, and calculate the hydrostatic pressure value at the ancient BSR at that time in combination with the current seabed depth, and obtain data such as sea level change and bottom water temperature in the work area;

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

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

[0071] S7: Carry out seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are seafloor leakage characteristics near the landward boundary of hydrates. By analyzing the reflection anomaly characteristics of the strata between ancient and modern BSRs, possible vertical and lateral fluid migration pathways and free gas accumulation can be explained;

[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 paths of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary in the study area, as well as the accumulation and destruction of free gas on the migration path, and 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 controlling factors of the different methane fates at the landward boundary of hydrates.

[0077] The 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 domain by interpreting and analyzing the present and ancient BSR (bottom boundary reflection interface) and related seismic data.

[0078] First, this method selects the landward distribution boundary of natural gas hydrates (LLGHSZ) in the study area, and performs seismic interpretation and attribute analysis on the present BSR to determine the specific location and related parameters of the BSR. If there is a residual BSR below the present BSR, further extract its seismic attributes to understand the development location of the ancient BSR in the area. If the characteristics of the ancient BSR are not obvious, the development depth of the ancient BSR during the last glacial period (LGM) can be predicted through numerical simulation tools, and the bottom boundary position of its stable domain can be calculated by combining the hydrate phase equilibrium formula to obtain the development depth information of the ancient BSR.

[0079] Secondly, by investigating the historical data and literature of the study area, we obtained the sea level change and seabed depth data of the work area, calculated the hydrostatic pressure value at the time of the ancient BSR, and thus inferred the influence of sea level change and bottom water temperature during that period. Then, based on parameters such as geothermal gradient and thermal conductivity, we further predicted the location of the ancient BSR through numerical simulation, and then divided the main area of ​​the hydrate stability domain, the landward distribution boundary area, and the unstable domain shallower than the boundary, providing a reference area for the fate of methane.

[0080] On this basis, seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs and the reflective layers between them are carried out to identify the seafloor leakage characteristics near the landward boundary of hydrates, analyze the reflection anomalies between ancient and modern BSRs, and explore potential vertical and lateral fluid migration channels and free gas accumulation. Then, the coherence body and dip-type seismic attributes are used to outline the reflection interruption and discontinuity structure, providing support for further understanding of the migration path of fluids.

[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 methane fate 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 the position of the current BSR is grasped.

[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 as 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 as 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, g is the gravitational acceleration, g = 9.81 m / s2;

[0092] Investigate the data and literature of the research 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 domain can be obtained by different methods. One is to use the bottom water temperature and the balanced and 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; it can also be calculated using a two-dimensional heat diffusion conduction model (4):

[0093]

[0094] Among them, ρb, Cb, Kx and Kz are sediment density, specific heat capacity and conductivity respectively. After determining the temperature distribution data in the sediment, the hydrate stability bottom boundary, that is, the development position of the ancient BSR in a certain geological history period can be predicted according to the hydrate phase equilibrium formula.

[0095] The S6 divides the study area into three main areas: the main area of ​​the hydrate stability domain, the landward distribution boundary area of ​​hydrates, and the shallow non-hydrate stability area at the boundary, such as Figure 5 shown.

[0096] The S7, by analyzing the attributes of the seabed such as 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 attributes of the strata between the 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 the methane released by decomposition, faults and gas chimneys and other channels provide vertical migration channels, the lithology and stratigraphic spatial configuration provide long-distance lateral migration paths, and the shallow stratigraphic and lithological closures 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, combined with the previous research results on the porosity, hydrate saturation and other parameters 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 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.

[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 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 channels, etc., according to 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), etc., 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 natural gas hydrate in the sea area, specifically comprising:

[0102] An input module is used to obtain two-dimensional and three-dimensional 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 limit of hydrate stability domain;

[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 and numerical simulation prediction results of ancient and modern BSR to divide the study area into three main areas: the main area of ​​hydrate stability, the landward distribution boundary area of ​​hydrate, and the shallow non-hydrate stability area at the boundary;

[0107] The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone lateral long-distance 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 migrates 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 in the figure, no obvious methane leakage characteristics are found at the boundary of the gas hydrate distribution in the study area. Instead, there are high-amplitude anomalies representing free gas accumulation in the non-hydrate stable domain shallow to the landward distribution boundary of the gas hydrate. The updip direction is blocked by carbonate rocks with chaotic reflection characteristics. This phenomenon represents another fate of the hydrate distribution boundary that is different from large-scale leakage - lateral migration of methane, that is, the methane decomposed from the hydrate migrates laterally and upward along the permeable sandstone layer (which can be used as a fluid channel) until it encounters a stratigraphic trap blocked by carbonate rocks. Then, the methane accumulates in the stratigraphic trap of the shallow non-hydrate stable domain. Ancient pits buried below the seafloor can be seen above some of the shallow free gas reservoirs, which may be methane leakage caused by overpressure in the later period. This pattern indicates that methane at the boundary of the hydrate stable domain can further migrate and accumulate laterally to the shallow part, delaying the direct leakage of methane to the seafloor or even the seawater at the boundary, but moving the methane to a shallower depth. Previous studies have shown that methane is more likely to escape into the atmosphere when it leaks in areas with a water depth of less than 150m. If the water depth is greater than 150m, methane is consumed in the seawater and cannot enter the atmosphere to affect the climate (McGinnis et al., 2006). Therefore, this methane fate may be more likely to cause methane to enter the atmosphere than methane leakage at the boundary of natural gas hydrate distribution, which may bring more serious 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 part can be implemented using dedicated logic; the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. It can be understood by a person of ordinary skill in the art that the above-mentioned devices and methods can be implemented using computer executable instructions and / or contained in a processor control code, 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. Such code is provided on the carrier medium. The device and its 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, etc., 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, and can also be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

[0113] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope 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 gas hydrate visible in seismic data, and conduct seismic interpretation and attribute analysis on the present BSR; S2: If there is an ancient BSR below the present BSR, conduct seismic interpretation and attribute extraction on the ancient BSR to understand the development position of the ancient BSR; 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: Investigate the data and literature of the study area, obtain the data of sea level change in the work area, and calculate the hydrostatic pressure value at the ancient BSR at that time in combination with the current seabed depth, and obtain data such as sea level change and bottom water temperature in the work area; S5: By adjusting parameters such as thermal conductivity and geothermal gradient, the location of the ancient BSR can be simulated and predicted, and the location of the landward distribution boundary of the ancient hydrate can be determined; S6: Combining the seismic interpretation and numerical simulation prediction results of ancient and modern BSR, the three main areas of the study area are divided into the main area of ​​hydrate stability, the landward distribution boundary area of ​​hydrate, and the shallow non-hydrate stability area at the boundary; S7: Carry out seismic interpretation and attribute analysis of the seafloor, ancient and modern BSRs, and the reflective layers between them to identify whether there are seafloor leakage characteristics near the landward boundary of hydrates. By analyzing the reflection anomaly characteristics of the strata between ancient and modern BSRs, possible vertical and lateral fluid migration pathways and free gas accumulation can be explained; 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 paths of fluids in the main area of ​​the hydrate stability domain, the landward distribution boundary of hydrates, and the shallow non-hydrate stability domain at the boundary in the study area, as well as the accumulation and destruction of free gas on the migration path, 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: 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; S12: Summarize the main controlling factors of 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: The S1 selects the work area with the intersection characteristics of BSR and seabed as the study area, conducts seismic interpretation and attribute analysis on the present BSR, and its seismic characteristics are strong amplitude, negative polarity, oblique cutting and 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 can be selected according to parameters such as gas composition, pore water salinity, sediment pore size, etc. in a 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, investigated the data and literature in the study area, obtained the sea level change data in the work area, combined with the current seabed depth, and calculated 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 / m3, g is the gravitational acceleration, g = 9.81 m / s2; The temperature at the bottom boundary of the gas hydrate stability domain can be obtained by different methods. One is to use the bottom water temperature and the balanced, stable one-dimensional thermal diffusion coefficient (such as 10-6 m2s-1 in the Mauritanian waters) and the geothermal gradient to calculate the temperature of the sedimentary strata; it can also be calculated using the two-dimensional heat diffusion conduction model (4): Among them, ρb, Cb, Kx and Kz are sediment density, specific heat capacity and conductivity respectively. By determining the temperature distribution data in the sediment, the stable bottom boundary of hydrate 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.

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, by analyzing the attributes of the seabed such as 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 attributes of the strata between the ancient and modern BSRs can reveal 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 discontinuous structures in seismic data, such as 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 the methane released by decomposition, faults and gas chimneys and other channels provide vertical migration channels, the lithology and stratigraphic spatial configuration provide long-distance lateral migration paths, and the shallow stratigraphic and lithological closures 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.

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, 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 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 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 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 channels, etc., according to 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), etc., 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: An input module is used to obtain two-dimensional and three-dimensional seismic data of the landward distribution boundary of hydrate in the developed sea area; Numerical simulation module, used for numerical simulation of the lower limit of hydrate stability domain; 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 and numerical simulation prediction results of ancient and modern BSR to divide the study area into three main areas: the main area of ​​hydrate stability, the landward distribution boundary area of ​​hydrate, and the shallow non-hydrate stability area at the boundary; The model building module is used to determine whether methane in the main area of ​​the stability domain has undergone lateral long-distance 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 migrates 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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