A method for evaluating the occurrence pattern of natural gas hydrate in natural gas hydrate reservoirs

Abstract

The present invention provides a method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir. The method includes: obtaining the P-wave velocity of a target natural gas hydrate reservoir; obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir; obtaining a P-wave velocity determination model for the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence pattern; and determining the occurrence pattern of natural gas hydrates in the target natural gas hydrate reservoir based on the P-wave velocity and the natural gas hydrate saturation of the target natural gas hydrate reservoir and using the P-wave velocity determination model for the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence pattern. The method provided by the present invention uses P-wave velocity and hydrate saturation data that can be directly obtained using well logging data to identify the occurrence pattern of a hydrate reservoir without relying on core sampling experiments.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrate occurrence pattern recognition, and in particular to a method for evaluating the occurrence pattern of natural gas hydrate in a natural gas hydrate reservoir. Background Art

[0002] Natural gas hydrates, an unconventional energy source, have total resources twice as large as the total proven resources of coal, oil, and natural gas combined. They represent a highly promising energy source for the 21st century, with broad prospects for development. Natural gas hydrate occurrence patterns vary significantly across regions and lithologic formations. These patterns indicate the gas source, fluid flux, and temperature conditions during hydrate formation, providing guidance for the study of hydrate-bearing formation properties and reservoir formation patterns.

[0003] Common natural gas hydrate occurrence modes include: (1) particle cementation mode: hydrates are formed near the contact points of sediment particles and cement the sediment skeleton; (2) encapsulation cementation mode: hydrates are formed by encapsulating sediment particles and cementing the sediment skeleton; (3) skeleton support mode: hydrates exist as part of the sediment skeleton, enhancing sediment stiffness but not participating in cementation; (4) pore suspension mode: hydrates are suspended and dispersed in the sediment pores and do not contact the sediment skeleton. Differences in hydrate occurrence modes can cause significant changes in the elastic properties of the formation. The acoustic properties of rocks play an important role in the identification of hydrate occurrence modes. Identifying hydrate occurrence modes is the basis for hydrate reservoir evaluation and resource development and utilization. Therefore, it is of great significance to study economical and effective quantitative identification methods for natural gas hydrate occurrence modes.

[0004] Existing methods for quantitatively identifying natural gas hydrate occurrence patterns can be divided into two main categories: one is to identify the boundaries of different materials in the sediment or detect the heat of the samples based on the results of CT scanning and thermal imaging analysis of drilling core samples, thereby determining the occurrence pattern of hydrates; the other is to establish reservoir rock physical acoustic models and discrimination parameters based on logging longitudinal and shear wave velocity data to determine the hydrate saturation of pore suspension and skeleton support modes.

[0005] Existing quantitative identification methods for hydrate occurrence patterns have many shortcomings, mainly reflected in the following aspects: ① The current methods that can accurately identify hydrate occurrence patterns rely on drilling coring, and it is difficult to identify hydrate occurrence patterns directly based on logging data; ② The identification methods based on logging data are only targeted at one or two hydrate occurrence patterns and cannot quantitatively evaluate reservoirs where multiple hydrate occurrence patterns coexist.

[0006] Based on this, there is still a need to further study economical and effective methods for quantitative identification of natural gas hydrate occurrence patterns. Summary of the Invention

[0007] The object of the present invention is to provide a method for effectively determining the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir without relying on core sampling experiments.

[0008] To achieve the above object, the present invention provides a method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir, wherein the method comprises:

[0009] Obtain the P-wave velocity of the target natural gas hydrate reservoir;

[0010] Obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir;

[0011] Obtaining a P-wave velocity determination model for a target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in various occurrence modes (i.e., a P-wave velocity determination model for the target natural gas hydrate reservoir under different occurrence conditions);

[0012] Based on the longitudinal wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is determined by utilizing a longitudinal wave velocity determination model of the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrates in each occurrence mode.

[0013] Among them, the longitudinal wave velocity determination model of the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence mode is the relationship model between the longitudinal wave velocity of the target natural gas hydrate reservoir and the content of natural gas hydrates in each occurrence mode, that is, the calculation model of the longitudinal wave velocity of the target natural gas hydrate reservoir with respect to the content of natural gas hydrates in each occurrence mode, which can determine the longitudinal wave velocity corresponding to the target natural gas hydrate reservoir when the content of natural gas hydrates in each occurrence mode is any value.

[0014] The method for evaluating the occurrence pattern of natural gas hydrates in natural gas hydrate reservoirs provided by the present invention uses more easily obtained P-wave velocity and hydrate saturation data (P-wave velocity and hydrate saturation data can be directly obtained using logging data such as resistivity logging and P-wave velocity logging) to identify the occurrence pattern of the hydrate reservoir without relying on core sampling experiments.

[0015] In the above evaluation method, preferably, the method further comprises:

[0016] Target natural gas hydrate reservoir identification steps: obtaining well logging data of the target study area; determining the natural gas hydrate reservoir in the target study area as the target natural gas hydrate reservoir based on the well logging data of the target study area;

[0017] Among them, determining the natural gas hydrate reservoir in the target study area based on the logging data of the target study area can be carried out in accordance with conventional methods in the field. Usually, the natural gas hydrate reservoir can be judged by the following characteristics: the natural gas hydrate reservoir exhibits low natural gamma value, low density value, high neutron porosity value, high resistivity value, high acoustic wave velocity value and other characteristics on the logging curve.

[0018] In the above evaluation method, preferably, obtaining the longitudinal wave velocity of the target natural gas hydrate reservoir includes:

[0019] P-wave velocity logging data of the target gas hydrate reservoir;

[0020] The P-wave velocity of the target natural gas hydrate reservoir is determined based on the P-wave velocity logging data of the target natural gas hydrate reservoir.

[0021] In the above evaluation method, given that resistivity data is affected by hydrate saturation, resistivity logging data can be used to determine hydrate saturation;

[0022] Preferably, obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir includes:

[0023] Obtain the mud content of the target natural gas hydrate reservoir;

[0024] Obtaining the reservoir porosity of the target natural gas hydrate reservoir;

[0025] Obtaining the resistivity of the target natural gas hydrate reservoir;

[0026] determining a gas hydrate saturation of the target gas hydrate reservoir based on a shale content of the target gas hydrate reservoir, a reservoir porosity of the target gas hydrate reservoir, and a resistivity of the target gas hydrate reservoir;

[0027] More preferably, obtaining the mud content of the target natural gas hydrate reservoir includes:

[0028] Obtain natural gamma ray logging data for target gas hydrate reservoirs;

[0029] Determine the shale content of the target gas hydrate reservoir based on the natural gamma logging data of the target gas hydrate reservoir;

[0030] In one embodiment, the mud content of the target natural gas hydrate reservoir is determined by the following formula: Where: GR is the natural gamma logging value of the target gas hydrate reservoir; GR max is the natural gamma logging value of pure sandstone; GR minis the natural gamma ray logging value of the pure mudstone section; GUCR is the regional empirical coefficient (usually around 2 for old formations and around 3.7 for new formations); V sh is the mud content of the target gas hydrate reservoir;

[0031] More preferably, obtaining the reservoir porosity of the target natural gas hydrate reservoir includes:

[0032] Acquiring density logging data of a target natural gas hydrate reservoir, and determining the density porosity of the target natural gas hydrate reservoir based on the density logging data of the target natural gas hydrate reservoir;

[0033] Acquiring neutron porosity logging data of a target natural gas hydrate reservoir, and determining the neutron porosity of the target natural gas hydrate reservoir based on the neutron porosity logging data of the target natural gas hydrate reservoir;

[0034] Performing shale correction on the neutron porosity and density porosity of the target gas hydrate reservoir in combination with the shale content of the target gas hydrate reservoir, respectively, to obtain the shale-corrected neutron porosity and shale-corrected density porosity of the target gas hydrate reservoir;

[0035] Determining the reservoir porosity of the target natural gas hydrate reservoir based on the neutron porosity corrected for the shale content of the target natural gas hydrate reservoir and the density porosity corrected for the shale content of the target natural gas hydrate reservoir;

[0036] In one embodiment, the density porosity of the target natural gas hydrate reservoir is determined by the following formula: Where: ρ ma is the stratum skeleton density; ρ b is the formation density logging value of the target natural gas hydrate reservoir; ρ f is the formation water density; φ D is the density porosity of the target gas hydrate reservoir;

[0037] In one embodiment, the neutron porosity of the target natural gas hydrate reservoir is corrected for shale using the following formula: Nc =φ N -V sh ·φ Nsh Where: φ N is the neutron porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Nsh is the apparent neutron porosity of mud; φ Nc Neutron porosity corrected for the mud quality of the target gas hydrate reservoir;

[0038] In one embodiment, the density porosity of the target natural gas hydrate reservoir is corrected for shale using the following formula: Dc =φ D -V sh ·φ Dsh Where: φ D is the density porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Dsh is the apparent density porosity of mud; φ Dc The density porosity corrected for the mud quality of the target gas hydrate reservoir;

[0039] In one embodiment, the reservoir porosity of the target natural gas hydrate reservoir is determined by the following formula: Where: φ Nc is the neutron porosity of the target gas hydrate reservoir after mud correction; φ Dc is the density porosity of the target natural gas hydrate reservoir after mud correction; φ is the reservoir porosity of the target natural gas hydrate reservoir;

[0040] More preferably, obtaining the resistivity of the target natural gas hydrate reservoir includes:

[0041] Obtain resistivity logging data for target natural gas hydrate reservoirs;

[0042] Based on the resistivity logging data of the target natural gas hydrate reservoir, the resistivity of the target natural gas hydrate reservoir is determined; more preferably, the natural gas hydrate saturation of the target natural gas hydrate reservoir is determined by the following formula:

[0043]

[0044] Where: φ is the reservoir porosity of the target natural gas hydrate reservoir; R w is the formation water resistivity; R sh is the resistivity of clay; V sh is the mud content of the target natural gas hydrate reservoir; R t is the resistivity of the target natural gas hydrate reservoir; a, m, and n are Archie constants (Archie constants a, m, and n can be determined in a conventional manner); the Archie constants a and m indicate the lithoelectric relationship of the formation and are usually obtained by fitting the resistivity and porosity of the surrounding rock; the Archie constant n depends on the reservoir lithology and is usually close to 2.

[0045] In the above evaluation method, preferably, each occurrence mode includes a particle cementation mode, an encapsulation cementation mode, a skeleton support mode and a pore suspension mode.

[0046] In the above evaluation method, preferably, obtaining a P-wave velocity determination model for a target natural gas hydrate reservoir having a specific content of natural gas hydrates in each occurrence mode includes:

[0047] Obtaining the reservoir porosity of the target natural gas hydrate reservoir;

[0048] Obtaining sedimentary mineral physical property parameters of the target natural gas hydrate reservoir and physical property parameters of the natural gas hydrate reservoir;

[0049] Obtaining a natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents (i.e., a natural gas hydrate reservoir rock physics model applicable to different occurrence conditions) and a calculation model for the relationship between the longitudinal wave velocity of the natural gas hydrate reservoir and the elastic modulus of the natural gas hydrate reservoir;

[0050] Determining a model for determining the longitudinal wave velocity of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode based on the reservoir porosity of the target gas hydrate reservoir, the physical property parameters of the sedimentary minerals in the target gas hydrate reservoir, the physical property parameters of the gas hydrate reservoir, a gas hydrate reservoir rock physics model applicable to gas hydrates in each occurrence mode with a specific content, and a calculation model for the longitudinal wave velocity of the gas hydrate reservoir with respect to the elastic modulus of the gas hydrate reservoir;

[0051] The natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents is a natural gas hydrate reservoir elastic modulus determination model applicable to natural gas hydrates of various occurrence modes with specific contents;

[0052] More preferably, the natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents includes:

[0053] A model for determining the elastic modulus of the sediment matrix in a natural gas hydrate reservoir under conditions of hydrate content in a specific skeleton support mode;

[0054] A model for determining the elastic modulus of the hydrate cementation layer in a natural gas hydrate reservoir based on the hydrate content of a specific particle cementation mode and the hydrate content of a specific encapsulation cementation mode of the sediment matrix elastic modulus;

[0055] A model for determining the elastic modulus of dry rock skeleton in natural gas hydrate reservoirs under hydrate content conditions based on a specific skeleton support model based on the elastic modulus of the sediment matrix and the elastic modulus of the hydrate cement layer;

[0056] A model is provided for determining the elastic modulus of the gas hydrate reservoir saturated with fluid (i.e., the gas hydrate reservoir elastic modulus) under specific pore suspension mode hydrate content conditions based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix;

[0057] In a specific embodiment, the model for determining the elastic modulus of the natural gas hydrate reservoir sediment matrix under the condition of a specific skeleton-supported hydrate content is obtained by introducing the skeleton-supported hydrate content parameter into the Viogt-Reuss-Hill sediment matrix elastic modulus calculation model; for example,

[0058]

[0059]

[0060] φ h =φ·S h ·f ms

[0061] Where K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir; K s is the bulk modulus of the sedimentary minerals in the natural gas hydrate reservoir (determined using conventional methods in the art); μ s is the shear modulus of the sedimentary minerals of the natural gas hydrate reservoir (determined using conventional methods in the art); ms is the volume fraction of skeleton-supported hydrates in the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir;

[0062] In a specific embodiment, the elastic modulus of the hydrate cementation layer in the natural gas hydrate reservoir under the conditions of the hydrate content of the specific particle cementation mode and the hydrate content of the specific encapsulation cementation mode based on the elastic modulus of the sediment matrix is ​​determined by introducing the particle cementation mode hydrate content parameter and the encapsulation cementation mode hydrate content parameter into the cementation model proposed by Dvorkin and Nur (1993); for example,

[0063]

[0064]

[0065]

[0066] in,

[0067] S n (β) = A n (Λ n )·β 2 +B n (Λ n )·β+C n (Λ n )

[0068] S t (β) = A t (Λ t ,v)·β 2 +B t (Λ t ,v)·β+C t (Λ t ,v)

[0069] A n (Λ n )=-0.024153·Λ n -1.3646

[0070] B n (Λ n )=0.20405·Λ n -0.89008

[0071] C n (Λ n )=0.00024649·Λ n -1.9864

[0072]

[0073]

[0074]

[0075]

[0076]

[0077] Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; φ cis the critical porosity of sediments. The critical porosity of sediments in the sea is usually 0.36-0.42; σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir; σ h is the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir; P eff is the effective pressure of the sediment in the gas hydrate reservoir, which can be calculated based on the depth of the gas hydrate reservoir and the density of the sedimentary minerals and formation water in the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrates in the gas hydrate reservoir; C is the coordination number of sediments in the gas hydrate reservoir, usually 4-11; τ is the friction coefficient indicating the grain roughness or slip degree, usually 0-1; f cc is the volume fraction of the particle-cemented hydrate in the natural gas hydrate of the natural gas hydrate reservoir (based on the total volume of the natural gas hydrate in the natural gas hydrate reservoir); f gc is the volume fraction of hydrates in the gas hydrate of the gas hydrate reservoir (based on the total volume of gas hydrates in the gas hydrate reservoir); β0 is the radius of the initial gas hydrate cementing layer of the gas hydrate reservoir; β is the thickness of the gas hydrate cementing layer of the gas hydrate reservoir; S h is the gas hydrate saturation of the gas hydrate reservoir;

[0078] The Poisson's ratio of the sediment matrix of the natural gas hydrate reservoir can be determined by the following formula: Where, σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir;

[0079] The Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir can be determined by the following formula: Where, σ h is the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir, K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir;

[0080] The effective pressure of the sediment in the natural gas hydrate reservoir can be determined by the following formula: eff =(ρ s-ρ f )·g·L; where P eff is the effective pressure of the sediment in the gas hydrate reservoir, ρ s is the density of sedimentary minerals in the gas hydrate reservoir, ρ f is the density of formation water, g is the acceleration of gravity (usually 9.8m / s 2 ), L is the depth of the hydrate layer;

[0081] In a specific embodiment, the model for determining the dry rock skeleton elastic modulus of the natural gas hydrate reservoir under the specific skeleton support mode hydrate content condition based on the elastic modulus of the sediment matrix and the elastic modulus of the hydrate cementing layer is a model obtained by introducing the skeleton support mode hydrate content parameter into the loose sediment model proposed by Hertz-Mindlin (1949) and the equivalent medium model proposed by Helgerud (1999); for example,

[0082]

[0083]

[0084] φ e =φ·(1-S h ·f ms )

[0085] Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; φ is the reservoir porosity of the gas hydrate reservoir; φ c is the critical porosity of sediments. The critical porosity of sediments in the sea is usually 0.36-0.42; f ms is the volume fraction of skeleton-supported hydrates in the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the gas hydrate reservoir;

[0086] In a specific embodiment, a model for determining the elastic modulus of a natural gas hydrate reservoir saturated with fluid (i.e., the elastic modulus of the natural gas hydrate reservoir) under specific pore suspension mode hydrate content conditions based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix is ​​a model obtained by introducing the pore suspension mode hydrate content parameter into the saturated formation modulus calculation model proposed by Gassmann (1951); for example,

[0087]

[0088] μ sat =μ dry

[0089] φ eff =φ·(1-S h +S h ·f pf )

[0090]

[0091] Where K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; f pf is the volume fraction of hydrates in the pore suspension mode in the natural gas hydrates of the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; K w is the bulk modulus of formation water; K fl is the bulk modulus of the pore fluid in the natural gas hydrate reservoir: K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ sat is the shear modulus of the fluid-saturated formation of the gas hydrate reservoir;

[0092] More preferably, the calculation model of the natural gas hydrate reservoir longitudinal wave velocity with respect to the natural gas hydrate reservoir elastic modulus includes:

[0093]

[0094] Where V p is the P-wave velocity of the natural gas hydrate reservoir; K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ satis the shear modulus of the fluid-saturated formation of the natural gas hydrate reservoir; ρ b is the density of the sediment in the natural gas hydrate reservoir (using conventional methods in this field);

[0095] The natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents (i.e., the natural gas hydrate reservoir rock physics model applicable under different occurrence conditions) and the calculation model of the natural gas hydrate reservoir longitudinal wave velocity with respect to the elastic modulus of the natural gas hydrate reservoir in this preferred technical solution are rock physics acoustic models applicable to hydrates of particle cementation, encapsulation cementation, skeleton support and pore suspension modes. They can simulate the reservoir longitudinal wave velocity under different mineral contents, porosities and hydrate saturations, and quantitatively calculate the hydrate saturation of each occurrence mode based on the determination of the main occurrence mode of the hydrate.

[0096] In the above evaluation method, preferably, based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, and utilizing a P-wave velocity determination model for the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrates in each occurrence mode, determining the gas hydrate occurrence mode in the target gas hydrate reservoir includes:

[0097] Determining a hydrate saturation-P-wave velocity petrophysical template for the target gas hydrate reservoir, which includes relationship curves between hydrate saturation and P-wave velocity for the target gas hydrate reservoir under each gas hydrate occurrence mode, using a P-wave velocity determination model for the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrate in each occurrence mode;

[0098] Intersecting the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir on a hydrate saturation-P-wave velocity petrophysical template of the target gas hydrate reservoir, thereby determining the occurrence pattern of the gas hydrate in the target gas hydrate reservoir;

[0099] In this preferred technical solution, a rock physics acoustic map of different hydrate occurrence modes (i.e., a hydrate saturation-P-wave velocity rock physics template of the target natural gas hydrate reservoir) was established. By intersecting the hydrate saturation and P-wave velocity data on the rock physics acoustic map, the main hydrate occurrence mode of the reservoir was determined.

[0100] More preferably, the hydrate saturation-P-wave velocity petrophysical template of the target gas hydrate reservoir, which includes the relationship curves between hydrate saturation and P-wave velocity of the target gas hydrate reservoir under each gas hydrate occurrence mode, is determined by using the P-wave velocity determination model of the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrate in each occurrence mode, by the following method:

[0101] Using a P-wave velocity determination model for a target natural gas hydrate reservoir having a specific content of natural gas hydrates in each occurrence mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under each natural gas hydrate occurrence mode is determined, for example, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a particle cementation mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under an encapsulation cementation mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a skeleton support mode, and a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a pore suspension mode.

[0102] The relationship curves between hydrate saturation and P-wave velocity of the target gas hydrate reservoir under each gas hydrate occurrence mode are combined to form a hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir;

[0103] More preferably, determining the occurrence pattern of natural gas hydrates in the target natural gas hydrate reservoir is achieved by:

[0104] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect on a certain hydrate saturation-P-wave velocity relationship curve in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the hydrate saturation-P-wave velocity relationship curve is the gas hydrate occurrence pattern in the target gas hydrate reservoir;

[0105] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect between two hydrate saturation-P-wave velocity relationship curves in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the occurrence pattern of the gas hydrate in the target gas hydrate reservoir includes the gas hydrate occurrence patterns corresponding to the two hydrate saturation-P-wave velocity relationship curves respectively;

[0106] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect above all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the uppermost relationship curve between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of gas hydrate in the target gas hydrate reservoir;

[0107] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect below all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the relationship curve between hydrate saturation and P-wave velocity at the bottom of the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of the gas hydrate in the target gas hydrate reservoir.

[0108] In the above evaluation method, preferably, the method further comprises:

[0109] Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is used for the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrates of each occurrence mode, to determine the content of gas hydrates of each occurrence mode in the target gas hydrate reservoir (for example, the saturation of gas hydrates of each occurrence mode in the target gas hydrate reservoir is used to represent the content of gas hydrates of each occurrence mode in the target gas hydrate reservoir);

[0110] More preferably, based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode is utilized, combined with the determined occurrence mode of the gas hydrates in the target gas hydrate reservoir, to inversely calculate the content of gas hydrates in each occurrence mode in the target gas hydrate reservoir (for example, the saturation of gas hydrates in each occurrence mode in the target gas hydrate reservoir is used to characterize the content of gas hydrates in each occurrence mode in the target gas hydrate reservoir);

[0111] In this preferred technical solution, quantitative evaluation of the saturation of hydrates in different occurrence modes is achieved.

[0112] P-wave velocity data is affected by both hydrate saturation and hydrate occurrence pattern. Based on this, the present invention proposes a method for evaluating the occurrence pattern of natural gas hydrates in natural gas hydrate reservoirs based on hydrate saturation and P-wave data. Compared to existing technologies, the technical solution provided by the present invention has the following beneficial effects:

[0113] 1. The method for evaluating the occurrence pattern of natural gas hydrates in natural gas hydrate reservoirs provided by the present invention uses P-wave velocity and hydrate saturation data to identify the occurrence pattern of hydrate reservoirs. Since P-wave velocity and hydrate saturation can be obtained directly from well logging data without relying on core sampling experiments, the technical solution provided by the present invention is more practical.

[0114] 2. The method for evaluating the occurrence pattern of natural gas hydrates in natural gas hydrate reservoirs provided by the present invention utilizes a longitudinal wave velocity determination model of the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence pattern to identify the occurrence pattern of natural gas hydrates, which helps to accurately identify natural gas hydrates in each occurrence pattern, and further helps to identify the content of natural gas hydrates in each occurrence pattern, providing a more economical and effective hydrate reservoir evaluation method for marine natural gas hydrate reservoir evaluation and resource development and utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0115] Figure 1 This is a schematic flow chart of the method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir in Example 1.

[0116] Figure 2 Schematic diagram of the hydrate saturation-compressional wave velocity rock physics template for the target natural gas hydrate reservoir in Example 1.

[0117] Figure 3 This is a diagram showing the evaluation results of the natural gas hydrate occurrence pattern in the target natural gas hydrate reservoir in Example 1. DETAILED DESCRIPTION

[0118] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0119] A specific embodiment of the present invention provides a method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir, wherein the method comprises:

[0120] Step S1: obtaining the longitudinal wave velocity of the target natural gas hydrate reservoir;

[0121] Step S2: obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir;

[0122] Step S3: obtaining a P-wave velocity determination model for the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence mode (i.e., a P-wave velocity determination model for the target natural gas hydrate reservoir under different occurrence conditions);

[0123] Step S4: Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, the P-wave velocity determination model of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode is used to determine the occurrence mode of the gas hydrate in the target gas hydrate reservoir.

[0124] Among them, the longitudinal wave velocity determination model of the target natural gas hydrate reservoir when it has a specific content of natural gas hydrates in each occurrence mode is the relationship model between the longitudinal wave velocity of the target natural gas hydrate reservoir and the content of natural gas hydrates in each occurrence mode, that is, the calculation model of the longitudinal wave velocity of the target natural gas hydrate reservoir with respect to the content of natural gas hydrates in each occurrence mode, which can determine the longitudinal wave velocity corresponding to the target natural gas hydrate reservoir when the content of natural gas hydrates in each occurrence mode is any value.

[0125] In some embodiments, the method further comprises:

[0126] Target natural gas hydrate reservoir identification steps: obtaining well logging data of the target study area; determining the natural gas hydrate reservoir in the target study area as the target natural gas hydrate reservoir based on the well logging data of the target study area;

[0127] Among them, determining the natural gas hydrate reservoir in the target study area based on the logging data of the target study area can be carried out in accordance with conventional methods in the field. Usually, the natural gas hydrate reservoir can be judged by the following characteristics: the natural gas hydrate reservoir exhibits low natural gamma value, low density value, high neutron porosity value, high resistivity value, high acoustic wave velocity value and other characteristics on the logging curve.

[0128] In some embodiments, in step S1, obtaining the longitudinal wave velocity of the target natural gas hydrate reservoir includes:

[0129] Step S11: P-wave velocity logging data of the target natural gas hydrate reservoir;

[0130] Step S12: Determine the P-wave velocity of the target natural gas hydrate reservoir based on the P-wave velocity logging data of the target natural gas hydrate reservoir.

[0131] In some embodiments, in step S2, obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir includes:

[0132] Step S21: obtaining the mud content of the target natural gas hydrate reservoir;

[0133] Step S22: obtaining the reservoir porosity of the target natural gas hydrate reservoir;

[0134] Step S23: obtaining the resistivity of the target natural gas hydrate reservoir;

[0135] Step S24: determining the gas hydrate saturation of the target gas hydrate reservoir based on the shale content of the target gas hydrate reservoir, the reservoir porosity of the target gas hydrate reservoir, and the resistivity of the target gas hydrate reservoir;

[0136] Furthermore, in step S21, obtaining the mud content of the target natural gas hydrate reservoir includes:

[0137] Step S211: acquiring natural gamma ray logging data of the target natural gas hydrate reservoir;

[0138] Step S211: determining the shale content of the target natural gas hydrate reservoir based on the natural gamma ray logging data of the target natural gas hydrate reservoir;

[0139] In step S211, the mud content of the target natural gas hydrate reservoir can be determined by the following formula: Where: GR is the natural gamma logging value of the target gas hydrate reservoir; GR max is the natural gamma logging value of pure sandstone; GR min is the natural gamma ray logging value of the pure mudstone section; GUCR is the regional empirical coefficient (usually around 2 for old formations and around 3.7 for new formations); V sh is the mud content of the target gas hydrate reservoir;

[0140] Furthermore, in step S22, obtaining the reservoir porosity of the target natural gas hydrate reservoir includes:

[0141] Step S221: acquiring density logging data of the target natural gas hydrate reservoir, and determining the density porosity of the target natural gas hydrate reservoir based on the density logging data of the target natural gas hydrate reservoir;

[0142] Step S222: acquiring neutron porosity logging data of the target natural gas hydrate reservoir, and determining the neutron porosity of the target natural gas hydrate reservoir based on the neutron porosity logging data of the target natural gas hydrate reservoir;

[0143] Step S223: performing shale correction on the neutron porosity and density porosity of the target natural gas hydrate reservoir in combination with the shale content of the target natural gas hydrate reservoir, to obtain the shale-corrected neutron porosity and shale-corrected density porosity of the target natural gas hydrate reservoir;

[0144] Step S224: determining the reservoir porosity of the target natural gas hydrate reservoir based on the neutron porosity corrected for the shale content of the target natural gas hydrate reservoir and the density porosity corrected for the shale content of the target natural gas hydrate reservoir;

[0145] In step S221, the density porosity of the target natural gas hydrate reservoir can be determined by the following formula: Where: ρ ma is the stratum skeleton density; ρ b is the formation density logging value of the target natural gas hydrate reservoir; ρ f is the formation water density; φ D is the density porosity of the target gas hydrate reservoir;

[0146] In step S223, the neutron porosity of the target natural gas hydrate reservoir can be corrected for shale using the following formula: Nc =φ N -V sh ·φ Nsh Where: φ N is the neutron porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Nsh is the apparent neutron porosity of mud; φ Nc Neutron porosity corrected for the mud quality of the target gas hydrate reservoir;

[0147] In step S223, the density porosity of the target natural gas hydrate reservoir can be corrected for shale using the following formula: Dc =φ D -V sh ·φ Dsh Where: φ D is the density porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Dsh is the apparent density porosity of mud; φ Dc The density porosity corrected for the mud quality of the target gas hydrate reservoir;

[0148] In step S224, the reservoir porosity of the target natural gas hydrate reservoir can be determined by the following formula: Where: φNc is the neutron porosity of the target gas hydrate reservoir after mud correction; φ Dc is the density porosity of the target natural gas hydrate reservoir after mud correction; φ is the reservoir porosity of the target natural gas hydrate reservoir;

[0149] Furthermore, in step S23, obtaining the resistivity of the target natural gas hydrate reservoir includes:

[0150] Step S231: Acquire resistivity logging data of the target natural gas hydrate reservoir;

[0151] Step S232: determining the resistivity of the target natural gas hydrate reservoir based on the resistivity logging data of the target natural gas hydrate reservoir;

[0152] Furthermore, in step S24, the natural gas hydrate saturation of the target natural gas hydrate reservoir can be determined by the following formula:

[0153]

[0154] Where: φ is the reservoir porosity of the target natural gas hydrate reservoir; R w is the formation water resistivity; R sh is the resistivity of clay; V sh is the mud content of the target natural gas hydrate reservoir; R t is the resistivity of the target natural gas hydrate reservoir; a, m, and n are Archie constants (the Archie constants a, m, and n can be determined in a conventional manner); wherein, the Archie constants a and m indicate the lithoelectric relationship of the formation and are usually obtained by fitting the resistivity and porosity of the surrounding rock; the Archie constant n depends on the reservoir lithology and is usually close to 2.

[0155] In some embodiments, in step S3, each occurrence mode includes a particle cementation mode, an encapsulation cementation mode, a skeleton support mode, and a pore suspension mode.

[0156] In some embodiments, in step S3, obtaining a P-wave velocity determination model for a target natural gas hydrate reservoir having a specific content of natural gas hydrates in each occurrence mode includes:

[0157] Step S31: obtaining the reservoir porosity of the target natural gas hydrate reservoir;

[0158] Step S32: obtaining the physical property parameters of the sedimentary minerals and the physical property parameters of the natural gas hydrate reservoir of the target natural gas hydrate reservoir;

[0159] Step S33: Obtaining a natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents (i.e., a natural gas hydrate reservoir rock physics model applicable to different occurrence conditions) and a calculation model of the natural gas hydrate reservoir longitudinal wave velocity with respect to the natural gas hydrate reservoir elastic modulus;

[0160] Step S34: Determine a P-wave velocity determination model for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode based on the reservoir porosity of the target gas hydrate reservoir, the physical properties of the sedimentary minerals in the target gas hydrate reservoir, the physical properties of the gas hydrate reservoir in the target gas hydrate reservoir, a gas hydrate reservoir rock physics model applicable to gas hydrates in each occurrence mode with a specific content, and a calculation model for the P-wave velocity of the gas hydrate reservoir with respect to the elastic modulus of the gas hydrate reservoir;

[0161] The natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents is a natural gas hydrate reservoir elastic modulus determination model applicable to natural gas hydrates of various occurrence modes with specific contents;

[0162] Furthermore, in step S33, the natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents includes:

[0163] A model for determining the elastic modulus of the sediment matrix in a natural gas hydrate reservoir under conditions of hydrate content in a specific skeleton support mode;

[0164] A model for determining the elastic modulus of the hydrate cementation layer in a natural gas hydrate reservoir based on the hydrate content of a specific particle cementation mode and the hydrate content of a specific encapsulation cementation mode of the sediment matrix elastic modulus;

[0165] A model for determining the elastic modulus of dry rock skeleton in natural gas hydrate reservoirs under hydrate content conditions based on a specific skeleton support model based on the elastic modulus of the sediment matrix and the elastic modulus of the hydrate cement layer;

[0166] A model is provided for determining the elastic modulus of the gas hydrate reservoir saturated with fluid (i.e., the gas hydrate reservoir elastic modulus) under specific pore suspension mode hydrate content conditions based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix;

[0167] Among them, the determination model of the matrix elastic modulus of the natural gas hydrate reservoir sediment under the condition of a specific skeleton-supported hydrate content can be obtained by introducing the skeleton-supported hydrate content parameter into the Viogt-Reuss-Hill sediment matrix elastic modulus calculation model; for example,

[0168]

[0169]

[0170] φ h =φ·S h ·f ms

[0171] Where K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir; K s is the bulk modulus of the sedimentary minerals of the natural gas hydrate reservoir (using conventional methods in this field); μ s is the shear modulus of the sedimentary minerals of the natural gas hydrate reservoir (using conventional methods in this field); ms is the volume fraction of skeleton-supported hydrates in the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir;

[0172] Among them, the model for determining the elastic modulus of the hydrate cementation layer in the natural gas hydrate reservoir under the conditions of the hydrate content of the specific particle cementation mode and the hydrate content of the specific encapsulation cementation mode based on the elastic modulus of the sediment matrix can be a model obtained by introducing the hydrate content parameters of the particle cementation mode and the hydrate content parameters of the encapsulation cementation mode into the cementation model proposed by Dvorkin and Nur (1993); for example,

[0173]

[0174]

[0175]

[0176] in,

[0177] S n (β) = A n (Λ n )·β 2 +B n (Λ n )·β+C n (Λ n )

[0178] St (β) = A t (Λ t ,v)·β 2 +B t (Λ t ,v)·β+C t (Λ t ,v)

[0179] A n (Λ n )=-0.024153·Λ n -1.3646

[0180] B n (Λ n )=0.20405·Λ n -0.89008

[0181] C n (Λ n )=0.00024649·Λ n -1.9864

[0182]

[0183]

[0184]

[0185]

[0186]

[0187] Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; φ c is the critical porosity of sediments. The critical porosity of sediments in the sea is usually 0.36-0.42; σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir; σ h is the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir; P eff is the effective pressure of the sediment in the gas hydrate reservoir, which can be calculated based on the depth of the gas hydrate reservoir and the density of the sedimentary minerals and formation water in the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ his the shear modulus of natural hydrates in the gas hydrate reservoir; C is the coordination number of sediments in the gas hydrate reservoir, usually 4-11; τ is the friction coefficient indicating the grain roughness or slip degree, usually 0-1; f cc is the volume fraction of the particle-cemented hydrate in the natural gas hydrate of the natural gas hydrate reservoir (based on the total volume of the natural gas hydrate in the natural gas hydrate reservoir); f gc is the volume fraction of hydrates in the gas hydrate of the gas hydrate reservoir (based on the total volume of gas hydrates in the gas hydrate reservoir); β0 is the radius of the initial gas hydrate cementing layer of the gas hydrate reservoir; β is the thickness of the gas hydrate cementing layer of the gas hydrate reservoir; S h is the gas hydrate saturation of the gas hydrate reservoir;

[0188] Wherein, the Poisson's ratio of the sediment matrix of the natural gas hydrate reservoir can be determined by the following formula: σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir;

[0189] Wherein, the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir can be determined by the following formula: σ h is the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir, K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir;

[0190] Where, the effective pressure of the sediment in the gas hydrate reservoir can be determined by the following formula: P eff =(ρ s -ρ f )·g·L;P eff is the effective pressure of the sediment in the gas hydrate reservoir, ρ s is the density of sedimentary minerals in the gas hydrate reservoir, ρ f is the density of formation water, g is the acceleration of gravity (usually 9.8m / s 2 ), L is the depth of the hydrate layer;

[0191] It can be seen that when the volume fraction of the particle-cemented hydrate in the natural gas hydrate reservoir is 0 (that is, when there is no particle-cemented hydrate),

[0192] When the volume fraction of the encapsulated cementation mode hydrate in the natural gas hydrate of the natural gas hydrate reservoir is 0 (that is, when there is no encapsulated cementation mode hydrate),

[0193] When the volume fraction of the particle-cementing mode hydrate in the gas hydrate of the gas hydrate reservoir is 0 and the volume fraction of the encapsulated-cementing mode hydrate in the gas hydrate of the gas hydrate reservoir is 0, β = β0;

[0194] Among them, the model for determining the dry rock skeleton elastic modulus of the natural gas hydrate reservoir under the specific skeleton support mode hydrate content condition based on the elastic modulus of the sediment matrix and the elastic modulus of the hydrate cementing layer can be a model obtained by introducing the skeleton support mode hydrate content parameter into the loose sediment model proposed by Hertz-Mindlin (1949) or the equivalent medium model proposed by Helgerud (1999); for example,

[0195]

[0196]

[0197] φ e =φ·(1-S h ·f ms )

[0198] Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; φ is the reservoir porosity of the gas hydrate reservoir; φ c is the critical porosity of sediments. The critical porosity of sediments in the sea is usually 0.36-0.42; f ms is the volume fraction of skeleton-supported hydrates in the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the gas hydrate reservoir;

[0199] Among them, the model for determining the elastic modulus of the saturated fluid formation of the natural gas hydrate reservoir (i.e., the elastic modulus of the natural gas hydrate reservoir) under the condition of a specific pore suspension mode hydrate content based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix can be a model obtained by introducing the pore suspension mode hydrate content parameter into the saturated formation modulus calculation model proposed by Gassmann (1951); for example,

[0200]

[0201] μ sat =μ dry

[0202] φ eff =φ·(1-S h +S h ·f pf )

[0203]

[0204] Where K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; f pf is the volume fraction of hydrates in the pore suspension mode in the natural gas hydrates of the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; K w is the bulk modulus of formation water; K fl is the bulk modulus of the pore fluid in the natural gas hydrate reservoir: K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ sat is the shear modulus of the fluid-saturated formation of the gas hydrate reservoir;

[0205] Furthermore, in step S33, the calculation model of the natural gas hydrate reservoir longitudinal wave velocity with respect to the natural gas hydrate reservoir elastic modulus includes:

[0206]

[0207] Where V p is the longitudinal wave velocity of the natural gas hydrate reservoir, in km / s; K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir, in GPa; μsat is the shear modulus of the fluid-saturated formation of the natural gas hydrate reservoir, in GPa; ρ b is the density of the sediment in the natural gas hydrate reservoir (conventional methods in this field can be used), in g / cm 3 .

[0208] The natural gas hydrate reservoir rock physics model applicable to natural gas hydrates of various occurrence modes with specific contents (i.e., the natural gas hydrate reservoir rock physics model applicable under different occurrence conditions) and the calculation model of the natural gas hydrate reservoir longitudinal wave velocity with respect to the elastic modulus of the natural gas hydrate reservoir in the above-mentioned embodiment are rock physics acoustic models applicable to hydrates of particle cementation, encapsulation cementation, skeleton support and pore suspension modes. They can simulate the reservoir longitudinal wave velocity under different mineral contents, porosities and hydrate saturations, and quantitatively calculate the hydrate saturation of each occurrence mode based on the determination of the main occurrence mode of the hydrate.

[0209] In some embodiments, step S4 further comprises:

[0210] Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is utilized for the target gas hydrate reservoir when it has a specific content of gas hydrates of each occurrence mode to determine the content of gas hydrates of each occurrence mode in the target gas hydrate reservoir (for example, the saturation of gas hydrates of each occurrence mode in the target gas hydrate reservoir is used to characterize the content of gas hydrates of each occurrence mode in the target gas hydrate reservoir).

[0211] In some embodiments, in step S4, based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, and using a P-wave velocity determination model for the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrates in each occurrence mode, determining the gas hydrate occurrence mode in the target gas hydrate reservoir includes:

[0212] Step S41: using a P-wave velocity determination model for a target natural gas hydrate reservoir having a specific content of natural gas hydrates in each occurrence mode, determining a hydrate saturation-P-wave velocity petrophysical template for the target natural gas hydrate reservoir, which includes relationship curves between hydrate saturation and P-wave velocity for the target natural gas hydrate reservoir in each natural gas hydrate occurrence mode;

[0213] Step S42: Intersecting the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir on the hydrate saturation-P-wave velocity petrophysical template of the target gas hydrate reservoir, thereby determining the occurrence mode of the gas hydrate in the target gas hydrate reservoir;

[0214] Furthermore, step S4 further includes:

[0215] Step S43: Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is used for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode, and the occurrence mode of the gas hydrates in the target gas hydrate reservoir is determined in combination with the determined gas hydrate content, and the content of the gas hydrates in each occurrence mode in the target gas hydrate reservoir is inverted and calculated (for example, the saturation of the gas hydrates in each occurrence mode in the target gas hydrate reservoir is used to represent the content of the gas hydrates in each occurrence mode in the target gas hydrate reservoir);

[0216] In step S41, the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, which includes the relationship curves between the hydrate saturation and the P-wave velocity of the target gas hydrate reservoir under each gas hydrate occurrence mode, is determined by using the P-wave velocity determination model of the target gas hydrate reservoir when the target gas hydrate reservoir has a specific content of gas hydrate in each occurrence mode. This can be achieved in the following manner:

[0217] Using a P-wave velocity determination model for a target natural gas hydrate reservoir having a specific content of natural gas hydrates in each occurrence mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under each natural gas hydrate occurrence mode is determined, for example, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a particle cementation mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under an encapsulation cementation mode, a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a skeleton support mode, and a relationship curve between hydrate saturation and P-wave velocity of the target natural gas hydrate reservoir under a pore suspension mode.

[0218] The relationship curves between hydrate saturation and P-wave velocity of the target gas hydrate reservoir under each gas hydrate occurrence mode are combined to form a hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir;

[0219] In step S42, the occurrence pattern of natural gas hydrates in the target natural gas hydrate reservoir is determined by:

[0220] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect on a certain hydrate saturation-P-wave velocity relationship curve in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the hydrate saturation-P-wave velocity relationship curve is the gas hydrate occurrence pattern in the target gas hydrate reservoir;

[0221] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect between two hydrate saturation-P-wave velocity relationship curves in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the occurrence pattern of the gas hydrate in the target gas hydrate reservoir includes the gas hydrate occurrence patterns corresponding to the two hydrate saturation-P-wave velocity relationship curves respectively;

[0222] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect above all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the uppermost relationship curve between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of gas hydrate in the target gas hydrate reservoir;

[0223] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect below all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the relationship curve between hydrate saturation and P-wave velocity at the bottom of the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of the gas hydrate in the target gas hydrate reservoir.

[0224] Example 1

[0225] This embodiment provides a method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir.

[0226] This method is used to determine the occurrence mode of natural gas hydrates in a certain sea area and the content of natural gas hydrates in each occurrence mode.

[0227] like Figure 1 As shown, the method includes:

[0228] Step 1: Obtain well logging data in the target study area, identify the target natural gas hydrate reservoir, and obtain geological data of the target natural gas hydrate reservoir; specifically:

[0229] Obtain natural gamma ray logging curves, density logging curves, neutron porosity logging curves, resistivity logging curves, and sonic velocity logging curves in the target study area;

[0230] Based on the well logging data of the target study area, the natural gas hydrate reservoir in the target study area is identified as the target natural gas hydrate reservoir; the natural gas hydrate reservoir is identified by the following characteristics: the natural gas hydrate reservoir has characteristics such as low natural gamma value, low density value, high neutron porosity value, high resistivity value, and high acoustic velocity value on the well logging curve;

[0231] The geological data of the target gas hydrate reservoir were obtained to determine the physical properties of the sedimentary minerals, formation water, and gas hydrate reservoir, including the mineral components of the target gas hydrate reservoir and the elastic modulus, density, and content of each mineral component. The elastic modulus and density of the formation water were determined, and the elastic modulus and density of the gas hydrate were determined. The results are shown in Table 1.

[0232] Table 1

[0233] Components Bulk modulus (GPa) Shear modulus (GPa) <![CDATA[Density (g / cm 3 )]]> Volume fraction (%) quartz 36.0 44.3 2.65 62 clay 20.9 6.5 2.58 20 calcite 76.8 32 2.71 18 formation water 2.5 -- 1.03 -- Hydrate 6.4 2.5 0.91 --

[0234] Step 2: Determine the reservoir porosity of the natural gas hydrate reservoir based on the well logging data of the natural gas hydrate reservoir; specifically:

[0235] Based on the natural gamma logging data of the target gas hydrate reservoir, the mud content of the target gas hydrate reservoir is determined by the following formula: Where: GR is the natural gamma logging value of the target gas hydrate reservoir; GR max is the natural gamma logging value of pure sandstone, which is 80API; GR min is the natural gamma logging value of the pure mudstone section, which is 22API; GUCR is the regional empirical coefficient, which is 3.7; V sh is the mud content of the target gas hydrate reservoir;

[0236] The density porosity of the target natural gas hydrate reservoir is determined based on the density logging data of the target natural gas hydrate reservoir using the following formula: Where: ρ ma is the stratum skeleton density, which is 2.63 g / cm 3 ρ b is the formation density logging value of the target natural gas hydrate reservoir; ρ f is the formation water density; φ Dis the density porosity of the target gas hydrate reservoir;

[0237] determining the neutron porosity of the target natural gas hydrate reservoir based on neutron porosity logging data of the target natural gas hydrate reservoir;

[0238] Combined with the shale content of the target gas hydrate reservoir, the neutron porosity of the target gas hydrate reservoir is corrected for shale content using the following formula to obtain the neutron porosity of the target gas hydrate reservoir after shale correction: Nc =φ N -V sh ·φ Nsh Where: φ N is the neutron porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Nsh is the apparent neutron porosity of mud, which is 0.3; φ Nc Neutron porosity corrected for the mud quality of the target gas hydrate reservoir;

[0239] Combined with the shale content of the target gas hydrate reservoir, the density porosity of the target gas hydrate reservoir is corrected for shale content using the following formula to obtain the density porosity of the target gas hydrate reservoir after shale correction: Dc =φ D -V sh ·φ Dsh Where: φ D is the density porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Dsh is the apparent porosity of mud, with a value of 0.07; φ Dc The density porosity corrected for the mud quality of the target gas hydrate reservoir;

[0240] Based on the neutron porosity and density porosity corrected for the shale content of the target gas hydrate reservoir, the reservoir porosity of the target gas hydrate reservoir is determined by the following formula: Where: φ Nc is the neutron porosity of the target gas hydrate reservoir after mud correction; φ Dc is the density porosity of the target natural gas hydrate reservoir after mud correction; φ is the reservoir porosity of the target natural gas hydrate reservoir;

[0241] The porosity of the target natural gas hydrate reservoir is different at different depths, and the reservoir porosity of the determined target natural gas hydrate reservoir is between 0.35 and 0.55.

[0242] Step 3: Determine the gas hydrate saturation of the target gas hydrate reservoir; specifically:

[0243] Based on the resistivity logging data of the target gas hydrate reservoir, the resistivity of the target gas hydrate reservoir is determined; based on the mud content, reservoir porosity and resistivity of the target gas hydrate reservoir, the gas hydrate saturation of the target gas hydrate reservoir is determined by the following formula:

[0244]

[0245] Where: φ is the reservoir porosity of the target natural gas hydrate reservoir; R w is the formation water resistivity, which is 0.19Ω·m; R sh is the resistivity of clay, which is 5Ω·m; V sh is the mud content of the target natural gas hydrate reservoir; R t is the resistivity of the target natural gas hydrate reservoir; a, m, and n are Archie constants, where a and m indicate the rock-electrical relationship of the formation and are 1.09 and 2.22, respectively; and n depends on the reservoir lithology and is 2 in this embodiment.

[0246] Step 4: Obtain a P-wave velocity determination model for the target gas hydrate reservoir with a specific content of gas hydrates in each occurrence mode; and then determine the hydrate saturation-P-wave velocity rock physics template for the target gas hydrate reservoir; specifically:

[0247] 4.1. Obtain a rock physics model for a gas hydrate reservoir applicable to gas hydrates of various occurrence modes with specific contents (i.e., a rock physics model for a gas hydrate reservoir applicable to different occurrence conditions) and a calculation model for the relationship between the P-wave velocity of the gas hydrate reservoir and the elastic modulus of the gas hydrate reservoir;

[0248] Among them, the natural gas hydrate reservoir rock physics model of natural gas hydrates of each occurrence mode with a specific content (i.e., the natural gas hydrate reservoir rock physics model applicable to different occurrence conditions) is:

[0249]

[0250]

[0251] φ h =φ·S h ·f ms

[0252]

[0253]

[0254]

[0255]

[0256]

[0257] f e =φ·(1-S h ·f ms )

[0258]

[0259] m sat =μ dry

[0260] f eff =φ·(1-S h +S h ·f pf )

[0261]

[0262] Among them,

[0263] S n (b)=A n (L n )·b 2 +B n (L n )·β+C n (L n )

[0264] S t (b)=A t (L t ,v)·b 2 +B t (L t ,v)·β+C t (L t ,v)

[0265] A n (L n )=-0.024153·L n -1.3646

[0266] B n (L n )=0.20405·L n -0.89008

[0267] C n (Ln )=0.00024649·Λ n -1.9864

[0268]

[0269]

[0270]

[0271]

[0272]

[0273]

[0274] Where K h is the bulk modulus of natural gas hydrate in the gas hydrate reservoir, in GPa; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir, in GPa; K s is the bulk modulus of sedimentary minerals in the gas hydrate reservoir, (f q 、f sh 、f c are the volume fractions of quartz, clay, and calcite, respectively, K q , K sh , K c are the bulk moduli of quartz, clay, and calcite, respectively), which is 37.3 GPa in this embodiment; μ s is the shear modulus of sedimentary minerals in the gas hydrate reservoir, (f q 、f sh 、f c are the volume fractions of quartz, clay, and calcite, respectively, μ q 、μ sh 、μ c are the shear moduli of quartz, clay, and calcite, respectively), which is 27.2 GPa in this embodiment; f ms is the volume fraction of skeleton-supported hydrates in the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir); S h is the natural gas hydrate saturation of the natural gas hydrate reservoir, unit is dimensionless; K ma is the bulk modulus of the sediment matrix of the natural gas hydrate reservoir, in GPa; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir, in GPa; K ectis the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir, in GPa; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir, in GPa; φ is the reservoir porosity of the natural gas hydrate reservoir, in dimensionless units; φ c is the critical porosity of sediment, unit is dimensionless, and in this embodiment, the value is 0.4; σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir, unit is dimensionless; σ h is the Poisson's ratio of the natural gas hydrate in the natural gas hydrate reservoir, in dimensionless units, In this embodiment, the value is 0.218; P eff is the effective pressure of the sediment in the gas hydrate reservoir, in GPa, which is calculated based on the depth of the gas hydrate reservoir and the density of the sedimentary minerals and formation water in the gas hydrate reservoir (P eff =(ρ s -ρ f )·g·L,ρ s is the density of sedimentary minerals in the gas hydrate reservoir, ρ f is the density of formation water, g is the acceleration of gravity (valued at 9.8m / s 2 ), L is the depth of the hydrate layer, which is 0.019 GPa in this embodiment; C is the coordination number of the sediment in the natural gas hydrate reservoir, which is dimensionless and is 6 in this embodiment; τ is the friction coefficient indicating the grain roughness or slip degree, which is dimensionless and is 0.3 in this embodiment; f cc is the volume fraction of hydrates in the particle cementation mode in the natural gas hydrates of the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir), and the unit is dimensionless; gc is the volume fraction of hydrates in the cementing mode encapsulated in the gas hydrates of the gas hydrate reservoir (based on the total volume of gas hydrates in the gas hydrate reservoir), unit is dimensionless; β0 is the radius of the initial gas hydrate cementing layer of the gas hydrate reservoir, unit is dimensionless; β is the thickness of the gas hydrate cementing layer of the gas hydrate reservoir, unit is dimensionless; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir, in GPa; μ dry is the shear modulus of the dry rock skeleton of the natural gas hydrate reservoir, in GPa; f pf is the volume fraction of hydrates in the pore suspension mode in the natural gas hydrates of the natural gas hydrate reservoir (based on the total volume of natural gas hydrates in the natural gas hydrate reservoir), and the unit is dimensionless; K w is the bulk modulus of formation water, in GPa; K fl is the bulk modulus of the pore fluid in the natural gas hydrate reservoir, in GPa:Ksat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir, in GPa; μ sat is the shear modulus of the fluid-saturated formation of the gas hydrate reservoir, in GPa;

[0275] Among them, the calculation model of the compressional wave velocity of the natural gas hydrate reservoir with respect to the elastic modulus of the natural gas hydrate reservoir is:

[0276]

[0277] Where K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ sat is the shear modulus of the fluid-saturated formation of the natural gas hydrate reservoir; V p is the P-wave velocity of the natural gas hydrate reservoir; ρ b is the density of the sediment in the gas hydrate reservoir, which is the weighted average of the density of each component in the gas hydrate reservoir;

[0278] 4.2. Determine a model for determining the P-wave velocity of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode, by combining the reservoir porosity of the target gas hydrate reservoir, the physical properties of the sedimentary minerals in the target gas hydrate reservoir, and the physical properties of the gas hydrate reservoir in the target gas hydrate reservoir, using a rock physics model applicable to gas hydrates in each occurrence mode with a specific content, and a calculation model for the P-wave velocity of the gas hydrate reservoir with respect to the elastic modulus of the gas hydrate reservoir;

[0279] 4.3. Utilizing a P-wave velocity determination model for target gas hydrate reservoirs with specific gas hydrate contents in various occurrence modes, by setting different hydrate saturations for the occurrence modes to simulate formation P-wave velocities under different conditions, the following relationship curves were determined: hydrate saturation versus P-wave velocity for the target gas hydrate reservoir in a particle-cemented pattern, hydrate saturation versus P-wave velocity for the target gas hydrate reservoir in an encapsulated-cemented pattern, hydrate saturation versus P-wave velocity for the target gas hydrate reservoir in a skeletal-supported pattern, and hydrate saturation versus P-wave velocity for the target gas hydrate reservoir in a pore-suspension pattern.

[0280] 4.4. The hydrate saturation-P-wave velocity relationship curves for the target gas hydrate reservoir under the particle cementation model, the hydrate saturation-P-wave velocity relationship curves for the target gas hydrate reservoir under the encapsulation cementation model, the hydrate saturation-P-wave velocity relationship curves for the target gas hydrate reservoir under the skeleton support model, and the hydrate saturation-P-wave velocity relationship curves for the target gas hydrate reservoir under the pore suspension model are combined to form a hydrate saturation-P-wave velocity rock physics template for the target gas hydrate reservoir; the results are shown in the figure below. Figure 2 shown.

[0281] Step 5: On the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, intersect the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir to determine the occurrence pattern of gas hydrates in the target gas hydrate reservoir; specifically:

[0282] On the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir are intersected to determine the occurrence pattern of gas hydrates in the target gas hydrate reservoir:

[0283] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect on the relationship curve between the hydrate saturation and P-wave velocity of the target gas hydrate reservoir under the particle cementation mode, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is the particle cementation mode;

[0284] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect on the relationship curve between the hydrate saturation and P-wave velocity of the target gas hydrate reservoir under the encapsulation cementation mode, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is the encapsulation cementation mode;

[0285] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect on the relationship curve between the hydrate saturation and P-wave velocity of the target gas hydrate reservoir under the skeleton support mode, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is the skeleton support mode;

[0286] When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect on the relationship curve between the hydrate saturation and P-wave velocity of the target gas hydrate reservoir under the pore suspension mode, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is the pore suspension mode;

[0287] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir converge in region I, the occurrence mode of gas hydrate in the target gas hydrate reservoir is the particle cementation mode;

[0288] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir converge in region II, the occurrence mode of gas hydrate in the target gas hydrate reservoir is the coexistence of particle cementation mode and encapsulation cementation mode;

[0289] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir converge in region III, the occurrence mode of gas hydrate in the target gas hydrate reservoir is the coexistence of the encapsulation cementation mode and the skeleton support mode;

[0290] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir converge in region IV, the occurrence mode of gas hydrate in the target gas hydrate reservoir is the coexistence of the skeleton support mode and the pore suspension mode;

[0291] When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir converge in region V, the occurrence mode of the gas hydrate in the target gas hydrate reservoir is the pore suspension mode;

[0292] Results see Figure 3 .

[0293] Step 6: Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is used for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode. Combined with the determined occurrence mode of the gas hydrate in the target gas hydrate reservoir, the content of gas hydrates in each occurrence mode in the target gas hydrate reservoir is inverted and calculated;

[0294] The results are as follows Figure 3 shown.

[0295] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for evaluating the occurrence pattern of natural gas hydrates in a natural gas hydrate reservoir, wherein: The method includes: Obtain the P-wave velocity of the target natural gas hydrate reservoir; Obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir; Obtaining a model for determining the longitudinal wave velocity of a target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode; including: obtaining the reservoir porosity of the target gas hydrate reservoir; obtaining the sedimentary mineral physical property parameters and the physical property parameters of the gas hydrate reservoir; obtaining a gas hydrate reservoir rock physics model applicable to gas hydrates in each occurrence mode with a specific content and a calculation model for the longitudinal wave velocity of the gas hydrate reservoir with respect to the elastic modulus of the gas hydrate reservoir; determining the model for determining the longitudinal wave velocity of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode based on the reservoir porosity of the target gas hydrate reservoir, the sedimentary mineral physical property parameters of the target gas hydrate reservoir, the physical property parameters of the target gas hydrate reservoir, the gas hydrate reservoir rock physics model applicable to gas hydrates in each occurrence mode with a specific content and the calculation model for the longitudinal wave velocity of the gas hydrate reservoir with respect to the elastic modulus of the gas hydrate reservoir; Based on the P-wave velocity and the P-wave saturation of the target gas hydrate reservoir, the P-wave velocity determination model of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode is used to determine the occurrence mode of the gas hydrate in the target gas hydrate reservoir; the method includes: using the P-wave velocity determination model of the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode to determine a hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir that includes a relationship curve between the hydrate saturation and P-wave velocity of the target gas hydrate reservoir under each gas hydrate occurrence mode; and intersecting the P-wave velocity and the P-wave saturation of the target gas hydrate reservoir on the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir to determine the occurrence mode of the gas hydrate in the target gas hydrate reservoir.

2. The evaluation method according to claim 1, wherein Obtaining the P-wave velocity of the target natural gas hydrate reservoir includes: P-wave velocity logging data of the target gas hydrate reservoir; The P-wave velocity of the target natural gas hydrate reservoir is determined based on the P-wave velocity logging data of the target natural gas hydrate reservoir.

3. The evaluation method according to claim 1, wherein: Obtaining the natural gas hydrate saturation of the target natural gas hydrate reservoir includes: Obtain the mud content of the target natural gas hydrate reservoir; Obtaining the reservoir porosity of the target natural gas hydrate reservoir; Obtaining the resistivity of the target natural gas hydrate reservoir; The gas hydrate saturation of the target gas hydrate reservoir is determined based on the mud content of the target gas hydrate reservoir, the reservoir porosity of the target gas hydrate reservoir, and the resistivity of the target gas hydrate reservoir.

4. The evaluation method according to claim 3, wherein: Obtaining the mud content of the target natural gas hydrate reservoir includes: Obtain natural gamma ray logging data for target gas hydrate reservoirs; The shale content of the target gas hydrate reservoir is determined based on the natural gamma ray logging data of the target gas hydrate reservoir.

5. The evaluation method according to claim 3, wherein: Obtaining the resistivity of the target natural gas hydrate reservoir includes: Obtain resistivity logging data for target natural gas hydrate reservoirs; The resistivity of the target natural gas hydrate reservoir is determined based on the resistivity logging data of the target natural gas hydrate reservoir.

6. The evaluation method according to claim 3, wherein: The natural gas hydrate saturation of the target natural gas hydrate reservoir is determined by the following formula: Where: φ is the reservoir porosity of the target natural gas hydrate reservoir; R w is the formation water resistivity; R sh is the resistivity of clay; V sh is the mud content of the target natural gas hydrate reservoir; R t is the resistivity of the target natural gas hydrate reservoir; a, m, and n are Archie constants.

7. The evaluation method according to claim 1 or 3, wherein Obtaining the reservoir porosity of the target natural gas hydrate reservoir includes: Acquiring density logging data of a target natural gas hydrate reservoir, and determining the density porosity of the target natural gas hydrate reservoir based on the density logging data of the target natural gas hydrate reservoir; Acquiring neutron porosity logging data of a target natural gas hydrate reservoir, and determining the neutron porosity of the target natural gas hydrate reservoir based on the neutron porosity logging data of the target natural gas hydrate reservoir; Performing shale correction on the neutron porosity and density porosity of the target gas hydrate reservoir in combination with the shale content of the target gas hydrate reservoir, respectively, to obtain the shale-corrected neutron porosity and shale-corrected density porosity of the target gas hydrate reservoir; The reservoir porosity of the target natural gas hydrate reservoir is determined based on the neutron porosity corrected for the shale content of the target natural gas hydrate reservoir and the density porosity corrected for the shale content of the target natural gas hydrate reservoir.

8. The evaluation method according to claim 7, wherein: The density porosity of the target natural gas hydrate reservoir is determined by the following formula: Where: ρ ma is the stratum skeleton density; ρ b is the formation density logging value of the target natural gas hydrate reservoir; ρ f is the formation water density; φ D is the density porosity of the target natural gas hydrate reservoir.

9. The evaluation method according to claim 7, wherein: The neutron porosity of the target gas hydrate reservoir is corrected for shale using the following formula: f Nc =φ N -V sh ·f Nsh Where: φ N is the neutron porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Nsh is the apparent neutron porosity of mud; φ Nc is the neutron porosity of the target gas hydrate reservoir after correction for mud quality.

10. The evaluation method according to claim 7, wherein: The density porosity of the target natural gas hydrate reservoir is corrected for shale using the following formula: f Dc =φ D -V sh ·f Dsh Where: φ D is the density porosity of the target natural gas hydrate reservoir; V sh is the mud content of the target natural gas hydrate reservoir; φ Dsh is the apparent density porosity of mud; φ Dc is the density porosity of the target gas hydrate reservoir after correction for mud quality.

11. The evaluation method according to claim 7, wherein: The reservoir porosity of the target natural gas hydrate reservoir is determined by the following formula: Where: φ Nc is the neutron porosity of the target gas hydrate reservoir after mud correction; φ Dc is the density porosity of the target natural gas hydrate reservoir after mud correction; φ is the reservoir porosity of the target natural gas hydrate reservoir.

12. The evaluation method according to claim 1, wherein: The rock physics models of natural gas hydrate reservoirs applicable to natural gas hydrates of various occurrence modes with specific contents include: a model for determining the elastic modulus of the sediment matrix of the natural gas hydrate reservoir under the condition of a specific skeleton-supported mode hydrate content; a model for determining the elastic modulus of the hydrate cementing layer in the natural gas hydrate reservoir under the conditions of a specific particle-cemented mode hydrate content and a specific encapsulated-cemented mode hydrate content based on the elastic modulus of the sediment matrix; a model for determining the elastic modulus of the dry rock skeleton of the natural gas hydrate reservoir under the condition of a specific skeleton-supported mode hydrate content based on the elastic modulus of the sediment matrix and the elastic modulus of the hydrate cementing layer; and a model for determining the elastic modulus of the saturated fluid formation of the natural gas hydrate reservoir under the condition of a specific pore-suspended mode hydrate content based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix.

13. The evaluation method according to claim 12, wherein: The various occurrence modes include particle cementation mode, encapsulation cementation mode, skeleton support mode and pore suspension mode.

14. The evaluation method according to claim 13, wherein: The model for determining the elastic modulus of the natural gas hydrate reservoir sediment matrix under the conditions of hydrate content in a specific skeleton support mode is: f h =φ·S h ·f ms Where K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir; K s is the bulk modulus of sedimentary minerals in the gas hydrate reservoir; μ s is the shear modulus of sedimentary minerals in the gas hydrate reservoir; f ms is the volume fraction of skeleton-supported hydrate in the natural gas hydrate reservoir; S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; The elastic modulus of the hydrate cementation layer in the natural gas hydrate reservoir is determined based on the hydrate content of the specific particle cementation mode and the hydrate content of the specific encapsulation cementation mode of the sediment matrix elastic modulus as follows: in, S n (b)=A n (L n )·b 2 +B n (L n )·β+C n (L n ) S t (b)=A t (L t ,v)·b 2 +B t (L t ,v)·β+C t (L t ,v) A n (L n )=-0.024153·L n -1.3646 B n (L n )=0.20405·L n -0.89008 C n (L n )=0.00024649·L n -1.9864 Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; φ c is the critical porosity of sediment; σ ma is the Poisson's ratio of the sediment matrix of the gas hydrate reservoir; σ h is the Poisson's ratio of natural gas hydrate in the natural gas hydrate reservoir; P eff is the effective pressure of the sediment in the gas hydrate reservoir, which is calculated based on the depth of the gas hydrate reservoir and the density of the sedimentary minerals and formation water in the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; μ h is the shear modulus of natural hydrate in the gas hydrate reservoir; C is the coordination number of sediment in the gas hydrate reservoir; τ is the friction coefficient indicating the grain roughness or slip degree; f cc is the volume fraction of hydrate in the particle cementation mode in the natural gas hydrate reservoir; f gc is the volume fraction of hydrates in the gas hydrate of the gas hydrate reservoir in the cementing mode; β0 is the radius of the initial gas hydrate cementing layer of the gas hydrate reservoir; β is the thickness of the gas hydrate cementing layer of the gas hydrate reservoir; S h is the gas hydrate saturation of the gas hydrate reservoir; The elastic modulus of dry rock skeleton of natural gas hydrate reservoir under hydrate content condition based on specific skeleton support mode of sediment matrix elastic modulus and hydrate cementing layer elastic modulus is determined as follows: f e =φ·(1-S h ·f ms ) Where K ect is the bulk modulus of the hydrate cementing layer in the natural gas hydrate reservoir; μ ect is the shear modulus of the hydrate cementing layer in the natural gas hydrate reservoir; K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; μ ma is the shear modulus of the sediment matrix of the gas hydrate reservoir; φ is the reservoir porosity of the gas hydrate reservoir; φ c is the critical porosity of sediment; f ms is the volume fraction of skeleton-supported hydrate in the natural gas hydrate reservoir; S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the gas hydrate reservoir; The model for determining the elastic modulus of the saturated fluid formation of the natural gas hydrate reservoir under the conditions of hydrate content in a specific pore suspension mode based on the elastic modulus of the dry rock skeleton and the elastic modulus of the sediment matrix is: m sat =μ dry φeff=φ·(1-Sh+Sh·fpf) Where K ma is the bulk modulus of the sediment matrix of the gas hydrate reservoir; K dry is the bulk modulus of the dry rock skeleton of the natural gas hydrate reservoir; μ dry is the shear modulus of the dry rock skeleton of the natural gas hydrate reservoir; φ is the reservoir porosity of the natural gas hydrate reservoir; f pf is the volume fraction of hydrate in pore suspension mode in the natural gas hydrate reservoir; S h is the natural gas hydrate saturation of the natural gas hydrate reservoir; K h is the bulk modulus of natural hydrate in the gas hydrate reservoir; K w is the bulk modulus of formation water; K fl is the bulk modulus of the pore fluid in the natural gas hydrate reservoir: K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ sat is the shear modulus of the fluid-saturated formation in the gas hydrate reservoir.

15. The evaluation method according to claim 12, wherein: The calculation model of the compressional wave velocity of the natural gas hydrate reservoir with respect to the elastic modulus of the natural gas hydrate reservoir includes: Where V p is the P-wave velocity of the natural gas hydrate reservoir; K sat is the bulk modulus of the fluid-saturated formation of the natural gas hydrate reservoir; μ sat is the shear modulus of the fluid-saturated formation of the natural gas hydrate reservoir; ρ b is the density of the sediment in the gas hydrate reservoir.

16. The evaluation method according to claim 1, wherein: The occurrence pattern of natural gas hydrates in the target natural gas hydrate reservoir is determined by: When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect on a certain hydrate saturation-P-wave velocity relationship curve in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the hydrate saturation-P-wave velocity relationship curve is the gas hydrate occurrence pattern in the target gas hydrate reservoir; When the P-wave velocity of the target gas hydrate reservoir and the gas hydrate saturation of the target gas hydrate reservoir intersect between two hydrate saturation-P-wave velocity relationship curves in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the occurrence pattern of the gas hydrate in the target gas hydrate reservoir includes the gas hydrate occurrence patterns corresponding to the two hydrate saturation-P-wave velocity relationship curves respectively; When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect above all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the uppermost relationship curve between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of gas hydrate in the target gas hydrate reservoir; When the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir intersect below all the relationship curves between hydrate saturation and P-wave velocity in the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir, the gas hydrate occurrence pattern corresponding to the relationship curve between hydrate saturation and P-wave velocity at the bottom of the hydrate saturation-P-wave velocity rock physics template of the target gas hydrate reservoir is the occurrence pattern of the gas hydrate in the target gas hydrate reservoir.

17. The evaluation method according to claim 1 or 16, wherein: The method further comprises: Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is used for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode to determine the content of gas hydrates in each occurrence mode in the target gas hydrate reservoir.

18. The evaluation method according to claim 17, wherein: Based on the P-wave velocity and the gas hydrate saturation of the target gas hydrate reservoir, a P-wave velocity determination model is used for the target gas hydrate reservoir when it has a specific content of gas hydrates in each occurrence mode. Combined with the determined occurrence mode of the gas hydrate in the target gas hydrate reservoir, the content of gas hydrates in each occurrence mode in the target gas hydrate reservoir is inverted and calculated.

Images