CN111856560B - Natural gas hydrate reservoir information evaluation method and application thereof - Google Patents

Natural gas hydrate reservoir information evaluation method and application thereof Download PDF

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CN111856560B
CN111856560B CN202010653235.6A CN202010653235A CN111856560B CN 111856560 B CN111856560 B CN 111856560B CN 202010653235 A CN202010653235 A CN 202010653235A CN 111856560 B CN111856560 B CN 111856560B
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张进
王尧
刘怀山
邢磊
王林飞
尹燕欣
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Abstract

The invention belongs to the technical field of natural gas hydrate reservoir exploration and discloses a natural gas hydrate reservoir information evaluation method and application thereof. Based on a two-phase medium theory, a plurality of natural gas hydrate reservoir models are generated from a micro-mode of the natural gas hydrate, reservoir porosity, hydrate saturation and reservoir thickness; and accurately depicting an AVO curve of the natural gas hydrate model by using a Brekhovski equation, and establishing indirect connection between AVO marks of a typical hydrate reservoir and rock physical parameters of the reservoir, thereby realizing prediction and evaluation of the hydrate reservoir. According to the invention, a Brekhovski equation considering the thickness of the thin layer is used as a basic formula, the limit of the semi-infinite medium assumption of the Zoeppritz equation is avoided, the calculation of the thin layer AVO curve is more accurate, and the prediction and evaluation accuracy of the hydrate reservoir is improved.

Description

Natural gas hydrate reservoir information evaluation method and application thereof
Technical Field
The invention belongs to the technical field of natural gas hydrate reservoir exploration, and particularly relates to a natural gas hydrate reservoir information evaluation method and application thereof.
Background
In the technology of identifying the natural gas hydrate reservoir by using the seismic AVO technology, AVO forward research is one of important links. The conventional AVO forward modeling analysis about a natural gas hydrate model is based on a Zoeppritz equation and an approximate expression thereof under the condition of infinite half-space assumption, a research object is a single stratum or a thick stratum, the influence of the thickness of a thin layer existing in the natural gas hydrate on a reflection coefficient is not considered, the reflection coefficient in the thin layer cannot be accurately calculated, and the actual AVO curve of the thin reservoir is difficult to describe. Therefore, traditional AVO analysis based on the Zoeppritz equation is not applicable to the prediction and evaluation of natural gas hydrate thin reservoirs. Aiming at the problem, the method adopts a Brekhovski equation which considers the influence of the thickness of the thin layer when calculating the reflection coefficient to carry out AVO forward analysis on the natural gas hydrate reservoir. And establishing a relation between the more accurate AVO mark of the reservoir and the rock physical parameters of the natural gas hydrate, thereby achieving the purpose of evaluating the thickness, the microscopic mode, the hydrate saturation and the porosity of the reservoir.
In summary, the problems of the prior art are as follows: AVO forward research plays an important role in identifying natural gas hydrate reservoirs by using a seismic exploration method. Since natural gas hydrates often exist in seabed thin interbed in various microscopic modes, the Zoeppritz equation based on semi-infinite medium assumption in the traditional AVO can not accurately describe the reflection coefficient curve of the reservoir.
The difficulty of solving the technical problems is as follows:
(1) the thickness of the thin layer has an influence on the calculation of the reflection coefficient.
(2) And establishing indirect relation between the hydrate reservoir thickness, the microscopic mode, the porosity, the hydrate saturation and the reservoir AVO mark.
The significance of solving the technical problems is as follows:
(1) in the process of AVO forward modeling, a Brekhovski equation in which the thin layer thickness factor is considered is applied, the defect of semi-infinite medium assumption of the traditional Zoeppritz equation is overcome, and a thin layer AVO curve is accurately drawn.
(2) And establishing a relation between the natural gas hydrate rock physical parameters and the thin-layer AVO mark, and evaluating the thickness, the microscopic mode, the porosity and the hydrate saturation of the hydrate reservoir through AVO forward analysis.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a natural gas hydrate reservoir information evaluation method and application thereof.
The invention is realized in such a way that a natural gas hydrate reservoir information evaluation method comprises the following steps:
the method comprises the steps of firstly, generating a plurality of natural gas hydrate reservoir models based on a classic fluid replacement thought-Gassmann equation, and exploring the change rule of the longitudinal and transverse wave speeds and densities of the hydrate reservoir with the porosity and the hydrate saturation of the reservoir in different micro modes.
The bulk and shear moduli of saturated fluid deposits are, according to the Gassmann equation:
Figure GDA0003517928170000021
there is no relative motion between the Gassmann fluid and the solid and the density is expressed as a weighted average of the biphasic densities:
ρ＝ρfφr+ρS(1-φr)；
Figure GDA0003517928170000022
in the formula, KsatAnd GsatBulk modulus, shear modulus, phi, of rock saturated with pore fluidrIs the rock porosity; kdryAnd GdryThe bulk and shear moduli of the dry skeleton, K and K, respectivelyfBulk modulus of the solid phase and pore fluid, respectively;
when the hydrate is part of the solid framework, the presence of natural gas hydrate reduces the porosity and changes the elastic modulus of the solid phase, the reduced porosity being:
φr＝φ(1-Sh)；
Shthe natural gas hydrate saturation degree is shown, and according to the average Hill, the changed solid-phase elastic modulus is as follows:
Figure GDA0003517928170000031
Figure GDA0003517928170000032
Ksand GsRespectively the bulk modulus and shear modulus of the pure rock solid phase, KhAnd GhThe bulk modulus and the shear modulus of the pure natural gas hydrate are respectively;
the compressional-shear velocity of the reservoir is thus calculated as:
Figure GDA0003517928170000033
the depth of the seawater is 300 meters, and a shallow sea common sedimentary layer is arranged below the seawater; the middle is a hydrate reservoir stratum; hydrate underburden is a typical marine sedimentary layer; the rock skeleton is composed of calcite, quartz and clay.
The longitudinal and transverse wave AVO curve changes along with the saturation of hydrate: the porosity of the rock is fixed to be 20%, the thickness of a hydrate reservoir is 10m, and the variation range of the saturation of the hydrate is 10% -70%.
The longitudinal and transverse wave AVO curve changes along with the porosity of the reservoir: the saturation of the hydrate is fixed to be 30%, the thickness of a hydrate reservoir is 10m, the critical porosity of the sedimentary rock is 0.4, and the variation range of the porosity of the reservoir in the model is 0.1-0.4
The longitudinal and transverse wave AVO curve changes along with the thickness of the reservoir: the porosity is 0.2, the hydrate saturation is 30%, and the variation range of the reservoir thickness is as follows: 1/8 λ,1/4 λ,1/2 λ, λ,2 λ,3 λ.
And secondly, calculating an AVO curve of the reservoir by using a Brekhovski equation. Procedure for Brekhovski equation to calculate AVO curves: knowing the longitudinal and transverse wave speeds and densities of the natural gas hydrate thin reservoir, the overburden stratum and the underburden, calculating the reflection coefficients of the thin reservoir and the underburden interface at different incidence angles, and then sequentially arranging the reflection coefficients from small to large according to the incidence angles to obtain the thin reservoir AVO curve.
And thirdly, performing AVO forward modeling analysis on the natural gas hydrate reservoir with the variable thickness by using a Brekhovski equation, establishing indirect relation between the rock physical parameters of the hydrate reservoir and AVO marks, and evaluating the thickness, the porosity, the microscopic mode and the hydrate saturation of the hydrate reservoir.
In summary, the advantages and positive effects of the invention are: the invention utilizes a Brekhovski equation which is more suitable for the calculation of the thin interbed reflection coefficient, and performs multi-wave AVO research aiming at 4 influencing factors of the micro mode, the reservoir porosity, the hydrate saturation and the reservoir thickness of the natural gas hydrate. The results show that: when AVO research is carried out on a thin layer, a Brekhovski equation is superior to a Zoeppritz equation; AVO marks of a typical hydrate reservoir established by using a Brekhovski equation can be used for evaluating parameters such as hydrate saturation, porosity and reservoir thickness of the reservoir.
Drawings
Fig. 1 is a flow chart of a natural gas hydrate reservoir information evaluation method provided by an embodiment of the invention.
FIG. 2 is a schematic diagram of reflection and transmission in a thin layer medium according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of a change of a longitudinal wave AVO curve with a thickness of a thin layer under a prosodic model according to an embodiment of the present invention.
Fig. 4 is a schematic diagram of a longitudinal-transverse wave AVO curve varying with a hydrate saturation according to an embodiment of the present invention.
FIG. 5 is a schematic diagram of an AVO curve of longitudinal and transverse waves as a function of reservoir porosity provided by an embodiment of the invention.
FIG. 6 is a schematic diagram of AVO curves of longitudinal and transverse waves as a function of reservoir thickness according to an embodiment of the present invention.
FIG. 7 is a schematic of an SH-2a well log provided by an embodiment of the present invention.
FIG. 8 is a comparison graph of the integrated model provided by the embodiment of the invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
In view of the problems in the prior art, the invention provides a natural gas hydrate reservoir information evaluation method, which is described in detail below with reference to the accompanying drawings.
As shown in fig. 1, a method for evaluating natural gas hydrate reservoir information provided by an embodiment of the present invention includes the following steps:
s101: starting from four influence factors of a micro mode of the natural gas hydrate, the porosity of a reservoir, the saturation of the hydrate and the thickness of the reservoir, generating a plurality of natural gas hydrate reservoir models through a classic fluid replacement thought-Gassmann equation;
s102: and accurately depicting the AVO curve of the natural gas hydrate model by using a Brekhovski equation to obtain the relation between the changes of the four influencing factors and the changes of the AVO curve.
The technical solution of the present invention is further described below with reference to the accompanying drawings.
The theory of reflection coefficient spectra of thin layers is to discuss reflection and transmission of waves in a layered medium from the viewpoint of the theory of elastic wave dynamics, and the theoretical basis is the Brekhovski equation. It assumes that a sandwich layer composed of many horizontal layered media is sandwiched between two thick layers, a plane simple harmonic wave is incident to the top interface of the sandwich layer from the medium 1, a reflected longitudinal wave and a reflected transverse wave are generated in the medium, and simultaneously the wave will also be transmitted into the sandwich layer, and the reflection and transmission will be continuously generated on each interface of the sandwich layer, and finally the wave passes through the sandwich layer, and a transmitted longitudinal wave and a transmitted transverse wave are generated in the medium N +1 below the sandwich layer. The following is a formula for elastic wave propagation in a classical three-layer medium.
The reflection and transmission of the longitudinal wave incident on the thin medium is assumed as shown in fig. 1. According to the conditions of displacement and stress continuity on the interface, the relation between the displacement component and the stress component on the bottom interface of the 1 st layer of medium and the top interface of the 3 rd layer of medium is deduced and obtained:
Figure GDA0003517928170000051
a11＝2sin2γcos P+cos 2γcos Q，
a12＝i(tanθcos 2γsin P-sin 2γsin Q)
Figure GDA0003517928170000052
a14＝-2iVS(tanθsinγsin P+cosγsin Q)
Figure GDA0003517928170000053
a22＝cos 2γcos P+2sin2γcos Q
Figure GDA0003517928170000054
a24＝2VSsinγ(cos Q-cos P)
a31＝2ρVSsinγcos 2γ(cos Q-cos P)
Figure GDA0003517928170000061
a33＝cos 2γcos P+2sin2γcos Q
Figure GDA0003517928170000062
Figure GDA0003517928170000063
Figure GDA0003517928170000064
Figure GDA0003517928170000065
a44＝2sin2γcos P+cos 2γcos Q
Figure GDA0003517928170000066
wherein, ai,jP is the phase shift factor of longitudinal wave on the 2 nd layer, Q is the phase shift factor of transverse wave on the 2 nd layer, d is the 2 nd layer thickness, and ω is the incident wave angular frequency.
Substituting the bit function relation of the 1 st layer medium and the 3 rd layer medium into the above formula (2-1) to make the reflection coefficient and the transmission coefficient as follows:
Figure GDA0003517928170000067
simplified and arranged to obtain
Figure GDA0003517928170000068
In the formula (2-2)
Figure GDA0003517928170000069
Figure GDA00035179281700000610
Figure GDA0003517928170000071
Figure GDA0003517928170000072
Figure GDA0003517928170000073
In the formula (2-2), the
Figure GDA0003517928170000074
Figure GDA0003517928170000075
Figure GDA0003517928170000076
Figure GDA0003517928170000077
The solution of the formula (2-2) is
Figure GDA0003517928170000078
1. Effect of layer thickness variation of the three-layer model on the AVO Curve
In contrast to the conventional Zoeppritz equation, the Brekhovski equation takes into account the effect of the thickness of the thin layer on the reflection coefficient.
The following experiments were performed in order to verify the superiority of Brekhovski's equation in thin layer studies. First, a three-layer media model was created, and in order to reduce unnecessary computation, the elastic parameters of the layer 1 media and the layer 3 media in the model were assumed to be the same, and the model parameters are shown in table 1. Then, AVO curves for different interlayer thicknesses were calculated using Brekhovski's equation. The intermediate layer has a thickness variation range of 10 times wavelength, 5 times wavelength, 2 times wavelength, 1 time wavelength, 1/2 wavelength, 1/4 wavelength, 1/8 wavelength, 1/16 wavelength, 1/32 wavelength. Finally, AVO curves for different layer thicknesses calculated using Brekhovski's equation were compared to the results calculated using Zoeppritz's equation. The comparative results are shown in FIG. 3.
TABLE 1 elastic parameters of layers in thin layer model
Figure GDA0003517928170000081
When the thickness of the thin layer is larger than 1/2 wave length, the trend of AVO curve calculated by the two is approximately consistent, but the result of calculation of Zoeppritz equation is not reflected due to the reflection coefficient fluctuation caused by the increase of the thickness.
When the thickness of the thin layer is between 1/4 and 1/2 wavelengths, the AVO curve calculated by the Brekhovski equation tends to be smooth, and the average error of the AVO curve calculated by the Brekhovski equation is 7.1% at 1/4 and 2.3% at 1/2, which is consistent with the trend of the result calculated by the Zoeppritz equation.
When the thickness of the thin layer is between 1/32 and 1/4 wavelengths, the reflection coefficient calculated by Brekhovski's equation is close to 0 at close offsets and differs greatly from the results obtained by Zoeppritz's equation. The elastic parameters of the first layer and the third layer of the model adopted by the experiment are the same[15]As the layer thickness decreases, there is no transmission or reflection of waves when the intermediate layer is not present. So it is more theoretical that the Brekhovski equation near offset reflection coefficient is close to 0 when the thickness of the thin layer is small.
To sum up, by comparison of the two, the Brekhovski equation has a higher accuracy than the Zoeppritz equation, which is more theoretical for the description of thin-layer AVO curves, at layer thicknesses less than 1/2 wavelengths.
2. Establishment of AVO model of cementing framework type natural gas hydrate
In order to research the influence of the porosity, saturation and thickness of a hydrate reservoir on a longitudinal wave AVO curve, a reservoir model of the natural gas hydrate is established based on a classic fluid replacement thought-Gassmann equation. The natural gas hydrate reservoir model designed in the invention is a seabed three-layer medium model: the depth of the seawater is 300 meters. The shallow sea common sedimentary layer with the thickness of 500 m is arranged under the sea water, the longitudinal wave speed is 1859.75m/s, the transverse wave speed is 951.81m/s, and the density is 2.2936g/cm3. The middle layer is a hydrate reservoir with the thickness of 15 meters. The reservoir rock skeleton is composed of hydrate, calcite, quartz and clay. The pore fluid consists of water and free gas. The petrophysical parameters of each component are shown in table 2. In the modeling process, the volume modulus, the shear modulus and the density of the middle reservoir are obtained by calculating the volume modulus, the shear modulus and the density of the pore fluid and the rock framework, and then the longitudinal and transverse wave speeds of the reservoir are obtained. The hydrate underburden is a general ocean sedimentary stratum with the longitudinal wave velocity of 1926.19m/s, the transverse wave velocity of 941.78m/s and the density of 2.53g/cm3。
TABLE 2 physical parameters of each component of rock in the model construction
Figure GDA0003517928170000091
3. AVO curve of cemented framework type natural gas hydrate reservoir during change of various physical parameters
3.1 variation of AVO Curve of longitudinal and transverse waves with hydrate saturation
In order to study the change of a longitudinal and transverse wave AVO curve along with the hydrate saturation, the hydrate saturation is the only variable in the experiment, and the change range is 10-70%. Assuming a hydrate reservoir thickness of 15m and a porosity of 0.2, the shear wave velocity and density change with changes in hydrate saturation as shown in table 3:
TABLE 3 reservoir compressional and shear velocities and Density at varying hydrate saturations
Figure GDA0003517928170000092
As can be seen from table 3, the compressional-shear velocity and density of the gas hydrate reservoir both exhibit increasing regularity as the hydrate saturation increases. The longitudinal and transverse wave AVO curve of the natural gas hydrate reservoir under the condition of different hydrate saturation degrees is calculated according to the Brekhovski equation and is shown in the figure 4:
the left graph describes the change rule of a longitudinal wave AVO curve, and the right graph describes the change rule of a transverse wave AVO curve. The longitudinal wave AVO curve shows a relatively obvious rule along with the change of the hydrate saturation: under the condition of different hydrate saturation degrees, longitudinal wave AVO curves show that the longitudinal wave AVO curves firstly rise slowly and then rise rapidly, and then fall slowly after reaching a peak value; with the increase of the saturation of the hydrate, the reflection coefficient of the 0 offset distance is gradually increased, and the angle of the AVO curve at the step point is gradually reduced; when the saturation degree of the hydrate is between 20% and 60%, the step of the AVO curve is obvious.
The shear wave AVO curve exhibits an'm' shape at hydrate saturations between 20-70%, and as the hydrate saturation increases, the AVO curve gradually increases and the angle at which minima occur gradually decreases. The hydrate saturation degree of 10% shows a form of increasing first and then decreasing.
3.2 variation of the longitudinal-transverse wave AVO Curve with reservoir porosity
In order to study the change of the longitudinal and transverse wave AVO curve along with the porosity of the reservoir, the porosity of the reservoir is taken as the only variable in the test. Assuming a hydrate reservoir thickness of 15m, the saturation is 30%. The critical porosity of the sedimentary rock is 0.4, the rock is the main propagation medium of vibration when the porosity is lower than the critical porosity, and the rock exists in a fluid in a suspension form when the porosity is higher than the critical porosity, and the fluid phase is the main propagation medium of vibration. The cementing skeleton mode takes rock as a main medium of vibration, so the porosity of the reservoir in the model is in a range of 0.1-0.4. The variation of the velocity and density of the longitudinal and transverse waves with the change of the porosity is shown in table 4:
TABLE 4 reservoir compressional and shear velocities and Density at varying porosity
Figure GDA0003517928170000101
As can be seen from table 4, both the compressional and shear wave velocities and the density of the reservoir gradually decrease as the reservoir porosity increases. The longitudinal and transverse wave AVO curve of the natural gas hydrate reservoir is calculated according to the Brekhovski formula under the condition of different reservoir porosities as shown in the figure 5:
the change rule of the longitudinal wave AVO curve is as follows: when the porosity is between 0.1 and 0.2, the shape is formed by firstly slowly descending, then gradually ascending and finally slowly descending; with the increase of the porosity, the angle of the AVO curve jump point is gradually increased; at a porosity of 0.3-0.4, the AVO curve shows a rising-falling-rising-falling morphology, and at a porosity of 0.3, the AVO curve finally jumps to a positive value; the above two forms are not satisfied when the porosity is 0.25, and the value increases first and then becomes negative.
The change rule of the transverse wave AVO curve is as follows: positive at a porosity of 0.1-0.2, in the shape of an'm', and gradually decreases as the porosity increases; negative at porosities between 0.3-0.4. The AVO curve becomes positive at a far offset, is bowl-shaped, and gradually increases in absolute value as the porosity increases; at a porosity of 0.25, the AVO curve decreases and increases, jumping to positive values around 80 degrees.
By observing the longitudinal and transverse wave AVO curves when the porosity is changed, the longitudinal and transverse wave AVO curves have two different morphologies and change rules when the porosity is between 0.1 and 0.2 and the porosity is between 0.3 and 0.4, and the longitudinal and transverse wave AVO curves do not meet the two morphologies when the porosity is 0.25. Since the relative impedance of the intermediate layer to the underlying formation changes as the porosity increases, the reflection coefficient changes from being positive overall to negative overall. It can be concluded that there is a transition zone with varying porosity between the two forms, and the AVO curve of longitudinal and transverse waves at a porosity of 0.25 is the index of the transition zone.
3.3 variation of the longitudinal-transverse wave AVO Curve with reservoir thickness
When the variation of a longitudinal-transverse wave AVO curve along with the thickness of a hydrate reservoir is explored, the reservoir thickness is the only variable in an experiment. The reservoir thickness is calculated according to the wavelength lambda, and the variation range is as follows: 1/8 λ,1/4 λ,1/2 λ, λ,2 λ,3 λ. Assuming a reservoir porosity of 0.2, hydrate saturation is 30%. Under the condition, the longitudinal wave velocity of the hydrate reservoir is 2100.37m/s, the transverse wave velocity is 1082.62m/s, and the density is 2.3887g/cm3. The variation of the longitudinal and transverse wave AVO curve with reservoir thickness calculated according to the Brekhovski equation is shown in FIG. 6:
the AVO curve of the longitudinal wave of the natural gas hydrate reservoir layer is in a form of firstly stabilizing and then rising, and firstly simplifying and then complicating at different thicknesses. When the thickness of the reservoir is less than 1 time of the wavelength, the longitudinal wave AVO curve is reduced after reaching the peak value. When the thickness is more than 2 times of the wavelength, the longitudinal wave AVO curve does not drop after the farther offset distance reaches an extreme value. The transverse wave AVO curve shows an unobvious change rule along with the thickness change, different reservoir thicknesses correspond to different transverse wave AVO curve forms, and the overall form shows a form of increasing firstly and then decreasing.
4. Forward AVO analysis of natural gas hydrate in south God fox sea area
The Shenfox sea area is located in the middle of the land slope in the north of the south China sea, is structurally located in the Zhujiang mouth basin Zhubie cave, and belongs to the key area of hydrate exploration in the south China sea. In the geological survey related to the early stage of the area, a large number of geophysical, geochemical and other related marks which indicate the existence of the natural gas hydrate are found. The natural gas hydrate sample was successfully drilled for the first time by the guangzhou marine geological survey in 2007, 5 months. And (3) carrying out natural gas hydrate forward study based on a Brekhovski equation based on SH-2a well logging data in the region to evaluate reservoir thickness, hydrate saturation and porosity reservoir parameters in the region. The velocity log and density log are shown in FIG. 7:
well logs with hydrate containing layers having high compressional velocity are abnormal. According to the theory and the well core, the reservoir containing the hydrate is positioned between the depths of 200m and 220m, namely the position of the dark gray strip is the reservoir containing the hydrate. And (4) synthesizing the seismic record according to the logging data, extracting an AVO curve of longitudinal and transverse waves, and comparing the AVO curve with the AVO curve of the longitudinal and transverse waves of the hydrate model. In order to comprehensively consider the above-mentioned influencing factors, a comprehensive model is established. The reservoir thickness in the model was 20 meters. The porosity of the reservoirs varied from 0.15, 0.20, 0.25,0.30, 0.35. The hydrate saturation of the reservoir ranges from 0.20, 0.25,0.30,0.35, 0.40. The AVO curves for reservoir porosities of 0.25,0.3,0.35 are shown in fig. 8 as a function of hydrate saturation.
Using the velocity and density information in the logs, AVO curves for the reservoir were calculated by Brekhovski's equation (shown in black dashed lines in fig. 8). Comparing the AVO curve obtained by logging with the AVO curve of the natural gas hydrate comprehensive model calculated by using the Brekhovski equation, the result shows that the goodness of fit of the AVO curve and the natural gas hydrate comprehensive model is higher when the porosity is 0.30 and the hydrate saturation is 30-35%. Therefore, the thickness of the natural gas hydrate near the SH-2a well is about 20m, the porosity of the reservoir is about 0.30, and the saturation of the hydrate is between 30 and 35 percent. Therefore, by establishing AVO signatures for typical natural gas hydrates through Brekhovski's equation, the thickness, porosity, and hydrate saturation of the hydrate reservoir can be approximated.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims (8)

1. A natural gas hydrate reservoir information evaluation method is characterized by comprising the following steps:
the method comprises the steps that firstly, a plurality of natural gas hydrate reservoir models are generated through a classic fluid replacement thought-Gassmann equation from four influence factors of a micro mode of the natural gas hydrate, reservoir porosity, hydrate saturation and reservoir thickness;
secondly, describing AVO curves of a plurality of natural gas hydrate reservoir models by using a Brekhovski equation to obtain indirect relation between changes of four influencing factors and AVO marks of typical hydrates;
and thirdly, extracting an AVO curve in actual data, observing curve characteristics and comparing the curve characteristics with typical hydrate reservoir AVO marks to achieve reservoir prediction and approximate evaluation of a microscopic mode, reservoir porosity, hydrate saturation and reservoir thickness.
2. The natural gas hydrate reservoir information evaluation method of claim 1, wherein the natural gas hydrate reservoir modeling method: when the hydrate is part of the solid framework, the hydrate affects the elastic modulus of the solid framework and changes the reservoir parameters; the bulk and shear moduli of saturated fluid deposits are, according to the Gassmann equation:
Figure FDA0003517928160000011
Gsat＝Gdry
there is no relative motion between the Gassmann fluid and the solid and the density is expressed as a weighted average of the biphasic densities:
ρ＝ρfφr+ρS(1-φr)；
Figure FDA0003517928160000012
in the formula, KsatAnd GsatBulk modulus, shear modulus, phi, of rock saturated with pore fluidrIs the rock porosity; kdryAnd GdryThe bulk and shear moduli of the dry skeleton, K and K, respectivelyfThe bulk modulus of the solid phase and the pore fluid, respectively.
3. The natural gas hydrate reservoir information evaluation method of claim 1, wherein the natural gas hydrate reservoir modeling method: when the hydrate is part of the solid framework, the presence of natural gas hydrate reduces the porosity and changes the elastic modulus of the solid phase, the reduced porosity being:
φr＝φ(1-Sh)；
Shthe natural gas hydrate saturation degree is shown, and according to the average Hill, the changed solid-phase elastic modulus is as follows:
Figure FDA0003517928160000021
Figure FDA0003517928160000022
Ksand GsRespectively the bulk modulus and shear modulus of the pure rock solid phase, KhAnd GhThe bulk modulus and the shear modulus of the pure natural gas hydrate are respectively;
the compressional-shear velocity of the reservoir is thus calculated as:
Figure FDA0003517928160000023
the depth of the seawater is 300 meters, and a shallow sea common sedimentary layer is arranged below the seawater; the middle is a hydrate reservoir stratum; the hydrate underburden is a marine sedimentary layer; the rock skeleton is composed of calcite, quartz and clay.
4. The natural gas hydrate reservoir information evaluation method of claim 1, wherein a compressional-shear wave AVO curve varies with hydrate saturation: the porosity of the rock is fixed to be 20%, the thickness of a hydrate reservoir is 10m, and the variation range of the saturation of the hydrate is 10% -70%.
5. The natural gas hydrate reservoir information evaluation method of claim 1, wherein a longitudinal-transverse wave AVO curve varies with reservoir porosity: the saturation of the hydrate is fixed to be 30%, the thickness of a hydrate reservoir is 10m, the critical porosity of the sedimentary rock is 0.4, and the change of the reservoir porosity in the model is 0.1-0.4.
6. The natural gas hydrate reservoir information evaluation method of claim 5, wherein the compressional-shear wave AVO curve varies with reservoir thickness: the porosity is 0.2, the hydrate saturation is 30%, and the variation range of the reservoir thickness is as follows: 1/8 λ,1/4 λ,1/2 λ, λ,2 λ,3 λ.
7. Use of the method for evaluating natural gas hydrate reservoir information according to any one of claims 1 to 6 in identification of a natural gas hydrate reservoir.
8. Use of a method for evaluating natural gas hydrate reservoir information according to any one of claims 1 to 6 in energy exploration.
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