A nuclear magnetic resonance logging probe, a logging apparatus and a probe construction method

Abstract

The application discloses a nuclear magnetic resonance logging probe, a logging instrument and a probe construction method. The method comprises the following steps: an electromagnetic field model comprising a permanent magnet region, a soft magnetic region, a radio frequency coil region and a target sensitive region is established, a first material distribution variable is used to represent the residual magnetic flux density distribution of the permanent magnet region, and a second material distribution variable is used to represent the magnetic permeability distribution of the soft magnetic region; the static magnetic field intensity, the static magnetic field gradient, the effective component of the radio frequency magnetic field and the nuclear magnetic response quantity in the target sensitive region are obtained based on the above distribution and the radio frequency coil region; the optimization target is to maximize the nuclear magnetic response quantity, and the permanent magnet solid region and the soft magnetic solid region are determined iteratively by taking the static magnetic field intensity and the static magnetic field gradient as the constraint conditions, and then the nuclear magnetic resonance logging probe is formed. The probe can form a nuclear magnetic response region towards the formation outside the well wall. The static magnetic field intensity and the static magnetic field gradient in the target sensitive region are matched with each other.

Description

Technical Field

[0001] This invention relates to the field of nuclear magnetic resonance logging technology, and in particular to a nuclear magnetic resonance logging probe, logging instrument, and probe construction method. Background Technology

[0002] Nuclear magnetic resonance logging is used to obtain information about the pore fluids in the formation around the well. It typically uses a downhole probe to generate a static magnetic field and a radio frequency magnetic field, which causes the hydrogen-containing fluids in the formation to produce a nuclear magnetic response. The reservoir parameters are then evaluated based on the echo signals.

[0003] Downhole single-sided nuclear magnetic resonance probes are limited by wellbore size, temperature resistance requirements, and space for component arrangement. They need to arrange permanent magnets, soft magnetic components, radio frequency coils, and support components within a limited outer diameter to form a detection area facing the formation outside the wellbore.

[0004] Existing probes mostly use permanent magnets and soft magnetic components of a preset shape. There is a lack of unified matching between the distribution of permanent magnet materials and the distribution of soft magnetic materials, which makes it difficult to take into account both the static magnetic field strength and the magnetic field gradient in the target detection area. Summary of the Invention

[0005] In view of the above, it is necessary to provide a nuclear magnetic resonance logging probe, logging instrument and probe construction method to solve the problem that the lack of unified matching between the distribution of permanent magnet materials and the distribution of soft magnetic materials in the existing technology makes it difficult to take into account the static magnetic field strength and magnetic field gradient in the target detection area.

[0006] To achieve the above objectives, the present invention provides a method for constructing a nuclear magnetic resonance logging probe, comprising: An electromagnetic field model is established that includes a permanent magnet region, a soft magnetic region, a radio frequency coil region, and a target sensitive region; a first material distribution variable is set for the permanent magnet region to characterize the residual magnetic flux density distribution, and a second material distribution variable is set for the soft magnetic region to characterize the permeability distribution; Based on the residual magnetic flux density distribution, magnetic permeability distribution, and radio frequency coil region, the static magnetic field strength, static magnetic field gradient, effective components of the radio frequency magnetic field, and NMR response determined by the static magnetic field strength and the effective components of the radio frequency magnetic field within the target sensitive area are obtained in the electromagnetic field model. The optimization objective is to maximize the nuclear magnetic response within the target sensitive region, and the static magnetic field strength and static magnetic field gradient within the target sensitive region are used as constraints. The first material distribution variables and the second material distribution variables are iteratively updated, and the permanent magnet solid region and the soft magnetic solid region are determined. The permanent magnet component is formed from the permanent magnet solid region, the soft magnetic component is formed from the soft magnetic solid region, and the nuclear magnetic resonance logging probe is constructed from the permanent magnet component and the soft magnetic component.

[0007] In some embodiments, the values ​​of the first material distribution variable and the second material distribution variable are both 0 to 1; The residual magnetic flux density distribution is obtained by multiplying the first material distribution variable by the residual magnetic flux density of the permanent magnet material using a penalty function; The permeability distribution is obtained by interpolating the permeability of non-soft magnetic materials and soft magnetic materials using a penalty function from the second material distribution variable; The penalty factor of the penalty function ranges from 1 to 5.

[0008] In some embodiments, the effective component of the radio frequency magnetic field is the component of the radio frequency magnetic field generated in the radio frequency coil region that is perpendicular to the direction of the static magnetic field, or it is the rotational component that matches the direction of hydrogen nucleus precession. The nuclear magnetic resonance response includes the integral or discrete summation of the product of the square of the static magnetic field strength and the effective component of the radio frequency magnetic field within the target sensitive area.

[0009] In some embodiments, the constraints include a preset field strength condition and a preset gradient condition; The preset field strength conditions include a hydrogen nucleus Larmor frequency of 500 kHz to 1500 kHz corresponding to the static magnetic field in the target sensitive area; The preset gradient conditions include that the static magnetic field gradient at at least one target location is no greater than 30 Gs / cm, and the target location includes a position 4.5 cm radially outward from the outer surface of the probe.

[0010] In some embodiments, the arrangement of the radio frequency coil components is determined based on the effective components of the radio frequency magnetic field, the eddy current effects of the permanent magnet components or conductive supports, and the flipping parameters of the target sensitive area. The radio frequency coil component includes two planar strip coils arranged opposite each other, with an inner angle between the two planar strip coils ranging from 0° to 60°; The flipping parameters include 90° pulse time or 180° pulse time.

[0011] In some embodiments, iteratively updating the first material distribution variable and the second material distribution variable includes updating them using an interior point method; The iteration stops when the change in material distribution variable between two consecutive iterations is less than 0.001, or when the number of iterations reaches 100. After stopping the iteration, at least one of density filtering, projection processing, thresholding processing, or contour smoothing processing is applied to the first material distribution variable and the second material distribution variable to determine the permanent magnet solid region and the soft magnetic solid region.

[0012] In some embodiments, the target sensitive area includes at least one of a point area, a line area, a surface area, a volume area, or a sector area located radially outside the probe; The detection depth of the target sensitive area is 4.5 cm to 14 cm radially outward from the outer surface of the probe; The magnetization direction of the permanent magnet component forms an angle of 0° to 45° with respect to the radial reference direction of the probe.

[0013] The present invention also provides a nuclear magnetic resonance logging probe, comprising a permanent magnet component, a soft magnetic component, a radio frequency coil component, and a skeleton component; The permanent magnet component includes a pair of permanent magnets. The cross-section of the permanent magnet has an outer arc edge, an inner concave edge, and a side edge connecting the outer arc edge and the inner concave edge. The outer arc edge faces the outer periphery of the probe, and the inner concave edge faces the inside of the probe or the skeleton component. The soft magnetic component is disposed on the side of the permanent magnet component facing the target sensitive area. The soft magnetic component includes a pair of soft magnetic parts arranged together. The soft magnetic parts include an extended portion facing the radio frequency coil component and a contracted portion facing the permanent magnet component. The radio frequency coil component is disposed outside or adjacent to the soft magnetic component, and the radio frequency coil component includes a double-layer overlapping planar strip coil; The frame component is used to fix the permanent magnet component, the soft magnetic component, and the radio frequency coil component; The permanent magnet component, soft magnetic component, and radio frequency coil component together form the nuclear magnetic response region of the formation facing the outer side of the wellbore.

[0014] In some embodiments, the solid outline of the permanent magnet component is formed by the solid region corresponding to the first material distribution variable within the permanent magnet region, and the solid outline of the soft magnetic component is formed by the solid region corresponding to the second material distribution variable within the soft magnetic region. The magnetization direction of the permanent magnet forms an angle of 0° to 45° with respect to the radial reference direction of the probe; The radio frequency coil component includes two planar strip coils arranged opposite each other, with an included angle of 0° to 60° between the two planar strip coils; The permanent magnet components are made of one of samarium cobalt, neodymium iron boron or alnico, and the soft magnetic components are made of soft magnetic materials with a relative permeability of 50 to 300.

[0015] The present invention also provides a nuclear magnetic resonance logging tool, including a pressure-resistant housing, an electronic sub, a radio frequency transmitting and receiving circuit, and the above-mentioned nuclear magnetic resonance logging probe; The nuclear magnetic resonance logging probe is housed within a pressure-resistant casing. The radio frequency (RF) transmitting and receiving circuit is electrically connected to the RF coil component. The RF transmitting and receiving circuit is used to apply RF pulses to the RF coil component and receive nuclear magnetic resonance echo signals. The frequency of the RF pulses matches the hydrogen nucleus Larmor frequency corresponding to the static magnetic field in the target sensitive area.

[0016] The nuclear magnetic resonance (NMR) logging probe construction method provided by this invention sets material distribution variables for the permanent magnet region and the soft magnetic region respectively, enabling the residual magnetic flux density distribution of the permanent magnet and the permeability distribution of the soft magnetic component to be matched in the same iteration process. Under the joint constraints of the NMR response quantity, static magnetic field strength, and static magnetic field gradient in the target sensitive area, both permanent magnet and soft magnetic structures are formed. Therefore, the probe can take into account both the static magnetic field strength and static magnetic field gradient of the target sensitive area within a limited downhole space, thereby forming an NMR response region suitable for NMR logging. Attached Figure Description

[0017] To more clearly illustrate the technical content of the embodiments of the present invention, the accompanying drawings used in the embodiments are briefly described below. The drawings described below are only embodiments of the present invention, and those skilled in the art can obtain other drawings based on the provided drawings without creative effort.

[0018] Figure 1 This is a flowchart of the nuclear magnetic resonance logging probe construction method provided in the embodiments of the present invention; Figure 2 This is a cross-sectional structural diagram of the nuclear magnetic resonance logging probe provided in an embodiment of the present invention; Figure 3 This is a graph showing the relationship between the magnetization angle and the Larmor frequency of hydrogen nuclei at different detection depths, provided in an embodiment of the present invention. Figure 4 This is a graph showing the relationship between the magnetization angle and the static magnetic field gradient at different detection depths, provided in an embodiment of the present invention. Figure 5 This is a graph showing the relationship between the magnetization angle and the magnetic response of the core at different orientations in an embodiment of the present invention. Figure 6 This is a graph showing the relationship between the magnetization angle and the proportion of the nuclear magnetic response at different orientations in the statistical open-angle configuration provided in this embodiment of the invention. Figure 7 This is a graph showing the relationship between the included angle of the radio frequency coil and the Larmor frequency of the hydrogen nucleus at different detection depths, provided in an embodiment of the present invention. Figure 8 This is a graph showing the relationship between the included angle of the radio frequency coil and the static magnetic field gradient at different detection depths, provided in an embodiment of the present invention. Figure 9 This is a graph showing the relationship between the included angle of the radio frequency coil and the magnetic response of the core at different orientations according to the statistical opening angles provided in this embodiment of the invention. Figure 10 This is a graph showing the relationship between the included angle of the radio frequency coil and the proportion of the core magnetic response in different orientations according to statistical open angles, provided in an embodiment of the present invention. Figure 11This is a graph showing the relationship between the included angle of the radio frequency coil and the magnetic response of the core at different orientations when considering the influence of eddy currents, as provided in an embodiment of the present invention. Figure 12 This is a graph showing the relationship between the included angle of the RF coil and the proportion of the core magnetic response in different orientations when considering the influence of eddy currents, provided in an embodiment of the present invention. Figure 13 This is a diagram showing the relationship between the detection depth and the static magnetic field strength before and after construction, provided in an embodiment of the present invention. Figure 14 This is a diagram showing the relationship between the detection depth and the static magnetic field gradient before and after construction, provided in an embodiment of the present invention. Figure 15 This is a distribution diagram of the signal-sensitive region of the constructed structure provided in the embodiment of the present invention; Figure 16 This is a distribution diagram of the signal-sensitive region of the initial structure provided in the embodiment of the present invention; Figure 17 This is a circumferential distribution diagram of the signal strength of the initial structure provided in the embodiment of the present invention; Figure 18 This is a circumferential distribution diagram of the signal intensity of the constructed structure provided in an embodiment of the present invention.

[0019] The meanings of the markings in the attached diagram are as follows: 1. Radio frequency coil component; 2. Soft magnetic component; 3. Insulating cylindrical barrel frame; 4. Permanent magnet component; 5. Non-magnetic steel plate frame; ρ1, First material distribution variable; ρ2, Second material distribution variable; θ, Magnetization angle; α, Inner angle of the radio frequency coil; β, azimuth statistical opening angle; B0, Static magnetic field; B1, Radio frequency magnetic field; B1c, Effective component of the radio frequency magnetic field; DOI, Detection depth. Detailed Implementation

[0020] The following is combined with Figures 1 to 18 The present invention will be further described below. The following embodiments are used to illustrate the implementation process of the present invention and are not intended to limit the scope of protection of the present invention. Without changing the permanent magnet region and the soft magnetic region to be represented by material distribution variables respectively, and obtaining the permanent magnet structure and the soft magnetic structure under the joint constraints of the NMR response quantity, static magnetic field strength and static magnetic field gradient of the target sensitive area, those skilled in the art can make substitutions for the component shape, material, parameters, solution method and application scenario.

[0021] This invention provides a method for constructing a nuclear magnetic resonance (NMR) logging probe. This method is used to obtain the arrangement of the permanent magnet component 4, soft magnetic component 2, and radio frequency coil component 1 in the downhole NMR logging probe. The NMR logging probe is used in the wellbore environment, with the probe located inside the wellbore and the target sensitive area located radially outward of the probe and facing the formation outside the wellbore. Hydrogen-containing fluids such as water and oil in the formation generate NMR responses under the influence of the static magnetic field B0 and the radio frequency magnetic field B1 generated by the probe. The radio frequency coil component 1 receives the NMR echo signals, and the logging tool obtains formation pore fluid information based on the NMR echo signals.

[0022] The static magnetic field B0 is mainly generated by the permanent magnet component 4 and distributed in the target sensitive area after being adjusted by the soft magnetic component 2. The radio frequency magnetic field B1 is generated by the radio frequency coil component 1 and is used to excite the hydrogen-containing fluid in the target sensitive area and receive nuclear magnetic resonance echoes. The static magnetic field strength in the target sensitive area needs to ensure that the hydrogen nuclear Larmor frequency is within the operating frequency range of the logging tool. In one embodiment, the hydrogen nuclear Larmor frequency in the target sensitive area is 500kHz to 1500kHz. The static magnetic field strength at a detection depth of 4.5cm can be 0.028T, corresponding to a hydrogen nuclear Larmor frequency of approximately 1192kHz. The static magnetic field gradient at a detection depth of 4.5cm can be limited to no more than 30Gs / cm.

[0023] like Figure 1 As shown, during probe construction, an initial structural model is first established based on the probe's outer diameter, shell thickness, initial permanent magnet region, initial soft magnetic region, initial position of the RF coil, and target sensitive area. Constraints and optimization objectives are then determined within this model. Subsequently, the permanent magnet region is meshed to form multiple permanent magnet elements, and a first material distribution variable ρ1 is assigned to each permanent magnet element. Similarly, the soft magnetic region is meshed to form multiple soft magnetic elements, and a second material distribution variable ρ2 is assigned to each soft magnetic element. Then, the material distribution variables of both the soft magnetic and permanent magnet elements are updated simultaneously using an interior-point algorithm. The optimal solution is output when the density change is less than 0.001 or the number of iterations reaches 100. If these conditions are not met, the process returns to the material distribution variable update step and continues calculation.

[0024] The electromagnetic field model includes a permanent magnet region, a soft magnetic region, a radio frequency coil region, a skeleton region, and an external medium region. The permanent magnet region is the region where permanent magnet materials can be placed; the soft magnetic region is the region where soft magnetic materials can be placed; the radio frequency coil region is the region where radio frequency coil component 1 can be placed; the skeleton region is a non-magnetic structural region used to accommodate and support the permanent magnet component 4, the soft magnetic component 2, and the radio frequency coil component 1; and the external medium region can include at least one of the following: wellbore mud region, wellbore wall region, formation region, and air equivalent region.

[0025] The electromagnetic field model can be either a two-dimensional cross-sectional model or a three-dimensional model. The two-dimensional cross-sectional model is used to determine the material distribution of the permanent magnet component 4 and the soft magnetic component 2 within the probe's cross-section. The three-dimensional model is used to verify the influence of the probe's axial length, end effect, skeleton components, outer shell components, and RF coil component 1 on B0 and B1. In the two-dimensional cross-sectional model, the probe axis is perpendicular to the cross-section, the probe radial direction is from the inside of the probe towards the formation outside the wellbore, and the probe circumferential direction is around the probe axis. For ease of description, the center direction of the target sensitive area can be used as the radial reference direction, and the magnetization direction of the permanent magnet forms a magnetization angle θ relative to this radial reference direction.

[0026] The target sensitive area can be determined according to logging requirements. The target sensitive area can be a point area, line area, surface area, volume area, or fan-shaped area located radially outward of the probe. The distance reference for the target sensitive area can be the probe center, the probe outer surface, the wellbore surface, or the wellbore center. In one embodiment, the detection depth of the target sensitive area is 4.5 cm to 14 cm radially outward from the probe outer surface. In other embodiments, the detection depth can be defined as the radial distance outward from the probe center, the wellbore surface, or the wellbore center, based on the probe assembly reference and logging instrument coordinates. The target sensitive area can be divided radially into multiple detection depth positions, such as 4.5 cm, 5 cm, 7 cm, 9 cm, and 14 cm.

[0027] The target sensitive area can also be divided into sector regions of 60°, 90°, or 120° according to the azimuth statistical opening angle β, which is used to statistically analyze the proportion of NMR response within different azimuth ranges. The opening angles 60°, 90°, and 120° shown in the attached figure all represent azimuth statistical opening angles β of 60°, 90°, and 120°, respectively. The azimuth statistical opening angle β is different from the included angle α of the RF coil component 1.

[0028] After establishing the electromagnetic field model, the permanent magnet region is discretized into multiple permanent magnet units, and the soft magnetic region is discretized into multiple soft magnetic units. Each permanent magnet unit is assigned a first material distribution variable ρ1, and each soft magnetic unit is assigned a second material distribution variable ρ2. The value of the first material distribution variable ρ1 ranges from 0 to 1, and the value of the second material distribution variable ρ2 also ranges from 0 to 1. When ρ1 is 0, the corresponding permanent magnet unit has no permanent magnet material. When ρ1 is 1, the corresponding permanent magnet unit has permanent magnet material. When ρ2 is 0, the corresponding soft magnetic unit has no soft magnetic material. When ρ2 is 1, the corresponding soft magnetic unit has soft magnetic material.

[0029] During the iteration process, ρ1 and ρ2 can have intermediate values ​​between 0 and 1. After the iteration, they are converted into a machinable solid structure through thresholding and contour trimming. The first material distribution variable ρ1 is used to determine the remanent magnetic flux density of the corresponding permanent magnet unit. The second material distribution variable ρ2 is used to determine the permeability of the corresponding soft magnetic unit. A penalty interpolation method for solid isotropic materials can be used. The penalty function can be written as: f(ρ)=ρ^ m ; In the formula, ρ represents the first material distribution variable ρ1 or the second material distribution variable ρ2, and m represents the penalty factor. m can be 1 to 5, and further can be 3 to 5. The penalty function can reduce the proportion of intermediate material distribution values ​​in the final result, making the material distribution tend to 0 or 1.

[0030] The residual magnetic flux density of a permanent magnet unit can be expressed as: B r ,i=ρ1,i^ m ×B r 0; In the formula, B r ,i is the remanent magnetic flux density of the i-th permanent magnet unit, ρ1,i is the first material distribution variable of the i-th permanent magnet unit, B r 0 represents the remanent magnetic flux density of the permanent magnet material. In one embodiment, the permanent magnet material is Sm2Co of the 350°C grade. 17 Samarium cobalt permanent magnet material, B r 0 is 1.12T. When ρ1,i is 0, B r When ρ1,i is 0T, B r i is 1.12T. Permanent magnet materials can also be samarium cobalt, neodymium iron boron, alnico, or other heat-resistant permanent magnet materials.

[0031] The permeability of a soft magnetic unit can be expressed as: μi=μ air +ρ2,i^ m ×μ soft -ρ2,i^ m ×μ air ; In the formula, μi is the permeability of the i-th soft magnetic unit, μ air The permeability of a non-soft magnetic medium, μ softLet ρi be the permeability of the soft magnetic material, and ρ2,i be the second material distribution variable of the i-th soft magnetic unit. The relative permeability of the soft magnetic material can be from 50 to 300, and in one embodiment, it is 100. The soft magnetic material can be ferrite, soft magnetic composite material, permalloy, silicon steel laminate, or other high permeability material. When ρ2,i is 0, μi is close to the permeability of a non-soft magnetic medium. When ρ2,i is 1, μi is close to the permeability of the soft magnetic material.

[0032] In the selection of soft magnetic material parameters, the relative permeability of the soft magnetic material affects the working efficiency of the RF coil component 1. Generally, increasing the relative permeability of the soft magnetic material enhances the magnetic flux guiding capability near the RF coil component 1, thereby increasing the equivalent excitation efficiency of the RF coil component 1. However, the downhole environment presents high-temperature conditions, causing changes in the magnetic properties of the soft magnetic material. Therefore, in this embodiment, the relative permeability of the soft magnetic material is set to 100 to balance the magnetic permeability of the soft magnetic component 2 with the material stability under downhole temperature conditions. In other embodiments, the relative permeability of the soft magnetic material can be selected within the range of 50 to 300, depending on the downhole temperature, operating frequency, and material grade.

[0033] The mapping relationship between the first material distribution variable ρ1 and the remanent magnetic flux density is not limited to the power function interpolation described above; linear interpolation, piecewise interpolation, or interpolation with a projection function can also be used. The mapping relationship between the second material distribution variable ρ2 and the permeability is also not limited to the above expression; nonlinear magnetization curve interpolation, equivalent relative permeability interpolation, or interpolation using BH curve parameters can be used. For soft magnetic materials, a linear equivalent model with a relative permeability of 100 can be used, or nonlinear calculations can be performed using the BH curve of the soft magnetic material.

[0034] When solving the finite element method, the boundaries of the electromagnetic field model can be magnetically insulated, open, or infinite element boundaries. The mesh can be locally refined in the permanent magnet region, soft magnetic region, RF coil region, and target sensitive area. The external medium region can be treated as equivalent to air, well drilling mud, or formation. Simulation material parameters can be input from material handbooks, manufacturer parameters, or measured parameters.

[0035] In the calculation process, the permanent magnet region, the soft magnetic region, and the radio frequency coil region jointly participate in the electromagnetic field model. First, the residual magnetic flux density distribution of the permanent magnet region is determined based on the current ρ1. Then, the permeability distribution of the soft magnetic region is determined based on the current ρ2. Next, the static magnetic field B0 and static magnetic field gradient within the target sensitive area are solved. The radio frequency coil region is used to solve for the effective component B1c of the radio frequency magnetic field within the target sensitive area. B1c can be the radio frequency magnetic field component perpendicular to B0, the effective rotational component of the radio frequency magnetic field, or the equivalent component determined based on the receiving sensitivity of radio frequency coil component 1. B1c can be calculated using unit radio frequency current normalization or based on actual radio frequency drive power; when calculated using unit radio frequency current, the nuclear magnetic resonance response is used to relatively evaluate different structures.

[0036] The objective function can be to maximize the NMR response within the target sensitive region. The NMR response Φ can be determined by B0 and B1c within the target sensitive region. In one implementation, Φ can be expressed as B0^ within the target sensitive region Ωs. 2 The integral of the product with B1c is: Φ= ΩsB0^ 2 ×B1c dΩ; When solving discretely, Φ can be expressed as: Φ=ΣB0,j^ 2 ×B1c,j×ΔΩj; In the formula, j is the sampling point number within the target sensitive area, B0,j is the static magnetic field strength at the j-th sampling point, B1c,j is the effective component of the radio frequency magnetic field at the j-th sampling point, and ΔΩj is the region weight corresponding to the j-th sampling point. The NMR response can also be composed of at least two of B0, B1c, sensitive volume, frequency bandwidth, target sensitive area weight, hydrogen content index, or receiver sensitivity.

[0037] During the iteration process, both static magnetic field strength constraints and static magnetic field gradient constraints are set simultaneously. The static magnetic field strength constraint can be set to a hydrogen nucleus Larmor frequency corresponding to the static magnetic field within the target sensitive area, ranging from 500 kHz to 1500 kHz. The static magnetic field gradient constraint can be set to ensure that the static magnetic field gradient at at least one target location does not exceed a preset gradient threshold. The preset gradient threshold can be from 20 Gs / cm to 50 Gs / cm, and in one embodiment, it is 30 Gs / cm. In one embodiment, at a detection depth of 4.5 cm, the static magnetic field strength is 0.028 T, and the static magnetic field gradient does not exceed 30 Gs / cm. The static magnetic field gradient can be a radial gradient, axial gradient, circumferential gradient, or gradient magnitude. The radial gradient is the rate of change of B0 along the probe's radial direction. The axial gradient is the rate of change of B0 along the probe's axial direction. The circumferential gradient is the rate of change of B0 along the probe's circumference. The gradient magnitude is the value synthesized from multiple directional gradient components.

[0038] When iteratively updating ρ1 and ρ2, constrained nonlinear optimization algorithms can be employed. In one implementation, the IPOPT interior-point method can be used. When using the IPOPT interior-point method, the constraints on the range of ρ1 and ρ2, the static magnetic field strength constraint, the static magnetic field gradient constraint, and the NMR response objective are all incorporated into the iterative solution. Besides the IPOPT interior-point method, sequential quadratic programming, moving asymptote methods, genetic algorithms, particle swarm optimization, or other solution methods capable of handling constrained nonlinear problems can also be used. During the iteration process, the relationship between the objective function and constraints and ρ1 and ρ2 can be obtained through the finite difference method, the adjoint method, or other sensitivity calculation methods.

[0039] Each iteration may include the following steps: Calculate the residual magnetic flux density distribution in the permanent magnet region based on the current ρ1, and calculate the permeability distribution in the soft magnetic region based on the current ρ2. Solve the electromagnetic field model to obtain B0, B1c, and the gradient of B0 within the target sensitive region. Calculate the NMR response based on B0 and B1c. Determine whether the static magnetic field strength constraint and static magnetic field gradient constraint meet preset conditions. Update ρ1 and ρ2 based on the NMR response and constraint conditions. Repeat the above process until the stopping condition is met.

[0040] The stopping condition can be that the change in ρ1 and ρ2 between two adjacent iterations is less than 0.001, the change in NMR response is less than a preset threshold, or the constraint residual or KKT residual is less than a preset threshold. The stopping condition can also be that the number of iterations reaches 100. In one embodiment, when the change in material distribution variable is less than 0.001, the iteration typically converges in about 80 iterations; if it does not converge, the iteration stops when the number of iterations reaches 100, and the permanent magnet structure and soft magnetic structure are output.

[0041] When using the IPOPT interior-point method for iterative solution, the upper and lower bound constraints of the material distribution variables and the static magnetic field gradient constraints can be transformed into equality constraints containing relaxation variables. Let x represent the variable vector composed of ρ1 and ρ2, xL represent the lower bound of the variables, xU represent the upper bound of the variables, and sL and sU represent the relaxation variables corresponding to the lower and upper bound constraints, respectively. Then, the upper and lower bound constraints of the variables can be transformed into: x-xL-sL=0; xU-x-sU=0; Let gx represent the static magnetic field gradient at the current target location, gmax represent the upper limit of the allowed static magnetic field gradient, and sg represent the relaxation variable corresponding to the static magnetic field gradient constraint. Then the static magnetic field gradient constraint can be transformed into: gmax - gx - sg = 0; Since the objective of this invention is to maximize the NMR response Φ, and the IPOPT interior-point method typically deals with minimization problems, the objective function can be negativeized and a logarithmic barrier term can be introduced, resulting in: min-Φ-ηlnsL-ηlnsU-ηlnsg; In the formula, η is the barrier parameter, which gradually decreases and approaches 0 during the iteration process. When the slack variable approaches 0, the logarithmic barrier term increases the objective function value, thus keeping the iteration process within the feasible range. During the solution process, a filter line search strategy and Newton's iteration can be combined to update the variable vector, so that ρ1 and ρ2 gradually satisfy the objective function and constraints.

[0042] During the iteration process, at least one of the following can be applied to the first material distribution variable ρ1 and the second material distribution variable ρ2: density filtering, projection processing, thresholding, and contour smoothing. Density filtering is used to reduce abrupt changes in material distribution between adjacent units. Projection processing is used to make ρ1 and ρ2 approach 0 or 1. Thresholding processing is used to convert continuous material distributions into solid and non-solid regions. The threshold can be between 0.3 and 0.7. Contour smoothing is used to remove isolated small regions, sharp corners, and excessively small recesses, ensuring that the permanent magnet structure and soft magnetic structure meet processing and assembly requirements.

[0043] Permanent magnet units with a first material distribution variable ρ1 greater than a threshold can be identified as permanent magnet solid regions, while permanent magnet units with a first material distribution variable ρ1 less than a threshold can be identified as non-permanent magnet regions. Similarly, soft magnetic units with a second material distribution variable ρ2 greater than a threshold can be identified as soft magnetic solid regions, while soft magnetic units with a second material distribution variable ρ2 less than a threshold can be identified as non-soft magnetic regions. After contour smoothing, permanent magnet structures with outer arc edges, inner concave edges, and side edges in their cross-sections, as well as soft magnetic structures with umbrella-shaped cross-sections, can be formed.

[0044] like Figure 2 As shown, the nuclear magnetic resonance logging probe includes a radio frequency coil component 1, a soft magnetic component 2, an insulating cylindrical barrel frame 3, a permanent magnet component 4, a non-magnetic steel plate frame 5, and a filling frame. Figure 2 The left side shows the relative positions of each component within the probe's cross-section. Figure 2 The right side illustrates the definition of the included angle α of the RF coil and the magnetization angle θ of the permanent magnet. The RF coil component 1 is located outside or adjacent to the soft magnetic component 2. The soft magnetic component 2 is located on the side of the permanent magnet component 4 facing the target sensitive area. The permanent magnet component 4 is located inside the probe and fixed by the frame component. The insulating cylindrical frame 3 is located on the outer periphery of the probe, forming an insulating shell and component housing space. The non-magnetic steel plate frame 5 is located inside the probe, supporting and positioning the components on both sides and reducing the impact on the magnetic field distribution. The filling frame is used to fill the empty space between the permanent magnet component 4, the soft magnetic component 2, and the RF coil component 1.

[0045] The permanent magnet component 4 may include pairs of permanent magnets arranged symmetrically about the probe axis or radial reference line. Each permanent magnet has a concave sector cross-section, with an outer arc edge, an inner concave edge, and a side connecting the outer arc edge and the inner concave edge. The outer arc edge may face the outer periphery of the probe, and the inner concave edge may face the inside of the probe or the frame component. The permanent magnet can be a monolithic structure or formed by splicing multiple permanent magnet blocks. When multiple permanent magnet blocks are spliced, each permanent magnet block may have the same or different magnetization directions to approximate the magnetic field distribution of a continuous concave sector permanent magnet.

[0046] The soft magnetic component 2 may include pairs of soft magnetic elements arranged symmetrically about the probe axis or radial reference line. Each soft magnetic element may have an umbrella-shaped cross-section. The umbrella-shaped cross-section may include an extended portion that expands towards the RF coil component 1 and a contracted portion that contracts towards the permanent magnet component 4. The extended portion may be positioned close to the RF coil component 1, and the contracted portion may be positioned close to the permanent magnet component 4. The soft magnetic element may be fitted directly onto the permanent magnet or spaced apart from it by non-magnetic spacers. The thickness of the non-magnetic spacers may be from 0.5 mm to 5 mm. The soft magnetic element may be a single ferrite component or formed by splicing together multiple ferrite blocks.

[0047] The permanent magnet component 4 and the soft magnetic component 2 are arranged in pairs within the probe's cross-section. Both permanent magnets in the permanent magnet component 4 have a concave fan-shaped cross-section and are symmetrical about the probe's central axis. Both soft magnetic components in the soft magnetic component 2 have an umbrella-shaped cross-section and are also symmetrical about the probe's central axis. The contracted portion of the soft magnetic component faces the corresponding concave fan-shaped permanent magnet, and the extended portion faces the RF coil component 1, so that the umbrella-shaped profile of the soft magnetic component corresponds radially to the concave fan-shaped profile of the permanent magnet. Through this relative arrangement, the static magnetic field B0 generated by the permanent magnet component 4 can be guided via the soft magnetic component 2 to the target sensitive area radially outside the probe.

[0048] In one embodiment, the permanent magnet component 4 is made of 350°C grade samarium cobalt permanent magnet material with a residual magnetic flux density of 1.12T. The soft magnetic component 2 is made of ferrite material with a relative permeability of 100. The probe outer diameter is 110mm, the shell thickness is 8mm, the permanent magnet outer diameter is 90mm, and the permanent magnet frame thickness is 8mm. The above values ​​are for illustrating one embodiment; in other embodiments, the probe outer diameter can be 80mm to 160mm, the shell thickness can be 3mm to 15mm, the permanent magnet outer diameter can be 60mm to 130mm, and the permanent magnet frame thickness can be 3mm to 15mm.

[0049] The permanent magnet component 4 can also be configured with a magnetization angle θ. The magnetization angle θ is the angle between the magnetization direction of the permanent magnet and the radial reference direction or the X-axis. The magnetization angle θ can range from 0° to 45°. Two permanent magnets arranged in pairs can be magnetized in a mirror manner, in which case the magnetization directions of the two permanent magnets form positive and negative angles relative to the radial reference direction, respectively. Two permanent magnets arranged in pairs can also be magnetized in the same direction or in opposite directions. The magnetization angle θ can be scanned, with a scanning range of 0° to 45° and a scanning step size of 1° to 5°. In one embodiment, when the magnetization angle θ varies within the range of 0° to 35°, the nuclear magnetic response within the target sensitive area changes with θ. When θ is 25°, the nuclear magnetic response within the target sensitive area is at a relatively large value; therefore, θ can be selected as 25°.

[0050] like Figure 3 As shown, at different detection depths (DOIs), the magnetization angle θ affects the Larmor frequency of the hydrogen nucleus. Figure 3 In the results, the four curves with DOIs of 4.5cm, 5cm, 7cm, and 9cm all show slight changes with the magnetization angle θ. At DOI of 4.5cm, the curve is in the higher frequency range, while at DOI of 9cm, it is in the lower frequency range. This indicates that the closer the detection depth is to the probe, the higher the static magnetic field strength within the target's sensitive area, corresponding to a higher Larmor frequency for hydrogen nuclei. By changing the magnetization angle θ, the resonant frequency at different detection depths can be adjusted.

[0051] like Figure 4 As shown, the magnetization angle θ affects the static magnetic field gradient at different detection depths (DOIs). The four curves for DOIs of 4.5cm, 5cm, 7cm, and 9cm show a slight decrease or maintain a similar trend as the magnetization angle θ increases. At DOI 4.5cm, the gradient value is in the higher range. At DOI 9cm, the gradient value is in the lower range. Figure 4 The results shown illustrate that by changing the magnetization angle θ of the permanent magnet, the static magnetic field gradient at the target location can be adjusted while maintaining the frequency distribution of the target sensitive area.

[0052] like Figure 5 As shown, the magnetization angle θ affects the NMR response within the statistical opening angle β in different orientations. Figure 5 In the study, when the azimuth statistical opening angle β is 60°, 90°, and 120°, the NMR response changes with θ. Specifically, when θ is in the range of 20° to 30°, all three curves are in the higher range. In one embodiment, when θ is 25°, the NMR response is at a relatively large value within different azimuth statistical opening angles β. Figure 5 To illustrate, the magnetization angle θ not only affects the B0 distribution, but also changes the NMR response through the combined effect of B0 and B1c.

[0053] like Figure 6 As shown, the magnetization angle θ affects the proportion of NMR response within the statistical opening angle β in different orientations. Figure 6 In the analysis, when the azimuth statistical opening angle β is 60°, 90°, and 120°, the signal proportion decreases slightly or maintains a similar trend as θ increases. When the azimuth statistical opening angle β is 120°, the corresponding signal proportion is higher than that at 90° and 60°. Therefore, the larger the azimuth statistical opening angle β, the wider the target azimuth region included in the statistics, and the higher the proportion of the NMR response. Figure 6 and Figure 5 When used in combination, the magnetization angle θ can be selected between the nuclear magnetic response quantity and the azimuth concentration.

[0054] The radio frequency (RF) coil component 1 can be further defined after the permanent magnet component 4 and the soft magnetic component 2 are determined. The RF coil component 1 can be disposed outside or adjacent to the soft magnetic component 2. The RF coil component 1 can be disposed on the side of the soft magnetic component 2 facing the target sensitive area, or it can be embedded in the coil slot of the skeleton component. The RF coil component 1 includes a double-layer overlapping planar strip coil. The double-layer overlapping planar strip coil includes two layers of conductors with an insulating layer between them. The two layers of conductors can completely overlap, or they can be offset to a certain extent along the probe axis or radial direction. The two layers of conductors can be connected in series or in parallel. The current directions of the two layers of conductors can be the same or opposite.

[0055] The RF coil component 1 may include two planar strip coils arranged opposite each other. The two planar strip coils are symmetrical about the probe axis or radial reference line. An internal angle α is formed between the two planar strip coils. The internal angle α can be from 0° to 60°. α is a structural angle used to characterize the degree to which the two planar strip coils are open to each other. α is different from the azimuth statistical opening angle β. α can be obtained by scanning or iteration, and α can be selected as 0°, 10°, 20°, 30°, 40°, 50°, or 60°.

[0056] Each planar strip coil of the RF coil component 1 may include 1 to 10 turns. The strip width may be 1 mm to 20 mm. The spacing between adjacent strips may be 0.5 mm to 10 mm. The copper layer thickness may be 0.02 mm to 2 mm. The interlayer spacing between two conductors may be 0.05 mm to 5 mm. The RF coil component 1 may be formed from copper foil, flexible circuit board, wound wire, plated conductor, or printed conductor. The RF coil component 1 may adopt an integrated transceiver structure or a separate structure for the transmitting coil and the receiving coil.

[0057] The operating frequency of the RF coil component 1 can be from 500kHz to 1500kHz. The RF coil component 1 can be connected to the RF transmitting and receiving circuit via a matching and tuning circuit. The matching and tuning circuit may include capacitors, inductors, transformers, switching components, or impedance matching networks. The RF coil component 1 can employ a circuit configuration with a medium-to-low Q value and medium-to-high bandwidth to cover the Larmor frequencies corresponding to different detection depths within the target's sensitive area. The Q value and bandwidth can be determined based on the coil inductance, resistance, matching capacitor, and operating frequency.

[0058] The magnetic field generated by the radio frequency coil component 1 is a radio frequency magnetic field B1, which is used to excite the hydrogen-containing fluid in the target sensitive area via radio frequency and to receive nuclear magnetic resonance echo signals. The double-layer overlapping planar strip coil, through its double-layer conductors and symmetrical arrangement, can improve the effective component B1c of the radio frequency magnetic field in the target sensitive area and the receiving sensitivity. The permanent magnet component 4 and the soft magnetic component 2 are mainly used to form and adjust the static magnetic field B0, while the radio frequency coil component 1 is mainly used to form and adjust the radio frequency magnetic field B1.

[0059] like Figure 7 As shown, the included angle α inside the RF coil has little effect on the hydrogen nucleus Larmor frequency at different detection depths (DOI). Figure 7 In the data, the four curves with DOIs of 4.5cm, 5cm, 7cm, and 9cm remain essentially horizontal, indicating that once the structures of the permanent magnet component 4 and the soft magnetic component 2 are determined, changing the inner angle α of the RF coil will not significantly alter the static magnetic field intensity distribution within the target sensitive area. Therefore, the inner angle α of the RF coil is primarily used to adjust the RF magnetic field B1, rather than the static magnetic field B0.

[0060] like Figure 8 As shown, the included angle α inside the RF coil has little effect on the static magnetic field gradient at different detection depths (DOI). Figure 8 In the data, the four curves with DOIs of 4.5cm, 5cm, 7cm, and 9cm remained essentially horizontal. This result is consistent with... Figure 7 The evidence corroborates that, given that the permanent magnet component 4 and the soft magnetic component 2 are already determined, the included angle α within the RF coil mainly affects B1c and the NMR response, without significantly altering the B0 gradient.

[0061] like Figure 9 As shown, the included angle α within the RF coil affects the NMR response within the statistical opening angle β at different orientations. Figure 9 In the study, as α increases from 10° to 60°, the NMR response at azimuth statistical opening angles β of 60°, 90°, and 120° all show an increasing trend. When the azimuth statistical opening angle β is 120°, the corresponding curve is located in the higher interval, while when the azimuth statistical opening angle β is 60°, the corresponding curve is located in the lower interval. Figure 9To illustrate, increasing the included angle α within the radio frequency coil can increase the participation of the effective component B1c of the radio frequency magnetic field in the target sensitive area, thereby improving the NMR response.

[0062] like Figure 10 As shown, the included angle α within the RF coil affects the proportion of NMR response within the statistical opening angle β in different orientations. Figure 10 In the data, when α increases from 10° to 60°, all three curves show a downward trend. This result indicates that although... Figure 9 The NMR response increases with increasing α, but Figure 10 The proportion of the directional statistics within the open angle β decreases as α increases. Therefore, when determining α, it is possible to simultaneously refer to... Figure 9 The NMR response and Figure 10 The percentage of nuclear magnetic response shown is used to adapt the radio frequency coil component 1 to the target orientation range.

[0063] In the arrangement of the RF coil component 1, the controlled variable method can be used. First, the initial structure of the RF coil component 1 is fixed, and the permanent magnet component 4 and the soft magnetic component 2 are iteratively solved under an approximate static field solver to obtain the permanent magnet structure and soft magnetic structure that satisfy the static magnetic field strength and static magnetic field gradient constraints of the target sensitive area. Subsequently, the RF coil component 1 is introduced into the obtained permanent magnet structure and soft magnetic structure, and the effective component B1c of the RF magnetic field is calculated in the operating frequency range of 500kHz to 1500kHz. Then, considering the eddy current effects generated by the permanent magnet component 4, the non-magnetic steel plate frame 5, and other conductive components, the t90, t180, and NMR response quantities are calculated in combination with the spin response to determine the inner angle α, strip width, interlayer spacing, and connection method of the RF coil component 1.

[0064] In the calculation of RF coil component 1, the effect of eddy currents can be considered. Eddy currents can originate from the permanent magnet component 4, the non-magnetic steel plate frame 5, the metal casing, other conductive support components, and the RF coil itself. During the calculation, the conductivity can be set for the samarium cobalt permanent magnet, the non-magnetic steel plate, the metal casing, and the copper conductor. Each conductivity can be input according to material handbooks, manufacturer parameters, or measured values, and the RF magnetic field distribution can be calculated within the frequency range of 500kHz to 1500kHz. After considering the eddy current effect, the resulting B1c distribution can be used to evaluate the excitation and reception capabilities of the RF coil component 1 for the target sensitive area under actual RF operating conditions.

[0065] like Figure 11 As shown, when considering the influence of eddy currents, the included angle α inside the RF coil affects the NMR response within the statistical opening angle β in different orientations. Figure 11In the study, as α increases from 10° to 60°, the NMR response at azimuth statistical opening angles β of 60°, 90°, and 120° all show an increasing trend. This trend indicates that, considering the influence of eddy currents, increasing the included angle α within the RF coil can still improve the RF excitation and reception response within the target's sensitive area.

[0066] like Figure 12 As shown, when considering the influence of eddy currents, the included angle α inside the RF coil affects the proportion of NMR response within the statistical opening angle β in different orientations. Figure 12 In the azimuth statistics, the three curves with azimuth angles β of 60°, 90°, and 120° are all in the higher proportion range, and show a decreasing trend as α increases. When the azimuth angle β is 120°, the corresponding curve is in the higher range. Figure 12 This indicates that, after considering the influence of eddy currents, the NMR response can be more concentrated within the target azimuth range. Figure 11 and Figure 12 By combining these methods, the range of the inner included angle α of the RF coil component 1 when considering the influence of eddy currents can be determined.

[0067] In some embodiments, the RF coil component 1 can be evaluated using a spin dynamics model. The spin dynamics model can be used to calculate the reversal parameters of the hydrogen-containing fluid within the target sensitive region under the influence of B0 and B1. The reversal parameters can include the 90° pulse time t90 and the 180° pulse time t180. t90 is the duration of the RF pulse required for the magnetization vector within the target sensitive region to reverse by 90°. t180 is the duration of the RF pulse required for the magnetization vector within the target sensitive region to reverse by 180°. t90 and t180 can be calculated based on the B1c distribution at different detection depths, and the inner angle α, number of turns, strip width, interlayer spacing, and matching tuning parameters of the RF coil component 1 can be adjusted accordingly.

[0068] The filler skeleton can be made of polytetrafluoroethylene (PTFE) and is used for filling gaps, insulation, and positioning. The filler skeleton can be placed in the space between the permanent magnet component 4, the soft magnetic component 2, and the radio frequency coil component 1. Multiple slots can be evenly arranged on the upper surface of the filler skeleton. These slots are used to accommodate or abut against the soft magnetic component 2, allowing the filler skeleton to contact the soft magnetic material, thereby providing positioning and support for the soft magnetic component 2.

[0069] The skeleton components are not limited to the materials mentioned above. The insulating cylindrical barrel skeleton 3 can also be made of polyetheretherketone, epoxy composite material, glass fiber reinforced material, or ceramic material. The non-magnetic steel plate skeleton 5 can also be made of other non-magnetic metals or non-magnetic alloys. The filling skeleton can also be made of ceramic, polyimide, polyetheretherketone, or other non-magnetic insulating materials. The skeleton components can be provided with limiting steps, slots, adhesive surfaces, clamping parts, and potting cavities. The permanent magnet component 4, the soft magnetic component 2, and the radio frequency coil component 1 can be fixed by snap-fitting, bonding, clamping, screwing, or potting.

[0070] The initial structure can be a regular magnet structure or an empirically laid-out structure without synchronous updates of ρ1 and ρ2. In one embodiment, the initial structure includes a permanent magnet of a preset shape, a soft magnetic component of a preset shape, and a radio frequency coil component in a fixed position. The initial structure and the topology-optimized structure have the same probe outer diameter, the same target sensitive area, and the same material parameters to compare the static magnetic field strength, static magnetic field gradient, and NMR response distribution.

[0071] like Figure 13 As shown, the magnetic field strength can be compared between the initial structure and the topology-optimized structure. Figure 13 In the figure, the horizontal axis represents the detection depth in cm, and the vertical axis represents the magnetic field strength in T. The magnetic field strength of the initial structure is higher in the near-probe region, but decreases rapidly with increasing detection depth. The magnetic field strength of the topology-optimized structure is lower than that of the initial structure in the shallow region, but crosses with the initial structure after about 4 to 5 cm, and remains above that of the initial structure in the deeper detection region. Therefore, the permanent magnet component 4 and the soft magnetic component 2 of the topology-optimized structure can improve the static magnetic field distribution in the detection depth range of 4.5 cm to 14 cm.

[0072] like Figure 14 As shown, the static magnetic field gradient can be compared between the initial structure and the topology-optimized structure. Figure 14 In the diagram, the horizontal axis represents the detection depth in cm, and the vertical axis represents the gradient in Gs / cm. The initial structure has a higher gradient near the probe, which decreases with increasing detection depth. The gradient curve of the topology-optimized structure is lower than that of the initial structure in the shallow region. This indicates that the permanent magnet structure and soft magnetic structure obtained after simultaneous updates of ρ1 and ρ2 can reduce the static magnetic field gradient at the target depth.

[0073] like Figure 15 and Figure 16 As shown, the distribution of signal-sensitive regions in the optimized structure can be compared with that in the initial structure. Figure 15 This is a diagram of the signal-sensitive region of the topology-optimized structure. Figure 16 This is a diagram of the signal-sensitive region of the initial structure. Both diagrams use X and Y coordinates to represent the probe's outer perimeter, in mm, and grayscale bars to represent the normalized NMR or RF response. A grayscale value closer to 1 indicates a higher response in the corresponding region; a grayscale value closer to 0 indicates a lower response; negative values ​​can represent normalized values ​​with opposite phase or direction. Figure 15 In the middle, the signal-sensitive area is concentrated in the target orientation area near the inner ring on the upper side of the probe, and the sensitive area is distributed in a short arc shape. Figure 16 In the first case, the signal-sensitive area is relatively wide and the distribution is more dispersed. The comparison shows that the optimized topology allows the NMR response to be more widely distributed in the target orientation region.

[0074] like Figure 17 and Figure 18 As shown, the circumferential distribution of signal amplitude can be compared between the initial structure and the structure after topology optimization. Figure 17 The signal amplitude circumferential distribution of the initial structure, Figure 18 The signal amplitude distribution is shown in the circumferential direction after topology optimization. Figure 17 In the middle, the relative signal strength forms a narrow main peak near the relative angle of 0°, and the lateral region has only low side lobes, indicating that the signal is concentrated in the target azimuth region. Figure 18 In the diagram, the relative signal strength forms a relatively wide plateau-like distribution near the relative angle of 0°, indicating that the initial structure has a wide signal distribution range. Combined with... Figures 15 to 18 Within a statistical opening angle β of 120°, the NMR response of the topology-optimized structure accounts for approximately 94.5%, while that of the initial structure within the same statistical opening angle β accounts for approximately 60%.

[0075] Since ρ1 and ρ2 represent the distribution of permanent magnet materials and soft magnetic materials within their respective deployable areas, after iteration, permanent magnet materials and soft magnetic materials are retained only in areas that meet the requirements of NMR response, static magnetic field strength, and static magnetic field gradient in the target sensitive area. Therefore, compared to the initial structure using regular blocky permanent magnets and regular blocky soft magnetic components, this invention is advantageous in reducing the amount of permanent magnet materials and soft magnetic materials that do not participate in the formation of an effective magnetic field, and in adapting the probe structure to limited downhole space. This probe can be used for NMR logging in small-diameter wells, long horizontal wells, ultra-deep wells, and complex well conditions, and can also be used for reservoir evaluation in shale oil, tight sandstone, and carbonate rocks.

[0076] The present invention also provides a nuclear magnetic resonance (NMR) logging tool. The NMR logging tool includes a pressure-resistant housing, an electronic sub, a radio frequency (RF) transmitter / receiver circuit, and the aforementioned NMR logging probe. The NMR logging probe is disposed within the pressure-resistant housing. The RF transmitter / receiver circuit is electrically connected to the RF coil component 1. The RF transmitter / receiver circuit is used to apply RF pulses with a frequency matching the hydrogen nucleus Larmor frequency corresponding to the static magnetic field within the target sensitive area to the RF coil component 1, and to receive the NMR echo signal.

[0077] The radio frequency (RF) transmitter and receiver circuit may include a pulse generation module, a power amplification module, a matching and tuning module, a low-noise receiver module, an acquisition module, and a processing module. The pulse generation module generates RF pulses. The power amplification module amplifies the RF pulses. The matching and tuning module matches the RF coil component 1 to the operating frequency. The low-noise receiver module receives NMR echo signals. The acquisition module acquires NMR echo signals. The processing module obtains the formation NMR response information based on the NMR echo signals. The RF pulses can be 90° pulses, 180° pulses, CPMG pulse sequences, or multi-frequency pulse sequences. The frequency of the RF pulses can be determined based on the hydrogen nucleus Larmor frequency corresponding to B0 within the target sensitive area. The operating frequency can cover 500kHz to 1500kHz.

[0078] Nuclear magnetic resonance (NMR) logging tools can be wireline logging tools, logging-while-drilling tools, through-the-pipe logging tools, or storage logging tools. The NMR logging probe can be housed within the probe section of the logging tool. The NMR logging tool may also include a centering mechanism, a wall-adhering mechanism, or an azimuth positioning mechanism to ensure the target sensitive area of ​​the NMR logging probe faces the formation outside the wellbore. After the NMR logging tool is lowered into the wellbore, the permanent magnet component 4 and the soft magnetic component 2 form a B0 facing the target sensitive area. The radio frequency (RF) transmitter / receiver circuit drives the RF coil component 1 to generate a B1. The hydrogen-containing fluid within the target sensitive area generates a nuclear magnetic response, which the RF coil component 1 receives. The electronic section acquires and processes the nuclear magnetic echo signal.

[0079] In different embodiments of the present invention, the relative positions of the permanent magnet component 4, the soft magnetic component 2, and the radio frequency coil component 1 can be adjusted according to the wellbore size and the location of the target sensitive area. The permanent magnet component 4 can be a concave fan-shaped, sector-shaped, arc-shaped, irregularly shaped block, or segmented spliced ​​structure. The soft magnetic component 2 can be an umbrella-shaped, arc-shaped, sheet-shaped, block-shaped, split, or continuous structure. The radio frequency coil component 1 can be a double-layer overlapping planar strip coil, or a single-layer planar strip coil, a multi-layer planar strip coil, a flexible circuit coil, a copper foil coil, or a wound coil. All of the above substitutions can be determined jointly by the first material distribution variable ρ1, the second material distribution variable ρ2, the nuclear magnetic resonance response of the target sensitive area, the static magnetic field strength constraint, and the static magnetic field gradient constraint.

[0080] In summary, this invention sets material distribution variables for the permanent magnet region and the soft magnetic region respectively, allowing the residual magnetic flux density distribution in the permanent magnet region and the permeability distribution in the soft magnetic region to be updated in the same iterative process. Furthermore, it obtains both permanent magnet and soft magnetic structures under the joint constraints of the nuclear magnetic resonance response, static magnetic field strength, and static magnetic field gradient in the target sensitive area. The resulting nuclear magnetic resonance logging probe can form a nuclear magnetic response region facing the formation outside the wellbore within a limited downhole outer diameter, and ensures that the static magnetic field strength and static magnetic field gradient within the target sensitive area are mutually matched.

[0081] It should be noted that in this specification, relational terms such as first and second are used only to distinguish one entity from several other entities, and do not necessarily require or imply any such actual relationship or order between these entities.

[0082] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the present invention.

Claims

1. A method for constructing a nuclear magnetic resonance logging probe, characterized in that, include: An electromagnetic field model is established that includes a permanent magnet region, a soft magnetic region, a radio frequency coil region, and a target sensitive region; a first material distribution variable is set for the permanent magnet region to characterize the residual magnetic flux density distribution, and a second material distribution variable is set for the soft magnetic region to characterize the permeability distribution; Based on the residual magnetic flux density distribution, the permeability distribution, and the radio frequency coil region, the static magnetic field strength, static magnetic field gradient, effective component of radio frequency magnetic field, and NMR response determined by the static magnetic field strength and the effective component of radio frequency magnetic field within the target sensitive area are obtained in the electromagnetic field model. The optimization objective is to maximize the nuclear magnetic response within the target sensitive region, and the static magnetic field strength and static magnetic field gradient within the target sensitive region are used as constraints. The first material distribution variable and the second material distribution variable are iteratively updated, and the permanent magnet solid region and the soft magnetic solid region are determined. A permanent magnet component is formed from the permanent magnet solid region, a soft magnetic component is formed from the soft magnetic solid region, and the nuclear magnetic resonance logging probe is constructed from the permanent magnet component and the soft magnetic component.

2. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, The values ​​of the first material distribution variable and the second material distribution variable are both between 0 and 1; The residual magnetic flux density distribution is obtained by multiplying the first material distribution variable by the residual magnetic flux density of the permanent magnet material using a penalty function; The permeability distribution is obtained by interpolating the second material distribution variable between the permeability of non-soft magnetic materials and soft magnetic materials using a penalty function; The penalty factor of the penalty function is between 1 and 5.

3. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, The effective component of the radio frequency magnetic field is either the component of the radio frequency magnetic field generated in the radio frequency coil region that is perpendicular to the direction of the static magnetic field, or a rotational component that matches the direction of hydrogen nucleus precession. The nuclear magnetic resonance response includes the integral or discrete summation of the product of the square of the static magnetic field strength and the effective component of the radio frequency magnetic field within the target sensitive area.

4. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, The constraints include preset field strength conditions and preset gradient conditions; The preset field strength conditions include a hydrogen nucleus Larmor frequency of 500 kHz to 1500 kHz corresponding to the static magnetic field in the target sensitive area; The preset gradient condition includes at least one target location where the static magnetic field gradient is no greater than 30 Gs / cm, and the target location includes a position 4.5 cm radially outward from the outer surface of the probe.

5. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, It also includes determining the arrangement of the radio frequency coil components based on the effective components of the radio frequency magnetic field, the eddy current effects of the permanent magnet components or conductive support components, and the flipping parameters of the target sensitive area; The radio frequency coil component includes two planar strip coils arranged opposite each other, with an included internal angle between the two planar strip coils ranging from 0° to 60°. The flipping parameters include 90° pulse time or 180° pulse time.

6. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, Iterative updates to the first material distribution variable and the second material distribution variable include updating using the interior point method; The iteration stops when the change in material distribution variable between two consecutive iterations is less than 0.001, or when the number of iterations reaches 100. After stopping the iteration, at least one of density filtering, projection processing, thresholding processing, or contour smoothing processing is performed on the first material distribution variable and the second material distribution variable to determine the permanent magnet solid region and the soft magnetic solid region.

7. The method for constructing a nuclear magnetic resonance logging probe according to claim 1, characterized in that, The target sensitive area includes at least one of a point area, a line area, a surface area, a volume area, or a sector area located radially outside the probe. The detection depth of the target sensitive area is 4.5 cm to 14 cm radially outward from the outer surface of the probe; The magnetization direction of the permanent magnet component forms an angle of 0° to 45° with respect to the radial reference direction of the probe.

8. A nuclear magnetic resonance logging probe, characterized in that, Includes permanent magnet components, soft magnetic components, radio frequency coil components, and frame components; The permanent magnet component includes a pair of permanent magnets arranged in a cross-section. The cross-section of the permanent magnet has an outer arc edge, an inner concave edge, and a side edge connecting the outer arc edge and the inner concave edge. The outer arc edge faces the outer periphery of the probe, and the inner concave edge faces the inside of the probe or the skeleton component. The soft magnetic component is disposed on the side of the permanent magnet component facing the target sensitive area. The soft magnetic component includes a pair of soft magnetic parts, each soft magnetic part including an extended portion facing the radio frequency coil component and a contracted portion facing the permanent magnet component. The radio frequency coil component is disposed outside or adjacent to the soft magnetic component, and the radio frequency coil component includes a double-layer overlapping planar strip coil; The skeleton component is used to fix the permanent magnet component, the soft magnetic component, and the radio frequency coil component; The permanent magnet component, the soft magnetic component, and the radio frequency coil component together form a nuclear magnetic response region facing the formation outside the wellbore.

9. The nuclear magnetic resonance logging probe according to claim 8, characterized in that, The solid outline of the permanent magnet component is formed by the solid region corresponding to the first material distribution variable within the permanent magnet region, and the solid outline of the soft magnetic component is formed by the solid region corresponding to the second material distribution variable within the soft magnetic region. The magnetization direction of the permanent magnet forms an angle of 0° to 45° with respect to the radial reference direction of the probe; The radio frequency coil component includes two planar strip coils arranged opposite each other, with an internal angle of 0° to 60° between the two planar strip coils; The permanent magnet component is made of one of samarium cobalt, neodymium iron boron or alnico, and the soft magnetic component is made of a soft magnetic material with a relative permeability of 50 to 300.

10. A nuclear magnetic resonance logging tool, characterized in that, Includes a pressure-resistant housing, an electronic sub, a radio frequency transmitting and receiving circuit, and the nuclear magnetic resonance logging probe as described in claim 8 or 9; The nuclear magnetic resonance logging probe is housed within the pressure-resistant housing; The radio frequency transmitting and receiving circuit is electrically connected to the radio frequency coil component. The radio frequency transmitting and receiving circuit is used to apply radio frequency pulses to the radio frequency coil component and receive nuclear magnetic resonance echo signals. The frequency of the radio frequency pulses matches the hydrogen nucleus Larmor frequency corresponding to the static magnetic field in the target sensitive area.

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