Transient electromagnetic wave through-casing resistivity measurement method

Abstract

The invention belongs to the technical field of oil-gas exploration and development, and relates to a transient electromagnetic wave through casing resistivity measurement method, which comprises the following steps of: establishing a cased well stratum model by considering a borehole, a casing and a stratum and combining transient electromagnetic wave logging characteristics; in a cased well formation model, the resistivity is measured by adopting an anomaly-background method based on a middle-term signal, and the method comprises the following steps: in the cased well formation model, setting a background environment, and measuring transient electromagnetic wave logging induced electromotive force as a background signal by adopting a single-transmitting single-receiving coil structure; under the same background environment, only stratum conditions are changed, the single-transmitting and single-receiving coil structure is lifted up along a borehole, and transient electromagnetic wave logging induced electromotive force measured by the single-transmitting and single-receiving coil serves as an actual response signal, namely an abnormal value; subtracting the actual response signal from the background signal at the same moment to obtain a difference signal; drawing a curve reflecting formation resistivity information according to the relationship between the difference signal and the formation resistivity; and converting the curve reflecting the formation resistivity information into a resistivity calibration chart. According to the method, the transient electromagnetic wave through casing logging response precision can be improved, and formation resistivity extraction under the influence of a high-conductivity casing is achieved.

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 name: Transient electromagnetic wave through casing resistivity measurement method Technical Field

[0005] The invention belongs to the technical field of oil and gas exploration and development, and relates to electric logging technology, in particular to a method for measuring resistivity of a transient electromagnetic wave through a casing. Background Art

[0006] In the exploration and development of oil and gas fields, the through-casing resistivity measurement technology has important application value, especially in casing drilling. Through-casing resistivity measurement is one of the important means for reservoir oil content evaluation, reservoir dynamic monitoring and residual oil distribution analysis. It is of great significance for casing drilling reservoir evaluation and production well oil well mining plan optimization and improving recovery rate. As a new drilling method for oil and gas exploration and development, casing drilling is difficult to directly apply conventional resistivity logging due to the existence of casing. How to achieve accurate and efficient through-casing resistivity measurement has become a key issue in the industry.

[0007] At present, the electrode method is usually used for casing resistivity logging. Its basic principle is to infer the 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: scaling, waxing, rust spots, etc. on the inner wall of the casing will interfere with the stability of the electrode method measurement, and it takes time to prepare for well washing (such as descaling, degreasing, etc.), which increases the complexity of logging. (3) The logging efficiency is low: The electrode method usually adopts a point measurement mode, which takes a long time to establish stable measurement conditions. It cannot meet the real-time response requirements for rapid changes in the downhole environment, and the overall logging efficiency is low.

[0008] Transient electromagnetic wave technology provides a new solution for resistivity measurement through casing. Its working principle is to arrange transmitting and receiving coils in the well, and send instantaneous current pulses to the formation through the transmitting coil. The instantaneous current pulse will generate an electromagnetic field that varies with time, stimulate the formation to produce induced eddy currents that decay with time, and diffuse at different speeds in the impassable medium. After the current is turned off, the decay characteristics of the secondary induced electromagnetic field generated by the eddy current are measured to extract the formation resistivity information. Transient electromagnetic wave technology has the following advantages: (1) Strong penetration: low-frequency signals can penetrate the casing shielding and can effectively collect the resistivity information of the formation. (2) Continuous measurement: transient electromagnetic waves have wide frequency domain characteristics and can work in multiple frequency bands simultaneously 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 ) and high relative magnetic permeability (50μ0-100μ0) result in a strong shielding effect on electromagnetic wave signals. The signal measured by the receiving antenna mainly comes from the casing contribution, and it is particularly difficult to extract the weak signal of the formation from it. (2) High engineering difficulty: The relevant instruments are still in the stage of theoretical research and experimental development, lack of mature engineering equipment, and there is still a large gap from practical application.

[0009] Therefore, clarifying the response law of transient electromagnetic wave logging in cased 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] In view of the problems existing in the prior art, the present invention provides a method for measuring resistivity of a transient electromagnetic wave through casing, which can improve the response accuracy of transient electromagnetic wave through casing logging and realize the extraction of formation resistivity under the influence of high-conductivity casing. In a first aspect, the present invention provides a method for measuring resistivity of a transient electromagnetic wave through casing, and the steps are as follows: a model building step: considering the wellbore, casing, and formation, and combining the transient electromagnetic wave logging characteristics, a cased well formation model is established;

[0011] Resistivity measurement steps: in the cased well formation model, the resistivity is measured by using an apparent resistivity extraction method based on late signals, or an abnormality-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 compensating coil system, the compensating 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 forward-wound, and the first receiving coil, as the compensating coil, is reversely wound; the transmitting coil and the compensating coil system are arranged in sequence in a front-to-back order;

[0012] The steps of measuring resistivity based on the apparent resistivity extraction method of late signal are as follows:

[0013] Induced electromotive force measurement steps: In the cased well formation model, simulate different formation resistivities and use a single-transmitter and single-receiver coil structure to measure the induced electromotive force of the zz component of late transient electromagnetic wave logging;

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

[0015]

[0016] In the formula, R a is the apparent resistivity, L is the source distance of the transmitting and receiving coils, t a is the time at the peak of the induced electromotive force curve, t is the time response, t l E is the late time. a is the acceptable error, 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 magnetic permeability, and m is the magnetic moment;

[0017] The steps for measuring resistivity based on the anomaly-background method of the mid-term signal are:

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

[0019] Outlier measurement steps: Under the same background environment, only the formation conditions are changed, and the single-transmitter and single-receiver coil structure is lifted up along the wellbore. At this time, the transient electromagnetic wave logging induced electromotive force measured by the single-transmitter and single-receiver coil is used as the actual response signal, that is, the outlier value;

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

[0021] Drawing step: drawing a curve reflecting formation resistivity information according to the relationship between the difference signal and the formation resistivity; Resistivity determination step: converting the curve reflecting formation resistivity information into a resistivity calibration plate;

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

[0023] Coil parameter determination steps: in the cased 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, and the coil parameters at this time are the final coil parameters of the compensation differential coil structure;

[0024] Signal measurement steps: lifting the compensation differential coil structure along the wellbore, and measuring the induced electromotive force at different formation positions through the compensation differential coil structure;

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

[0026] Resistivity determination steps: Convert the curve reflecting the formation resistivity information into a resistivity calibration plate.

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

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

[0029]

[0030] Assume t l E is the late time. a is an acceptable error, according to The Taylor expansion of is:

[0031] Where, σ is the formation conductivity;

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

[0033]

[0034] Where r is the distance from any point in space to the origin; u(t) is a step function, and the response time t is greater than 0 and the response time t is less than 0 and the response time t is 0;

[0035] The time t at the peak of the induced electromotive force curve is obtained by taking the derivative of t in the time response Green's function formula. a , define R tm is the intermediate-term apparent resistivity of transient electromagnetic wave logging, and its algebraic relationship is:

[0036]

[0037] According to the inverse relationship between resistivity and conductivity, by substituting equation (4) into equation (2), the late time is approximately determined as:

[0038]

[0039] The apparent resistivity is defined as:

[0040]

[0041] In some embodiments, the compensation differential coil structure includes a transmitting coil and a group of compensation coil systems, the transmitting coil is forward-wound, and the compensation coil system includes a first receiving coil, a second receiving coil, and a third receiving coil arranged in sequence, and the first receiving coil is adjacent to the transmitting coil; the first receiving coil and the third receiving coil are used as compensation coils and are reverse-wound, and the second receiving coil is forward-wound. In some embodiments, the compensation differential coil structure includes a transmitting coil and N groups of compensation coil systems, N≥2, the transmitting coil, the first group of compensation coil systems to the Nth group of compensation coil systems are arranged in sequence, the transmitting coil is forward-wound, and each group of compensation coil systems includes a first receiving coil, a second receiving coil, and a third receiving coil arranged in sequence, the first receiving coil and the third receiving coil are used as compensation coils and are reverse-wound, and the second receiving coil is forward-wound, and the first receiving coil of the first group of compensation coil systems is adjacent to the transmitting coil; in two adjacent groups of compensation coil systems, the third receiving coil of the previous group of compensation coil systems is adjacent to the first receiving coil of the next group of compensation coil systems.

[0042] In some embodiments, the compensated differential coil structure also 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 forward wound. The fifth receiving coil serves as a compensation coil and is reverse wound.

[0043] In some embodiments, the method further comprises a bed interface identification step, wherein the bed interface identification step further comprises:

[0044] Signal measurement steps: lift the compensation differential coil structure along the wellbore, and measure the induced electromotive force at different formation positions through the symmetrical compensation coil system;

[0045] Identification steps: Draw a curve of the measured induced electromotive force versus depth, and 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 group of compensating coils, the transmitting coil is wound in the forward direction, the compensating coil system includes a first receiving coil and a second receiving coil, the first receiving coil is adjacent to the transmitting coil, the first receiving coil serves as a compensating coil and is wound in the reverse direction, and 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 groups of compensation coil systems, N≥2, the transmitting coil, the first group of compensation coil systems to the Nth group of compensation coil systems are arranged in sequence, the transmitting coil is forward wound, each group of compensation coil systems includes a first receiving coil and a second receiving coil connected in sequence, the first receiving coil serves as a compensation coil and is reversely wound, the second receiving coil is forward wound, the first receiving coil of the first group of compensation coil systems is adjacent to the transmitting coil; in two adjacent groups of compensation coil systems, the second receiving coil of the previous group of compensation coil systems is adjacent to the first receiving coil of the next group of compensation coil systems.

[0048] In some embodiments, the compensated differential coil structure also 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:

[0050] (1) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention establishes a cased well formation model that takes into account the conditions of the wellbore, casing, formation, etc. 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, a late signal-based apparent resistivity extraction method is used to measure the formation resistivity, which can improve the transient electromagnetic wave through-casing resistivity logging response accuracy, realize the formation resistivity extraction under the influence of high-conductivity casing, and the formation resistivity measurement is accurate and high, 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 cased well formation model that takes into account conditions such as the wellbore, casing, and formation. According to the transient electromagnetic wave through-casing resistivity logging response law, when the signal magnitude during the transient electromagnetic wave logging period is small and it is difficult to effectively extract useful signals in actual engineering, an abnormal-background method based on the mid-term signal is used to measure the formation resistivity. This can improve the transient electromagnetic wave through-casing resistivity logging response accuracy, realize the extraction of formation resistivity under the influence of high-conductivity casing, and the measurement of formation resistivity is highly accurate, which fills the gap in transient electromagnetic wave through-casing resistivity logging.

[0052] (3) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention establishes a cased well formation model that takes into account the conditions of the wellbore, casing, formation, etc., and uses a compensated differential coil structure to measure the formation resistivity according to the transient electromagnetic wave through-casing resistivity logging response law. The compensated differential coil structure of the present invention suppresses the mid-term background signal through the compensated differential measurement method, highlights the useful signal (i.e., the induced electromotive force related to the formation resistivity), can improve the transient electromagnetic wave through-casing logging response accuracy, realize the formation resistivity extraction under the influence of high-conductivity casing, and the formation resistivity measurement is highly accurate, which fills the gap in transient electromagnetic wave through-casing resistivity logging.

[0053] (4) The transient electromagnetic wave through-casing resistivity measurement method provided by the present invention adopts an array design for the compensation coil system of the compensation differential coil structure, which can realize near-far well and near-mid-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 can realize the identification of formation interfaces by setting up a symmetrical compensation coil system. The formation resistivity can be determined based on the identified formation interface, providing a technical reference for the development of transient electromagnetic wave through-casing logging instruments and data processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is a flow chart of the method for measuring resistivity of a transient electromagnetic wave through a casing according to the embodiment of the first aspect of the present invention;

[0056] Figure 2 This is a flow chart of measuring resistivity by using a method for extracting apparent resistivity based on late signals according to an embodiment of the first aspect of the present invention;

[0057] Figure 3 It is a schematic structural diagram of a three-layer columnar stratified formation model of a cased well according to an embodiment of the first aspect of the present invention;

[0058] Figure 4 It is a schematic diagram of the resistivity response characteristics of different formations in the embodiment of the first aspect of the present invention;

[0059] Figure 5 It is a schematic diagram of apparent resistivity curves of a three-layer columnar formation model of a cased well with different formation resistivities according to an embodiment of the first aspect of the present invention;

[0060] Figure 6 This is a flow chart of the method for measuring resistivity of a transient electromagnetic wave through a casing according to an embodiment of the second aspect of the present invention;

[0061] Figure 7 This is a flow chart of measuring resistivity based on the abnormal-background method of the mid-term signal according to the second aspect of the present invention;

[0062] Figure 8 It is a schematic structural diagram of a three-layer columnar stratified formation model of a cased well according to an embodiment of the second aspect of the present invention;

[0063] Fig. 9 A formation depth map drawn when the formation condition of the three-layer columnar formation model of the cased well in the second embodiment of the present invention is 1-2-10Ω·m;

[0064] Fig.10 A formation depth map drawn when the formation condition of the three-layer columnar formation model of the cased well in the second embodiment of the present invention is 1-10-1Ω·m;

[0065] Fig.11 A formation depth map drawn when the formation condition of the three-layer columnar formation model of the cased well in the second embodiment of the present invention is 10-2-1Ω·m;

[0066] Fig.12 This is a flow chart of the method for measuring resistivity of a transient electromagnetic wave through a casing according to an embodiment of the third aspect of the present invention;

[0067] Fig.13 A flow chart of measuring resistivity using a compensated differential coil structure in an embodiment of the third aspect of the present invention;

[0068] Fig.14 A schematic diagram of the structure of a compensation differential coil structure used in an embodiment of the third aspect of the present invention;

[0069] Fig.15 This is a schematic diagram of a third aspect of the present invention using a compensation differential type coil structure to measure resistivity;

[0070] Fig.16 It is a schematic diagram of the measurement results of resistivity measured by using a compensation differential type coil structure according to the third aspect of the embodiment of the present invention;

[0071] Fig.17 A schematic diagram of a resistivity curve obtained by measuring resistivity using a compensated differential coil structure in an embodiment of the third aspect of the present invention;

[0072] Fig.18 A schematic structural diagram of a compensation differential coil structure used in an embodiment of the fourth aspect of the present invention;

[0073] Fig.19 A schematic diagram of a stratum interface structure identified by a symmetrical compensation coil system in accordance with a fourth aspect of the present invention;

[0074] Fig. 20 A schematic structural diagram of a compensation differential coil structure used in an embodiment of the fifth aspect of the present invention;

[0075] Fig.21 A schematic structural diagram of a compensation differential coil structure used in an embodiment of a sixth aspect of the present invention;

[0076] Fig. 22 A schematic diagram of the structure of a compensation differential coil structure used in an embodiment of the seventh aspect of the present invention;

[0077] Fig.23 A comparison chart of measurement results of using the compensation differential type coil structure described in the third aspect of the present invention and using the compensation differential type coil structure described in the seventh aspect of the present invention;

[0078] Fig.24 A schematic structural diagram of a compensation differential coil structure used in an embodiment of an eighth aspect of the present invention;

[0079] Fig.25 A schematic structural diagram of a compensation differential coil structure used in an embodiment of a ninth aspect of the present invention;

[0080] Fig.26 This is a schematic structural diagram of the compensation differential coil structure adopted in the embodiment of the tenth aspect of the present invention.

[0081] In the figure, 1, borehole mud, 2, casing, 3, formation, 31, first formation, 32, second formation, 33, third formation. DETAILED DESCRIPTION

[0082] The present invention will be described in detail below by way of exemplary embodiments in conjunction with the accompanying drawings. However, it should be understood that, without further description, elements, structures and features in one embodiment may also be beneficially combined in other embodiments.

[0083] See also Figure 1 The first embodiment of the present invention provides a method for measuring the resistivity of a casing by using transient electromagnetic waves.

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

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

[0086] Specifically, see Figure 2 The steps of measuring resistivity based on the apparent resistivity extraction method of late signal are: S21. Induced electromotive force measurement steps: In the cased well formation model, simulate different formation resistivities and use a single-transmitter and single-receiver coil structure to measure the induced electromotive force of the zz component of late transient electromagnetic wave logging.

[0087] S22, resistivity extraction step: extract the apparent resistivity through the extraction formula according to the induced electromotive force signal of the zz component of the late transient electromagnetic wave logging. The extracted apparent resistivity is the resistivity of the transient electromagnetic wave through the casing. The extraction formula is expressed as:

[0088]

[0089] In the formula, R a is the apparent resistivity, L is the source distance of the transmitting and receiving coils, t a is the time at the peak of the induced electromotive force curve, t is the time response, t l E is the late time, a is the acceptable error, 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 magnetic permeability, and m is the magnetic moment.

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

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

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

[0093]

[0094] Where 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 magnetic permeability, and m is the magnetic moment.

[0095] Assume t l E is the late time. a is an acceptable error, according to The Taylor expansion of is:

[0096] Where, σ is the formation conductivity;

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

[0098]

[0099] Where r is the distance from any point in space to the origin; u(t) is a step function, and the response time t is greater than 0 and the response time t is less than 0 and the response time t is 0;

[0100] The time t at the peak of the induced electromotive force curve is obtained by taking the derivative of t in the time response Green's function formula.a , define R tm is the intermediate-term apparent resistivity of transient electromagnetic wave logging, and its algebraic relationship is:

[0101]

[0102] According to the inverse relationship between resistivity and conductivity, by substituting equation (4) into equation (2), the late time is approximately determined as:

[0103]

[0104] Since the existence of casing will delay the oblique straight line signal reflecting the formation information, the early and middle curve segments are not considered, and only the late curve segment is considered. The apparent resistivity is defined as:

[0105]

[0106] The effectiveness of the method for measuring resistivity of a casing using transient electromagnetic waves as described in the first aspect of the present invention will be verified in conjunction with specific examples below.

[0107] See also Figure 3 , a three-layer columnar formation model of a cased well is established. Among them, the wellbore radius is 3-11 inches, that is, 7.5-28 cm, the conductivity of the wellbore mud 1 depends on the mud type, the relative magnetic permeability of the casing 2 is 1-200, the casing thickness is 0.5-1.5 cm, and the formation 3 is a uniform formation. In the figure, r i is the inner diameter of the casing, r o is the outer diameter of the casing, ρ m is the mud resistivity, ρ ca is the casing resistivity, ρ t is the formation resistivity.

[0108] Simulate different formation resistivities. For example, assume the borehole radius is 10 cm and the casing conductivity is 10 6 S / m, casing relative permeability is 100, casing thickness is 1 cm, wellbore mud resistivity is 1 Ω·m, coil spacing is 0.3 m, magnetic dipole source emission in z direction, formation resistivity is set between 1-15 Ω·m, response characteristics are as follows Figure 4 As shown, the resistivity curve is converted into Figure 5 It can be seen that after conversion to apparent resistivity, the curve can intuitively reflect the characterization of the formation information by the measurement signal. The late apparent resistivity is a smooth straight line and is moderately consistent with the real resistance, which verifies the feasibility and effectiveness of the apparent resistivity extraction method based on the late signal to measure resistivity.

[0109] See also Figure 6 The second aspect of the present invention provides a method for measuring the resistivity of a casing by using a transient electromagnetic wave, the steps of which are as follows:

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

[0111] S2. Resistivity measurement steps: 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 of measuring resistivity based on the anomaly-background method of the mid-term signal are:

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

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

[0115] S22. Outlier value measurement steps: Under the same background environment (i.e. the borehole, borehole mud and casing parameters remain unchanged), only the formation conditions are changed (i.e. the formation outside the casing is a layered formation, and the formation resistivity of two adjacent layers is different). The single-transmitter and single-receiver coil structure is lifted up along the borehole. At this time, the transient electromagnetic wave logging induced electromotive force measured by the single-transmitter and single-receiver coil is used as the actual response signal, i.e. the outlier value.

[0116] S23, calculation step: subtract the actual response signal from the background signal at the same time to obtain a difference signal.

[0117] S24, drawing step: drawing a curve reflecting the formation resistivity information according to the relationship between the difference signal and the formation resistivity.

[0118] S25, resistivity determination step: convert the curve reflecting the formation resistivity information into a resistivity calibration plate. It should be noted that the abnormal value (i.e., transient electromagnetic wave logging induced electromotive force) obtained from the cased well formation model is subtracted from the background value (i.e., background signal) at the same time, because the comprehensive response of the casing and formation in the background environment is considered as a whole. Therefore, the result after subtraction mainly reflects the difference between the formation resistivity in the cased well formation model and the uniform formation.

[0119] The above-mentioned transient electromagnetic wave through-casing resistivity measurement method of this embodiment is suitable for the scenario where the signal magnitude in the late stage of transient electromagnetic wave logging is small and it is difficult to effectively extract the signal in actual engineering.

[0120] The effectiveness of the method for measuring resistivity of a casing using transient electromagnetic waves as described in the second aspect of the present invention will be verified in conjunction with specific examples below.

[0121] See also Figure 8 , a three-layer columnar stratified formation model of a cased well is established. Among them, the wellbore radius is 3-11 inches, that is, 7.5-28 cm, the conductivity of the wellbore mud 1 depends on the mud type, the relative magnetic permeability of the casing 2 is 1-200, the casing thickness is 0.5-1.5 cm, and the formation 3 is a three-layer stratified formation, including the first formation 31, the second formation 32, and the third formation 33. In the figure, r i is the inner diameter of the casing, r o is the outer diameter of the casing, ρ1 is the resistivity of the first layer, ρ2 is the resistivity of the second layer, and ρ3 is the resistivity of the third layer. The interface positions of the first layer 31 and the second layer 32, and the interface positions of the second layer 32 and the third layer 33 are l1=1.5m and l2=-1.5m respectively, and the coil distance between the transmitting coil and the receiving coil is 0.3m. The formation conditions are changed for the first time, and the formation conditions are changed to: ρ1=1Ω·m, ρ2=2Ω·m, ρ3=10Ω·m. The single-transmitting and single-receiving coil structure is lifted up along the wellbore. According to the time of different lifting positions, the formation depth map is drawn with the difference signal as the horizontal coordinate and the measurement point position as the vertical coordinate as shown in the figure Fig. 9 The second time the formation conditions are changed to: ρ1 = 1Ω·m, ρ2 = 10Ω·m, ρ3 = 1Ω·m, the single-transmitting and single-receiving coil structure is lifted up along the wellbore, and according to the time of different lifting positions, the formation depth map is drawn with the difference signal as the horizontal coordinate and the measurement point position as the vertical coordinate as shown in the figure below. Fig.10 The third time the formation conditions were changed to: ρ1 = 10Ω·m, ρ2 = 2Ω·m, ρ3 = 1Ω·m, the single-transmitting and single-receiving coil structure was lifted up along the wellbore, and according to the time of different lifting positions, the formation depth map was drawn with the difference signal as the horizontal coordinate and the measurement point position as the vertical coordinate, as shown in the figure below. Fig.11 As shown in the figure. It can be seen that under different formation conditions, the difference signal curve can reflect the change of formation resistivity. When the focus time is near the peak, the larger the signal value reflecting the change of formation resistivity, the weaker the signal will be as time goes by. The feasibility and effectiveness of extracting formation resistivity from the medium-term induced electromotive force are proved, which provides an important basis for the design and practical application of transient through-casing resistivity logging instruments.

[0122] See also Fig.12 The third aspect of the present invention provides a method for measuring the resistivity of a casing by using a transient electromagnetic wave, the steps of which are as follows:

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

[0124] S2. Resistivity measurement steps: In the cased well formation model, the resistivity is measured using a compensated differential coil structure.

[0125] Specifically, see Fig.13 , the method for measuring resistivity using a compensated differential coil structure is:

[0126] S21, coil parameter determination step: in the cased well formation model, under the set background environment, adjusting the coil parameters of the compensation differential type coil structure so that the casing background signal in the measured signal is zero, and the coil parameters at this time are the final coil parameters of the compensation differential type coil structure;

[0127] S22, signal measurement step: lifting the compensation differential coil structure along the wellbore, and measuring the induced electromotive force at different formation positions through the compensation differential coil structure;

[0128] S23, drawing step: drawing a curve reflecting formation resistivity information according to the induced electromotive force;

[0129] S24, resistivity determination step: converting the curve reflecting the formation resistivity information into a resistivity calibration chart. Fig.14 The compensated differential coil structure includes a transmitting coil T and a group of compensating coils. The transmitting coil T is forward-wound. The compensating coil system includes 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 compensation coils and are reverse-wound. The second receiving coil R3 is forward-wound. Fig.14 In the figure, 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, V R2 is the induced electromotive force of the first receiving coil R2, V R3 is the induced electromotive force of the second receiving coil R3, V R4 is the induced electromotive force of the third receiving coil R4, N2 is the number of turns of the first receiving coil, N3 is the number of turns of the second receiving coil R3, N4 is the number of turns of the third receiving coil R4, and f is the differential induced electromotive force. During the resistivity measurement process, the second receiving coil R3 wound in the forward direction is differentiated from the compensation coil wound in the reverse direction (i.e., the first receiving coil R2 and the third receiving coil R4). By using the offset effect of the compensation coil, the influence of the casing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) can be highlighted.

[0130] according to Fig.14 The coil structure shown is in the wellbore 1Ω·m, casing 10-6 The coil parameters are determined under the background environment of 10Ω·m and 1Ω·m of the formation, that is, the casing background signal in the signal measured by the coil structure in a certain period of time is made as small as possible (zero), so as to achieve the compensation suppression of the casing signal and the differential measurement of the formation signal. The time domain response signals of the simulated formation resistivity of 10Ω·m, 2Ω·m and 1Ω·m are digitally processed with the difference of the 1Ω·m background signal. Fig.15 As shown in the figure, the solid black line is the compensation differential measurement result of the formation resistivity of 10Ω·m, corresponding to the logarithmic coordinate axis on the left; the purple and blue dotted lines are the difference processing results between the measurement results and the background value, corresponding to the linear coordinate axis on the right. -4 At s, the difference signal begins to increase, indicating that the electromagnetic wave signal propagates to the formation; at the same time, the measurement results using the compensated differential method are generally small near the peak of the difference signal. At this time, the influence of the casing is almost completely suppressed, which can reflect the formation resistivity information. Therefore, if detection is performed during this time period, the influence of the casing can be effectively suppressed, greatly improving the signal-to-noise ratio of the formation signal. According to the above-mentioned compensated differential measurement method, the following is established: Figure 8 The three-layer columnar formation model of the cased well is shown in the figure. Assume that the formation resistivity is 10Ω·m, 5Ω·m, 1Ω·m and 100Ω·m, 10Ω·m, 1Ω·m from top to bottom, the middle layer is 3m thick, the positions of the layer interfaces are l1=1.5m, l2=-1.5m, the distance between the transmitting coil and the receiving coil is 0.3m, the instrument is lifted up along the wellbore, and a transient electromagnetic signal is given at a certain moment, where the horizontal axis is the measured induction signal size, and the vertical axis is the measurement point position. The measurement results are shown in Fig.16 As shown. Fig.16 It can be seen that the trend of the signal curve after the compensated differential measurement can reflect the change of the formation resistivity. At the same time, the values ​​of the three-layer model with casing in two different formation conditions at 1Ω·m and 10Ω·m are similar, indicating that they are less affected by the model change and can be used to calibrate the formation resistivity. The value of 1Ω·m is set as the background base value, and the difference between the resistivity of different formations and the background base value is calculated respectively, and the resistivity curve is drawn. The results are shown in Fig.17 As shown. It can be seen that when the formation resistivity is below 50Ω·m, the difference signal changes significantly, indicating that the transient electromagnetic wave through-casing logging response is more sensitive to low-resistance formations. Between 0 and 200Ω·m, the formation resistivity and the difference signal show a good logarithmic relationship, which can effectively calibrate the formation resistivity.

[0131] The above-mentioned transient electromagnetic wave through-casing resistivity measurement method of this embodiment is applicable to the measurement of formation resistivity during well logging at any position.

[0132] The fourth embodiment of the present invention provides a method for measuring the resistivity of a casing through transient electromagnetic waves, which is basically the same as the method for measuring the resistivity of a casing through transient electromagnetic waves described in the third embodiment of the present invention. The difference from the third embodiment of the present invention is that the compensation coil structure used is different.

[0133] Specifically, see Fig.18 The compensation differential coil structure includes a transmitting coil T, a group of compensation coils and a symmetrical compensation coil system, and the transmitting coil T is forward-wound. The compensation coil system includes a first receiving coil R2, a second receiving coil R3 and a third receiving coil R4 arranged in sequence, and 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 compensation coils and are reversely wound, and the second receiving coil R3 is forward-wound. The symmetrical compensation coil system includes a receiving coil R1 and a receiving coil R1', and 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 forward-wound, and the receiving coil R1' is used as a compensation coil and is reversely wound. During the resistivity measurement process, the second receiving coil R3 wound in the forward direction is differentially wound with the compensation coil (i.e., the first receiving coil R2 and the third receiving coil R4) wound in the reverse direction. By using the offset effect of the compensation coil, the influence of the casing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) can be highlighted.

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

[0135] Identification steps: Draw a curve of the measured induced electromotive force versus depth, and the peak value of the induced electromotive force is the formation interface.

[0136] Taking the 10-5-1Ω·m formation model as an example, the formation interface is identified according to the above formation interface identification steps. The identification results are shown in Fig.19 .Depend on Fig.19 It can be seen that the signals at the positions of l1=1.5m and l2=-1.5m at the layer interface are significantly higher than those at other positions. This indicates that the total signal received by the two symmetrical receiving coils depends on the existence of the formation interface, because the signal should be 0 in a uniform infinite medium. Therefore, when the formation resistivity changes, the signals received by the symmetrical coils will be different. At the same time, the amplitude and time of the signal are controlled by the resistivity of the medium and the resistivity contrast of the medium on both sides of the interface. Therefore, the signal can also be used in combination with the compensated differential measurement method to determine the change in formation resistivity distribution.

[0137] The transient electromagnetic wave through-casing resistivity measurement method of this embodiment is applicable to the measurement of formation resistivity during well logging at any position. A symmetrical compensation coil system is also provided to identify formation interfaces and determine changes in formation resistivity distribution in combination with a compensation differential measurement method.

[0138] The fifth embodiment of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the third embodiment of the present invention. The difference from the third embodiment of the present invention is that the compensation coil structure used is different.

[0139] In this embodiment, see Fig. 20 The compensation differential coil structure includes a transmitting coil T and two groups of compensation coil systems. The transmitting coil T, the first group of compensation coil systems, and the second group of compensation coil systems are arranged in sequence, and the transmitting coil T is forward-wound. The first group of compensation coil systems includes R2, the second receiving coil R3, and the third receiving coil R4, which are arranged in sequence. The first receiving coil R2 and the third receiving coil R4 are used as compensation coils and are reverse-wound. The second receiving coil R3 is forward-wound. The first receiving coil R2 is adjacent to the transmitting coil T. The second group of compensation coil systems includes the fourth receiving coil R5, the fifth receiving coil R6, and the sixth receiving coil R7, which are arranged in sequence. The fourth receiving coil R5 and the sixth receiving coil R7 are used as compensation coils and are reverse-wound. The fifth receiving coil R6 is forward-wound. The fourth receiving coil R5 is adjacent to the third receiving coil R4.

[0140] according to Fig.18 The coil structure shown is in the wellbore 1Ω·m, casing 10 -6 Ω·m, and the formation 1Ω·m background environment, each compensation coil system determines the coil parameters of each compensation coil system, that is, different compensation coil systems select their own corresponding time periods, so that the casing background signal in the total signal of each compensation coil system is as small as possible (zero), thereby realizing the compensation suppression of the casing signal and the differential measurement of the formation signal. The time period is preferably set from the middle to the late stage, with high resolution and shallow detection depth in the front and large detection depth in the back.

[0141] In this embodiment, two sets of compensation coil systems are provided to achieve near-far wellbore formation resistivity measurement.

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

[0143] The sixth aspect of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the third aspect of the present invention. The difference from the third aspect of the present invention is that the compensation coil structure used is different.

[0144] In this embodiment, see Fig.21 The compensation differential coil structure includes a transmitting coil T, two groups of compensation coil systems and a symmetrical coil compensation system. The transmitting coil T, the first group of compensation coil systems and the second group of compensation coil systems are arranged in sequence, and the transmitting coil T is forward-wound. The first group of compensation coil systems includes R2, the second receiving coil R3 and the third receiving coil R4 arranged in sequence. The first receiving coil R2 and the third receiving coil R4 are used as compensation coils and are reverse-wound. The second receiving coil R3 is forward-wound. The first receiving coil R2 is adjacent to the transmitting coil T. The second group of compensation coil systems includes the fourth receiving coil R5, the fifth receiving coil R6 and the sixth receiving coil R7 arranged in sequence. The fourth receiving coil R5 and the sixth receiving coil R7 are used as compensation coils and are reverse-wound. The fifth receiving coil R6 is forward-wound. 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 forwardly wound. The receiving coil R1' is used as a compensation coil and is reversely wound.

[0145] according to Fig.21 The coil structure shown is in the wellbore 1Ω·m, casing 10 -6 Ω·m, and the formation 1Ω·m background environment, each compensation coil system determines the coil parameters of each compensation coil system, that is, different compensation coil systems select their own corresponding time periods, so that the casing background signal in the total signal of each compensation coil system is as small as possible (zero), thereby realizing the compensation suppression of the casing signal and the differential measurement of the formation signal. The time period is preferably set from the middle to the late stage, with high resolution and shallow detection depth in the front and large detection depth in the back.

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

[0147] Identification steps: Draw a curve of the measured induced electromotive force versus depth, and the peak value of the induced electromotive force is the formation interface.

[0148] In this embodiment, two sets of compensation coil systems are provided to achieve near-far well formation resistivity measurement. A symmetrical compensation coil system is also provided to identify formation interfaces and determine formation resistivity distribution changes in combination with a compensated differential measurement method.

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

[0150] The seventh aspect of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the third aspect of the present invention. The difference from the third aspect of the present invention is that the compensation coil structure used is different.

[0151] In this embodiment, see Fig. 22 The compensated differential coil structure includes a transmitting coil T and a group of compensating coils. The transmitting coil T is forward-wound. The compensating coil system includes 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 is used as a compensating coil and is reverse-wound. The second receiving coil R3 is forward-wound. Fig. 22 In the equation, 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, V R2 is the induced electromotive force of the first receiving coil R2, V R3 is the induced electromotive force of the second receiving coil R3, N2 is the number of turns of the first receiving coil, N3 is the number of turns of the second receiving coil R3, and f is the differential induced electromotive force. During the resistivity measurement process, the second receiving coil R3 wound in the forward direction is differentiated from the compensation coil (i.e., the first receiving coil R2) wound in the reverse direction. By using the offset effect of the compensation coil, the influence of the casing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) can be highlighted.

[0152] In this embodiment, a compensation coil system consisting of two coils is used for compensation, and only one receiving coil is used for compensation, that is, a single compensation coil is used for compensation. When measuring the formation resistivity, the coil parameters are determined according to the background environment of the third aspect of the present invention, and the formation resistivity information is extracted through the compensation coil system of this embodiment. Fig.23 Compared with the compensation coil system with double compensation coils as described in the embodiment of the third aspect of the present invention, the measurable time is reduced, but the coil structure is simplified.

[0153] The above-mentioned transient electromagnetic wave through-casing resistivity measurement method of this embodiment is applicable to the measurement of formation resistivity during well logging at any position.

[0154] The eighth embodiment of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the seventh embodiment of the present invention. The difference from the seventh embodiment of the present invention is that the compensation coil structure used is different.

[0155] See also Fig.24 The compensation differential coil structure includes a transmitting coil T, a group of compensation coils and a symmetrical compensation coil system, and the transmitting coil T is forward-wound. The compensation coil system includes a first receiving coil R2 and a second receiving coil R3 arranged in sequence, and the first receiving coil R2 is adjacent to the transmitting coil T; the first receiving coil R2 is used as a compensation coil and is reversely wound, and the second receiving coil R3 is forward-wound. The symmetrical compensation coil system includes a receiving coil R1 and a receiving coil R1', and 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 forward-wound, and the receiving coil R1' is used as a compensation coil and is reversely wound. During the resistivity measurement process, the second receiving coil R3 wound in the forward direction is differentially wound with the compensation coil (i.e., the first receiving coil R2) wound in the reverse direction. By using the offset effect of the compensation coil, the influence of the casing can be effectively suppressed, and the useful signal in the receiving coil (i.e., the induced electromotive force related to the formation resistivity) can be highlighted.

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

[0157] Identification steps: Draw a curve of the measured induced electromotive force versus depth, and the peak value of the induced electromotive force is the formation interface.

[0158] The transient electromagnetic wave through-casing resistivity measurement method of this embodiment is applicable to the measurement of formation resistivity during well logging at any position. A symmetrical compensation coil system is also provided to identify formation interfaces and determine changes in formation resistivity distribution in combination with a compensation differential measurement method.

[0159] The ninth aspect of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the seventh aspect of the present invention. The difference from the seventh aspect of the present invention is that the compensation coil structure used is different.

[0160] In this embodiment, see Fig.25 The compensation differential coil structure includes a transmitting coil T and two groups of compensation coil systems. The transmitting coil T, the first group of compensation coil systems, and the second group of compensation coil systems are arranged in sequence, and the transmitting coil T is forward-wound. The first group of compensation coil systems includes R2 and the second receiving coil R3 arranged in sequence. The first receiving coil R2 is used as a compensation coil and is reversely wound. The second receiving coil R3 is forward-wound. The first receiving coil R2 is adjacent to the transmitting coil T. The second group of compensation coil systems includes a third receiving coil R4 and a fourth receiving coil R5 arranged in sequence. The third receiving coil R4 is used as a compensation coil and is reversely wound. The fourth receiving coil R5 is forward-wound. The second receiving coil R3 is adjacent to the third receiving coil R4.

[0161] according to Fig.25 The coil structure shown is in the wellbore 1Ω·m, casing 10 -6 Ω·m, and the formation 1Ω·m background environment, each compensation coil system determines the coil parameters of each compensation coil system, that is, different compensation coil systems select their own corresponding time periods, so that the casing background signal in the total signal of each compensation coil system is as small as possible (zero), thereby realizing the compensation suppression of the casing signal and the differential measurement of the formation signal. The time period is preferably set from the middle to the late stage, with high resolution and shallow detection depth in the front and large detection depth in the back.

[0162] In this embodiment, two sets of compensation coil systems are provided to achieve near-far wellbore formation resistivity measurement.

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

[0164] The tenth aspect of the present invention provides a method for measuring the resistivity of a transient electromagnetic wave through a casing, which is basically the same as the method for measuring the resistivity of a transient electromagnetic wave through a casing described in the ninth aspect of the present invention. The difference from the ninth aspect of the present invention is that the compensation coil structure used is different.

[0165] In this embodiment, see Fig.26The compensation differential coil structure includes a transmitting coil T, two groups of compensation coil systems and a symmetrical compensation coil system. The transmitting coil T, the first group of compensation coil systems and the second group of compensation coil systems are arranged in sequence, and the transmitting coil T is forward-wound. The first group of compensation coil systems includes R2 and the second receiving coil R3 arranged in sequence. The first receiving coil R2 is used as a compensation coil and is reverse-wound. The second receiving coil R3 is forward-wound. The first receiving coil R2 is adjacent to the transmitting coil T. The second group of compensation coil systems includes the third receiving coil R4 and the fourth receiving coil R5 arranged in sequence. The third receiving coil R4 is used as a compensation coil and is reverse-wound. The fourth receiving coil R5 is forward-wound. The second receiving coil R3 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 forward-wound. The receiving coil R1' is used as a compensation coil and is reverse-wound.

[0166] according to Fig.26 The coil structure shown is in the wellbore 1Ω·m, casing 10 -6 Ω·m, and the formation 1Ω·m background environment, each compensation coil system determines the coil parameters of each compensation coil system, that is, different compensation coil systems select their own corresponding time periods, so that the casing background signal in the total signal of each compensation coil system is as small as possible (zero), thereby realizing the compensation suppression of the casing signal and the differential measurement of the formation signal. The time period is preferably set from the middle to the late stage, with high resolution and shallow detection depth in the front and large detection depth in the back.

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

[0168] Identification steps: Draw a curve of the measured induced electromotive force versus depth, and the peak value of the induced electromotive force is the formation interface.

[0169] In this embodiment, two sets of compensation coil systems are provided to achieve near-far well formation resistivity measurement. A symmetrical compensation coil system is also provided to identify formation interfaces and determine formation resistivity distribution changes in combination with a compensated differential measurement method.

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

[0171] The above embodiments are used to explain the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for measuring resistivity of a casing using transient electromagnetic waves, characterized in that: The steps are: Model construction steps: Considering the borehole, casing, and formation, and combining the transient electromagnetic wave logging characteristics, a cased well formation model is established; Resistivity measurement steps: In the cased hole formation model, the resistivity is measured using the anomaly-background method based on the mid-term signal; The steps for measuring resistivity based on the anomaly-background method of the mid-term signal are: Background signal measurement steps: in the cased well formation model, set the background environment, and use a single-transmitter and single-receiver coil structure to measure the transient electromagnetic wave logging induced electromotive force as the background signal; Outlier measurement steps: Under the same background environment, only the formation conditions are changed, and the single-transmitter and single-receiver coil structure is lifted up along the wellbore. At this time, the transient electromagnetic wave logging induced electromotive force measured by the single-transmitter and single-receiver coil is used as the actual response signal, that is, the outlier value; Calculation steps: subtract the actual response signal from the background signal at the same time to obtain the difference signal; Drawing steps: drawing a curve reflecting the formation resistivity information according to the relationship between the difference signal and the formation resistivity; resistivity determination steps: converting the curve reflecting the formation resistivity information into a resistivity calibration plate.

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