Transient electromagnetic wave through casing resistivity measurement method

Abstract

The present application belongs to the technical field of oil and gas exploration and development, and relates to a transient electromagnetic wave through casing resistivity measurement method, which comprises the following steps: considering the wellbore, casing and formation, and combining with the transient electromagnetic wave logging characteristics, a casing well formation model is established; in the casing well formation model, a late signal-based apparent resistivity extraction method is used to measure the resistivity, and the steps of the late signal-based apparent resistivity extraction method for measuring the resistivity are as follows: in the casing well formation model, different formation resistivities are simulated, and a single-shot single-receiving coil structure is used to measure the late transient electromagnetic wave logging zz component induced electromotive force; the apparent resistivity is extracted according to the late transient electromagnetic wave logging zz component induced electromotive force signal, and the extracted apparent resistivity is the transient electromagnetic wave through casing resistivity. The present application can improve the transient electromagnetic wave through casing logging response accuracy and realize the extraction of formation resistivity under the influence of high-conductivity casing.

Description

[0001] This application is a divisional application of the following invention patent application.

[0002] Application Number: 202510000378X

[0003] Application date: January 2, 2025

[0004] Invention Title: Method for Measuring Resistivity of Transient Electromagnetic Waves Through a Bushing Technical Field

[0005] This invention belongs to the field of oil and gas exploration and development technology, and relates to electrical logging technology. Specifically, it relates to a method for measuring the resistivity of transient electromagnetic waves through casing. Background Technology

[0006] In oil and gas field exploration and development, through-casing resistivity measurement technology has significant application value, especially in casing drilling. Through-casing resistivity measurement is one of the important means for evaluating reservoir oil-bearing capacity, monitoring reservoir dynamics, and analyzing remaining oil distribution. It is of great significance for reservoir evaluation in casing drilling, optimizing production well development plans, and improving oil recovery. As a new drilling method in oil and gas exploration and development, casing drilling presents challenges because conventional resistivity logging cannot be directly applied due to the presence of the casing. Therefore, achieving accurate and efficient through-casing resistivity measurement has become a key issue in the industry.

[0007] Currently, the electrode method is commonly used for through-casing resistivity logging. Its basic principle is to infer formation resistivity by measuring the weak current leaking inside and outside the casing. However, the electrode method has the following limitations: (1) It is greatly affected by the quality of the casing: corrosion, thinning or deformation of the casing will significantly affect the contact quality between the electrode and the casing, resulting in inaccurate measurement data. (2) It is affected by the downhole environment: scale, wax, rust and other deposits on the inner wall of the casing will interfere with the stability of the electrode method measurement, requiring time for well cleaning (such as descaling, deoiling, etc.) and other preparatory work, which increases the complexity of logging. (3) The logging efficiency is low: the electrode method usually adopts a point measurement mode, which requires a long time to establish stable measurement conditions, and cannot meet the real-time response requirements for rapid changes in the downhole environment, resulting in low overall logging efficiency.

[0008] Transient electromagnetic wave technology provides a new solution for through-casing resistivity measurement. Its working principle is to arrange transmitting and receiving coils in the well. Transmitting current pulses are sent to the formation through the transmitting coil. The instantaneous current pulses generate electromagnetic fields that change with time, which excite the formation to generate induced eddy currents that decay with time and spread at different speeds in different media. After the current is turned off, the decay characteristics of the secondary induced electromagnetic field generated by the eddy currents over time are measured to extract formation resistivity information. Transient electromagnetic wave technology has the following advantages: (1) Strong penetration: Low frequency signals can penetrate the casing shield and can effectively collect formation resistivity information. (2) Continuous measurement: Transient electromagnetic waves have wide frequency domain characteristics and can work simultaneously in multiple frequency bands to obtain rich formation information and realize continuous logging. However, transient electromagnetic wave logging also faces the following problems in practical applications: (1) Strong shielding effect of metal casing: The high conductivity of steel casing (10 7 -10 10 (1) The high relative permeability (50μ0-100μ0) results in a strong shielding effect on electromagnetic wave signals. The signal measured by the receiving antenna mainly comes from the contribution of the casing, making it particularly difficult to extract the weak signals of the strata. (2) High engineering difficulty: The relevant instruments are still in the theoretical research and experimental development stage, lacking mature engineering equipment, and there is still a large gap between them and practical applications.

[0009] Therefore, clarifying the response law of transient electromagnetic wave logging in casing wells, suppressing the influence of metal casing, and designing a reasonable instrument structure are of great theoretical value and practical significance for promoting the practical application of transient electromagnetic wave technology in through-casing resistivity measurement. Summary of the Invention

[0010] This invention addresses the problems existing in the prior art by providing a method for measuring the resistivity of transient electromagnetic waves through casing, which can improve the accuracy of transient electromagnetic wave logging response through casing and realize the extraction of formation resistivity under the influence of high-conductivity casing. The first aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves through casing, the steps of which are: Model construction step: Considering the wellbore, casing, and formation, and combining the characteristics of transient electromagnetic wave logging, a casing well formation model is established;

[0011] Resistivity measurement steps: In the cased well formation model, resistivity is measured using a late-signal-based apparent resistivity extraction method, an anomaly-background method based on mid-term signals, or a compensated differential coil structure. The compensated differential coil structure includes a transmitting coil and at least one compensation coil system. The compensation coil system includes at least a first receiving coil and a second receiving coil arranged in sequence. The transmitting coil and the second receiving coil are wound in the forward direction, while the first receiving coil, as a compensation coil, is wound in the reverse direction. The transmitting coil and the compensation coil system are arranged in a sequential order.

[0012] The steps for measuring resistivity using the apparent resistivity extraction method based on late signals are as follows:

[0013] Induced electromotive force measurement steps: In the cased well formation model, different formation resistivities are simulated, and the induced electromotive force of the late transient electromagnetic wave logging zz component is measured using a single-transmitter single-receiver coil structure.

[0014] Resistivity extraction steps: Based on the induced electromotive force signal of the zz component of the late transient electromagnetic wave logging, extract the apparent resistivity using the following formula. The extracted apparent resistivity is the transient electromagnetic wave resistivity through the casing.

[0015]

[0016] In the formula, R a Let L be the apparent resistivity, L be the source-source distance between the transmitting and receiving coils, and t be the source-source distance. a The time at which the induced electromotive force curve reaches its peak is t, and the time response is t0. l For the late period, E a For acceptable error, V(t) is the induced electromotive force of the zz component of the late transient electromagnetic wave logging, N is the number of coil turns, S is the coil area, μ is the formation permeability, and m is the magnetic moment.

[0017] The steps for measuring resistivity based on the anomaly-background method of intermediate-term signals are as follows:

[0018] Background signal measurement steps: In the cased well formation model, set the background environment, and use a single-transmitter single-receiver coil structure to measure the transient electromagnetic wave logging induced electromotive force as the background signal;

[0019] Anomaly measurement steps: Under the same background environment, only the formation conditions are changed. The single-transmitter single-receiver coil structure is pulled up along the wellbore. At this time, the transient electromagnetic wave logging induced electromotive force measured by the single-transmitter single-receiver coil is used as the actual response signal, i.e., the anomaly.

[0020] Calculation steps: Simultaneously subtract the actual response signal from the background signal to obtain the difference signal;

[0021] Plotting steps: Plot a curve reflecting formation resistivity information based on the relationship between the difference signal and formation resistivity; Resistivity determination steps: Convert the curve reflecting formation resistivity information into a resistivity calibration chart;

[0022] The method for measuring resistivity using a compensated differential coil structure is as follows:

[0023] Coil parameter determination steps: In the cased well formation model, under the set background environment, adjust the coil parameters of the compensated differential coil structure so that the casing background signal in the measured signal is zero. The coil parameters at this time are the final coil parameters of the compensated differential coil structure.

[0024] Signal measurement steps: Raise the compensated differential coil structure along the wellbore and measure the induced electromotive force at different formation locations through the compensated differential coil structure;

[0025] Plotting steps: Plot a curve reflecting formation resistivity information based on the induced electromotive force;

[0026] Resistivity determination steps: Convert the curves reflecting formation resistivity information into resistivity calibration charts.

[0027] In some embodiments, the method for extracting apparent resistivity in the resistivity extraction step is as follows:

[0028] Define R tl For the apparent resistivity in the late stage of transient electromagnetic wave logging, its algebraic relationship is:

[0029]

[0030] Let t l For the late period, E a For acceptable error, according to The Taylor expansion yields:

[0031] In the formula, σ is the formation conductivity;

[0032] Introducing the Green's function for time response:

[0033]

[0034] In the formula, r is the distance from any point in space to the origin; u(t) is the step function, which is 1 when the response time t is greater than 0 and 0 when the response time t is less than 0.

[0035] Differentiating t in the Green's function formula for time response yields the time t at the peak of the induced electromotive force curve. a Define R tm For the apparent resistivity in transient electromagnetic wave logging, its algebraic relationship is:

[0036]

[0037] Based on the reciprocal relationship between resistivity and conductivity, substituting equation (4) into equation (2), the late-stage time is approximately determined as:

[0038]

[0039] The apparent resistivity is defined as:

[0040]

[0041] In some embodiments, the compensated differential coil structure includes a transmitting coil and a set of compensated coil systems. The transmitting coil is wound in the forward direction, and the compensated coil system includes a first receiving coil, a second receiving coil, and a third receiving coil arranged in sequence. The first receiving coil is adjacent to the transmitting coil. The first and third receiving coils, as compensated coils, are wound in the reverse direction, while the second receiving coil is wound in the forward direction. In some embodiments, the compensated differential coil structure includes a transmitting coil and N sets of compensated coil systems, where N ≥ 2. The transmitting coil and the first to Nth sets of compensated coil systems are arranged in sequence. The transmitting coil is wound in the forward direction, and each set of compensated coil systems includes a first receiving coil, a second receiving coil, and a third receiving coil arranged in sequence. The first and third receiving coils, as compensated coils, are wound in the reverse direction, while the second receiving coil is wound in the forward direction. The first receiving coil of the first set of compensated coil systems is adjacent to the transmitting coil. In two adjacent sets of compensated coil systems, the third receiving coil of the first set of compensated coil systems is adjacent to the first receiving coil of the second set of compensated coil systems.

[0042] In some embodiments, the compensated differential coil structure further includes a symmetrical compensation coil system, which includes a fourth receiving coil and a fifth receiving coil. The two receiving coils are symmetrically arranged on both sides of the transmitting coil. The fourth receiving coil is located between the transmitting coil and the first receiving coil and is wound in the forward direction. The fifth receiving coil, as a compensation coil, is wound in the reverse direction.

[0043] In some embodiments, the method further includes a formation interface identification step, the formation interface identification step further including:

[0044] Signal measurement steps: Raise the compensated differential coil structure along the wellbore and measure the induced electromotive force at different formation locations through the symmetrical compensated coil system;

[0045] Identification steps: Plot the curve of the induced electromotive force as a function of depth based on the measured induced electromotive force. The peak value of the induced electromotive force is the formation interface.

[0046] In some embodiments, the compensated differential coil structure includes a transmitting coil and a set of compensated coils. The transmitting coil is wound in the forward direction, and the compensated coils include a first receiving coil and a second receiving coil. The first receiving coil is adjacent to the transmitting coil and serves as a compensated coil, being wound in the reverse direction. The second receiving coil is wound in the forward direction.

[0047] In some embodiments, the compensated differential coil structure includes a transmitting coil and N sets of compensated coil systems, where N≥2. The transmitting coil and the first set of compensated coil systems to the Nth set of compensated coil systems are arranged sequentially. The transmitting coil is wound in the forward direction. Each set of compensated coil systems includes a first receiving coil and a second receiving coil connected in sequence. The first receiving coil serves as a compensated coil and is wound in the reverse direction. The second receiving coil is wound in the forward direction. The first receiving coil of the first set of compensated coil systems is adjacent to the transmitting coil. In two adjacent sets of compensated coil systems, the second receiving coil of the first set of compensated coil systems is adjacent to the first receiving coil of the second set of compensated coil systems.

[0048] In some embodiments, the compensated differential coil structure further includes a symmetrical compensation coil system, which includes a third receiving coil and a fourth receiving coil. The two receiving coils are symmetrically arranged on both sides of the transmitting coil. The third receiving coil is located between the transmitting coil and the first receiving coil and is wound in the forward direction. The fourth receiving coil serves as a compensation coil and is wound in the reverse direction.

[0049] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0050] (1) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention establishes a casing well formation model considering wellbore, casing, formation and other conditions. According to the transient electromagnetic wave through-casing resistivity logging response law, when the signal magnitude during the transient electromagnetic wave logging period is large, the apparent resistivity extraction method based on the late signal is used to measure the formation resistivity. This can improve the accuracy of transient electromagnetic wave through-casing logging response, realize the formation resistivity extraction under the influence of high-conductivity casing, and the formation resistivity measurement is highly accurate, filling the gap in transient electromagnetic wave through-casing resistivity logging.

[0051] (2) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention establishes a casing well formation model considering wellbore, casing, formation and other conditions. According to the transient electromagnetic wave through-casing resistivity logging response law, when the signal magnitude is small during the transient electromagnetic wave logging period and it is difficult to effectively extract useful signals in actual engineering, the anomaly-background method based on the mid-term signal is used to measure the formation resistivity. This can improve the accuracy of transient electromagnetic wave through-casing logging response, realize the extraction of formation resistivity under the influence of high-conductivity casing, and the formation resistivity measurement accuracy is high, filling the gap in transient electromagnetic wave through-casing resistivity logging.

[0052] (3) The transient electromagnetic wave resistivity measurement method through casing provided by this invention establishes a casing well formation model considering wellbore, casing, and formation conditions. Based on the transient electromagnetic wave resistivity logging response law through casing, a compensated differential coil structure is used to measure formation resistivity. The compensated differential coil structure of this invention suppresses the mid-term background signal and highlights the useful signal (i.e., the induced electromotive force related to formation resistivity) through the compensated differential measurement method, which can improve the accuracy of transient electromagnetic wave logging response through casing, realize formation resistivity extraction under the influence of high-conductivity casing, and achieve high accuracy in formation resistivity measurement, thus filling the gap in transient electromagnetic wave resistivity logging through casing.

[0053] (4) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention uses an array design for the compensation coil system of the compensation differential coil structure, which can realize near-far well and near-middle-far well formation resistivity measurement, thereby improving the efficiency of transient electromagnetic wave through-casing logging.

[0054] (5) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention, through the set symmetrical compensation coil system of the compensation differential coil structure, can realize the identification of the formation interface. Based on the identified formation interface, the formation resistivity can be determined, providing technical reference for the development of transient electromagnetic wave through-casing logging instruments and data processing. Attached Figure Description

[0055] Figure 1 This is a flowchart of the transient electromagnetic wave resistivity measurement method through a bushing according to the first aspect of the present invention;

[0056] Figure 2 This is a flowchart illustrating the resistivity measurement method based on late-signal apparent resistivity extraction according to a first aspect embodiment of the present invention.

[0057] Figure 3 This is a schematic diagram of the structure of the three-layer columnar stratigraphic model of the casing well according to the first aspect of the present invention;

[0058] Figure 4 This is a schematic diagram illustrating the resistivity response characteristics of different formations according to the first aspect of the present invention;

[0059] Figure 5 This is a schematic diagram of the apparent resistivity curves of a three-layer columnar stratigraphic model of a casing well with different formation resistivity according to an embodiment of the first aspect of the present invention.

[0060] Figure 6 This is a flowchart of the transient electromagnetic wave resistivity measurement method through a bushing according to a second aspect embodiment of the present invention;

[0061] Figure 7 This is a flowchart illustrating the resistivity measurement method based on an anomaly-background method using intermediate signals according to a second aspect embodiment of the present invention.

[0062] Figure 8 This is a schematic diagram of the structure of a three-layer columnar stratigraphic model of a casing well according to a second aspect embodiment of the present invention;

[0063] Figure 9 This is a formation depth map drawn for a three-layer columnar stratigraphic model of a casing well according to a second aspect embodiment of the present invention, when the formation conditions are 1-2-10 Ω·m.

[0064] Figure 10 This is a formation depth map drawn for a three-layer columnar stratigraphic model of a casing well according to a second aspect embodiment of the present invention, when the formation conditions are 1-10-1 Ω·m.

[0065] Figure 11 This is a formation depth map drawn for a three-layer columnar stratigraphic model of a casing well according to a second aspect embodiment of the present invention, when the formation condition is 10⁻²⁻¹ Ω·m.

[0066] Figure 12 This is a flowchart of the transient electromagnetic wave resistivity measurement method through a bushing according to a third aspect embodiment of the present invention;

[0067] Figure 13 This is a flowchart illustrating the measurement of resistivity using a compensated differential coil structure according to a third aspect embodiment of the present invention.

[0068] Figure 14 This is a schematic diagram of the structure of the compensated differential coil used in the third aspect embodiment of the present invention;

[0069] Figure 15 This is a schematic diagram illustrating the principle of measuring resistivity using a compensated differential coil structure according to a third aspect embodiment of the present invention.

[0070] Figure 16 This is a schematic diagram of the resistivity measurement results using a compensated differential coil structure according to a third aspect embodiment of the present invention;

[0071] Figure 17 This is a schematic diagram of the resistivity curve obtained by measuring resistivity using a compensated differential coil structure according to a third aspect embodiment of the present invention.

[0072] Figure 18 This is a schematic diagram of the structure of the compensated differential coil used in the fourth aspect embodiment of the present invention;

[0073] Figure 19 This is a schematic diagram of the formation interface structure identified by a symmetrical compensation coil system according to a fourth aspect embodiment of the present invention;

[0074] Figure 20 This is a schematic diagram of the structure of the compensated differential coil used in the fifth aspect embodiment of the present invention;

[0075] Figure 21 This is a schematic diagram of the structure of the compensated differential coil used in the sixth aspect embodiment of the present invention;

[0076] Figure 22 This is a schematic diagram of the structure of the compensated differential coil used in the seventh aspect embodiment of the present invention;

[0077] Figure 23 This is a comparison chart of measurement results using the compensation differential coil structure described in the third aspect embodiment of the present invention and the compensation differential coil structure described in the seventh aspect embodiment of the present invention;

[0078] Figure 24 This is a schematic diagram of the structure of the compensated differential coil used in the eighth aspect embodiment of the present invention;

[0079] Figure 25 This is a schematic diagram of the structure of the compensated differential coil used in the ninth aspect embodiment of the present invention;

[0080] Figure 26 This is a schematic diagram of the compensated differential coil structure used in the tenth aspect embodiment of the present invention.

[0081] In the diagram, 1 is the wellbore mud, 2 is the casing, 3 is the formation, 31 is the first formation, 32 is the second formation, and 33 is the third formation. Detailed Implementation

[0082] The present invention will now be described in detail with reference to the accompanying drawings through exemplary embodiments. However, it should be understood that, without further description, elements, structures, and features in one embodiment may be advantageously incorporated into other embodiments.

[0083] See Figure 1 The first aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves passing through a bushing.

[0084] S1. Model construction steps: Considering the wellbore, casing, and formation, and combining the characteristics of transient electromagnetic wave logging, establish a casing well formation model.

[0085] S2. Resistivity Measurement Steps: In the cased well formation model, the resistivity is measured using the apparent resistivity extraction method based on late signals.

[0086] Specifically, see Figure 2 The steps for measuring resistivity based on the late-signal apparent resistivity extraction method are as follows: S21, Induction electromotive force measurement steps: In the cased well formation model, different formation resistivities are simulated, and the induced electromotive force of the late transient electromagnetic wave logging zz component is measured using a single-transmitter single-receiver coil structure.

[0087] S22. Resistivity Extraction Steps: Based on the induced electromotive force signal of the late transient electromagnetic wave logging zz component, the apparent resistivity is extracted using the extraction formula. The extracted apparent resistivity is the transient electromagnetic wave resistivity through the casing. The extraction formula is expressed as:

[0088]

[0089] In the formula, R a Let L be the apparent resistivity, L be the source-source distance between the transmitting and receiving coils, and t be the source-source distance. a The time at which the induced electromotive force curve reaches its peak is t, and the time response is t0. l For the late period, E a For acceptable error, V(t) is the induced electromotive force of the zz component of the late transient electromagnetic wave logging, N is the number of coil turns, S is the coil area, μ is the formation permeability, and m is the magnetic moment.

[0090] Specifically, the method for extracting apparent resistivity is as follows:

[0091] First, it should be noted that the induced electromotive force of the zz component in late transient electromagnetic wave logging is a sloping straight line in a double logarithmic coordinate system, and the intercept of this line has an algebraic relationship with the formation resistivity.

[0092] Define R tl For the apparent resistivity in the late stage of transient electromagnetic wave logging, its algebraic relationship is:

[0093]

[0094] In the formula, V(t) is the induced electromotive force of the zz component of late transient electromagnetic wave logging, N is the number of coil turns, S is the coil area, μ is the formation permeability, and m is the magnetic moment.

[0095] Let t l For the late period, E a For acceptable error, according to The Taylor expansion yields:

[0096] In the formula, σ is the formation conductivity;

[0097] To determine the late time t l Introducing the Green's function for time response:

[0098]

[0099] In the formula, r is the distance from any point in space to the origin; u(t) is the step function, which is 1 when the response time t is greater than 0 and 0 when the response time t is less than 0.

[0100] Differentiating t in the Green's function formula for time response yields the time t at the peak of the induced electromotive force curve.a Define R tm For the apparent resistivity in transient electromagnetic wave logging, its algebraic relationship is:

[0101]

[0102] Based on the reciprocal relationship between resistivity and conductivity, substituting equation (4) into equation (2), the late-stage time is approximately determined as:

[0103]

[0104] Because the presence of casing delays the linear signal reflecting formation information, the early and middle-stage curve segments are disregarded; only the late-stage curve segment is considered. Therefore, the apparent resistivity is defined as:

[0105]

[0106] The effectiveness of the transient electromagnetic wave resistivity measurement method through a bushing described in the first aspect of the present invention will be verified below with reference to specific embodiments.

[0107] See Figure 3 A three-layer columnar stratigraphic model of a cased well was established. The wellbore radius ranged from 3 to 11 inches (7.5 to 28 cm). The conductivity of the wellbore mud 1 depended on the mud type. The relative permeability of the casing 2 was 1-200, and the casing thickness was 0.5-1.5 cm. Formation 3 was a homogeneous formation. In the figure, r... i r is the inner diameter of the casing. o ρ is the outer diameter of the casing. m ρ is the resistivity of the mud. ca ρ is the resistivity of the bushing. t The resistivity is the formation resistivity.

[0108] Simulations were performed for different formation resistivities, for example, assuming a wellbore radius of 10 cm and a casing conductivity of 10. 6 S / m, casing relative permeability 100, casing thickness 1cm, wellbore mud resistivity 1Ω·m, coil spacing 0.3m, z-direction magnetic dipole source emission, formation resistivity set between 1-15Ω·m, response characteristics as follows Figure 4 As shown, the resistivity curve is converted to... Figure 5 As shown, after conversion to apparent resistivity, the curve can intuitively reflect the representation of formation information by the measured signal. The late apparent resistivity is a stable straight line and is close to the true resistivity, verifying the feasibility and effectiveness of the apparent resistivity extraction method based on late signals for measuring resistivity.

[0109] See Figure 6 The second aspect of the present invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, the steps of which are as follows:

[0110] S1. Model construction steps: Considering the wellbore, casing, and formation, and combining the characteristics of transient electromagnetic wave logging, establish a casing well formation model.

[0111] S2. Resistivity Measurement Procedure: In the cased well formation model, the resistivity is measured using the anomaly-background method based on the mid-term signal.

[0112] Specifically, see Figure 7 The steps for measuring resistivity based on the anomaly-background method of intermediate-term signals are as follows:

[0113] S21. Background signal measurement steps: In the cased well formation model, set the background environment and use a single-transmitter, single-receiver coil structure to measure the transient electromagnetic wave logging induced electromotive force as the background signal.

[0114] Specifically, the background environment includes the wellbore, wellbore mud, and casing parameters. When measuring the background signal, the formation outside the casing is uniform, and the formation resistivity is set to 1 Ω·m. At this time, the transient electromagnetic wave logging induced electromotive force measured is the background signal (i.e., the background value).

[0115] S22. Anomaly Measurement Procedure: Under the same background environment (i.e., the wellbore, wellbore mud, and casing parameters remain unchanged), only the formation conditions are changed (i.e., the formation outside the casing is a layered formation with different resistivity between adjacent layers). The single-transmitter single-receiver coil structure is raised along the wellbore. At this time, the transient electromagnetic wave logging induced electromotive force measured by the single-transmitter single-receiver coil is used as the actual response signal, i.e., the anomaly.

[0116] S23. Calculation steps: Subtract the actual response signal from the background signal at the same time to obtain the difference signal.

[0117] S24. Drawing steps: Draw a curve reflecting the formation resistivity information based on the relationship between the difference signal and the formation resistivity.

[0118] S25. Resistivity Determination Steps: Convert the curves reflecting formation resistivity information into resistivity calibration charts. It should be noted that the outlier values ​​(i.e., transient electromagnetic logging induced electromotive force) obtained from the casing well formation model are subtracted from the background values ​​(i.e., background signals) simultaneously. Since the combined response of the casing and formation in the background environment is considered as a whole, the result after subtraction mainly reflects the difference between the formation resistivity in the casing well formation model and that of a homogeneous formation.

[0119] The transient electromagnetic wave resistivity measurement method described in this embodiment is applicable to scenarios where the signal magnitude is small in the late stage of transient electromagnetic wave logging, making it difficult to effectively extract the signal in practical engineering.

[0120] The effectiveness of the transient electromagnetic wave resistivity measurement method through a bushing described in the second aspect of the present invention will be verified below with reference to specific embodiments.

[0121] See Figure 8 A three-layer columnar stratigraphic model of a casing well was established. The wellbore radius ranged from 3 to 11 inches (7.5 to 28 cm). The conductivity of the wellbore mud 1 depended on the mud type. The relative permeability of the casing 2 was 1-200, and the casing thickness was 0.5-1.5 cm. Formation 3 consisted of three layers: layer 31, layer 32, and layer 33. (See figure, r...) i r is the inner diameter of the casing. o ρ is the outer diameter of the casing, ρ1 is the resistivity of the first formation, ρ2 is the resistivity of the second formation, and ρ3 is the resistivity of the third formation. The interface positions between the first formation 31 and the second formation 32, and between the second formation 32 and the third formation 33, are l1 = 1.5m and l2 = -1.5m, respectively. The coil distance between the transmitting coil and the receiving coil is 0.3m. The formation conditions are changed for the first time to: ρ1 = 1Ω·m, ρ2 = 2Ω·m, and ρ3 = 10Ω·m. The single-transmitter, single-receiver coil structure is raised along the wellbore. Based on the time taken to raise the coil to different positions, a formation depth map is plotted with the difference signal as the abscissa and the measurement point position as the ordinate, as shown below. Figure 9 As shown. The formation conditions were changed a second time, becoming: ρ1 = 1 Ω·m, ρ2 = 10 Ω·m, ρ3 = 1 Ω·m. A single-transmitter, single-receiver coil was pulled up along the wellbore. Based on the time taken to different pulling positions, a formation depth map was plotted with the difference signal as the x-axis and the measurement point position as the y-axis, as shown. Figure 10 As shown. The formation conditions were changed for the third time, becoming: ρ1 = 10 Ω·m, ρ2 = 2 Ω·m, ρ3 = 1 Ω·m. A single-transmitter, single-receiver coil was pulled up along the wellbore. Based on the time taken to different pulling positions, a formation depth map was plotted with the difference signal as the abscissa and the measurement point position as the ordinate, as shown. Figure 11 As shown, the difference signal curves can reflect changes in formation resistivity under different formation conditions. When the time of interest is near the peak, the signal value reflecting changes in formation resistivity is larger, and the signal gradually weakens with increasing time. This demonstrates the feasibility and effectiveness of extracting formation resistivity from medium-term induced electromotive force, providing an important basis for the design and practical application of transient through-casing resistivity logging instruments.

[0122] See Figure 12 The third aspect of the present invention provides a method for measuring the resistivity of transient electromagnetic waves passing through a bushing, the steps of which are as follows:

[0123] S1. Model construction steps: Considering the wellbore, casing, and formation, and combining the characteristics of transient electromagnetic wave logging, establish a casing well formation model.

[0124] S2. Resistivity Measurement Procedure: In the cased well formation model, a compensated differential coil structure is used to measure the resistivity.

[0125] Specifically, see Figure 13 The method for measuring resistivity using a compensated differential coil structure is as follows:

[0126] S21. Coil parameter determination steps: In the casing well formation model, under the set background environment, adjust the coil parameters of the compensation differential coil structure so that the casing background signal in the measured signal is zero. The coil parameters at this time are the final coil parameters of the compensation differential coil structure.

[0127] S22. Signal measurement steps: Raise the compensation differential coil structure along the wellbore and measure the induced electromotive force at different formation locations through the compensation differential coil structure;

[0128] S23. Drawing steps: Draw a curve reflecting the formation resistivity information based on the induced electromotive force;

[0129] S24. Resistivity Determination Steps: Convert the curves reflecting formation resistivity information into resistivity calibration charts. For details, see... Figure 14 The compensated differential coil structure includes a transmitting coil T and a set of compensated coils. The transmitting coil T is wound in the forward direction. The compensated coils include a first receiving coil R2, a second receiving coil R3, and a third receiving coil R4 arranged in sequence. The first receiving coil R2 is adjacent to the transmitting coil T. The first receiving coil R2 and the third receiving coil R4 are used as compensated coils and are wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. Figure 14 In the diagram, T2 is the coil distance between the transmitting coil T and the first receiving coil R2, T3 is the coil distance between the transmitting coil T and the second receiving coil R3, T4 is the coil distance between the transmitting coil T and the third receiving coil R4, and V... R2 V is the induced electromotive force of the first receiving coil R2. R3 V is the induced electromotive force of the second receiving coil R3. R4 Let N2 be the induced electromotive force of the third receiving coil R4, N3 be the number of turns of the first receiving coil R3, N4 be the number of turns of the third receiving coil R4, and f be the differential induced electromotive force. During resistivity measurement, the forward-wound second receiving coil R3 is differentially induced with the reverse-wound compensation coils (i.e., the first receiving coil R2 and the third receiving coil R4). Utilizing the cancellation effect of the compensation coils, the influence of the bushing can be effectively suppressed, and the useful signal in the receiving coils (i.e., the induced electromotive force related to formation resistivity) is highlighted.

[0130] according to Figure 14 The coil structure shown is used in a wellbore with a diameter of 1 Ω·m and a casing diameter of 10 mm.-6 The coil parameters are determined under Ω·m and 1Ω·m formation background conditions. This means minimizing (or eliminating) the casing background signal in the measured signal from the coil structure within a certain time period, thereby achieving casing signal compensation and differential formation signal measurement. Time-domain response signals with simulated formation resistivity of 10Ω·m, 2Ω·m, and 1Ω·m are used, and digital difference processing is performed between these signals and the 1Ω·m background signal. Figure 15 As shown, the solid black line represents the measurement results using the compensated differential method with a formation resistivity of 10 Ω·m, corresponding to the logarithmic axis on the left; the dashed purple and blue lines represent the results of subtracting the measurement results from the background values, corresponding to the linear axis on the right. (From 2×10) -4 Starting from point s, the difference signal begins to increase, indicating that the electromagnetic wave signal has propagated into the formation. Simultaneously, the measurement results using the compensated differential method show a relatively small difference signal near its peak, almost completely suppressing the casing's influence and reflecting formation resistivity information. Therefore, if detection is performed during this time period, the casing's influence can be effectively suppressed, significantly improving the signal-to-noise ratio of the formation signal. Following the above compensated differential measurement method, a system is established as follows... Figure 8 The diagram shows a three-layer columnar stratigraphic model of a casing well. Assume the formation resistivity from top to bottom is 10 Ω·m, 5 Ω·m, 1 Ω·m and 100 Ω·m, 10 Ω·m, 1 Ω·m respectively, the middle layer is 3 m thick, and the layer interfaces are l1 = 1.5 m and l2 = -1.5 m respectively. The distance between the transmitting and receiving coils is 0.3 m. The instrument is raised along the wellbore to provide the transient electromagnetic signal at a certain moment. The horizontal axis represents the magnitude of the induced signal, and the vertical axis represents the location of the measurement point. The measurement results are shown below. Figure 16 As shown. From Figure 16 It can be seen that the trend of the signal curve after compensated differential measurement can reflect the change in formation resistivity. Meanwhile, the values ​​of the three-layer model with casing under two different formation conditions are similar at 1 Ω·m and 10 Ω·m, indicating that the influence of model changes is small and it can be used to calibrate formation resistivity. Setting the 1 Ω·m value as the background baseline, the difference between the resistivity of different formations and this background baseline value is calculated, and resistivity curves are plotted. The results are as follows... Figure 17 As shown, when the formation resistivity is below 50 Ω·m, the difference signal changes significantly, indicating that the transient electromagnetic wave logging response through the casing is more sensitive to low-resistivity formations. Between 0 and 200 Ω·m, the formation resistivity and the difference signal exhibit a good logarithmic relationship, which can effectively calibrate the formation resistivity.

[0131] The transient electromagnetic wave resistivity measurement method described in this embodiment is applicable to the measurement of formation resistivity during well logging at any location.

[0132] A fourth aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is essentially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in a third aspect of this invention. The difference lies in the compensation coil structure used.

[0133] Specifically, see Figure 18 The compensated differential coil structure includes a transmitting coil T, a set of compensated coils, and a symmetrical compensated coil system. The transmitting coil T is wound in the forward direction. The compensated coil system includes a first receiving coil R2, a second receiving coil R3, and a third receiving coil R4 arranged sequentially. The first receiving coil R2 is adjacent to the transmitting coil T. The first receiving coil R2 and the third receiving coil R4 serve as compensated coils and are wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. The symmetrical compensated coil system includes a receiving coil R1 and a receiving coil R1', which are symmetrically arranged on both sides of the transmitting coil T. The receiving coil R1 is located between the transmitting coil T and the first receiving coil R2 and is wound in the forward direction. The receiving coil R1' serves as a compensated coil and is wound in the reverse direction. During resistivity measurement, the forward-wound second receiving coil R3 and the reverse-wound compensated coils (i.e., the first receiving coil R2 and the third receiving coil R4) are differentially coupled. Utilizing the cancellation effect of the compensated coils, the influence of the bushing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) is highlighted.

[0134] In this embodiment, a formation interface identification step is also included, which further includes a signal measurement step: raising the compensation differential coil structure along the wellbore and measuring the induced electromotive force at different formation locations through a symmetrical compensation coil system (i.e., receiving coil R1 and receiving coil R1').

[0135] Identification steps: Plot the curve of the induced electromotive force as a function of depth based on the measured induced electromotive force. The peak value of the induced electromotive force is the formation interface.

[0136] Taking the 10⁻⁵-1Ω·m stratigraphic model as an example, stratigraphic interfaces were identified according to the above-described stratigraphic interface identification steps. The identification results are shown in [link to relevant documentation]. Figure 19 .Depend on Figure 19 It can be seen that the signal at the layer interface locations l1 = 1.5m and l2 = -1.5m is significantly higher than at other locations. This indicates that the total signal received by the two symmetrical receiving coils depends on the presence of the formation interface, because this signal should be 0 in a homogeneous infinite medium. Therefore, when the formation resistivity changes, the signals received by the symmetrical coils will differ. Furthermore, the amplitude and timing of this signal are controlled by the resistivity of the medium and the contrast in resistivity between the two sides of the interface. Therefore, this signal can also be used in conjunction with the compensated differential measurement method to determine changes in formation resistivity distribution.

[0137] The transient electromagnetic wave resistivity measurement method described in this embodiment is applicable to formation resistivity measurement during logging at any location. It also includes a symmetrical compensation coil system capable of identifying formation interfaces and can be combined with a compensation differential measurement method to determine changes in formation resistivity distribution.

[0138] A fifth aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is essentially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in a third aspect of this invention. The difference lies in the compensation coil structure used.

[0139] In this embodiment, see Figure 20 The compensated differential coil structure includes a transmitting coil T and two sets of compensated coil systems. The transmitting coil T, the first set of compensated coil systems, and the second set of compensated coil systems are arranged sequentially. The transmitting coil T is wound in the forward direction. The first set of compensated coil systems includes R2, a second receiving coil R3, and a third receiving coil R4 arranged sequentially. The first receiving coil R2 and the third receiving coil R4 serve as compensated coils and are wound in the reverse direction. The second receiving coil R3 is wound in the forward direction. The first receiving coil R2 is adjacent to the transmitting coil T. The second set of compensated coil systems includes a fourth receiving coil R5, a fifth receiving coil R6, and a sixth receiving coil R7 arranged sequentially. The fourth receiving coil R5 and the sixth receiving coil R7 serve as compensated coils and are wound in the reverse direction. The fifth receiving coil R6 is wound in the forward direction. The fourth receiving coil R5 is adjacent to the third receiving coil R4.

[0140] according to Figure 18 The coil structure shown is used in a wellbore with a diameter of 1 Ω·m and a casing diameter of 10 mm. -6 Under a background environment of Ω·m and 1Ω·m formation, the coil parameters of each compensation coil system are determined. Specifically, different compensation coil systems select their respective time periods to minimize (or eliminate) the casing background signal in the total signal of each system, thereby achieving casing signal compensation and differential formation signal measurement. The preferred time periods follow a progression from the middle to the late stages, with higher resolution and shallower detection depth in the earlier periods and greater detection depth in the later periods.

[0141] In this embodiment, two sets of compensation coil systems are provided, which can realize near-well and far-well formation resistivity measurement.

[0142] It should be noted that the above-mentioned compensation coil system can also be set up with three sets, and its measurement principle is the same as that of the two-set compensation coil system, which can realize formation resistivity measurement in near-well, middle-well, and far-well sections. Specifically, the number of compensation coil sets can be set according to actual needs to realize formation resistivity measurement in different well sections.

[0143] A sixth aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is essentially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in the third aspect of this invention. The difference lies in the compensation coil structure used.

[0144] In this embodiment, see Figure 21 The compensated differential coil structure includes a transmitting coil T, two sets of compensated coil systems, and a symmetrical coil compensation system. The transmitting coil T, the first set of compensated coil systems, and the second set of compensated coil systems are arranged sequentially. The transmitting coil T is wound in the forward direction. The first set of compensated coil systems includes R2, a second receiving coil R3, and a third receiving coil R4 arranged sequentially. The first receiving coil R2 and the third receiving coil R4 serve as compensated coils and are wound in the reverse direction. The second receiving coil R3 is wound in the forward direction. The first receiving coil R2 is adjacent to the transmitting coil T. The second set of compensated coil systems includes a fourth receiving coil R5, a fifth receiving coil R6, and a sixth receiving coil R7 arranged sequentially. The fourth receiving coil R5 and the sixth receiving coil R7 serve as compensated coils and are wound in the reverse direction. The fifth receiving coil R6 is wound in the forward direction. The fourth receiving coil R5 is adjacent to the third receiving coil R4. The symmetrical compensation coil system includes a receiving coil R1 and a receiving coil R1'. The two receiving coils are symmetrically arranged on both sides of the transmitting coil T. The receiving coil R1 is located between the transmitting coil T and the first receiving coil R2 and is wound in the forward direction. The receiving coil R1' serves as a compensation coil and is wound in the reverse direction.

[0145] according to Figure 21 The coil structure shown is used in a wellbore with a diameter of 1 Ω·m and a casing diameter of 10 mm. -6 Under a background environment of Ω·m and 1Ω·m formation, the coil parameters of each compensation coil system are determined. Specifically, different compensation coil systems select their respective time periods to minimize (or eliminate) the casing background signal in the total signal of each system, thereby achieving casing signal compensation and differential formation signal measurement. The preferred time periods follow a progression from the middle to the late stages, with higher resolution and shallower detection depth in the earlier periods and greater detection depth in the later periods.

[0146] In this embodiment, a formation interface identification step is also included, which further includes an electromagnetic signal measurement step: the compensation differential coil structure is pulled up along the wellbore, and the induced electromotive force at different formation locations is measured through a symmetrical compensation coil system (i.e., receiving coil R1 and receiving coil R1').

[0147] Identification steps: Plot the curve of the induced electromotive force as a function of depth based on the measured induced electromotive force. The peak value of the induced electromotive force is the formation interface.

[0148] In this embodiment, two sets of compensation coil systems are provided, enabling near-well and far-well formation resistivity measurements. A symmetrical compensation coil system is also included, capable of identifying formation interfaces and, in conjunction with a compensation differential measurement method, determining changes in formation resistivity distribution.

[0149] It should be noted that the above-mentioned compensation coil system can also be set up with three sets, and its measurement principle is the same as that of the two-set compensation coil system, which can realize formation resistivity measurement in near-well, middle-well, and far-well sections. Specifically, the number of compensation coil sets can be set according to actual needs to realize formation resistivity measurement in different well sections.

[0150] A seventh aspect embodiment of the present invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is basically the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in the third aspect embodiment of the present invention. The difference from the third aspect embodiment is the use of a different compensation coil structure.

[0151] In this embodiment, see Figure 22 The compensated differential coil structure includes a transmitting coil T and a set of compensated coils. The transmitting coil T is wound in the forward direction. The compensated coils include a first receiving coil R2 and a second receiving coil R3 arranged in sequence. The first receiving coil R2 is adjacent to the transmitting coil T. The first receiving coil R2 serves as a compensated coil and is wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. Figure 22 In the diagram, T2 is the coil distance between the transmitting coil T and the first receiving coil R2, T3 is the coil distance between the transmitting coil T and the second receiving coil R3, and V... R2 V is the induced electromotive force of the first receiving coil R2. R3 Let N2 be the induced electromotive force of the second receiving coil R3, N3 be the number of turns of the first receiving coil, and f be the differential induced electromotive force. During resistivity measurement, the forward-wound second receiving coil R3 and the reverse-wound compensation coil (i.e., the first receiving coil R2) are differentially induced. Utilizing the canceling effect of the compensation coil, the influence of the bushing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) is highlighted.

[0152] In this embodiment, a compensation coil system consisting of two coils is used for compensation, with only one receiving coil performing the compensation, i.e., single compensation coil compensation is employed. When measuring formation resistivity, the coil parameters are determined based on the background environment of the third aspect embodiment of the present invention, and formation resistivity information is extracted using the compensation coil system described above in this embodiment. See also Figure 23 Compared with the compensation coil system with dual compensation coils described in the third aspect embodiment of the present invention, the measurable time is reduced, but the coil structure is simplified.

[0153] The transient electromagnetic wave resistivity measurement method described in this embodiment is applicable to the measurement of formation resistivity during well logging at any location.

[0154] An eighth aspect embodiment of the present invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is substantially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in the seventh aspect embodiment of the present invention. The difference from the seventh aspect embodiment is the use of a different compensation coil structure.

[0155] See Figure 24 The compensated differential coil structure includes a transmitting coil T, a set of compensated coils, and a symmetrical compensated coil system. The transmitting coil T is wound in the forward direction. The compensated coil system includes a first receiving coil R2 and a second receiving coil R3 arranged sequentially. The first receiving coil R2 is adjacent to the transmitting coil T; the first receiving coil R2 serves as a compensated coil and is wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. The symmetrical compensated coil system includes a receiving coil R1 and a receiving coil R1', which are symmetrically arranged on both sides of the transmitting coil T. The receiving coil R1 is located between the transmitting coil T and the first receiving coil R2 and is wound in the forward direction, while the receiving coil R1' serves as a compensated coil and is wound in the reverse direction. During resistivity measurement, the forward-wound second receiving coil R3 and the reverse-wound compensated coil (i.e., the first receiving coil R2) are differentially coupled. Utilizing the cancellation effect of the compensated coils, the influence of the bushing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) is highlighted.

[0156] In this embodiment, a formation interface identification step is also included, which further includes an electromagnetic signal measurement step: the compensation differential coil structure is pulled up along the wellbore, and the induced electromotive force at different formation locations is measured through a symmetrical compensation coil system (i.e., receiving coil R1 and receiving coil R1').

[0157] Identification steps: Plot the curve of the induced electromotive force as a function of depth based on the measured induced electromotive force. The peak value of the induced electromotive force is the formation interface.

[0158] The transient electromagnetic wave resistivity measurement method described in this embodiment is applicable to formation resistivity measurement during logging at any location. It also includes a symmetrical compensation coil system capable of identifying formation interfaces and can be combined with a compensation differential measurement method to determine changes in formation resistivity distribution.

[0159] A ninth aspect embodiment of the present invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is substantially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in the seventh aspect embodiment of the present invention. The difference from the seventh aspect embodiment is the use of a different compensation coil structure.

[0160] In this embodiment, see Figure 25 The compensated differential coil structure includes a transmitting coil T and two sets of compensated coil systems. The transmitting coil T, the first set of compensated coil systems, and the second set of compensated coil systems are arranged sequentially. The transmitting coil T is wound in the forward direction. The first set of compensated coil systems includes an R2 and a second receiving coil R3 arranged sequentially. The first receiving coil R2 serves as a compensated coil and is wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. The first receiving coil R2 is adjacent to the transmitting coil T. The second set of compensated coil systems includes a third receiving coil R4 and a fourth receiving coil R5 arranged sequentially. The third receiving coil R4 serves as a compensated coil and is wound in the reverse direction, while the fourth receiving coil R5 is wound in the forward direction. The second receiving coil R3 is adjacent to the third receiving coil R4.

[0161] according to Figure 25 The coil structure shown is used in a wellbore with a diameter of 1 Ω·m and a casing diameter of 10 mm. -6 Under a background environment of Ω·m and 1Ω·m formation, the coil parameters of each compensation coil system are determined. Specifically, different compensation coil systems select their respective time periods to minimize (or eliminate) the casing background signal in the total signal of each system, thereby achieving casing signal compensation and differential formation signal measurement. The preferred time periods follow a progression from the middle to the late stages, with higher resolution and shallower detection depth in the earlier periods and greater detection depth in the later periods.

[0162] In this embodiment, two sets of compensation coil systems are provided, which can realize near-well and far-well formation resistivity measurement.

[0163] It should be noted that the above-mentioned compensation coil system can also be set up with three sets, and its measurement principle is the same as that of the two-set compensation coil system, which can realize formation resistivity measurement in near-well, middle-well, and far-well sections. Specifically, the number of compensation coil sets can be set according to actual needs to realize formation resistivity measurement in different well sections.

[0164] The tenth aspect of this invention provides a method for measuring the resistivity of transient electromagnetic waves through a bushing, which is essentially the same as the method for measuring the resistivity of transient electromagnetic waves through a bushing described in the ninth aspect of this invention. The difference from the ninth aspect is the use of a different compensation coil structure.

[0165] In this embodiment, see Figure 26The compensated differential coil structure includes a transmitting coil T, two sets of compensated coil systems, and a symmetrical compensated coil system. The transmitting coil T, the first set of compensated coil systems, and the second set of compensated coil systems are arranged sequentially. The transmitting coil T is wound in the forward direction. The first set of compensated coil systems includes R2 and a second receiving coil R3 arranged sequentially. The first receiving coil R2 serves as a compensated coil and is wound in the reverse direction, while the second receiving coil R3 is wound in the forward direction. The first receiving coil R2 is adjacent to the transmitting coil T. The second set of compensated coil systems includes a third receiving coil R4 and a fourth receiving coil R5 arranged sequentially. The third receiving coil R4 serves as a compensated coil and is wound in the reverse direction, while the fourth receiving coil R5 is wound in the forward direction. The second receiving coil R3 is adjacent to the third receiving coil R4. The symmetrical compensated coil system includes a receiving coil R1 and a receiving coil R1'. The two receiving coils are symmetrically arranged on both sides of the transmitting coil T. The receiving coil R1 is located between the transmitting coil T and the first receiving coil R2 and is wound in the forward direction. The receiving coil R1' serves as a compensated coil and is wound in the reverse direction.

[0166] according to Figure 26 The coil structure shown is used in a wellbore with a diameter of 1 Ω·m and a casing diameter of 10 mm. -6 Under a background environment of Ω·m and 1Ω·m formation, the coil parameters of each compensation coil system are determined. Specifically, different compensation coil systems select their respective time periods to minimize (or eliminate) the casing background signal in the total signal of each system, thereby achieving casing signal compensation and differential formation signal measurement. The preferred time periods follow a progression from the middle to the late stages, with higher resolution and shallower detection depth in the earlier periods and greater detection depth in the later periods.

[0167] In this embodiment, a formation interface identification step is also included, which further includes an electromagnetic signal measurement step: the compensation differential coil structure is pulled up along the wellbore, and the induced electromotive force at different formation locations is measured through a symmetrical compensation coil system (i.e., receiving coil R1 and receiving coil R1').

[0168] Identification steps: Plot the curve of the induced electromotive force as a function of depth based on the measured induced electromotive force. The peak value of the induced electromotive force is the formation interface.

[0169] In this embodiment, two sets of compensation coil systems are provided, enabling near-well and far-well formation resistivity measurements. A symmetrical compensation coil system is also included, capable of identifying formation interfaces and, in conjunction with a compensation differential measurement method, determining changes in formation resistivity distribution.

[0170] It should be noted that the above-mentioned compensation coil system can also be set up with three sets, and its measurement principle is the same as that of the two-set compensation coil system, which can realize formation resistivity measurement in near-well, middle-well, and far-well sections. Specifically, the number of compensation coil sets can be set according to actual needs to realize formation resistivity measurement in different well sections.

[0171] The above embodiments are used to explain the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for measuring the resistivity of transient electromagnetic waves passing through a bushing, characterized in that, The steps are as follows: Model construction steps: Considering the wellbore, casing, and formation, and combining the characteristics of transient electromagnetic wave logging, establish a casing well formation model; Resistivity measurement steps: In the cased well formation model, the resistivity is measured using the apparent resistivity extraction method based on late signals; The steps for measuring resistivity using the apparent resistivity extraction method based on late signals are as follows: Induced electromotive force measurement steps: In the cased well formation model, different formation resistivities are simulated, and the induced electromotive force of the late transient electromagnetic wave logging zz component is measured using a single-transmitter single-receiver coil structure. Resistivity extraction steps: Based on the induced electromotive force signal of the zz component of the late transient electromagnetic wave logging, extract the apparent resistivity using the following formula. The extracted apparent resistivity is the transient electromagnetic wave resistivity through the casing. In the formula, R a Let L be the apparent resistivity, L be the source-source distance between the transmitting and receiving coils, and t be the source-source distance. a The time at which the induced electromotive force curve reaches its peak is t, and the time response is t0. l For the late period, E a For acceptable error, V(t) is the induced electromotive force of the zz component of the late transient electromagnetic wave logging, N is the number of coil turns, S is the coil area, μ is the formation permeability, and m is the magnetic moment. The method for extracting apparent resistivity is as follows: Define R tl For the apparent resistivity in the late stage of transient electromagnetic wave logging, its algebraic relationship is: Let t l For the late period, E a For acceptable error, according to The Taylor expansion yields: In the formula, σ is the formation conductivity; Introducing the Green's function for time response: In the formula, r is the distance from any point in space to the origin; u(t) is the step function, which is 1 when the response time t is greater than 0 and 0 when the response time t is less than 0. Differentiating t in the Green's function formula for time response yields the time t at the peak of the induced electromotive force curve. a Define R tm For the apparent resistivity in transient electromagnetic wave logging, its algebraic relationship is: Based on the reciprocal relationship between resistivity and conductivity, substituting equation (4) into equation (2), the late-stage time is approximately determined as: The apparent resistivity is defined as:

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