Well-ground electromagnetic method target layer apparent resistivity calculation method, storage medium and device

Abstract

The invention belongs to the technical field of geophysical exploration, and particularly discloses a well-to-ground electromagnetic method target layer apparent resistivity calculation method, a storage medium and a device, and the calculation method sequentially comprises the following steps: S1, carrying out the difference of an upper excitation source and a lower excitation source of a target layer, and carrying out the difference of the real part and the imaginary part of an upper excitation electric field and a lower excitation electric field of the target layer; and S2, solving the apparent resistivity by adopting a negative gradient method based on source difference in combination with a theoretical electric field calculation formula excited by a limited-length vertical conductor source in the well. The apparent resistivity obtained by the calculation method provided by the invention has obvious reflection on the electrical property of the target layer, is close to the resistivity of the actual stratum in numerical value, and has a better reflection effect on the electrical property of the target layer. The method is suitable for calculating the apparent resistivity of the target layer through the well-ground electromagnetic method in geophysical exploration.

Description

Technical Field

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

[0002] The borehole-ground electromagnetic method is an electromagnetic exploration method that excites the electromagnetic field in the well and receives the electromagnetic field on the ground. Due to the use of in-well excitation, the detection target can be optimally excited and illuminated. Compared with the ground electromagnetic method, this method has higher detection accuracy and has been applied in the delineation of oil and gas reservoir boundaries and water injection monitoring in many regions at home and abroad. At present, the borehole electromagnetic method generally adopts the following acquisition, processing and interpretation methods: a vertical long wire source is used to excite the upper and lower parts of the target layer (one excitation electrode is located on the ground, and the other excitation electrode is located above and below the target in the well), and a survey line or survey network is deployed on the ground 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 by the upper and lower excitations are filtered, Fourier transformed and superimposed to obtain the electric field spectrum (amplitude, phase or real part, imaginary part); then the electric field amplitude and phase (or real part, imaginary part) are used to obtain the differential amplitude, differential phase, dual-frequency amplitude, dual-frequency phase, or the apparent resistivity is calculated from the processed electric field spectrum, and the resistivity is inverted to obtain the inversion resistivity. Based on the above electrical parameters, the oil content and water injection boundary of the target reservoir are analyzed and interpreted. The apparent resistivity is the earth resistivity calculated by the theoretical formula under the uniform half-space condition based on the observed data, which is a general reflection of the electrical properties of the formation. Apparent resistivity reflects formation resistivity more intuitively and has no multiple solutions (inversion resistivity has multiple solutions). It is an important parameter for qualitative interpretation of electromagnetic data.

[0003] However, for the borehole-to-ground electromagnetic method, the apparent resistivity calculated by exciting the electric field above or below the target layer is a comprehensive reflection of the entire underground half-space, and does not reflect the electrical characteristics of the target layer obviously, which is not conducive to the analysis and evaluation of the target layer. Therefore, it is necessary to further study the calculation method of the apparent resistivity 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 in a borehole-ground electromagnetic method, which can clearly reflect the electrical properties of the target layer, and the value is close to the resistivity of the actual formation, so as to better reflect the electrical properties of the target layer;

[0005] The second object of the present invention is to provide a computer-readable storage medium; used to store a corresponding computer program for calculating the apparent resistivity of a target layer using a borehole-to-ground 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-to-ground electromagnetic method.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for calculating the apparent resistivity of a target layer by 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, differentiate 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 excitation of a 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 the ground excitation point A, the excitation point B1 above the target layer and the excitation point B2 below the target layer; excite and measure the electric field E1 of the ground point P through the excitation source AB1 above the target layer, and excite and measure the electric field E2 of the ground point P through the excitation source AB2 below the target layer, and the excitation currents of both times are 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 steps:

[0015] S21, taking the wellhead as the origin 0 and downward as the positive direction of the Z coordinate axis, establish a coordinate system according to the right-hand screw rule and set the coordinates of point P 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 expressed by the field excited by n electric dipoles:

[0017]

[0018] Where 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 It is calculated as follows:

[0020] Setting dl mis the mth electric dipole between the excitation point B1 above the target layer and the excitation point B2 below the target layer, and its coordinates are (0,0,z). Starting from Maxwell's equations, we can get the vertical electric dipole dl m The radial electric field at the ground point P(x,y,0) 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 conductivity is σ0 according to formula ①;

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

[0027]

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

[0029] T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity And end the calculation process, otherwise execute step T26;

[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, bringing the earth conductivity σ obtained after correction in step T29 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] A first excitation source, serving as an excitation source above the target layer;

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

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

[0042] A ground measurement module, disposed on the ground, for measuring the electric field generated on the ground after being excited by the first excitation source and the second excitation source, and outputting the measurement result to the first data processing module;

[0043] The 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 outputs it to the second data processing module; performs difference on the electric field observed at the same point of 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 according to 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 is used to implement the above-mentioned method for calculating the apparent resistivity of a target layer by well-to-ground electromagnetic method when executed by a processor.

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

[0047] (1) The apparent resistivity obtained by the calculation method provided by the present invention has an obvious reflection on the electrical properties of the target layer, and is numerically close to the resistivity of the actual formation, and 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 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 picture:

[0052] Figure 1 This is an overall flow chart of Embodiment 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 the excitation source for monitoring the water injection well of the target layer in a certain work area according to Example 1 of the present invention;

[0057] Figure 6 The electric field amplitude diagram of the excitation observation above the target layer in Example 1 of the present invention;

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

[0059] Figure 8 The amplitude diagram of the differential electric field of the target layer under the excitation and the target layer under the excitation in Example 1 of the present invention is twice observed;

[0060] Fig. 9 This is an 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 apparent resistivity of target layer using well-ground electromagnetic method

[0063] like Figure 1 As shown, this embodiment is performed 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, differentiate 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 excitation of a 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, and both excitation currents are I;

[0068] S12, differentially performing the second long wire source and the first long wire source to obtain a finite length vertical wire source; differentially performing 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 obtaining 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 expressed by the field excited by n electric dipoles:

[0073]

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

[0075] Among them, E m It is calculated as follows:

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

[0077]

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

[0079] like Figure 4 As shown, solving the 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 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 And end the calculation process, otherwise execute step T25;

[0085] T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity And end the calculation process, otherwise execute step T26;

[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, bringing the earth conductivity σ obtained after correction in step T29 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 with 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 with a frequency of 1.95 Hz observed at 900 measuring points on the ground under the measured target layer; Figure 8 It 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 approximate 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. Fig. 9 The final apparent resistivity map based on source differential calculation is shown in Figure 1. The number of iterations at each measuring point is no more than 10, which means that the set error of 1.E-6% is achieved. The total calculation time of 900 measuring points is less than 1s. The results show that the present invention not only has a good effect on the electrical properties of the target layer, but also has a fast calculation speed and high accuracy.

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

[0097] This embodiment is used to implement Embodiment 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 is used as an excitation source above the target layer; the second excitation source is used as an excitation source below the target layer; the signal output end of the ground measurement module is connected to the signal input end of the first data processing module, and the signal output end of the first signal processing module is connected to the signal input end of the second signal processing module.

[0098] The ground measurement module is set 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 the measurement results are output to the first data processing module. The 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 it to the second data processing module; the electric field observed at the same point by the second excitation source and the first excitation source is differentiated 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 according to the received finite length vertical wire source and radial differential electric field data after source difference.

[0099] In this embodiment, the first excitation source and the second excitation source are power supply cables that are successively lowered to the top and bottom 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 a processing computer.

[0100] Embodiment 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-to-ground 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 a component 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 or special-purpose computer.

Claims

1. A method for calculating apparent resistivity of a target layer using a well-ground 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, differentiate 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 for the excitation of a finite length vertical wire source in the well, the negative gradient method is used based on source difference to solve the apparent resistivity.

2. The method for calculating apparent resistivity of target layer by well-ground electromagnetic method according to claim 1, characterized in that: Step S1 is performed in the following order: S11, select the ground excitation point A, the excitation point B1 above the target layer and the excitation point B2 below the target layer; excite and measure the electric field E1 of the ground point P through the excitation source AB1 above the target layer, and excite and measure the electric field E2 of the ground point P through the excitation source AB2 below the target layer, and the excitation currents of both times are I; 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.

3. The method for calculating apparent resistivity of target layer by well-ground electromagnetic method according to claim 2, characterized in that: 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 steps: S21, taking the wellhead as the origin 0 and downward as the positive direction of the Z coordinate axis, establish a coordinate system according to the right-hand screw rule and set the coordinates of point P 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. The radial electric field at point P is numerically expressed by the field excited by n electric dipoles: Where m∈[1,n], E m represents the radial electric field of the mth electric dipole at point P.

4. The method for calculating apparent resistivity of target layer by well-ground electromagnetic method according to claim 3, characterized in that: E m It is calculated as follows: Setting dl m is the mth electric dipole between the excitation point B1 above the target layer and the excitation point B2 below the target layer, and its coordinates are (0,0,z). Starting from Maxwell's equations, we can get the vertical electric dipole dl m The radial electric field at the ground point P(x,y,0) is 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, 5. The method for calculating apparent resistivity of target layer by well-ground electromagnetic method according to claim 4, characterized in that: The solution of apparent resistivity using the negative gradient method based on source difference includes the following steps: T21, select the initial apparent conductivity σ0, set the number of iterations t and the expected error Err0; T22. Calculate the theoretical electric field E at point P when the earth conductivity is σ0 according to formula ①; T23. Calculate the relative error Err between the theoretical electric field E amplitude and the differential electric field Ed amplitude according to formula ③ T24, if the relative error Err is less than the expected error Err0, output apparent resistivity And end the calculation process, otherwise execute step T25; T25, determine whether the number of iterations has reached t, if so, output the apparent resistivity And end the calculation process, otherwise execute step T26; T26. According to formula ①, the earth conductivity is calculated as Theoretical electric field E t ; T27. According to formula ④, the electric field amplitude derivative at point P is calculated by differential calculation when the earth conductivity is σ0. G = [lg(|E t |) - lg(|E|)] / 0.01 Equation ④; T28. Calculate the conductivity correction value according to formula ⑤ Δ=[lg(|Ed|)-lg(|E|)] / (2G) Formula ⑤; T29. Correct the earth conductivity according to formula ⑥ T30, bringing the earth conductivity σ obtained after correction in step T29 into step T22 as the initial apparent conductivity.

6. A device for calculating apparent resistivity of target layer by well-ground electromagnetic method, 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; A first excitation source, serving as an excitation source above the target layer; The second excitation source serves as an excitation source below the target layer; The signal output end of the ground measurement module is connected to the signal input end of the first data processing module, and the signal output end of the first signal processing module is connected to the signal input end of the second signal processing module; A ground measurement module, disposed on the ground, for measuring the electric field generated on the ground after the first excitation source and the second excitation source are excited, and outputting the measurement result 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 it to the second data processing module; Differentiating the electric fields observed at the same point by the second excitation source and the first excitation source 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 according to the received finite length vertical wire source and radial differential electric field data after source differential.

7. 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-ground electromagnetic method as described in any one of claims 1 to 5.

Images