Forward physical simulation method for seismic response characteristics of natural gas hydrate system in sea area

Abstract

The present invention belongs to the technical field of marine exploration, and discloses a forward physical simulation method for the seismic response characteristics of a marine natural gas hydrate system. A physical model is established according to the distribution characteristics of the hydrate system in the study area to determine the seismic response characteristics of natural gas hydrates and underlying free gas. The seismic interpretation results of the natural gas hydrate system are corrected according to the results of the forward physical simulation, realizing the forward physical simulation of the marine natural gas hydrate system. The present invention analyzes the relationships between the amplitude, waveform, etc. of seismic waves and the saturation, thickness, and occurrence area of natural gas hydrates, explores the seismic response characteristics and patterns of different geological models of natural gas hydrate systems, and guides the identification and characterization of natural gas hydrate systems in seismic interpretation work according to the results of forward physical simulation. It is a preliminary exploration and research carried out under the background that the forward physical simulation of natural gas hydrate systems at home and abroad is in its initial stage, and has important guiding significance.

Description

Technical Field

[0001] The present invention belongs to the technical field of marine exploration, and particularly relates to a forward physical simulation method for seismic response characteristics of a marine natural gas hydrate system. Background Technique

[0002] Natural gas hydrates contain a large amount of methane resources and are considered to contain twice the amount of resources of traditional conventional energy sources. They are a type of clean future new energy with great potential, and thus have been widely studied by scholars at home and abroad. The resource volume of natural gas hydrates in the seas of our country is as high as more than 80 billion tons of oil equivalent. In 2017, natural gas hydrates were listed by the Ministry of Natural Resources as the 173rd mineral species in our country. Their commercial exploration, development and effective utilization are of great significance for alleviating the energy pressure in our country as a potential alternative energy type.

[0003] In current research, seismic data is the most important data type used in the study of natural gas hydrate systems. Due to its characteristics such as covering three-dimensional space, low cost, and high speed, it can provide high-cost-effective exploration services for hydrate development areas and is the most widely used in the current field of natural gas hydrate research. The seismic research of natural gas hydrate systems provides seismic interpretation, attribute analysis, inversion prediction, etc. for staff to identify natural gas hydrate systems, including hydrate-bearing reservoirs, the bottom boundary BSR of the gas hydrate stability zone, and the underlying free gas zone FGZ. It is generally considered that the bottom-simulating reflection layer BSR (Bottomsimulating reflection) is the seismic marker of the bottom boundary of the natural gas hydrate stability zone, with characteristics such as strong amplitude, negative polarity, being roughly parallel to the seabed, and cutting obliquely through isochronous strata. Marine natural gas hydrates usually exist in fine-grained, loose and unconsolidated sediments, mostly muddy siltstones and fine siltstones. Due to the cementation effect of natural gas hydrates and the existence of underlying free gas, there are significant differences in the petrophysical properties of the sedimentary strata where the natural gas hydrate system is located from the surrounding strata. Generally, hydrate-bearing reservoirs are considered to have characteristics such as high resistivity, high shear wave and longitudinal wave velocities, and low density, and they are usually considered to exhibit certain seismic response characteristics in seismic data, such as high amplitude, amplitude blanking zone, BSR, etc., while the underlying free gas zone FGZ is manifested as a high-amplitude abnormal reflection directly blocked by BSR. The above-mentioned characteristics are widely used in the geophysical identification of natural gas hydrates.

[0004] However, in fact, there is still a controversy over whether there is a one-to-one correspondence between the seismic reflection event axes on the seismic profiles and the true phase interfaces of the hydrate system (such as the top and bottom interfaces of the hydrate-bearing formation and the underlying free gas), and whether the position of the BSR directly indicates the bottom boundary of the natural gas hydrate stability zone. In addition, due to differences in geological structures, gas source conditions, temperature and pressure during the hydrocarbon accumulation process, the distribution patterns of natural gas hydrates in different sedimentary strata are not the same. Therefore, it is necessary to explore the seismic response characteristics corresponding to the geological models of hydrates / free gas with different saturations, and clarify the corresponding relationship between the seismic response characteristics and the reservoir physical properties of the hydrate system (hydrate reservoir and free gas reservoir).

[0005] At present, the forward modeling research in the exploration field is mainly carried out for conventional oil and gas reservoirs, aiming to establish a one-to-one correspondence between the seismic reflection event axes and the true subsurface formation interfaces. Through investigating a large number of domestic and foreign literature materials, it is found that there is less forward modeling research on the natural gas hydrate system. At present, a small amount of forward modeling on the natural gas hydrate system is mainly numerical simulation, and the forward physical simulation for discovering the seismic response characteristics of natural gas hydrates is basically a research blank. In addition to cost factors, another factor restricting the forward physical simulation of hydrates is the production of cores. Usually, natural gas hydrates exist in highly porous, semi-consolidated loose sediments, mostly argillaceous-cemented siltstones, silty fine sandstones, with shallow burial depths, poor diagenesis, low cementation degree, and very loose structures. However, the porosity of the core samples produced by the existing artificial sandstone technology is generally below 30%, and they are fully consolidated samples, which have deficiencies such as large differences, small sizes, and poor pore uniformity compared with the physical property parameters of the in-situ formation. Therefore, to solve the above problems, it is necessary to explore the preparation technology of high-porosity and weakly cemented artificial sandstones.

[0006] Based on the above analysis, the problems and defects existing in the prior art are summarized as follows:

[0007] (1) It is still not clear whether there is a one-to-one correspondence between the seismic reflection event axes on the seismic profiles and the true phase interfaces of the hydrate system, and whether the position of the BSR directly indicates the phase interface separating the hydrate-bearing formation and the underlying free gas natural gas hydrate stability zone.

[0008] (2) There is less forward modeling research on the natural gas hydrate system at present. At present, a small amount of forward modeling on the natural gas hydrate system is also mainly numerical simulation.

[0009] (3) The porosity of the core samples produced by the existing artificial sandstone technology is below 30%, and they are fully consolidated samples, which have large differences, small sizes, and poor pore uniformity compared with the physical property parameters of the in-situ formation. Summary of the Invention

[0010] In view of the problems existing in the prior art, the present invention provides a forward physical simulation method for the seismic response characteristics of a marine natural gas hydrate system.

[0011] The present invention is implemented as follows. A forward physical simulation method for the seismic response characteristics of a marine natural gas hydrate system includes: comprehensively interpreting various data for different research areas of the natural gas hydrate system, establishing a physical model based on the interpretation results of the distribution characteristics of the hydrate system, imitating the emission and reception of seismic shot points, simulating the seismic response characteristics of natural gas hydrates and underlying free gas, and correcting the seismic interpretation results of the natural gas hydrate system according to the results of the forward physical simulation to achieve the forward physical simulation of the marine natural gas hydrate system.

[0012] Furthermore, the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system further includes:

[0013] Carrying out seismic forward simulation by establishing a physical model that conforms to the geophysical characteristics of the natural gas hydrate system reservoir, imitating the emission and reception of seismic shot points, and establishing the relationship between each interface within the natural gas hydrate system and the seismic response characteristics;

[0014] Among them, the seismic response characteristics include the seismic response characteristics of the top and bottom interfaces of the hydrate-bearing reservoir, the seismic response characteristics of the top and bottom interfaces of the free gas-bearing reservoir, and whether the bottom boundary of the natural gas hydrate stability zone between the hydrate-bearing reservoir and the underlying free gas reservoir strictly corresponds to the BSR seismic reflection characteristics.

[0015] Furthermore, the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system includes the following steps:

[0016] Step 1, select a specific research area, conduct comprehensive interpretation and analysis based on real seismic, geochemical, and geological data, carry out comprehensive identification of the natural gas hydrate system, and establish an initial geological model of the natural gas hydrate system;

[0017] Step 2, use the high-porosity weak-cementation artificial sandstone preparation technology to make cores that conform to the geophysical characteristic parameters of the natural gas hydrate and free gas-bearing reservoirs;

[0018] Step 3, respectively make cores of the natural gas hydrate reservoir and the free gas reservoir according to the initial geological model of the natural gas hydrate system established in Step 1, and analyze the reservoir velocity and density parameters;

[0019] Step 4, conduct tests on the repeatability of artificial core porosity, sample homogeneity, and sample stability for the two prepared cores;

[0020] Step 5, respectively set relevant physical simulation parameters and other parameters, and set the size of the formation containing natural gas hydrates and free gas and the overall size of the model;

[0021] Step 6: Establish a model with a natural gas hydrate-bearing formation on the upper part and a free gas-bearing formation on the lower part in a water tank equipped with a device for simulating seismic shot point emission and reception.

[0022] Step 7: Conduct forward seismic modeling to simulate seismic shot point emission and reception, and obtain the seismic response characteristics corresponding to the specific physical model of the hydrate system, which are used to guide the seismic interpretation scheme of the actual seismic data in a specific work area.

[0023] Furthermore, in Step 2, according to the characteristics that hydrates are organic crystal materials, are in solid form under normal temperature and pressure, can be prepared into powder form, have elastic parameters similar to those of hydrates, and have high velocity and low density, select substitute materials highly similar to the characteristics of natural gas hydrates, and analyze the velocity and density parameters of the reservoir.

[0024] Among them, the loose sediment is characterized by good porosity and relatively low P-wave and S-wave velocities. After multiple tests, it is finally determined to conduct diagenesis under the conditions of a small diagenetic pressure of 0.5 - 1.0 MPa, a low cement content of 5%, and the presence of formation water to produce cores meeting the requirements of hydrate reservoirs.

[0025] Furthermore, the method for fabricating the core of the natural gas hydrate reservoir in Step 3 includes:

[0026] Mix quartz sand and cementing agent evenly, and then add an aqueous solution of the hydrate substitute material and stir. Bake in an oven at 90 °C for at least 48 h to ensure that the moisture in the core sample is completely evaporated and the single crystal organic material in the water is completely precipitated. Specifically, it includes stirring, pressing, firing, demolding, and baking to complete the diagenesis process.

[0027] Compared with the method for fabricating the artificial rock sample of the free gas-bearing layer, the single crystal material is not added, and the diagenesis process is completed through stirring, pressing, firing, demolding, and baking.

[0028] Furthermore, in Step 5, according to the seismic dominant frequency and wavelet length parameters of a specific work area, set relevant physical simulation parameters. For example: the P-wave velocity of the core of the hydrate reservoir is 2780 m / s, the S-wave velocity is 1790 m / s, the P-wave velocity of the free gas reservoir is 1780 m / s, the S-wave velocity is 1190 m / s, the scale ratio factor is set to 1:10000, the velocity ratio factor is 1:1, and the frequency ratio factor is 10000:1. For Sediment 1: the P-wave velocity is 2000 m / s and the S-wave velocity is 1010 m / s; for Sediment 2: the P-wave velocity is 2650 m / s and the S-wave velocity is 1350 m / s.

[0029] Other parameter settings are based on the data of most natural gas hydrate formations in the study area. For example, they can be set as follows: water depth of 80 mm, corresponding to an actual 800 m; main frequency of 17 Hz; number of shot points of 200; number of receiving channels of 221; channel spacing of 1 mm, corresponding to an actual 10 m.

[0030] In the example, the size of the formation containing natural gas hydrates and free gas is set to 110 mm * 30 mm, corresponding to an actual 1100 m * 300 m; the overall size of the model is 300 mm * 90 mm, corresponding to an actual 3000 m * 900 m.

[0031] Another object of the present invention is to provide a forward physical simulation system for the seismic response characteristics of a marine natural gas hydrate system applying the forward physical simulation method of the seismic response characteristics of the marine natural gas hydrate system. The system includes:

[0032] An initial geological model construction module for selecting a specific study area, interpreting and analyzing based on real seismic, geochemical and geological data, conducting comprehensive identification of the natural gas hydrate system, and establishing an initial geological model of the natural gas hydrate system;

[0033] A reservoir core production module for using the high-porosity weak-cementation artificial sandstone preparation technology to produce cores that meet the geophysical characteristic parameters of the reservoir containing natural gas hydrates and free gas; respectively producing natural gas hydrate reservoir cores and free gas reservoir cores according to the established initial geological model of the natural gas hydrate system, and analyzing the reservoir velocity and density parameters;

[0034] An artificial core testing module for testing the repeatability of artificial core porosity, sample homogeneity, and sample stability of the two prepared cores;

[0035] A parameter setting module for respectively setting relevant physical simulation parameters and other parameters, and setting the size of the formation containing natural gas hydrates and free gas and the overall size of the model;

[0036] A model establishment module for establishing a model with a natural gas hydrate-bearing formation in the upper part and a free gas-bearing formation in the lower part in a water tank with a device imitating seismic shot point emission and reception;

[0037] A seismic forward simulation module for conducting seismic forward simulation, imitating seismic shot point emission and reception to obtain the seismic response characteristics corresponding to the specific physical model of the hydrate system, and used to guide the seismic interpretation scheme of the actual seismic data in a specific work area.

[0038] Another object of the present invention is to provide a computer device. The computer device includes a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor performs the following steps:

[0039] Establish different physical models for the distribution characteristics of hydrate systems in different research areas, simulate the seismic source and receiver to determine the seismic response characteristics of natural gas hydrates and underlying free gas, and correct the seismic interpretation results of the natural gas hydrate system according to the results of forward physical simulation, so as to realize the forward physical simulation of the natural gas hydrate system in the sea area.

[0040] Another object of the present invention is to provide a computer-readable storage medium storing a computer program, which when executed by a processor causes the processor to perform the following steps:

[0041] Establish different physical models for the distribution characteristics of hydrate systems in different research areas, determine the seismic response characteristics of natural gas hydrates and underlying free gas, and correct the seismic interpretation results of the natural gas hydrate system according to the results of forward physical simulation, so as to realize the forward physical simulation of the natural gas hydrate system in the sea area.

[0042] Another object of the present invention is to provide an information data processing terminal for realizing the forward physical simulation system of the seismic response characteristics of the natural gas hydrate system in the sea area.

[0043] Combined with the above technical solutions and the technical problems solved, please analyze the advantages and positive effects of the technical solution to be protected by the present invention from the following aspects:

[0044] First, in view of the technical problems existing in the above prior art and the difficulty of solving the problems, closely combined with the technical solution to be protected by the present invention and the results and data in the R & D process, analyze in detail and deeply how the technical solution of the present invention solves the technical problems and the creative technical effects brought after solving the problems. The specific description is as follows:

[0045] The method for forward predicting the seismic response characteristics of natural gas hydrates by using laboratory physical simulation provided by the present invention analyzes the relationships between the amplitude, waveform, etc. of seismic waves and the saturation, thickness, and occurrence area of natural gas hydrates, explores the seismic response characteristics and patterns of different geological models of natural gas hydrate systems, and guides the identification and characterization of natural gas hydrate systems in seismic interpretation work according to the results of forward physical simulation.

[0046] The present invention provides a method for conducting forward physical simulation research on the seismic response characteristics of marine natural gas hydrate systems. In view of the special rock geophysical properties of natural gas hydrates and underlying free gas reservoirs, a high-porosity, weakly cemented artificial sandstone preparation technology has been developed. Experiments are mainly carried out from the aspects of diagenetic pressure, rock composition, particle size, cement type and content, formation water content, etc., to select suitable cores that meet the requirements of hydrate reservoirs. First, after conducting porosity repeatability, sample homogeneity, and sample stability tests using the high-porosity, weakly cemented artificial sandstone preparation technology, natural gas hydrate reservoir cores and free gas reservoir cores that meet the requirements can be established. Secondly, scale scaling factors, velocity scaling factors, frequency scaling factors, etc. are set, and specific parameters of the sedimentary formations are set according to the study area, so as to establish a physical model of the natural gas hydrate and the underlying free gas formation. Finally, a sedimentary model is established in a water tank with a launch and receiving device that simulates seismic shot points, so as to perform seismic forward physical simulation. The method provided by the present invention can be used to analyze the seismic response characteristics of natural gas hydrate / underlying free gas geological models with different saturations. The geological model is established based on the actual geophysical parameters of a specific work area. The arrangement of the seismic source and the detector is close to the actual field acquisition method. Piezoelectric ultrasonic transducers are used to carry out physical simulation work. Based on the forward simulation results of the seismic response characteristics of the natural gas hydrate and free gas models, the seismic interpretation results of the natural gas hydrate system in the corresponding work area are corrected to improve the interpretation accuracy.

[0047] The seismic response characteristics obtained by the physical model provided by the present invention are as follows: the BSR interface shows obvious negative polarity, high amplitude, and oblique formation opposite to the seabed, representing the phase interface between the hydrate layer and the free gas layer; the top interface of the upper hydrate layer has positive polarity and strong amplitude; while the amplitude of the underlying free gas bottom interface is weak.

[0048] Second, considering the technical solution as a whole or from the perspective of the product, the technical effects and advantages of the technical solution to be protected by the present invention are described in detail as follows:

[0049] The forward physical simulation method for the seismic response characteristics of marine natural gas hydrate systems provided by the present invention is a preliminary exploration and research conducted under the background that the forward physical simulation of natural gas hydrate systems at home and abroad is in its early stages, and has important guiding significance.

[0050] Third, as auxiliary evidence for the inventiveness of the claims of the present invention, it is also reflected in the following important aspects:

[0051] (1) The expected benefits and commercial value of the technical solution of the present invention after transformation are:

[0052] After the technical solution of the present invention is transformed, forward physical simulation research can be carried out in different regions for the distribution and accumulation patterns of hydrates, improving the interpretation accuracy of the specific distribution range of the natural gas hydrate system, and playing an important role in future exploration, development and determination of favorable target areas.

[0053] (2) The technical solution of the present invention fills the technical gaps in the industry at home and abroad:

[0054] At present, the forward simulation research on the natural gas hydrate system is in its infancy. To a certain extent, the present invention fills the technical gaps in the industry at home and abroad.

[0055] (3) Does the technical solution of the present invention solve the technical problems that people have been eager to solve but have never succeeded in obtaining?

[0056] Since the natural gas hydrate system is located in shallow unconsolidated sediments, the cementation of hydrates and the existence of underlying free gas endow the hydrate system with special petrophysical properties, thus posing certain challenges to the preparation of artificial cores. The solution of the present invention proposes a high-porosity and weak-cementation artificial sandstone preparation technology to prepare physical models that conform to the petrophysical characteristics of the hydrate system. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments of the present invention. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0058] Figure 1 is a flow chart of the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system provided by the embodiment of the present invention;

[0059] Figure 2 is a block diagram of the structure of the forward physical simulation system for the seismic response characteristics of the marine natural gas hydrate system provided by the embodiment of the present invention;

[0060] Figure 3(a) is a flow chart of the core preparation of the layer containing natural gas hydrates provided by the embodiment of the present invention;

[0061] Figure 3(b) is a schematic diagram of the core of the layer containing natural gas hydrates provided by the embodiment of the present invention;

[0062] Figure 4 is a schematic diagram of the process of preparing an artificial core provided by the embodiment of the present invention;

[0063] Figure 5(a) is a schematic diagram of the CT scan of the natural gas hydrate core provided by the embodiment of the present invention;

[0064] Figure 5(b) is a schematic diagram of the homogeneity test of natural gas hydrate cores provided by an embodiment of the present invention;

[0065] Figure 5(c) is a schematic diagram of the stability test of natural gas hydrate cores provided by an embodiment of the present invention;

[0066] Figure 6 is a schematic diagram of the laboratory physical simulation model provided by an embodiment of the present invention;

[0067] Figure 6(a) is a schematic diagram of the designed model shape and size provided by an embodiment of the present invention;

[0068] Figure 6(b) is a schematic diagram of the actually made sedimentary formation model provided by an embodiment of the present invention;

[0069] Figure 7 is a schematic diagram of the device for conducting laboratory physical simulation provided by an embodiment of the present invention;

[0070] Figure 8 is a schematic diagram of the laboratory physical simulation results provided by an embodiment of the present invention;

[0071] Figure 8(a) is a schematic diagram of a single-shot record provided by an embodiment of the present invention;

[0072] Figure 8(b) is a schematic diagram of a self-exciting and self-receiving profile provided by an embodiment of the present invention;

[0073] Figure 8(c) is a schematic diagram of a single-trace record provided by an embodiment of the present invention;

[0074] Figure 8(d) is a schematic diagram of a post-stack seismic profile provided by an embodiment of the present invention;

[0075] In the figure: 1. Initial geological model construction module; 2. Reservoir core production module; 3. Artificial core test module; 4. Parameter setting module; 5. Model establishment module; 6. Seismic forward modeling module. Detailed implementation manners

[0076] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with 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.

[0077] In view of the problems existing in the prior art, the present invention provides a forward physical simulation method for the seismic response characteristics of a marine natural gas hydrate system. The present invention will be described in detail below with reference to the accompanying drawings.

[0078] I. Explanation of the embodiment. This part is an explanatory embodiment that expands and explains the technical solution of the claims in order to enable those skilled in the art to fully understand how the present invention is specifically implemented.

[0079] Glossary: Natural gas hydrate: It is an ice-like crystalline substance formed by hydrocarbon gases such as methane and water under high pressure and low temperature conditions. Due to its low permeability, it can act as a caprock to seal free gas in the lower part; Bottom Simulating Reflection (BSR), which is considered to be the seismic marker of the bottom boundary of the natural gas hydrate stability zone, has characteristics such as strong amplitude, negative polarity, being roughly parallel to the seabed, and isochronous strata with oblique cutting; Forward modeling, in geophysical exploration research, according to the shape of the target geological body, physical property parameters, acoustic velocity and other geophysical parameters, by establishing a numerical model or physical model, calculating the theoretical seismic response characteristics, or observing the actual seismic response characteristics generated by it is called forward modeling.

[0080] As Figure 1 shown, the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system provided by the embodiment of the present invention includes the following steps:

[0081] S101, Select a specific study area, interpret and analyze according to real seismic, geochemical and geological data, conduct comprehensive identification of the natural gas hydrate system, and establish an initial geological model of the natural gas hydrate system;

[0082] S102, Use the high-porosity and weakly cemented artificial sandstone preparation technology to produce cores that meet the geophysical characteristic parameters of the reservoir containing natural gas hydrates and free gas;

[0083] S103, Respectively produce cores of the natural gas hydrate reservoir and the free gas reservoir according to the initial geological model of the natural gas hydrate system established in S101, and analyze the reservoir velocity and density parameters;

[0084] S104, Conduct tests on the repeatability of artificial core porosity, sample homogeneity, and sample stability of the two prepared cores;

[0085] S105, Respectively set relevant physical simulation parameters and other parameters, and set the size of the formation containing natural gas hydrates and free gas and the overall size of the model;

[0086] S106, Establish a model with a natural gas hydrate-bearing formation on the upper part and a free gas-bearing formation on the lower part in a water tank with a device imitating seismic shot point emission and reception;

[0087] S107, Conduct seismic forward modeling to obtain the seismic response characteristics corresponding to the specific hydrate system physical model, which is used to guide the seismic interpretation scheme of the actual seismic data in the specific work area.

[0088] As Figure 2 shown, the forward physical simulation system for the seismic response characteristics of the marine natural gas hydrate system provided by the embodiment of the present invention includes:

[0089] The initial geological model construction module 1 is used to select a specific study area, interpret and analyze based on real seismic, geochemical, and geological data, conduct comprehensive identification of the natural gas hydrate system, and establish an initial geological model of the natural gas hydrate system;

[0090] The reservoir core production module 2 is used to produce cores that meet the geophysical characteristic parameters of the reservoir containing natural gas hydrates and free gas by using the high-porosity and weakly cemented artificial sandstone preparation technology; produce natural gas hydrate reservoir cores and free gas reservoir cores respectively according to the established initial geological model of the natural gas hydrate system, and analyze the reservoir velocity and density parameters;

[0091] The artificial core testing module 3 is used to test the repeatability of the porosity of the prepared two types of cores, the homogeneity of the samples, and the stability of the samples;

[0092] The parameter setting module 4 is used to set relevant physical simulation parameters and other parameters respectively, and set the size of the formation containing natural gas hydrates and free gas and the overall size of the model;

[0093] The model establishment module 5 is used to establish a model with a natural gas hydrate-bearing formation on the upper part and a free gas-bearing formation on the lower part in a water tank with a device imitating seismic shot points and receivers;

[0094] The seismic forward simulation module 6 is used to conduct seismic forward simulation to obtain the seismic response characteristics corresponding to the physical model of the specific hydrate system, and is used to guide the seismic interpretation scheme of the actual seismic data in the specific work area.

[0095] The present invention provides a forward physical simulation method for studying the seismic response characteristics of the natural gas hydrate system in the sea area. By establishing a physical model that conforms to the geophysical characteristics of the natural gas hydrate system reservoir, seismic forward simulation is carried out to establish the relationship between each interface in the natural gas hydrate system and the seismic response characteristics. Specifically, it includes (1) the seismic response characteristics of the top and bottom interfaces of the hydrate-bearing reservoir; (2) the seismic response characteristics of the top and bottom interfaces of the free gas-bearing reservoir; and (3) whether the bottom boundary of the natural gas hydrate stable zone between the hydrate-bearing reservoir and the underlying free gas reservoir strictly corresponds to the BSR seismic reflection characteristics. This method can establish different physical models according to the distribution characteristics of the hydrate system in different study areas, conduct research on the seismic response characteristics of natural gas hydrates and the underlying free gas, and correct the previous seismic interpretation results of the natural gas hydrate system according to the results of the forward physical simulation. The specific method steps are as follows:

[0096] Step 1: Select a specific study area, interpret and analyze based on real seismic, geochemical, geological, etc. data, conduct comprehensive identification of the natural gas hydrate system, and establish an initial geological model of the natural gas hydrate system.

[0097] Step 2: Using the preparation technology of highly porous weakly cemented artificial sandstone, prepare cores that conform to the geophysical characteristic parameters of natural gas hydrate and free gas reservoirs (see Figure 3). According to the characteristics of hydrates as organic crystal materials, being solid at normal temperature and pressure, being able to be prepared into powder form, having elastic parameters similar to hydrates, and low density with high velocity, select alternative materials that are highly similar to the characteristics of natural gas hydrates, considering parameters such as reservoir velocity and density (see Table 1). Loose sediments are characterized by good porosity and low P-wave and S-wave velocities. After multiple tests, it is finally determined that diagenesis is carried out under conditions of low diagenetic pressure (0.5 MPa - 1 MPa), low cement content (about 5%), and containing formation water, and cores that meet the requirements of hydrate reservoirs can be made.

[0098] Table 1 Comparison of parameters of natural gas hydrates and alternative materials

[0099]

[0100] Step 3: Prepare cores of natural gas hydrate reservoirs according to the initial geological model of the natural gas hydrate system established in Step 1. First, mix quartz sand and cementing agent evenly, and then add the aqueous solution of the hydrate alternative material and stir; bake in an oven at 90 °C for at least 48 hours to ensure that the moisture in the core samples is completely evaporated and the single-crystal organic materials in the water are completely precipitated (the rock samples are highly porous and highly permeable). Specifically, it includes processes such as stirring, pressing, firing, demolding, and baking to complete diagenesis (see Figure 4 ).

[0101] Step 4: Prepare cores of free gas reservoirs according to the initial geological model of the natural gas hydrate system established in Step 1, considering parameters such as reservoir velocity and density. Compared with the method of making artificial rock samples of hydrate reservoirs, no single-crystal material is added, and other steps are the same, including processes such as stirring, pressing, firing, demolding, and baking to complete diagenesis.

[0102] Step 5: Conduct tests on the repeatability of artificial core porosity, sample homogeneity, and sample stability of the two prepared cores (see Figure )), and the results show that the artificial core samples used in this example have good homogeneity and stability.

[0103] Step 6: Refer to parameters such as the seismic dominant frequency and wavelet length of a specific work area, and set relevant physical simulation parameters. The P-wave velocity of the hydrate reservoir core is 2780 m / s, the S-wave velocity is 1790 m / s, the P-wave velocity of the free gas reservoir is 1780 m / s, the S-wave velocity is 1190 m / s, the scale ratio factor is set to 1:10000, the velocity ratio factor is 1:1, and the frequency ratio factor is 10000:1. Sediment 1: P-wave velocity is 2000 m / s, S-wave velocity is 1010 m / s; Sediment 2: P-wave velocity is 2650 m / s, S-wave velocity is 1350 m / s.

[0104] Step 7: Other parameters are set according to the data of most natural gas hydrate formations in the study area as follows: water depth 80 mm (equivalent to 800 m in actual measurement); main frequency 17 Hz; number of shot points 200; number of receiving channels 221; channel spacing 1 mm (equivalent to 10 m in actual measurement).

[0105] Step 8: The size of the formation containing natural gas hydrates and free gas is set to 110 mm*30 mm (equivalent to 1100 m*300 m in reality), and the overall size of the model is 300 mm*90 mm (equivalent to 3000 m*900 m in reality, see Figure 6).

[0106] Step 9: A model with a natural gas hydrate-containing formation at the top and a free gas-containing formation at the bottom is built in a water tank with a device for simulating seismic shot point emission and reception (see Figure 7).

[0107] Step 10: Perform seismic forward modeling to obtain the seismic response characteristics corresponding to the specific hydrate system physical model. The results can be used to guide the seismic interpretation plan of actual seismic data in the specific work area (see Figure 8).

[0108] The seismic response characteristics obtained from this physical model show that the BSR interface shows a clear negative polarity, high amplitude, and oblique strata, opposite to the seafloor. This represents the phase interface between the hydrate layer and the free gas layer. The top interface of the upper hydrate layer has positive polarity and high amplitude, while the underlying free gas bottom interface has a weaker amplitude.

[0109] 2. Application Examples: In order to demonstrate the creativity and technical value of the technical solution of the present invention, this section provides application examples of the claimed technical solution on specific products or related technologies.

[0110] The physical model established in this embodiment of the present invention is shown in Figure 6. The model water depth is 80 mm (equivalent to 800 m in actual depth). The size of the strata containing natural gas hydrates and free gas is set to 110 mm by 30 mm (equivalent to 1100 m by 300 m in actual depth). The overall model size is 300 mm by 90 mm (equivalent to 3000 m by 900 m in actual depth). The phase transition interface between the hydrates and free gas is horizontal. When simulating the seismic response characteristics of this geological model, the following parameters were set: main frequency 17 Hz; number of shot points 200; number of receiving channels 221; channel spacing 1 mm (equivalent to 10 m in actual depth); scale factor 1:10,000; velocity scale factor 1:1; and frequency scale factor 10,000:1.

[0111] Finally, the seismic response characteristics obtained from the physical model in this study show that the BSR interface is horizontally distributed, consistent with the phase interface between the hydrate layer and the free gas layer in the actual geological model. The BSR characteristics are manifested as obvious negative polarity, high amplitude, and oblique cutting of the strata, opposite to the seabed. The top interface of the upper hydrate layer has positive polarity and relatively strong amplitude; while the bottom interface of the underlying free gas has relatively weak amplitude.

[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 designed hardware. Those of ordinary skill in the art can understand that the above-mentioned devices and methods can be implemented using computer-executable instructions and / or included in processor control code, such as provided on 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 of programmable hardware devices such as very large scale integrated circuits or gate arrays, semiconductors such as logic chips and transistors, or programmable logic devices such as field programmable gate arrays, or can be implemented by software executed by various types of processors, or can be implemented by a combination of the above hardware circuits and software, such as firmware.

[0113] III. Evidence of related effects of the embodiments. Some positive effects have been achieved during the research and development or use of the embodiments of the present invention, and it indeed has great advantages compared with the prior art. The following content will be described in combination with the data, charts, etc. in the test process.

[0114] The forward physical simulation results of the embodiments show that the seismic response characteristics obtained from the physical model are that the BSR interface is horizontally distributed, consistent with the phase interface between the hydrate layer and the free gas layer in the actual geological model. In addition, the seismic reflection characteristics of the top and bottom interfaces of the hydrate reservoir in the hydrate system are obvious. The BSR characteristics are manifested as obvious negative polarity, high amplitude, and oblique cutting of the strata, opposite to the seabed. The top interface of the upper hydrate layer has positive polarity and relatively strong amplitude, and no blank reflection band proposed by predecessors appears; while the bottom interface of the underlying free gas has relatively weak amplitude, which may be related to the setting of the petrophysical parameters of the free gas.

[0115] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention shall be covered by the protection scope of the present invention.

Claims

1. A forward physical simulation method for seismic response characteristics of a marine natural gas hydrate system, characterized in that, The forward physical simulation method for the seismic response characteristics of the submarine gas hydrate system includes: Establish different physical models according to the distribution characteristics of the gas hydrate systems in different areas, determine the seismic response characteristics of gas hydrates and underlying free gas, and correct the seismic interpretation results of the gas hydrate system based on the results of forward physical simulation to achieve the forward physical simulation of the submarine gas hydrate system; The forward physical simulation method for the seismic response characteristics of the submarine gas hydrate system includes the following steps: Step 1: Select a specific study area, conduct interpretation and analysis based on real seismic, geochemical and geological data, carry out comprehensive identification of the gas hydrate system, and establish an initial geological model of the gas hydrate system; Step 2: Use the high-porosity and weakly cemented artificial sandstone preparation technology to produce cores that conform to the geophysical characteristic parameters of the reservoir containing gas hydrates and free gas; Step 3: Respectively produce gas hydrate reservoir cores and free gas reservoir cores according to the initial geological model of the gas hydrate system established in Step 1, and analyze the reservoir velocity and density parameters; Step 4: Conduct tests on the repeatability of artificial core porosity, sample homogeneity, and sample stability for the two prepared cores; Step 5: Respectively set relevant physical simulation parameters and other parameters, and set the size of the formation containing gas hydrates and free gas and the overall size of the model; Step 6: Establish a model with a gas hydrate-bearing formation on the upper part and a free gas-bearing formation on the lower part in a water tank with a device imitating seismic shot points and receivers; Step 7: Conduct seismic forward simulation to obtain the seismic response characteristics corresponding to the physical model of the specific gas hydrate system, which is used to guide the seismic interpretation scheme of the actual seismic data in the specific work area; In Step 2, according to the characteristics that gas hydrates are organic crystal materials, are in solid form under normal temperature and pressure, can be prepared into powder form, have elastic parameters similar to gas hydrates, and have high velocity and low density, select a substitute material highly similar to the characteristics of gas hydrates, and analyze the reservoir velocity and density parameters; Among them, the loose sediment is characterized by good porosity and low P-wave and S-wave velocities. After multiple tests, it is finally decided to carry out diagenesis under the conditions of a small diagenetic pressure of 0.5 - 1.0 MPa, a low cement content of 5%, and containing formation water to produce cores that meet the requirements of the gas hydrate reservoir; 2. The forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system according to claim 1, wherein The forward physical simulation method for the seismic response characteristics of the submarine gas hydrate system further includes: Carry out seismic forward simulation by establishing a physical model that conforms to the geophysical characteristics of the gas hydrate system reservoir, imitate seismic shot point emission and reception, and establish the relationship between each interface in the gas hydrate system and the seismic response characteristics; Among them, the seismic response characteristics include the seismic response characteristics of the top and bottom interfaces of the hydrate-bearing reservoir, the seismic response characteristics of the top and bottom interfaces of the free gas-bearing reservoir, and whether the bottom boundary of the gas hydrate stable zone between the hydrate-bearing reservoir and the underlying free gas reservoir strictly corresponds to the BSR seismic reflection characteristics.

3. The forward physical simulation method for the seismic response characteristics of the natural gas hydrate system in the sea area as described in claim 1, wherein The method for producing the gas hydrate reservoir cores in Step 3 includes: After mixing quartz sand and cementing agent evenly, add an aqueous solution of the hydrate substitute material and stir; bake in an oven at 90 °C for at least 48 h to ensure that the moisture in the core sample is completely evaporated and the single-crystal organic material in the water is completely precipitated. Specifically, it includes stirring, pressing, firing, demolding, and baking to complete the diagenetic process; Compared with the method for making artificial rock samples of the free gas-bearing layer, the single-crystal material is not added, including stirring, pressing, firing, demolding, and baking to complete the diagenetic process.

4. The forward physical simulation method for seismic response characteristics of the natural gas hydrate system in the sea area according to claim 1, characterized in that, In step five, according to the seismic dominant frequency and wavelet length parameters of the specific work area, set relevant physical simulation parameters. The longitudinal wave velocity of the hydrate reservoir core is 2780 m / s, the transverse wave velocity is 1790 m / s, the longitudinal wave velocity of the free gas reservoir is 1780 m / s, the transverse wave velocity is 1190 m / s, the scale factor is set to 1:10000, the velocity scale factor is 1:1, and the frequency scale factor is 10000:1; for sediment 1: the longitudinal wave velocity is 2000 m / s, the transverse wave velocity is 1010 m / s; for sediment 2: the longitudinal wave velocity is 2650 m / s, the transverse wave velocity is 1350 m / s; Other parameter settings are based on the data of most natural gas hydrate formations in the study area: water depth 80 mm, equivalent to 800 m in reality; dominant frequency 17 Hz; number of shot points 200; number of receiving channels 221; channel spacing 1 mm, equivalent to 10 m in reality; The size of the formation containing natural gas hydrate and free gas is set to 110 mm * 30 mm, equivalent to 1100 m * 300 m in reality; the overall size of the model is 300 mm * 90 mm, equivalent to 3000 m * 900 m in reality.

5. A forward physical simulation system for seismic response characteristics of a marine natural gas hydrate system, which applies the forward physical simulation method for seismic response characteristics of the marine natural gas hydrate system described in any one of claims 1 to 4, characterized in that The forward physical simulation system for the seismic response characteristics of the marine natural gas hydrate system includes: An initial geological model construction module, used to select a specific study area, interpret and analyze based on real seismic, geochemical, and geological data, conduct comprehensive identification of the natural gas hydrate system, and establish an initial geological model of the natural gas hydrate system; A reservoir core production module, used to use the high-porosity weak-cement artificial sandstone preparation technology to produce cores that meet the geophysical characteristic parameters of the natural gas hydrate-bearing and free gas-bearing reservoirs; respectively produce natural gas hydrate reservoir cores and free gas reservoir cores according to the established initial geological model of the natural gas hydrate system, and analyze the reservoir velocity and density parameters; An artificial core testing module, used to test the repeatability of artificial core porosity, sample homogeneity, and sample stability of the two prepared cores; A parameter setting module, used to set relevant physical simulation parameters and other parameters respectively, and set the size of the formation containing natural gas hydrate and free gas and the overall size of the model; A model establishment module, used to establish a model with a natural gas hydrate-bearing formation on the upper part and a free gas-bearing formation on the lower part in a water tank with a device imitating seismic shot point emission and reception; A seismic forward simulation module, imitating seismic shot point emission and reception, used to conduct seismic forward simulation to obtain the seismic response characteristics corresponding to the specific hydrate system physical model, and used to guide the seismic interpretation scheme of the actual seismic data in the specific work area.

6. A computer device, characterized in that, The computer device includes a memory and a processor. The memory stores a computer program. When the computer program is executed by the processor, the processor is caused to execute the steps of the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system according to any one of claims 1 to 4.

7. A computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor is caused to execute the steps of the forward physical simulation method for the seismic response characteristics of the marine natural gas hydrate system according to any one of claims 1 to 4.

8. An information data processing terminal, characterized in that, The information data processing terminal is used to implement the forward physical simulation system for the seismic response characteristics of the marine natural gas hydrate system according to claim 5.

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