Underwater pore medium stratum bubble size calculation method

Abstract

The invention discloses an underwater pore medium stratum bubble size calculation method, and relates to the technical field of underwater pore medium stratum geophysical exploration, and the method comprises the following steps: S1, obtaining rock physical parameters of an underwater stratum to be detected; s2, a sound wave transmitting transducer and a receiver are deployed at a point to be measured, the sound wave transmitting transducer transmits a set signal to determine a measurement frequency range, and the receiver records a sound wave speed and an attenuation coefficient of each frequency point; s3, judging whether adjacent frequency points meeting the requirements of sound wave speed reduction and attenuation coefficient increase exist or not, and if yes, re-measuring and extracting the sound wave speed and attenuation coefficient corresponding to the bubble resonance characteristics; otherwise, recording the bubble-free characteristic times, and if the times do not reach the threshold value, continuing to adjust the number of the frequency points and returning to the step 2; s4, constructing a target function; and S5, under the condition that the angular frequency and the sound wave speed are known, solving the independent variable, the bubble radius and the bubble saturation which enable the objective function to be minimum, and determining the final bubble size and saturation.

Description

Technical Field

[0001] The invention belongs to the technical field of geophysical detection of underwater porous medium strata, and in particular relates to a method for calculating bubble size in underwater porous medium strata. Background Art

[0002] The size of bubbles in underwater porous media has important scientific significance and practical application value. The size of bubbles in seafloor porous media can reveal the stability and decomposition dynamics of natural gas hydrates, providing key parameters for resource reserve assessment; at the same time, the bubble size characteristics can indicate the deep oil and gas leakage path, optimize acoustic exploration technology, improve the accuracy of seafloor resource detection, and provide a scientific basis for carbon sequestration safety assessment. The size of bubbles directly regulates the release flux of greenhouse gases such as methane: small bubbles dissolve in water to slow down emissions, and large bubbles go directly to the atmosphere to intensify warming. Studying its distribution law can improve the global carbon cycle model, and at the same time reveal the impact of bubble release on seafloor acidification, microbial activity and ecosystems, providing theoretical support for ecological protection. Bubble aggregation can reduce sediment strength and induce submarine landslides. Its size monitoring can build a disaster warning system; in the engineering field, bubble size affects pipeline cavitation rate, platform foundation stability and resource extraction efficiency. The research results can provide key technical parameters for deep-sea engineering safety design and ecological risk prevention and control.

[0003] In summary, the study of bubble size in underwater porous medium formations is a core topic in the fields of geophysical exploration, environmental assessment and marine engineering safety. The existing technical system and bottlenecks mainly include: experimental characterization technology: using micro-CT imaging combined with image segmentation algorithm to extract bubble morphology, or using acoustic inversion (such as Biot-Stoll model) to infer the bubble equivalent radius, but limited by laboratory conditions and model parameters are difficult to obtain, it is impossible to simulate the underwater formation environment; numerical simulation method: simulating bubble migration based on pore network model or discrete bubble dynamics, but the cross-scale coupling algorithm is not yet mature; in-situ observation technology: relying on laser diffraction sensors, resistivity tomography (ERT) and other equipment to obtain bubble distribution, but the underwater formation environment causes severe attenuation of optical signals and complex interference, and there is a lack of long-term continuous observation data. The above limitations have seriously hindered the in-depth study of the stability assessment of submarine natural gas hydrates, the analysis of greenhouse gas release mechanisms and the risk prevention and control of submarine engineering. The existing patents analyze the bubble saturation of underwater formations, but do not involve bubble size assessment. Therefore, it is urgent to develop new technical solutions to overcome the difficulties in accurate characterization and dynamic monitoring of bubble size in underwater formations.

[0004] The study found that when the formation acoustic signal is close to the bubble resonance frequency, the sound velocity first drops sharply to a minimum value as the frequency increases, and then rises sharply to reach a peak value. The sound wave attenuation coefficient shows a significant peak at the bubble resonance frequency, with a sharp peak shape and the largest amplitude, and the frequency at the sound velocity peak is always lower than the frequency at the attenuation peak. This phenomenon is directly related to the change in the phase response of the bubble. The bubble resonance frequency is extremely sensitive to the bubble radius response. Therefore, the present invention proposes a method for calculating the bubble size in the underwater porous medium formation, which mainly solves the technical problem that the existing technology cannot evaluate the bubble size in the seabed formation. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a method for calculating the bubble size in underwater porous medium formations, which quickly identifies bubble-free areas through a threshold judgment mechanism, reduces invalid exploration costs, and improves operational efficiency.

[0006] To achieve the above object, the present invention provides a method for calculating bubble size in underwater porous medium formation, comprising: S1. Obtain rock physical parameters of the underwater strata to be tested through geological survey; S2. Deploy an acoustic wave transmitting transducer and a receiver at the point to be measured. The acoustic wave transmitting transducer transmits a set signal to determine the measurement frequency range, and the receiver records the acoustic wave velocity and attenuation coefficient of each frequency point; S3, determine whether there are adjacent frequency points that satisfy the acoustic wave velocity decrease and the attenuation coefficient increase. If so, reset the maximum frequency point and reduce the step size to 1 / 10 times of the original step size, re-measure and extract the acoustic wave velocity and attenuation coefficient corresponding to the bubble resonance feature; otherwise, record the number of times without bubble features. If the number does not reach the threshold, continue to adjust the number of frequency points and return to step 2; S4, construct objective function; S5. Under the condition that the angular frequency and the acoustic wave velocity are known, the independent variable, the bubble radius and the bubble saturation that minimize the objective function are solved by the grid generation method to determine the final bubble size and saturation.

[0007] Optionally, the measurement frequency range is , , ,in is the step length, is the number of frequency points.

[0008] Optionally, the objective function includes: ; in, is the objective function; and are weights respectively; It is the frequency corresponding to the starting point of the attenuation coefficient under the current working condition, that is, the place where the attenuation coefficient suddenly increases; It is the theoretical formula for calculating the bubble resonance frequency under the current working conditions; It is the theoretical formula for calculating the velocity of the underwater formation under the current working conditions; is the speed of sound waves with bubble resonance characteristics.

[0009] Optionally, the theoretical formula for calculating the bubble resonance frequency under the current working conditions includes: ; in, is the specific heat ratio of the gas in the bubble; is the hydrostatic pressure of the formation; is the current formation equivalent density; is the equivalent density of the gas in the bubble; is the pore fluid density; is the stratum skeleton density; is the current formation shear modulus; is the angular frequency, is the specific heat of the gas at constant pressure, is the thermal conductivity of the gas.

[0010] Optionally, the theoretical formula for calculating the water bottom formation velocity under the current working conditions includes: ; ; ; ; ; ; ; ; ; ; ; ; ; in, is the bubble saturation; is the bulk modulus of the gas in the bubble; is the bulk modulus of the pore liquid; is the bulk modulus of particles constituting the stratum skeleton; is the bulk modulus of the stratum skeleton.

[0011] Optionally, when meshing, the independent variable with the smallest objective function, the bubble radius and the bubble saturation are divided into equal steps, each variable is divided into N equal parts of not less than 20, and the objective function is calculated at the nodes, and the node corresponding to the minimum value is selected as the initial solution.

[0012] Optionally, eight adjacent cells of the grid where the initial solution is located are further subdivided into N equal parts, and after recalculating the objective function, the minimum node is selected as the final solution.

[0013] Optionally, in S3, if bubble resonance characteristics exist at all frequency points, the formation velocity takes the maximum value of the measured value as the average acoustic wave velocity.

[0014] Technical effect of the invention: The present invention discloses a method for calculating the size of bubbles in underwater porous medium formations. A specific pulse signal is emitted by an acoustic wave transmitting transducer to measure the acoustic wave velocity and attenuation coefficient at different frequencies. The characteristics that the sound velocity first drops sharply near the bubble resonance frequency and then rises back, and the attenuation coefficient changes in the opposite direction, are used to construct an objective function including the theoretical resonance frequency and the theoretical formation velocity. Through a grid subdivision optimization algorithm, the bubble saturation, bubble radius and resonance frequency are simultaneously inverted under the condition of minimizing the error of the objective function. The present invention innovatively combines acoustic characteristics with porous medium theory, providing a reliable means of quantitative evaluation of bubble parameters for seabed resource exploration, seabed gas storage and geological disaster warning. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 A schematic diagram of a flow chart of a method for calculating bubble size in a water bottom porous medium formation according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the change characteristics of the normalized sound wave velocity, attenuation coefficient and resonance frequency with frequency in the bubble resonance frequency range according to an embodiment of the present invention. DETAILED DESCRIPTION

[0016] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0017] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0018] like Figure 1 As shown, this embodiment provides a method for calculating bubble size in a water bottom porous medium formation, including: Step 1: Conduct geological survey to determine the rock physical parameters of the underwater strata to be tested; Step 2: At the underwater stratum test point, the acoustic wave transmitting transducer transmits a pulse with a width of , the amplitude is The pulse signal is measured in the frequency range , the step length is , the number of frequency points is greater than 3; , ; The acoustic signal receiver is placed in the underwater formation pointed by the acoustic wave transmitting transducer, with a distance from the transmitter greater than one wavelength. The received signal at each frequency point is recorded to obtain the acoustic wave speed. and attenuation coefficient ; Step 3, determine whether there are adjacent frequency points where the sound wave velocity decreases and the attenuation coefficient increases at the same time, where the adjacent frequency points refer to the frequency points whose serial numbers differ by 1; If it exists, concatenate all adjacent frequency points together in sequence to find the maximum frequency point and the minimum frequency point, return to step 2, and assign the maximum frequency point to the value in step 2. , reduce the step size to increase the number of frequency points tenfold, re-measure and calculate, and extract the sound wave velocity with bubble resonance characteristics and attenuation coefficient , extract the frequency corresponding to the attenuation coefficient starting point , the sound wave velocity without bubble resonance characteristics is averaged and recorded as , if all frequencies have bubble resonance characteristics, then Take the maximum value of all measured speeds and go to step 4; If it does not exist, record the number of nongas non-existences. If nongas is less than the given threshold Maxnongas, return to step 2 and increase the number of frequency points to 10 times. If the number of nongas non-existences is equal to the given threshold Maxnongas, end the measurement. There are no bubbles in the seabed formation within the measured frequency range.

[0019] Step 4: construct error and objective functions, where the errors include: the error between the measured speed and the theoretical speed, and the error between the bubble resonance frequency and the theoretical frequency; The objective function is defined as: ; in, It is the frequency corresponding to the starting point of the attenuation coefficient under the current working conditions, that is, the place where the attenuation coefficient suddenly increases. It is the theoretical formula for calculating the bubble resonance frequency under the current working conditions; It is the theoretical formula for calculating the velocity of the underwater formation under the current working conditions. and is the weight; The theoretical formula of bubble resonance frequency under current working conditions is: ; in, is the specific heat ratio of the gas in the bubble, is the hydrostatic pressure of the formation; is the current formation equivalent density; is the equivalent density of the gas in the bubble; is the pore fluid density; is the stratum skeleton density; is the current formation shear modulus; angular frequency , is the specific heat of the gas at constant pressure, is the thermal conductivity of the gas; The theoretical formula of formation velocity under current working conditions is: ; ; ; ; ; ; ; ; ; ; ; ; ; in, is the bubble saturation; is the bulk modulus of the gas in the bubble, is the bulk modulus of the pore liquid; is the bulk modulus of particles constituting the stratum skeleton; is the bulk modulus of the stratum skeleton; Step 5: Given the known data and Based on this, solve the independent variable under the condition that the objective function value Error is minimized. and ; Given a reasonable range of values ​​for the independent variable, and , divide each variable into N equal parts with equal step size, and calculate the objective function value at the grid nodes , select the node corresponding to the minimum value , if there are multiple nodes with the minimum value, select the node close to the center of the region; For the grid where the minimum node is located, the eight adjacent cells are regarded as a whole, each edge is divided into N equal parts, and the objective function value is calculated on the grid node. , select the node corresponding to the minimum value ( ) is recorded as the final solution, so the bubble saturation of the active water bottom formation is , and the size of the bubble .

[0020] Furthermore, the measurement frequency range in step 2 is , according to the bubble size range of interest, set the upper and lower bounds of the frequency to improve the calculation efficiency.

[0021] Furthermore, in step 3, it is determined whether the formation acoustic signal has a bubble resonance feature, and the frequency sweep grid is refined based on the feature to extract acoustic information.

[0022] Furthermore, in step 4, the dimension is eliminated by setting weights, so that the two errors become dimensionless and close in size.

[0023] A specific application embodiment of the present invention is as follows: This embodiment provides a method for calculating the size of bubbles in a porous medium formation under water, the steps of which are as follows: Step 1: Conduct geological survey to determine the rock physical parameters of the underwater strata to be tested; Step 2: At the underwater stratum test point, the acoustic wave transmitting transducer transmits a pulse with a width of , the amplitude is =1[V] CW pulse signal, the measurement frequency range is , the step length is , the number of frequency points is 91; , ; The acoustic signal receiver is placed in the underwater formation pointed by the acoustic wave transmitting transducer, with a distance of 0.4 [m] from the transmitter, and the received signal of each frequency point is recorded to obtain the acoustic wave velocity. and attenuation coefficient ; Step 3: Determine whether there are adjacent frequency points where the sound wave velocity decreases and the attenuation coefficient increases at the same time. Figure 2 , here adjacent frequency points refer to the frequency point numbers that differ by 1; If it exists, concatenate all adjacent frequency points together in sequence to find the maximum frequency point and the minimum frequency point, return to step 2, and assign the maximum frequency point to the value in step 2. , divide the frequency range equally and increase the number of frequency points tenfold, re-measure and calculate, and extract the sound wave velocity with bubble resonance characteristics and attenuation coefficient , extract the frequency corresponding to the starting point of the attenuation coefficient , the sound wave velocity without bubble resonance characteristics is averaged and recorded as , if all frequencies have bubble resonance characteristics, then Take the maximum value of all measured speeds and go to step 4; If it does not exist, record the number of nongas non-existences. If nongas is less than the given threshold Maxnongas, return to step 2 and change the number of frequency points to 10m. If the number of nongas non-existences is equal to the given threshold Maxnongas, end the measurement. There are no bubbles in the seabed formation within the measured frequency range.

[0024] Step 4: construct error and objective functions, where the errors include: the error between the measured speed and the theoretical speed, and the error between the bubble resonance frequency and the theoretical frequency; Preferably, the objective function here is defined as ; in, is the bubble radius under the current working condition The corresponding actual bubble resonance frequency, It is the theoretical formula for calculating the bubble resonance frequency under the current working conditions. It is the theoretical formula for calculating the velocity of the underwater formation under the current working conditions. and is the weight; Preferably, the theoretical formula of the bubble resonance frequency under the current working condition is taken as: ; in, is the specific heat ratio of the gas in the bubble, [Pa] Hydrostatic pressure of the formation, is the current formation equivalent density; [kg / ] is the equivalent density of the gas in the bubble, =1013[kg / ] pore fluid density, =2300[kg / ]Skeleton density of the formation, [Pa] is the current formation shear modulus, angular frequency , =2.19[J / C] specific heat of gas at constant pressure, =0.03[J / smC] thermal conductivity of gas; Preferably, the theoretical formula for formation velocity under current working conditions is taken as: ; ; ; ; ; ; ; ; ; ; ; ; ; in, Bubble saturation, Bulk modulus of the gas inside the bubble, The bulk modulus of the pore liquid, The bulk modulus of the particles that make up the stratum skeleton, Bulk modulus of the stratum skeleton; Step 5: Given the known data and Based on this, solve the independent variable under the condition that the objective function value Error is minimized. and ; Preferably, given a reasonable range of values ​​for the independent variable, , and , divide each variable into N=10 equal parts with equal step size, and calculate the objective function value at the grid nodes , select the node corresponding to the minimum value ( ), if there are multiple nodes with the minimum value, select the node close to the center of the area; Furthermore, for the grid where the minimum node is located, the eight adjacent cells are regarded as a whole, and each edge is divided into N=10 equal parts, and the objective function value is calculated on the grid node , select the node corresponding to the minimum value ( ) is recorded as the final solution, so the bubble saturation of the active water bottom formation is , and the size of the bubble .

[0025] According to the provided invention content, the advantages and positive effects of the present invention are summarized as follows: 1. Technical advantages: (1) Multi-parameter simultaneous inversion capability: Through the attenuation characteristics of the acoustic signal in frequency changes, the bubble saturation, bubble radius and resonance frequency in the underwater formation can be estimated simultaneously. This feature breaks through the limitation of single parameter evaluation of traditional methods.

[0026] (2) Adaptive measurement optimization mechanism: Adopting a dynamic frequency adjustment strategy (step 3), by judging the changing characteristics of the acoustic wave velocity and attenuation coefficient, the measurement frequency range and step size are automatically optimized, which significantly improves data acquisition efficiency and avoids invalid measurements.

[0027] (3) Deep integration of theoretical model and measured data: By constructing a theoretical formula including the bubble resonance frequency ( ) and the formation velocity theory formula ( ) (step 4) to achieve accurate matching of acoustic characteristics with the physical model of porous media.

[0028] (4) High-precision grid optimization algorithm: Adopting grid generation and layered optimization strategy (step 5), by adjusting the variables 、r、 Multi-level grid search is performed to ensure that the inversion result is the global optimal solution within a reasonable value range.

[0029] 2. Positive effects: (1) Improving the accuracy of marine resource exploration: Providing accurate bubble size parameters for natural gas hydrate exploration, seabed leakage monitoring and other scenarios, and assisting in determining the gas-bearing characteristics of the formation.

[0030] (2) Wide application in engineering: The method is compatible with the rock physical parameters of different formations (step 1) and is applicable to fields such as marine geological surveys, oil and gas field development, and environmental monitoring.

[0031] (3) Technological innovation breakthrough: For the first time, the sound speed-attenuation coefficient phase difference characteristic (the sound speed peak-valley frequency lags behind the attenuation coefficient peak) was used for inversion calculation, expanding the application boundaries of acoustic detection technology.

[0032] (4) Significant economic benefits: Through the threshold judgment mechanism (Maxnongas), bubble-free areas can be quickly identified, reducing ineffective exploration costs and improving operational efficiency.

[0033] The present invention provides a solution with both theoretical depth and engineering practicality for the assessment of gas content in underwater formations through the collaborative innovation of acoustic signal processing and theoretical models.

[0034] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for calculating bubble size in underwater porous medium formation, characterized in that: include: S1. Obtain rock physical parameters of the underwater strata to be tested through geological survey; S2. Deploy an acoustic wave transmitting transducer and a receiver at the point to be measured. The acoustic wave transmitting transducer transmits a set signal to determine the measurement frequency range, and the receiver records the acoustic wave velocity and attenuation coefficient of each frequency point; S3, determine whether there are adjacent frequency points that satisfy the acoustic wave velocity decrease and the attenuation coefficient increase. If so, reset the maximum frequency point and reduce the step size to 1 / 10 times of the original step size, re-measure and extract the acoustic wave velocity and attenuation coefficient corresponding to the bubble resonance feature; otherwise, record the number of times without bubble features. If the number does not reach the threshold, continue to adjust the number of frequency points and return to step 2; S4, construct objective function; S5. Under the condition that the angular frequency and the acoustic wave velocity are known, the independent variable, the bubble radius and the bubble saturation that minimize the objective function are solved by the grid generation method to determine the final bubble size and saturation.

2. The method for calculating bubble size in underwater porous medium formation according to claim 1, characterized in that: The measurement frequency range is , , ,in is the step length, is the number of frequency points.

3. The method for calculating bubble size in underwater porous medium formation according to claim 1, characterized in that: The objective function includes: ； in, is the objective function; and are weights respectively; is the resonance frequency corresponding to the starting point of the attenuation coefficient under the current working condition; It is the theoretical formula for calculating the bubble resonance frequency under the current working conditions; It is the theoretical formula for calculating the velocity of the underwater formation under the current working conditions; is the speed of sound waves with bubble resonance characteristics.

4. The method for calculating bubble size in underwater porous medium formation according to claim 3, characterized in that: The theoretical formula for calculating the bubble resonance frequency under the current working conditions includes: ； in, is the specific heat ratio of the gas in the bubble; is the hydrostatic pressure of the formation; is the current formation equivalent density; is the equivalent density of the gas in the bubble; is the pore fluid density; is the stratum skeleton density; is the current formation shear modulus; is the angular frequency, is the specific heat of the gas at constant pressure, is the thermal conductivity of the gas.

5. The method for calculating bubble size in underwater porous medium formation according to claim 3, characterized in that: The theoretical formulas for calculating the bottom formation velocity under current working conditions include: ； ； ； ； ； ； ； ； ； ； ； ； ； in, is the bubble saturation; is the bulk modulus of the gas in the bubble; is the bulk modulus of the pore liquid; is the bulk modulus of particles constituting the stratum skeleton; is the bulk modulus of the stratum skeleton.

6. The method for calculating bubble size in underwater porous medium formation according to claim 1, characterized in that: When meshing, the independent variable with the smallest objective function, bubble radius and bubble saturation are divided into equal steps respectively, each variable is divided into N equal parts of not less than 20, and the objective function is calculated at the node, and the node corresponding to the minimum value is selected as the initial solution.

7. The method for calculating bubble size in underwater porous medium formation according to claim 6, characterized in that: The eight adjacent cells of the grid where the initial solution is located are further subdivided into N equal parts, and the minimum node is selected as the final solution after recalculating the objective function.

8. The method for calculating bubble size in underwater porous medium formation according to claim 1, characterized in that: In S3, if all frequency points have bubble resonance characteristics, the maximum value of the measured formation velocity is taken as the average acoustic wave velocity.

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