Inversion method and application of seabed deposition rate and formation porosity distribution

Abstract

The invention belongs to the field of marine geology, and particularly discloses an inversion method and application of seabed deposition rate and formation porosity distribution. The method comprises the following steps: obtaining a possible distribution range of the stratum porosity n changing along with the depth z in a to-be-researched region; obtaining the porosity of the surface sediment sample; according to the distribution range and the porosity of the surface sediment, assuming m possible change relations of the stratum porosity along with the depth at the position where research is planned to be carried out; calculating a corresponding average mass deposition rate Rsi for each assumed relationship that the porosity of the stratum changes along with the depth, and then simulating the distribution condition of the porosity of the seabed stratum under the deposition rate Rsi in numerical software; and comparing the formation porosity obtained by simulation under each Rsi condition with the corresponding assumed formation porosity distribution along with the depth, and if the two are consistent, determining that the formation porosity distribution along with the depth obtained by simulation is the actual formation porosity distribution at the position. The method is low in cost, convenient, fast and convenient to popularize.

Description

Technical Field

[0001] The present application belongs to the field of marine geology, and more specifically, relates to an inversion method for seafloor sedimentation rate and formation porosity distribution and its application. Background Art

[0002] Accurately obtaining the seabed sedimentation rate and formation porosity distribution is of great significance to the fields of seabed geological hazard risk assessment, offshore oil and gas resource assessment, and marine engineering construction.

[0003] Dating and geotechnical testing of seafloor core samples are the primary methods for obtaining seafloor sedimentation rates and formation porosity distribution. Although this method is relatively accurate in obtaining sedimentation rates and formation porosity distribution, it has the following shortcomings: 1) Seafloor core samples often need to be obtained by drilling with ocean drilling ships. Currently, there are few ocean drilling ships, and drilling costs increase exponentially with drilling water depth and drilling depth, resulting in a small audience; 2) After obtaining the core samples, a large amount of sediment testing and geotechnical testing is required, which is time-consuming and labor-intensive.

[0004] Therefore, it is necessary to provide a method for calculating seabed sedimentation rate and formation porosity distribution that is low in implementation cost and easy to promote. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this application is to provide an inversion method and application of seabed sedimentation rate and formation porosity distribution, which is low-cost, convenient and fast, and easy to promote.

[0006] To achieve the above objectives, in a first aspect, the present application provides an inversion method for seafloor sedimentation rate and formation porosity distribution, comprising the following steps:

[0007] S10, collecting and obtaining the possible distribution range of formation porosity n as a function of depth z in this area based on ocean drilling core test data near the proposed research location;

[0008] S20, collect surface sediment samples at the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples;

[0009] S30, based on the distribution range of formation porosity n with depth z in this area and porosity n0, assume m possible formation porosity variation with depth at the proposed research location n = n i (z), i∈[1:m];

[0010] S40, for each assumed relationship between formation porosity and depth n = n i (z), calculate the corresponding average mass deposition rate R si, and then simulate the deposition rate R in the numerical software si The porosity distribution of the submarine strata below;

[0011] S50, will be available in various R si The formation porosity distribution with depth simulated under the conditions is compared with the corresponding assumed formation porosity distribution with depth. If the two are consistent, the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location. Otherwise, different R si Continue to simulate the distribution of formation porosity with depth and compare it with the corresponding assumed distribution of formation porosity with depth.

[0012] The beneficial effects of the present application are as follows: the present application provides an inversion method for seafloor sedimentation rate and formation porosity distribution. This method, based on existing core data in the area near the study point, only requires obtaining seafloor surface sediment samples and conducting a small amount of geotechnical testing to determine model calculation parameters. The method can then invert the average mass sedimentation rate at the study point and the relationship between formation porosity and depth. Compared to traditional technologies, the present application does not require deep-sea core drilling by ocean drilling vessels or extensive geotechnical testing, resulting in low cost and easy promotion.

[0013] As a further preferred embodiment, in step S40, the average mass deposition rate R si The calculation formula is:

[0014]

[0015] Where, d is the depth of the seabed stratum; T is the age of the stratum at the depth d; G s is the specific gravity of sediment particles; ρ w is the density of seawater.

[0016] As a further preference, the stratigraphic age T of the seabed stratum at the buried depth d is determined by collecting a three-dimensional seismic profile at the location where the study is to be conducted and combining it with a publicly available stratigraphic framework at a nearby location.

[0017] As a further preference, the stratigraphic framework is a framework structure established by analyzing and integrating various information of seabed strata to reflect the temporal and spatial distribution and mutual relationships of strata.

[0018] As a further preferred embodiment, in step S40, the deposition rate R is simulated in numerical software. si The steps for determining the porosity distribution of the seabed strata under investigation are as follows:

[0019] Build a geometric model of the sedimentary layer and then divide the seabed into layers;

[0020] According to the sedimentation layer thickness h0 and mass deposition rate R divided in the sedimentation layer geometric model s , calculate the analysis step length;

[0021] Assigning values ​​to the seepage parameters and modified Cambridge model parameters of each layer of material in the sedimentary layer geometric model based on the geotechnical test results of the surface sediment sample taken in step S20;

[0022] In the initial analysis step, the birth and death unit command in the numerical software is used to remove all sedimentary layers except the first sedimentary layer. Then, the birth and death unit command is used to activate each sedimentary layer from bottom to top in the subsequent analysis steps, and the corresponding gravity load is applied to realize the layer-by-layer deposition process.

[0023] In the initial analysis step, the displacement of the bottom slope base is fixed to 0 in the horizontal and vertical directions, and the displacement of the lateral boundary of the sediment layer is fixed to 0 in the horizontal direction. In each subsequent analysis step, the surface sediment is set as a permeable layer, and its excess pore water pressure is kept at 0 in the corresponding analysis step.

[0024] The four-node plane strain unit is used for the substrate and the quadratic reduced integration unit is used for the deposition layer. si The distribution of formation porosity with depth obtained by simulation under the conditions of 2.

[0025] As a further preferred embodiment, the modified Cambridge model parameters include the slope M of the critical state line of the surface sediment on the p′-q plane, the slope λ of the isotropic consolidation curve of the surface sediment on the e-lnp′ plane, and the slope k of the rebound curve of the surface sediment on the e-lnp′ plane.

[0026] As further preferred, the modified Cambridge model parameters are measured by triaxial test.

[0027] As a further preference, the analysis step adopts a Soil analysis step for simulating fluid-solid coupling of porous media.

[0028] As a further preference, in step S40, the numerical software is Abaqus numerical software.

[0029] Secondly, the present application provides an application of the inversion method of seabed sedimentation rate and formation porosity distribution as described above, which is applied to the fields of seabed geological disaster risk assessment, offshore oil and gas resource assessment, and marine engineering construction.

[0030] It can be understood that the beneficial effects of the second aspect mentioned above can be found in the relevant description of the first aspect mentioned above, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1This is a flow chart of the inversion method for seafloor sedimentation rate and formation porosity distribution provided in an embodiment of the present application;

[0032] Figure 2 This is a diagram showing the relationship between the porosity of submarine formations and their depth distribution near the research point provided in the embodiments of the present application;

[0033] Figure 3 This is a diagram of the hypothetical formation porosity distribution and the formation porosity distribution obtained by simulation provided in the embodiments of the present application. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0035] It should be understood that, in the description of this application, the term "several" means at least one, such as one, two, etc., unless otherwise clearly and specifically defined; the term "plurality" means two or more, unless otherwise clearly and specifically defined; the terms "first" and "second" etc. are used to distinguish different objects, rather than to describe a specific order of objects; the term "and / or" includes any and all combinations of one or more related listed items.

[0036] Additionally, references throughout this specification to "one embodiment," "one embodiment," "an example," or similar language indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Thus, appearances of the phrase "in one embodiment," "in one embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[0037] It should be noted that the sedimentation rate is the mass of sediment particles deposited per unit time and per unit area. The ratio of the pore volume to the total volume of the submarine stratum is called the formation porosity, usually expressed as a percentage, and its calculation formula is: V v is the pore volume, and V is the total volume of the formation (including the volume occupied by solid particles and pores).

[0038] This application analyzes the porosity data of sediment drilled samples from four stations in the northern South China Sea and draws two objective facts: (1) Sediments gradually become denser with burial depth, and the porosity of the seabed strata varies logarithmically with stratum depth; (2) Although the relationship between the stratum porosity at different locations in a certain area and its variation with depth is relatively discrete, they are all distributed within a certain range.

[0039] Based on the above two points, this application proposes an inversion method for seafloor sedimentation rate and formation porosity distribution, which specifically includes steps S10 to S50, as detailed below:

[0040] S10, collect and obtain the possible distribution range of formation porosity n as a function of depth z in this area based on ocean drilling core test data near the proposed research location.

[0041] S20: Collect surface sediment samples at the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples.

[0042] S30, based on the distribution range of formation porosity n with depth z in this area and porosity n0, assume m (m ≥ 1) possible formation porosity variation relationships with depth n = n i (z), i∈[1:m].

[0043] S40, for each assumed relationship between formation porosity and depth n = n i (z), calculate the corresponding average mass deposition rate R si , and then simulate the deposition rate R in the numerical software si The porosity distribution of the submarine strata below.

[0044] S50, will be available in various R si The formation porosity distribution with depth simulated under the conditions is compared with the corresponding assumed formation porosity distribution with depth. If the two are consistent, it means that the mass deposition rate is the average deposition rate at the location to be studied since time T, and the corresponding simulated formation porosity distribution with depth is the actual formation porosity distribution at that location; otherwise, different R si Continue to simulate the distribution of formation porosity with depth and compare it with the corresponding assumed distribution of formation porosity with depth.

[0045] The inversion method for seafloor sedimentation rate and formation porosity distribution provided in this embodiment is based on existing core data in the area near the study point. This method only requires obtaining seafloor surface sediment samples and conducting a small amount of geotechnical testing to determine model calculation parameters. It can invert the average mass sedimentation rate and formation porosity at the study point to obtain the relationship between depth and formation porosity. Compared with traditional technologies, this application does not require deep-sea core drilling by ocean drilling vessels or extensive geotechnical testing, which is low-cost and easy to promote.

[0046] In a specific embodiment, the technical solution for achieving the above-mentioned purpose is: a method for inverting seafloor sedimentation rate and formation porosity distribution, specifically comprising the following steps:

[0047] Step 1: Collect published ocean drilling core test data near the proposed research location to obtain the possible distribution range of formation porosity n as a function of depth z in this area.

[0048] Step 2: Collect surface sediment samples at the proposed research location and conduct geotechnical tests to obtain the dry density ρ of the surface sediment samples. d0 , porosity n0, modified Cambridge model parameters, the relationship between permeability coefficient and porosity (i.e. permeability parameter) k=k(e).

[0049] In this embodiment, the modified Cambridge model is used as the sediment constitutive model. Based on the Cambridge model, the modified Cambridge model adopts the adaptive flow law, and the unit total volume strain increment dε v Equal to the unit elastic body strain increment and the element plastic body strain increment The sum can be calculated by the following formula

[0050]

[0051] Where p′ is the average effective stress; q is the deviatoric stress; M is the slope of the critical state line of the surface sediment on the p′-q plane; e is the porosity; λ is the slope of the isotropic consolidation curve of the surface sediment on the e-lnp′ plane; k is the slope of the rebound curve of the surface sediment on the e-lnp′ plane; and η is the stress ratio.

[0052] In this embodiment, the main parameters of the modified Cambridge model include λ, k, and M, which can be determined through conventional triaxial testing. By determining the model parameters, the constructed sediment constitutive model can be used to invert the relationship between the sedimentation rate and formation porosity at the study point as a function of depth. Under the action of sedimentary load, as the sediment layer continues to increase, when the sedimentation rate is not much different, the main factor affecting the distribution of formation porosity along the depth is sediment compressibility. Sediment compressibility is related to the permeability coefficient: the smaller the permeability coefficient, the greater the compressibility, and vice versa.

[0053] Step 3: Collect a 3D seismic profile at the proposed study location and, in combination with a publicly available stratigraphic framework at a nearby location (a framework that reflects the temporal and spatial distribution and interrelationships of strata, established by analyzing and integrating various information on seabed strata), determine the stratigraphic age T at the depth d of the seabed strata.

[0054] Step 4: Based on the measured porosity n0 of the surface sediment sample and the distribution range of the formation porosity n with depth z in this area, reasonably assume m possible relationships of formation porosity with depth n=n at the proposed research location. i (z), i∈[1:m].

[0055] Step 5: For each assumed relationship between formation porosity and depth, n = n i (z), the corresponding average mass deposition rate R can be calculated by the following formula (2): si (mass of sediment particles deposited per unit time and per unit area):

[0056]

[0057] Where G s is the specific gravity of sediment particles; ρ w is the density of seawater.

[0058] Step 6: Simulate the deposition rate R in Abaqus numerical software si The porosity distribution of seabed formations under different conditions.

[0059] In this embodiment, step 6 may specifically be:

[0060] Establish a geometric model of the sediment layer. According to the dry density of the surface sediment ρ d0 Calculate the thickness of the unconsolidated stratum H, A geometric model of the sedimentary layer is established and then divided into layers. Considering that the surface sediments of the seabed are relatively loose and do not accumulate excess pore pressure, the excess pore pressure of the surface sediments must be set to 0 at each analysis step when setting the seepage boundary conditions. The thickness g0 of the surface sediments is determined based on actual conditions and serves as the layer thickness of the geometric model.

[0061] Assignment of material parameters: Based on the geotechnical test results of the surface sediments, the seepage parameters k = k(e) and the modified Cambridge model parameters of each layer of material in the model are assigned.

[0062] Analysis step settings. The analysis step uses the Soil analysis step that simulates the fluid-solid coupling of porous media. According to the sediment layer thickness h0 and mass deposition rate R divided in the sediment layer geometry model, s , the analysis step length Δt=g0ρ is calculated d0 / R s .

[0063] Sediment layer removal and activation. First, in the initial analysis step, all sediment layers except the first one are removed using the birth and death unit command provided by the Abaqus Interaction module. Then, in subsequent analysis steps, each sediment layer is activated from bottom to top using the birth and death unit command and the corresponding gravity load is applied to achieve the layer-by-layer deposition process.

[0064] Boundary condition settings. In the initial analysis step, the base displacement of the submarine slope is fixed to zero in both the horizontal and vertical directions, and the lateral displacement of the sediment layer is fixed to zero in the horizontal direction. In each subsequent analysis step, the surface sediment is set as a permeable layer, and its excess pore water pressure is maintained at zero in the corresponding analysis step.

[0065] Meshing and calculation. The four-node plane strain element (CPE4) is used for the base part, and the quadratic reduced integration element (CPE8RP) is used for the sedimentary layer. si The distribution of formation porosity with depth obtained by simulation under the conditions of 2.

[0066] Step 7: Place the si Compare the simulated porosity distribution with depth under these conditions with the corresponding hypothetical porosity distribution with depth. If the two are consistent, it means that the mass deposition rate is the average deposition rate at the proposed study location since time T, and the simulated porosity distribution with depth is the actual porosity distribution at that location.

[0067] The following describes in detail the inversion method for seafloor sedimentation rate and formation porosity distribution provided in this embodiment, taking the seafloor strata at a certain location in the shelf break zone of the northern continental slope of the South China Sea as an example.

[0068] An inversion method for seafloor sedimentation rate and formation porosity distribution includes the following steps:

[0069] Step S1: Collect the published ocean drilling core test data near the location and obtain the distribution range of the formation porosity n in this area with the change of depth z, see Figure 2 .

[0070] Step S2: Dry density ρ of surface sediment sample at the research point d0 , porosity n0, modified Cambridge model parameters, the relationship between permeability coefficient and porosity k = k(e), see Table 1.

[0071] Table 1

[0072] Calculation parameters value unit <![CDATA[ρ d0 ]]> 1.028 <![CDATA[g / cm 3 ]]> <![CDATA[n0]]> 0.62 / Modified Cambridge model parameter λ 0.148 / Modified Cambridge model parameter κ 0.030 / Modified Cambridge model parameter M 0.734 / k=k(e) <![CDATA[k=4.3×10 -8 ×exp[(e-1.63) / 0.27]]]> m / s

[0073] Step S3: The stratigraphic age at the seabed depth of 101.9 m at the research site is 306 ka.

[0074] Step S4: Four possible relationships of formation porosity with depth are assumed at the proposed study location, see Table 2.

[0075] Table 2

[0076] Hypothetical relationship 1 n=0.62-0.0230ln[(z+0.92417) / 0.92417] Hypothetical relationship 2 n=0.62-0.0365ln[(z+0.92417) / 0.92417] Hypothetical relationship 3 n=0.62-0.0495ln[(z+0.92417) / 0.92417] Hypothetical relationship 4 n=0.62-0.0565ln[(z+0.92417) / 0.92417]

[0077] Step S5: For each assumed relationship between formation porosity and depth, the corresponding average mass deposition rate R can be calculated by formula (2): si , see Table 3.

[0078] Table 3

[0079]

[0080] Step S6: Simulate the deposition rate R in the Abaqus numerical software si The porosity distribution of the seabed strata under the conditions of Figure 3 .

[0081] Step S7: By comparing the simulated formation porosity distribution with depth with the corresponding assumed formation porosity distribution with depth, it can be considered that the average mass deposition rate at the study point since 306 ka is about 52.70 g / (cm 2 ·ka), the porosity variation of seabed formation along the depth direction can be expressed as n=n0-0.0495·ln((h+0.92417) / 0.92417).

[0082] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A method for inverting seafloor sedimentation rate and formation porosity distribution, characterized in that: The steps include: S10, collecting and obtaining the possible distribution range of formation porosity n as a function of depth z in this area based on ocean drilling core test data near the proposed research location; S20, collect surface sediment samples at the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples; S30, based on the distribution range of formation porosity n with depth z in this area and porosity n0, assume m possible formation porosity variation with depth at the proposed research location n = n i (z), i∈[1:m]; S40, for each assumed relationship between formation porosity and depth n = n i (z), calculate the corresponding average mass deposition rate R si , and then simulate the deposition rate R in the numerical software si The porosity distribution of the submarine strata below; S50, will be available in various R si The simulated formation porosity distribution with depth under the conditions is compared with the corresponding assumed formation porosity distribution with depth. If the comparison between the two is consistent, the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location. Otherwise, different R si Continue to simulate the distribution of formation porosity with depth and compare it with the corresponding assumed distribution of formation porosity with depth.

2. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 1, characterized in that: In step S40, the average mass deposition rate R si The calculation formula is: Where, d is the depth of the seabed stratum; T is the age of the stratum at the depth d; G s is the specific gravity of sediment particles; ρ w is the density of seawater.

3. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 2, characterized in that: The stratigraphic age T of the seabed strata at the depth d is determined by collecting three-dimensional seismic profiles at the proposed study location and combining them with the published stratigraphic framework at a nearby location.

4. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 3, characterized in that: The stratigraphic framework is a framework structure that is established by analyzing and integrating various information of submarine strata and can reflect the temporal and spatial distribution and mutual relationship of strata.

5. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 1, characterized in that: In step S40, the deposition rate R is simulated in numerical software. si The steps for determining the porosity distribution of the seabed strata under investigation are as follows: Build a geometric model of the sedimentary layer and then divide the seabed into layers; According to the sedimentation layer thickness h0 and mass deposition rate R divided in the sedimentation layer geometric model s , calculate the analysis step length; Assigning values ​​to the seepage parameters and modified Cambridge model parameters of each layer of material in the sedimentary layer geometric model based on the geotechnical test results of the surface sediment sample taken in step S20; In the initial analysis step, the birth and death unit command in the numerical software is used to remove all sedimentary layers except the first sedimentary layer. Then, the birth and death unit command is used to activate each sedimentary layer from bottom to top in the subsequent analysis steps, and the corresponding gravity load is applied to realize the layer-by-layer deposition process. In the initial analysis step, the displacement of the bottom slope base is fixed to 0 in the horizontal and vertical directions, and the displacement of the lateral boundary of the sediment layer is fixed to 0 in the horizontal direction. In each subsequent analysis step, the surface sediment is set as a permeable layer, and its excess pore water pressure is kept at 0 in the corresponding analysis step. The four-node plane strain unit is used for the substrate and the quadratic reduced integration unit is used for the deposition layer. si The distribution of formation porosity with depth obtained by simulation under the conditions of 2.

6. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 5, characterized in that: The modified Cambridge model parameters include the slope M of the critical state line of the surface sediment sample on the p′-q plane, the slope λ of the isotropic consolidation curve of the surface sediment sample on the e-lnp′ plane, and the slope k of the rebound curve of the surface sediment sample on the e-lnp′ plane.

7. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 5, characterized in that: The modified Cambridge model parameters are determined by triaxial testing.

8. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 5, characterized in that: The analysis step adopts the Soil analysis step for simulating fluid-solid coupling of porous media.

9. The inversion method for seafloor sedimentation rate and formation porosity distribution according to claim 1, characterized in that: In step S40, the numerical software is Abaqus numerical software.

10. An application of the inversion method for seafloor sedimentation rate and formation porosity distribution according to any one of claims 1 to 9, characterized in that: It is used in the fields of submarine geological disaster risk assessment, offshore oil and gas resource evaluation, and marine engineering construction.

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