Target layer apparent resistivity calculation method, storage medium and device for borehole ground electromagnetic method

Abstract

The application belongs to the technical field of geophysical exploration, and particularly discloses a borehole-to-ground electromagnetic method target layer apparent resistivity calculation method, a storage medium and a device, wherein the calculation method is performed according to the following steps: S1, differentiating the upper and lower excited sources of the target layer, and simultaneously differentiating the real and imaginary parts of the electric field excited by the upper and lower sources of the target layer; S2, combining a theoretical electric field calculation formula of a finite-length vertical wire source excited in a well, and solving the apparent resistivity based on the source differentiation by using a negative gradient method. The apparent resistivity obtained by the calculation method is obviously reflected on the electrical property of the target layer, and is close to the actual resistivity of the stratum in the numerical value, so that the reflection effect on the electrical property of the target layer is better. The application is suitable for calculating the apparent resistivity of the target layer in the borehole-to-ground electromagnetic method in geophysical exploration.

Description

Technical Field

[0001] The present invention belongs to the technical field of geophysical exploration and relates to a method, storage medium and device for calculating apparent resistivity of a target layer using a well-to-ground electromagnetic method. Background Art

[0002] Borehole-to-surface electromagnetics (BSE) is an electromagnetic exploration method that excites electromagnetic fields in a well and receives them on the surface. Because it uses in-well excitation, it achieves optimal excitation and illumination of the target. Compared to surface electromagnetics, this method offers higher detection accuracy and has been applied in oil and gas reservoir delineation and water injection monitoring in numerous regions around the world. Current borehole-to-ground electromagnetic methods generally employ the following acquisition, processing, and interpretation methods: A vertical long-wire source is used to excite the target layer above and below (one excitation electrode is located on the surface, and the other excitation electrodes are located above and below the target in the well, respectively). Survey lines or networks are deployed on the surface to receive the radial horizontal electric field (the electric field along the direction from the wellhead to the measuring point). The electric field time series data obtained from these two excitations are filtered, Fourier transformed, and superimposed to obtain the electric field spectrum (amplitude, phase, or real and imaginary parts). The electric field amplitude and phase (or real and imaginary parts) are then used to obtain differential amplitude, differential phase, dual-frequency amplitude, and dual-frequency phase. Alternatively, the processed electric field spectrum is used to calculate apparent resistivity and perform resistivity inversion to obtain inverted resistivity. These electrical parameters are then used to analyze and interpret the oil content and water injection boundaries of the target reservoir. Apparent resistivity is the earth's resistivity calculated from the observed data according to a theoretical formula under uniform half-space conditions and is a comprehensive reflection of the formation's electrical properties. Apparent resistivity reflects formation resistivity more intuitively and does not have multiple solutions (inversion resistivity has multiple solutions). It is an important parameter for the qualitative interpretation of electromagnetic data.

[0003] However, in borehole-to-surface electromagnetic methods, the apparent resistivity calculated by stimulating the electric field above or below the target layer is a comprehensive reflection of the entire underground half-space. It does not clearly reflect the electrical characteristics of the target layer, which is not conducive to the analysis and evaluation of the target layer. Therefore, further research is needed to calculate the apparent resistivity method that reflects the electrical characteristics of the target layer. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for calculating the apparent resistivity of a target layer by using a borehole-ground electromagnetic method, which can clearly reflect the electrical properties of the target layer and has a numerical value close to the resistivity of the actual formation, thereby better reflecting the electrical properties of the target layer.

[0005] A second object of the present invention is to provide a computer-readable storage medium for storing a computer program for calculating the apparent resistivity of a target layer using a borehole electromagnetic method.

[0006] The third object of the present invention is to provide a device for calculating the apparent resistivity of a target layer using the well-surface electromagnetic method.

[0007] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0008] A method for calculating the apparent resistivity of a target layer using a borehole-ground electromagnetic method is performed in the following order:

[0009] S1. Differentiate the sources of the upper and lower excitations of the target layer, and at the same time, perform the difference of the real and imaginary parts of the electric field of the upper and lower excitations of the target layer;

[0010] S2. Combined with the theoretical electric field calculation formula for the finite length vertical wire source in the well, the negative gradient method is used based on source difference to solve the apparent resistivity.

[0011] As a limitation, step S1 is performed in the following order:

[0012] S11, select a ground excitation point A, an excitation point B1 above the target layer, and an excitation point B2 below the target layer; use an excitation source AB1 above the target layer to excite and measure the electric field E1 at the ground point P, and use an excitation source AB2 below the target layer to excite and measure the electric field E2 at the ground point P, with both excitation currents being I;

[0013] S12. Differentiate the excitation source AB2 below the target layer and the excitation source AB1 above the target layer to obtain a finite length vertical wire source B1B2; differentiate the electric field E1 and the electric field E2 to obtain the difference between the real and imaginary parts of the electric field as Ed=E2-E1, and obtain the differential electric field Ed.

[0014] As a further limitation, the calculation process of the theoretical electric field calculation formula excited by a finite length vertical wire source in a well is carried out in the following order:

[0015] S21. With the wellhead as the origin 0 and the downward direction as the positive direction of the Z coordinate axis, a coordinate system is established according to the right-hand screw rule, and the coordinates of point P are set to (x, y, 0);

[0016] S22, divide the finite length vertical wire source B1B2 into n segments, each segment is less than Each segment is regarded as an electric dipole, excited by a finite length vertical wire source B1B2. The radial electric field at point P is numerically integrated by the field excited by n electric dipoles and is expressed as

[0017]

[0018] Among them, m∈[1,n], E m represents the radial electric field of the mth electric dipole at point P;

[0019] As a further qualification, E m Calculated as follows:

[0020] Set dl mThe mth electric dipole between the excitation point B1 above the target layer and the excitation point B2 below the target layer has coordinates (0,0,z). Starting from Maxwell's equations, we can get the vertical electric dipole dl m The radial electric field at point P(x,y,0) on the ground is

[0021]

[0022] Where σ represents the earth's electrical conductivity, i represents the imaginary unit, u0 represents the air magnetic permeability, ω represents the circular frequency of the excitation source, λ represents the integral variable, J1 represents the first-order Bessel function,

[0023] As a further limitation, solving for apparent resistivity using the negative gradient method based on source differentials includes the following steps, performed in sequence:

[0024] T21, select the initial apparent conductivity σ0, set the number of iterations t and the expected error Err0;

[0025] T22. Calculate the theoretical electric field E at point P when the earth's conductivity is σ0 according to formula ①;

[0026] T23. Calculate the relative error Err between the theoretical electric field E amplitude and the differential electric field Ed amplitude according to formula ③

[0027]

[0028] T24, if the relative error Err is less than the expected error Err0, output apparent resistivity The calculation process ends, otherwise, step T25 is executed;

[0029] T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity The calculation process ends, otherwise, step T26 is executed;

[0030] T26. According to formula ①, the earth conductivity is calculated as The theoretical electric field E at point P is t ;

[0031] T27. According to formula ④, the electric field amplitude derivative at point P is calculated by differential calculation when the earth conductivity is σ0.

[0032] G=[lg(|E t |)-lg(|E|)] / 0.01Formula ④;

[0033] T28. Calculate the conductivity correction value according to formula ⑤

[0034] Δ=[lg(|Ed|)-lg(|E|)] / (2G) Formula ⑤;

[0035] T29. Correct the earth conductivity according to formula ⑥

[0036]

[0037] T30. The earth conductivity σ obtained after correction in step T29 is brought into step T22 as the initial apparent conductivity.

[0038] A target layer apparent resistivity calculation device based on source difference, comprising: a first excitation source, a second excitation source, a ground measurement module, a first data processing module and a second data processing module;

[0039] The first excitation source serves as an excitation source above the target layer;

[0040] The second excitation source serves as the excitation source below the target layer;

[0041] The signal output terminal of the ground measurement module is connected to the signal input terminal of the first data processing module, and the signal output terminal of the first signal processing module is connected to the signal input terminal of the second signal processing module;

[0042] A ground measurement module is provided on the ground and is used to measure the electric field generated on the ground after the first excitation source and the second excitation source are excited, and output the measurement results to the first data processing module;

[0043] The first data processing module performs source differentiation on the second excitation source and the first excitation source to obtain a finite length vertical wire source and outputs it to the second data processing module; and performs differentiation on the electric field observed at the same point by the second excitation source and the first excitation source to obtain a radial differential electric field and outputs it to the second data processing module;

[0044] The second data processing module solves the apparent resistivity based on the received finite length vertical wire source and radial differential electric field data after source differential.

[0045] A computer-readable storage medium stores a computer program, which, when executed by a processor, is used to implement the above-mentioned method for calculating the apparent resistivity of a target layer using the well-surface electromagnetic method.

[0046] Due to the adoption of the above technical solution, the present invention has achieved the following technical advancements compared with the prior art:

[0047] (1) The apparent resistivity obtained by the calculation method provided by the present invention has a significant reflection on the electrical properties of the target layer and is numerically close to the resistivity of the actual formation, which has a better reflection effect on the electrical properties of the target layer;

[0048] (2) The calculation method of the present invention is simple, easy to implement, and convenient for large-scale promotion.

[0049] The present invention belongs to the technical field of geophysical exploration and can improve the accuracy of apparent resistivity calculation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0051] In the attached figure:

[0052] Figure 1 This is an overall flow chart of Example 1 of the present invention;

[0053] Figure 2 This is a schematic diagram of an excitation source in a well according to Example 1 of the present invention;

[0054] Figure 3 A coordinate system diagram for calculating the radial electric field on the ground excited by a finite-length vertical wire source in Example 1 of the present invention;

[0055] Figure 4 This is a flow chart of apparent resistivity calculation in Example 1 of the present invention;

[0056] Figure 5 This is a schematic diagram of the arrangement of excitation sources for geo-electromagnetic monitoring of water injection wells in a target layer of a certain work area according to Example 1 of the present invention;

[0057] Figure 6 This is an electric field amplitude diagram observed by excitation above the target layer in Example 1 of the present invention;

[0058] Figure 7 This is an electric field amplitude diagram of the excitation observation below the target layer in Example 1 of the present invention;

[0059] Figure 8 : This is an amplitude diagram of the differential electric field observed twice when the target layer is excited below and above in Example 1 of the present invention;

[0060] Figure 9 This is the apparent resistivity map calculated based on source differential in Example 1 of the present invention. DETAILED DESCRIPTION

[0061] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0062] Example 1 A method for calculating the apparent resistivity of a target layer using well-surface electromagnetic method

[0063] like Figure 1 As shown, this embodiment is carried out in the following steps in sequence:

[0064] S1. Differentiate the sources of the upper and lower excitations of the target layer, and at the same time, perform the difference of the real and imaginary parts of the electric field of the upper and lower excitations of the target layer;

[0065] S2. Combined with the theoretical electric field calculation formula for the finite length vertical wire source in the well, the negative gradient method is used based on source difference to solve the apparent resistivity.

[0066] Wherein, step S1 is performed in the following order:

[0067] S11, such as Figure 2 As shown, a ground excitation point A, an excitation point B1 above the target layer, and an excitation point B2 below the target layer are selected; an excitation source AB1 is used above the target layer to measure the electric field E1 at the ground point P, and an excitation source AB2 is used below the target layer to excite and measure the electric field E2 at the ground point P. The excitation current for both times is I;

[0068] S12. Differentiate the second long wire source and the first long wire source to obtain a finite length vertical wire source; differentiate the electric field E1 and the electric field E2 to obtain the difference between the real and imaginary parts of the electric field as Ed = E2 - E1, and obtain the differential electric field Ed;

[0069] The second long wire source is the excitation source AB2 below the target layer, the first long wire source is the excitation source AB1 above the target layer, and the resulting finite length vertical wire source is B1B2.

[0070] In step S2, the calculation process of the theoretical electric field calculation formula excited by the finite length vertical wire source in the well is carried out in the following steps:

[0071] S21, such as Figure 3 As shown, with the wellhead as the origin 0, downward as the positive direction of the Z coordinate axis, the coordinate system is established according to the right-hand screw rule, and the coordinates of point P are set to (x, y, 0);

[0072] S22, divide the finite length vertical wire source B1B2 into n segments, each segment is less than Each segment is regarded as an electric dipole, excited by a finite length vertical wire source B1B2. The radial electric field at point P is numerically integrated by the field excited by n electric dipoles and is expressed as

[0073]

[0074] Among them, m∈[1,n], E m represents the radial electric field of the mth electric dipole at point P;

[0075] Among them, E m Calculated as follows:

[0076] Set dl mThe mth electric dipole between the excitation point B1 above the target layer and the excitation point B2 below the target layer has coordinates (0,0,z). Starting from Maxwell's equations, we can get the vertical point dipole dl m The radial electric field at point P(x,y,0) on the ground is

[0077]

[0078] Where ω represents the circular frequency of the excitation source, σ represents the earth conductivity, u0 represents the magnetic permeability of air, i represents the imaginary unit, J1 represents the first-order Bessel function, and λ represents the integral variable.

[0079] like Figure 4 As shown in the figure, the solution of apparent resistivity using the negative gradient method based on source difference includes the following steps:

[0080] T21, select the initial apparent conductivity σ0, set the number of iterations t and the expected error Err0;

[0081] T22. Calculate the theoretical electric field E at point P when the earth's conductivity is σ0 according to formula ①;

[0082] T23. Calculate the relative error Err between the theoretical electric field E amplitude and the differential electric field Ed amplitude according to formula ③

[0083]

[0084] T24, if the relative error Err is less than the error Err0, output apparent resistivity The calculation process ends, otherwise, step T25 is executed;

[0085] T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity The calculation process ends, otherwise, step T26 is executed;

[0086] T26. According to formula ①, the earth conductivity is calculated as The theoretical electric field E at point P is t ;

[0087] T27. According to formula ④, the electric field amplitude derivative at point P is calculated by differential calculation when the earth conductivity is σ0.

[0088] G=[lg(|E t |)-lg(|E|)] / 0.01Formula ④;

[0089] T28. Calculate the conductivity correction value according to formula ⑤

[0090] Δ=[lg(|Ed|)-lg(|E|)] / (2G) Formula ⑤;

[0091] T29. Correct the earth conductivity according to formula ⑥

[0092]

[0093] T30. The earth conductivity σ obtained after correction in step T29 is brought into step T22 as the initial apparent conductivity.

[0094] The method provided in this embodiment is used to calculate the apparent resistivity of the target layer in a certain work area, such as Figure 5 The figure shows the layout of the excitation source for geomagnetic monitoring of water injection wells. Figure 6 The figure shows the plane diagram of the electric field amplitude at a frequency of 1.95 Hz observed at 900 measuring points on the ground above the target layer. Figure 7 The plane diagram of the electric field amplitude at a frequency of 1.95 Hz observed at 900 measuring points on the ground below the measured target layer; Figure 8 is the amplitude of the differential electric field observed twice when the target layer is excited below and above.

[0095] According to the method provided in this embodiment, the initial conductivity σ0 is selected as the average conductivity of the formation in the measurement area, which is set to 0.01 S / m, the number of iterations t is set to 10,000 times, and the expected error Err0 is set to 1.E-6%. The test is performed on a laptop computer Precision 7750. Figure 9 This is the final apparent resistivity map obtained based on source differential calculation. The number of iterations at each measuring point did not exceed 10, achieving a set error of 1.E-6%. The total calculation time for 900 measuring points was less than 1 second. These results demonstrate that this method not only effectively reflects the electrical properties of the target layer, but also achieves fast calculation speed and high accuracy.

[0096] Example 2 A device for calculating apparent resistivity of a target layer based on source differential

[0097] This embodiment is used to implement Example 1 and specifically includes: a first excitation source, a second excitation source, a ground measurement module, a first data processing module, and a second data processing module. The first excitation source serves as an excitation source above the target layer; the second excitation source serves as an excitation source below the target layer; the signal output terminal of the ground measurement module is connected to the signal input terminal of the first data processing module, and the signal output terminal of the first signal processing module is connected to the signal input terminal of the second signal processing module.

[0098] The ground measurement module, located on the ground, measures the electric field generated by the first and second excitation sources, and outputs the measurement results to the first data processing module. The first data processing module performs source differentiation on the second excitation source and the first excitation source to obtain a finite-length vertical wire source, which is output to the second data processing module. It also performs source differentiation on the electric field observed at the same point by the second and first excitation sources to obtain a radial differential electric field, which is also output to the second data processing module. The second data processing module calculates the apparent resistivity based on the received finite-length vertical wire source and radial differential electric field data after source differentiation.

[0099] In this embodiment, the first excitation source and the second excitation source are power supply cables that are successively lowered to the upper and lower parts of the target layer in the well. The cables are connected to a dedicated transmitter, which supplies power to the formation through the power supply point at the wellhead and the end point of the cable in the well; the ground measurement module is a receiving instrument for receiving radial electric fields on the ground; the first data processing module and the second data processing module are processing programs stored on the processing computer.

[0100] Example 3 A computer-readable storage medium

[0101] The computer-readable storage medium in this embodiment stores a computer program, and when the computer program is executed by a processor, it is used to implement the method for calculating the apparent resistivity of the target layer using the well-surface electromagnetic method of Example 1.

[0102] Among them, the computer-readable storage medium can be a computer storage medium or a communication medium. The communication medium includes any medium that facilitates the transmission of a computer program from one place to another. The computer storage medium can be any available medium that can be accessed by a general-purpose or special-purpose computer. For example, a computer-readable storage medium is coupled to a processor so that the processor can read information from the computer-readable storage medium and write information to the computer-readable storage medium. Of course, the computer-readable storage medium can also be an integral part of the processor. The processor and the computer-readable storage medium can be located in an application-specific integrated circuit (ASIC). In addition, the ASIC can be located in a user device. Of course, the processor and the computer-readable storage medium can also exist in a communication device as discrete components. Specifically, the computer-readable storage medium can be implemented by any type of volatile or non-volatile storage device or a combination thereof, such as static random-access memory (SRAM), electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), programmable read-only memory (PROM), read-only memory (ROM), magnetic storage, flash memory, magnetic disk or optical disk. The storage medium can be any available medium that can be accessed by a general-purpose or special-purpose computer.

Claims

1. A method for calculating apparent resistivity of a target layer using a borehole-to-surface electromagnetic method, characterized in that: Follow these steps in order: S1. Differentiate the sources of the upper and lower excitations of the target layer, and at the same time, perform the difference of the real and imaginary parts of the electric field of the upper and lower excitations of the target layer; S2. Combined with the theoretical electric field calculation formula of the finite length vertical wire source in the well, the negative gradient method is used based on the source difference to solve the apparent resistivity; Step S1 is performed in the following order: S11, select a ground excitation point A, an excitation point B1 above the target layer, and an excitation point B2 below the target layer; use an excitation source AB1 above the target layer to excite and measure the electric field E1 at the ground point P, and use an excitation source AB2 below the target layer to excite and measure the electric field E2 at the ground point P, with both excitation currents being I; S12, differential the excitation source AB2 below the target layer and the excitation source AB1 above the target layer to obtain a finite length vertical wire source B1B2; differential the electric field E1 and the electric field E2 to obtain the difference between the real and imaginary parts of the electric field as Ed=E2-E1, and obtain the differential electric field Ed; The calculation process of the theoretical electric field calculation formula excited by a finite-length vertical wire source in a well is carried out in the following order: S21. With the wellhead as the origin 0 and the downward direction as the positive direction of the Z coordinate axis, a coordinate system is established according to the right-hand screw rule, and the coordinates of point P are set to (x, y, 0); S22, divide the finite length vertical wire source B1B2 into n segments, each segment is less than , each segment is regarded as an electric dipole, excited by a finite length vertical wire source B1B2, and the radial electric field at point P is numerically integrated by the field excited by n electric dipoles: Formula ① in, , Indicates the The radial electric field of an electric dipole at point P; The solution of apparent resistivity using the negative gradient method based on source difference includes the following steps: T21. Select initial apparent conductivity , set the number of iterations t and the expected error ; T22. According to formula ①, the earth conductivity is calculated as The theoretical electric field at point P when ; T23. Calculate the relative error between the theoretical electric field E amplitude and the differential electric field Ed amplitude according to formula ③ Formula ③; T24. If the relative error Smaller than expected error , output apparent resistivity The calculation process ends, otherwise, step T25 is executed; T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity The calculation process ends, otherwise, step T26 is executed; T26. According to formula ①, the earth conductivity is calculated as Theoretical electric field ; T27. According to formula ④, the earth conductivity is calculated by differential calculation. When the electric field amplitude derivative at point P is Formula ④; T28. Calculate the conductivity correction value according to formula ⑤ Formula ⑤; T29. Correct the earth conductivity according to formula ⑥ Formula ⑥; T30, the earth conductivity obtained after correction in step T29 This is taken into step T22 as the initial apparent conductivity.

2. The method for calculating apparent resistivity of target layer by well-surface electromagnetic method according to claim 1, characterized in that: Calculated as follows: set up The first excitation point between the target layer upper excitation point B1 and the target layer lower excitation point B2 An electric dipole with coordinates (0,0,z) is obtained from Maxwell's equations. The radial electric field at point P(x,y,0) on the ground is Formula ② in, represents the earth's electrical conductivity, , , To represent the imaginary unit, represents the magnetic permeability of air, represents the circular frequency of the excitation source, represents the integration variable, represents the first-order Bessel function, .

3. A device for calculating apparent resistivity of a target layer using a well-surface electromagnetic method, for implementing the method for calculating apparent resistivity of a target layer using a well-surface electromagnetic method according to claim 1 or 2, characterized in that: include: a first excitation source, a second excitation source, a ground measurement module, a first data processing module, and a second data processing module; The first excitation source serves as an excitation source above the target layer; The second excitation source serves as the excitation source below the target layer; The signal output terminal of the ground measurement module is connected to the signal input terminal of the first data processing module, and the signal output terminal of the first signal processing module is connected to the signal input terminal of the second signal processing module; A ground measurement module is provided on the ground and is used to measure the electric field generated on the ground after the first excitation source and the second excitation source are excited, and output the measurement results to the first data processing module; A first data processing module performs source difference on the second excitation source and the first excitation source to obtain a finite length vertical wire source and output the result to the second data processing module; Differentiating the electric fields of the second excitation source and the first excitation source observed at the same point to obtain a radial differential electric field and output it to the second data processing module; The second data processing module solves the apparent resistivity based on the received finite length vertical wire source and radial differential electric field data after source differential.

4. A computer-readable storage medium, characterized in that A computer program is stored in the computer-readable storage medium, and when the computer program is executed by the processor, it is used to implement the method for calculating the apparent resistivity of the target layer by the well-surface electromagnetic method according to claim 1 or 2.

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