Karst reservoir fluid detection method and system

Abstract

The invention relates to a karst reservoir fluid detection method and system, and belongs to the field of geophysical exploration, and the method comprises the following steps: obtaining an angle superposition gather of a karst reservoir to be detected; performing frequency decomposition on the angle superposition gather by using time-frequency transformation to obtain different frequency distribution conditions of each angle, and selecting a frequency sensitive factor corresponding to each angle, and according to the depth of the karst reservoir to be detected, the change condition of the frequency sensitive factor along with the frequency and the change condition of the frequency sensitive factor along with the frequency and the angle, calculating the value of the fluid-containing attribute of the reservoir. Due to the fact that the variable quantity of the reservoir gradient is in direct proportion to the possibility of containing the fluid, the frequency sensitive factor corresponding to the angle is selected according to the variable quantity of the gradient, the value of the fluid-containing attribute is calculated through the reservoir depth and the frequency sensitive factor, it is guaranteed that calculation of the value of the fluid-containing attribute is more accurate, and people can know the overall state of the reservoir more clearly; and the risk of well position design is reduced.

Description

Technical Field

[0001] The invention belongs to the field of geophysical exploration, and in particular relates to a karst reservoir fluid detection method and system. Background Art

[0002] When artificially excited seismic waves encounter a stratum interface during their propagation underground, they generate seismic waves that are reflected upward. The received seismic signals contain a wealth of information such as amplitude, frequency, phase, and travel time. After processing the seismic signals, we can obtain amplitude information relative to the incident angle and frequency changes. When the stratum contains fluids, such as water, oil, or gas, the amplitude value will change significantly with the incident angle and the frequency of the seismic signal. Therefore, we can use this amplitude change to identify oil and gas information in the stratum.

[0003] Patent authorization announcement number CN105259576B discloses a method for identifying oil and gas reservoirs using seismic statistical characteristics. This method uses post-stack fluid detection technology to detect the presence of fluid-containing reservoirs. Due to the diverse rock types, complex reservoir space, and strong heterogeneity of karst reservoirs, reservoir prediction is very difficult. Conventional post-stack fluid detection methods have low accuracy, are easily affected by the quality of seismic data, and cannot fully utilize amplitude and frequency information, resulting in inaccurate fluid detection results for karst reservoirs.

[0004] To solve the above problems, the patent patent publication number CN111856564A discloses a reservoir fluid detection method, which is based on angle superposition to identify fluids. This solution fully utilizes the incident angle information to solve the above problems. However, when identifying fluids, the influence of the frequency in the seismic signal on the fluid properties is not considered, resulting in low accuracy in identifying the fluid properties and an unclear understanding of the overall reservoir. Summary of the invention

[0005] The purpose of the present invention is to provide a karst reservoir fluid detection method and system to solve the technical problem in the prior art that the recognition accuracy of fluid properties is low, resulting in an insufficient overall understanding of the reservoir.

[0006] To achieve the above-mentioned purpose, the technical solution of a karst reservoir fluid detection method and system provided by the present invention is:

[0007] A karst reservoir fluid detection method comprises the following steps: obtaining an angle stacking gather of a karst reservoir to be detected; performing frequency decomposition on the angle stacking gather by using time-frequency transformation to obtain different frequency distributions of each angle, selecting a frequency sensitive factor corresponding to each angle according to the variation of each frequency gradient at each angle, and constructing a frequency sensitive factor set according to the frequency sensitive factors selected at each angle; calculating the value of the reservoir's fluid-containing attribute according to the depth of the karst reservoir to be detected, the variation of the frequency sensitive factor with frequency, and the variation of the frequency sensitive factor with frequency and angle; and judging whether the karst reservoir contains fluid according to the obtained value of the fluid-containing attribute.

[0008] The beneficial effect is that since the change in the reservoir frequency gradient is proportional to the possibility of containing fluid, the frequency sensitivity factor corresponding to the angle is selected according to the gradient change, which improves the accuracy of obtaining the frequency sensitivity factor. The value of the fluid-containing attribute is calculated by the reservoir depth and frequency sensitivity factor, which ensures that the calculation of the value of the fluid-containing attribute is more accurate and improves the accuracy of judging the fluid-containing karst reservoir.

[0009] As a further improvement, the calculation formula containing fluid properties is:

[0010]

[0011] Among them, Atr is the value of reservoir fluid properties, is the first-order derivative of the frequency sensitivity factor F, The second-order derivative of the sensitive frequency factor F, f is the frequency, and a, b, and c are constants related to the reservoir depth.

[0012] The beneficial effect is that the value Atr of the fluid property in the reservoir is calculated by the relationship between the karst reservoir depth and the sensitive frequency factor and the relationship between the frequency and the angle. The Atr value can accurately obtain the fluid property in the karst reservoir.

[0013] As a further improvement, the method for selecting the frequency sensitivity factor is as follows: the frequency with the largest frequency gradient change is selected as the frequency sensitivity factor of the corresponding angle.

[0014] The beneficial effect is: since the greater the frequency gradient change, the greater the possibility that the corresponding karst reservoir contains fluid, the value with the largest frequency gradient change is selected as the frequency sensitivity factor to provide data support for the subsequent calculation of the value of the fluid-containing attribute, thereby improving the accuracy of reservoir fluid detection.

[0015] As a further improvement, the method for obtaining the angle stacking gather is as follows: obtain the original gather of the common reflection point, convert the offset distance into the corresponding angle using a conversion formula, and superimpose the original gather with different angles to obtain the angle stacking gather.

[0016] The beneficial effect is: firstly obtain the original gather, obtain the angle superposition gather through the conversion formula, and obtain the angle data, so that the data of the gather is richer.

[0017] As a further improvement, the conversion formula is: Where θ is the incident angle of the gather, x is the offset, V stk is the velocity model, and t0 is the travel time of the seismic signal at the current moment.

[0018] The beneficial effect is that the offset distance data in the original gather is converted into angle data through the relationship between the distance and the incident angle.

[0019] As a further improvement, the time-frequency transform formula is: Where R(θ) is the residual signal, σ is the decomposition coefficient, f is the frequency, θ is the angle, D(θ) is the gather signal in the angle stack gather, S(θ,(f1,f2,...,f m )) is the gather signal in the angle superposition gather after frequency decomposition, and m represents the number of frequencies obtained by decomposition.

[0020] The beneficial effect is that the frequency corresponding to the angle is decomposed through the time-frequency transformation formula to obtain angle data, making the data of the gather richer.

[0021] The present invention also discloses a karst reservoir fluid detection system, which comprises a processor, and the processor is used to process any one embodiment of the karst reservoir fluid detection method. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a flow chart of the karst reservoir fluid detection method of the present invention;

[0023] Figure 2 It is a statistical result diagram of actual offset and azimuth in the karst reservoir fluid detection method of the present invention;

[0024] Figure 3a The time-frequency spectrum decomposition of the angle stacking gather in the karst reservoir fluid detection method of the present invention is Figure 1 ;

[0025] Figure 3b The time-frequency spectrum decomposition of the angle stacking gather in the karst reservoir fluid detection method of the present invention is Figure 2 ;

[0026] Figure 3c FIG3 is a time-frequency spectrum decomposition diagram of the angle stacking gather in the karst reservoir fluid detection method of the present invention;

[0027] Figure 4 It is a frequency sensitivity factor diagram in the karst reservoir fluid detection method of the present invention;

[0028] Figure 5 This is a diagram showing the calculation results of fluid properties in the karst reservoir fluid detection method of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0030] Karst reservoir fluid detection method embodiment:

[0031] This embodiment takes the 3D seismic data of a certain area as an example. The area of ​​the 3D seismic data of the area is 1600km 2 , and several exploration wells have been drilled, and oil reservoirs have been found in karst reservoirs. In addition to oil reservoirs, there are also coal seams in this reservoir, and the stratigraphic conditions are relatively complex. The existing identification methods cannot effectively identify the oil and gas reservoirs in the karst reservoirs under the influence of the overlying medium.

[0032] like Figure 1 As shown, the karst reservoir fluid detection method includes the following steps:

[0033] 1) Obtain angle stack gathers.

[0034] Get the original gathers of the common reflection point D = {D1(x1), D2(x2), ..., D l (x l )}where x is the offset and D represents the gather signal.

[0035] Use the conversion formula to convert the offset distance into the corresponding angle. The conversion formula is:

[0036]

[0037] Where x is the offset, V stk is the velocity model, and t0 is the travel time of the seismic signal at the current moment. Figure 2 The distribution of angles is shown. Figure 2 ] are the offset information and azimuth information in the original seismic data. The offset information is converted into incident angle information by the formula. The present invention does not consider the azimuth information and averages the azimuth during the stacking process.

[0038] The angle is superimposed with the original gather to obtain the angle superimposed gather, which is:

[0039] D={D1(θ1),D2(θ2),...,D n (θ n )}

[0040] n represents the number of superimposed gather angles, and D represents the gather signal.

[0041] 2) Perform frequency decomposition on the angle stacking gather to obtain the angle gather.

[0042] The angle stack gather is decomposed according to the time-frequency transform to obtain the different frequency distributions of each angle, such as Figure 3a , Figure 3b , Figure 3c The figure shows the result of time-frequency decomposition, where the left side shows the position of the extracted single-channel seismic signal, and the middle shows the result of the calculated time-frequency spectrum. The time-frequency transformation formula is:

[0043]

[0044] Among them, R(θ) is the residual signal, σ is the decomposition coefficient, f is the frequency, θ is the angle, and D represents the gather signal.

[0045] The angle stack gather after frequency decomposition is:

[0046] S={S1(θ1,(f1,f2,...,f m )),S2(θ2,(f1,f2,...,f m )),...,S n (θ n ,(f1,f2,...,f m ))}

[0047] Among them, f is the frequency, θ is the angle, and m represents the number of frequencies obtained by decomposition.

[0048] 3) Obtain a set of frequency sensitivity factors.

[0049] At each angle, a frequency sensitivity factor corresponding to each angle is selected according to the variation of each frequency gradient, and a frequency sensitivity factor set is constructed according to the frequency sensitivity factors selected at each angle.

[0050] Since the greater the change in frequency gradient, the greater the possibility of fluid, when selecting the frequency sensitivity factor, it is necessary to select the frequency with the largest frequency gradient change as the frequency sensitivity factor for the corresponding angle. Figure 4 The result of the frequency sensitivity factor is shown. The set of frequency sensitivity factors is:

[0051] F={F1(θ1,f),F2(θ2,f),...,F n (θ n,f)}

[0052] Among them, f is the frequency, θ is the angle, and F is the set.

[0053] Since the change in reservoir gradient is proportional to the possibility of containing fluid, the frequency sensitivity factor corresponding to the angle is selected according to the change in gradient, thereby improving the accuracy of obtaining the frequency sensitivity factor.

[0054] 4) Obtain the values ​​of fluid properties of the karst reservoir.

[0055] At the same angle, the frequency change when there is fluid in the reservoir is greater than when there is no fluid in the reservoir. First, based on the calculation result of the sensitive frequency factor F(θ,f), the first term in the fluid-containing attribute formula is output. and the second Then, according to the reservoir depth, the values ​​of a, b, and c are determined, and finally, the fluid properties are calculated using the approximate formula. Specifically, the volume-plate relationship constructed by the reservoir depth, frequency sensitivity factor set, and angle gather is expressed as follows:

[0056]

[0057] Among them, Atr is the possibility of fluid properties in the reservoir, f is the frequency, θ is the angle, F is the set, and a, b, and c are constants. Atr consists of three items, which are: And constant terms. The larger the value of Atr, the greater the possibility that there is fluid in the reservoir. In the formula, the first-order derivative represents the change of the frequency sensitivity factor F with frequency, that is, to find the relationship between the reservoir fluid content and frequency. The second-order derivative represents the change of the sensitive frequency factor F with frequency and angle, that is, the relationship between the reservoir fluid content and the frequency and angle. The constant term is related to the depth of the reservoir. When Atr changes less with frequency, the value of a is smaller and the value of b is larger. When Atr changes more with frequency, the value of a is larger and the value of b is smaller. When the depth of the reservoir is deeper, the values ​​of a and b are larger, and the value of c is smaller. The value range of a is 0.2-0.5, the value of b is 0.8-1.3, and the value of c is between 0.2 and 1.

[0058] The volume-plate relationship constructed in this way can accurately predict the values ​​of fluid properties.

[0059] Calculate the fluid properties of the karst reservoir in the entire work area, set the threshold range of the fluid properties, perform fluid detection on the reservoir, and output the final results. Figure 5 The figure shows the calculation results of the fluid properties in the work area. Based on the calculation results, the function of identifying the oil and gas in the karst reservoir is realized.

[0060] 5) Utilize the values ​​of fluid properties of karst reservoirs.

[0061] The spatial distribution of the reservoir is characterized by the value of the fluid-containing attribute of the karst reservoir. When the value of the fluid-containing attribute is within the threshold range, the value of the fluid-containing attribute is retained, otherwise, the value of the fluid-containing attribute is set to a null value, and the spatial distribution of the reservoir is characterized according to the statistical value of the fluid-containing attribute. The value of the fluid-containing attribute can be arbitrarily transformed as needed to obtain the state of the reservoir containing fluid.

[0062] If the value is within the threshold range, it means that the reservoir is more likely to contain fluid. The values ​​within the threshold range will be retained, and the values ​​with low possibility of containing fluid attributes will be directly set to null values. When characterizing the reservoir, the areas with high possibility of containing fluid can be highlighted, so that people can clearly identify the distribution of fluid in the reservoir through the spatial distribution of the reservoir, reduce the risk of well location design, and improve the efficiency of exploration and development.

[0063] Based on the values ​​of fluid properties obtained by this method, multiple exploratory wells were deployed in the work area, and all of them encountered oil reservoirs. This method for identifying oil and gas reservoirs in karst reservoirs has achieved good application results in this area, demonstrating the reliability of the use of prestack seismic data proposed in the present invention to identify oil and gas reservoirs. The application of this method greatly reduces the risk of well deployment and improves the economic benefits of oil field exploration and development.

[0064] Karst reservoir fluid detection system embodiment:

[0065] The karst reservoir fluid detection system includes a processor, which is used to process any one embodiment of the karst reservoir fluid detection method. The specific implementation of this embodiment refers to the embodiment of the karst reservoir fluid detection method, which will not be repeated here.

[0066] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments without creative work, or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A karst reservoir fluid detection method, characterized in that: The method comprises the following steps: Obtain angle stacking gathers of the karst reservoir to be detected; The angle stack gathers are decomposed by time-frequency transformation to obtain the frequency distribution of each angle. The frequency sensitivity factor corresponding to each angle is selected according to the change of each frequency gradient at each angle, and the frequency sensitivity factor set is constructed according to the frequency sensitivity factors selected at each angle. Calculate the value of the fluid-containing property of the reservoir according to the depth of the karst reservoir to be detected, the change of the frequency sensitivity factor with frequency, and the change of the frequency sensitivity factor with frequency and angle; Whether the karst reservoir contains fluid is determined based on the obtained value of the fluid-containing attribute.

2. The karst reservoir fluid detection method according to claim 1, characterized in that: The formula used for calculation of fluid properties is: Among them, Atr is the value of the fluid-containing property of the reservoir, is the first-order derivative of the frequency sensitivity factor F, The second-order derivative of the sensitive frequency factor F, f is the frequency, and a, b, and c are constants related to the reservoir depth.

3. The karst reservoir fluid detection method according to claim 1, characterized in that: Method for selecting frequency sensitivity factor: Select the frequency with the largest frequency gradient change as the frequency sensitivity factor of the corresponding angle.

4. The karst reservoir fluid detection method according to claim 2, characterized in that: The method for obtaining angle stacking gathers is as follows: obtain the original gathers of the same reflection point, convert the offset distance into the corresponding angle using the conversion formula of the gather offset distance and angle, and superimpose them with the original gathers according to different angles to obtain angle stacking gathers.

5. The karst reservoir fluid detection method according to claim 4, characterized in that: The conversion formula is: Where θ is the incident angle of the gather, x is the offset, V stk is the velocity model, and t0 is the travel time of the seismic signal at the current moment.

6. The karst reservoir fluid detection method according to claim 4, characterized in that: The time-frequency transformation formula is: Where R(θ) is the residual signal, σ is the decomposition coefficient, f is the frequency, θ is the angle, D(θ) is the gather signal in the angle stack gather, S(θ,(f1,f2,...,f m )) is the gather signal in the angle superposition gather after frequency decomposition, and m represents the number of frequencies obtained by decomposition.

7. A karst reservoir fluid detection system, characterized in that: The system comprises a processor, and the processor is used to process the karst reservoir fluid detection method according to any one of claims 1 to 6.

Images