Method and system for evaluating microscopic damage of rock core based on low-field nuclear magnetic resonance
The detection and inversion of the core through low-field nuclear magnetic resonance technology solves the problem of difficulty in quantitative characterizing the core damage of drilling fluid in the prior art, and realizes accurate quantitative evaluation and rapid evaluation of core microscopic damage.
Patent Information
- Application Number
- CN202311039852.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2023-08-17
- Publication Date
- 2025-02-21
AI Technical Summary
The prior art is difficult to accurately characterize the damage of drilling fluid to cores, especially in terms of the expansion of microfissures in mud shale and the quantitative description of core microscopic damage.
Using a low-field nuclear magnetic resonance method, the immersed core was detected through the spin echo magnetic resonance pulse sequence, and the spin echo string attenuation signal was recorded to obtain the attenuation distribution of T2, and the porosity component and T2 inversion spectrum, which then determines the quantitative characterization of fluid content in different pore sizes and core microscopic damage.
Accurate quantitative characterization of the microscopic damage of drilling fluid to the core is achieved, and the damage of the liquid to the core can be quickly and batched, without additional experiments or treatments, and only through soaking.
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Figure CN119492767A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the field of exploration technology, and in particular relates to a method and system for evaluating core microscopic damage based on low-field nuclear magnetic resonance. Background Art
[0002] In the exploration and development of oil and gas resources, wellbore instability is a prominent and difficult problem. Wellbore instability is common in the drilling process. At present, about 90% of wellbore instability and collapse are believed to be related to the instability of shale. Therefore, in-depth understanding of the physical and chemical properties and microscopic mechanism research of shale is of great significance for understanding the wellbore instability mechanism, effectively implementing anti-collapse technology, and realizing oil and gas resource development.
[0003] Studies have shown that microcracks developed in shale are one of the important reasons for wellbore instability. Affected by geological tectonic action, the stratification and microcracks of shale are relatively developed. Under the action of stress, the loose cementation of mineral particles will cause slippage, further causing crack extension. Secondly, during the drilling process, shale will interact with various fluids, greatly changing its internal microscopic physical characteristics. Drilling fluid filtrate invades the formation through the stratification and microcracks of shale, and reacts with clay minerals to further accelerate the growth and expansion of cracks, which can easily reduce the strength of the rock and is not conducive to the stability of the wellbore. Therefore, it is particularly important to effectively evaluate the damage of drilling fluid filtrate to the invaded formation and quantitatively describe the microscopic damage of shale.
[0004] At present, the damage of drilling fluid to core is basically characterized by porosity and permeability. The traditional method of determining nuclear magnetic permeability is based on the T2 distribution spectrum. The nuclear magnetic permeability is calculated by selecting and calculating the T2 cutoff value and the relative volume of the bound fluid through empirical formulas such as the Coates model, SDR model, and Coates extended model. However, the calculation results of these empirical formulas often deviate from the actual data. The reason is that the first selected T2 cutoff value has an error, which leads to inaccuracy of movable fluid and bound fluid; the other is that the T2 distribution spectrum itself has a fitting error. The steady-state method for measuring porosity and permeability has requirements for the size of the core. It is difficult to measure some cores of unconventional sizes, and it is impossible to accurately and quantitatively characterize the damage of drilling fluid to the core. At the same time, there is a lack of intuitive quantitative description of the microscopic damage of microcracks in the core, and there is little analytical data on the damage of different liquids to the core in the existing technology. Summary of the invention
[0005] In view of the above problems, on the one hand, the present invention proposes a method for evaluating core microscopic damage based on low-field nuclear magnetic resonance, comprising the following steps:
[0006] The cores after immersion were tested by nuclear magnetic resonance using a spin echo magnetic resonance pulse sequence, and the spin echo train attenuation signal was recorded to obtain the attenuation distribution of T2.
[0007] Performing fitting inversion on the spin echo train to obtain a porosity component of T2 attenuation distribution and a T2 inversion spectrum during the inversion process;
[0008] Determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, wherein the fluid content is expressed as a value of the total cumulative porosity component occupied;
[0009] The immersion time of the core was adjusted to obtain the T2 inversion spectrum under different immersion times, and a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time was established;
[0010] The core microscopic damage is quantitatively characterized according to the slope of the linear relationship.
[0011] Furthermore, the method further includes the following steps before performing nuclear magnetic resonance testing on the soaked core:
[0012] Determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core;
[0013] The dried cores were placed in three liquids: clean water, oil-based solution and water-based solution for immersion experiments.
[0014] Furthermore, the attenuation distribution of T2 can be expressed by the following formula:
[0015]
[0016] Where M(t) is the magnetization vector at time t, M i (0) is the initial value of the magnetization vector of the i-th relaxation component, T 2i is the decay time of the i-th transverse relaxation component.
[0017] Further, obtaining the porosity component of the attenuation distribution of each T2 during the inversion process comprises the following steps:
[0018] Assume that each T2 decay distribution contains n independent relaxation times T 2i And pre-set T 2i The value of
[0019] An expression for the porosity component of the decay distribution of T2 during the inversion process is established based on the relaxation time.
[0020] Furthermore, the expression of the porosity component of the attenuation distribution of T2 during the inversion process is as follows:
[0021]
[0022] In the formula, t(k)=iTE, i=1,…,k, which indicates the time when the i-th echo is collected, where TE indicates the echo time, echo(k) indicates the echo string of the i-th echo, and noise indicates the noise; n indicates the number of components, ranging from 2 to 100; the number of echoes k ranges from 10 to 20000; are relaxation time T 21 、T 22 …T 2n The corresponding porosity component.
[0023] Furthermore, the inversion curve is obtained according to the following method:
[0024] An algorithm is used to iterate the expression for each porosity component and determine the optimal solution;
[0025] Set the number of iterations to control the inversion accuracy;
[0026] The optimal solution algorithm is used to iterate to obtain a final inversion curve.
[0027] Furthermore, the expression of the fluid content is as follows:
[0028]
[0029] Where C is the cumulative porosity component; f(T2) is the final inversion curve, T2 is the transverse relaxation time, and ΔT2 is the infinitesimal quantity in the integral.
[0030] Furthermore, the establishment of a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time comprises the following steps:
[0031] Establish the relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time;
[0032] The relationship is processed in logarithmic coordinates to obtain a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time.
[0033] Furthermore, the linear relationship is as follows:
[0034] lg(C)=blg(h)+c
[0035] Among them, h is the immersion time, C is the cumulative porosity component, b is the slope, the absolute value of b is used to judge the degree of damage, and c is the intercept.
[0036] In a second aspect, the present invention proposes a system for evaluating core microscopic damage based on low-field nuclear magnetic resonance, comprising:
[0037] A nuclear magnetic resonance detection unit is used to perform nuclear magnetic resonance detection on the soaked core using a spin echo magnetic resonance pulse sequence, and record the spin echo train attenuation signal to obtain the T2 attenuation distribution;
[0038] An inversion unit, used for performing fitting inversion on the spin echo train to obtain a porosity component of the T2 attenuation distribution and a T2 inversion spectrum during the inversion process;
[0039] A cumulative porosity component determination unit, used to determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, wherein the fluid content is represented by the value of the total cumulative porosity component occupied;
[0040] Establishing a unit for adjusting the immersion time of the core to obtain the T2 inversion spectrum under different immersion times, and establishing a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time;
[0041] The characterization unit is used to quantitatively characterize the microscopic damage of the core according to the slope in the linear relationship.
[0042] Furthermore, the system further comprises a pre-processing unit, which is used to perform the following steps before performing nuclear magnetic resonance testing on the soaked core:
[0043] Determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core;
[0044] The dried cores were placed in three liquids: clean water, oil-based solution and water-based solution for immersion experiments.
[0045] In a third aspect, the present invention provides an electronic device, comprising a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other via the communication bus;
[0046] a memory storing a computer program;
[0047] The processor is used to implement the method for evaluating core microscopic damage based on low-field nuclear magnetic resonance when executing the program stored in the memory.
[0048] In a fourth aspect, the present invention proposes a computer-readable storage medium storing a computer program, which, when executed, executes the method for evaluating core microscopic damage based on low-field nuclear magnetic resonance.
[0049] Beneficial effects of the present invention:
[0050] The present invention uses a CPMG sequence to perform nuclear magnetic resonance detection on the soaked core, records the spin echo train attenuation signal to obtain the attenuation distribution of T2; then performs fitting inversion to obtain the porosity component and inversion curve of the attenuation distribution of T2 during the inversion process; determines the fluid content in different pore sizes according to the position of the T2 inversion spectrum peak; then establishes a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the soaking time; and the slope in the linear relationship can be used to quantitatively characterize the microscopic damage of the core. The above method can solve the problem that the existing technology cannot accurately and quantitatively characterize the damage of the drilling fluid to the core, and the present invention examines the situation of the core in different solutions, which can be used to evaluate the degree of damage of the drilling fluid to the core, and can quickly and batch evaluate the damage of the liquid to the core, without the need for additional experiments or other treatments of the core, and only needs to be soaked to quantitatively evaluate the damage of the liquid to the core, and at the same time, the experimental operation steps are simple.
[0051] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS
[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.
[0053] Figure 1 A flow chart of a method for evaluating core microscopic damage based on low-field nuclear magnetic resonance proposed by the present invention is shown;
[0054] Figure 2 The T2 attenuation spectra of shale in the water-based solution according to the embodiment of the present invention are shown when the shale is immersed in the water-based solution for 0.5h, 30h and 120h respectively;
[0055] Figure 3 The T2 attenuation spectra of shale in the embodiment of the present invention when the oil-based solution is immersed for 0.5h, 30h and 120h respectively are shown;
[0056] Figure 4 The T2 attenuation spectra of shale in the embodiment of the present invention when immersed in water for 0.5h, 30h and 120h respectively are shown;
[0057] Figure 5T2 inversion spectra of shale when immersed in water-based solution for 0.5h, 30h and 120h respectively in the embodiment of the present invention;
[0058] Figure 6 The T2 inversion spectra of the shale in the embodiment of the present invention when the oil-based solution is immersed for 0.5h, 30h and 120h respectively are shown;
[0059] Figure 7 The T2 inversion spectra of the shale in the embodiment of the present invention when immersed in water for 0.5h, 30h and 120h respectively are shown;
[0060] Figure 8 A graph showing the relationship between the immersion time and the cumulative pore component percentage in an embodiment of the present invention is shown;
[0061] Fig. 9 A schematic diagram of the architecture of a system for evaluating core microscopic damage based on low-field nuclear magnetic resonance proposed in an embodiment of the present invention is shown;
[0062] Fig.10 A diagram of an electronic device for evaluating core microscopic damage based on low-field nuclear magnetic resonance proposed in an embodiment of the present invention is shown. DETAILED DESCRIPTION
[0063] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.
[0064] The purpose of the present invention is to provide a method for evaluating core microscopic damage based on low-field nuclear magnetic resonance. Figure 1 As shown, the method comprises the following steps:
[0065] S1: Determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core;
[0066] S2: The core was placed in different liquids for immersion experiments. After immersion for different times, the core was taken out and dried for nuclear magnetic resonance detection. The spin echo train attenuation signal was recorded using a CPMG (spin echo magnetic pulse) sequence to obtain the attenuation distribution of T2, which can be expressed by the following formula:
[0067]
[0068] Where M(t) is the magnetization vector at time t, M i(0) is the initial value of the magnetization vector of the i-th relaxation component, T 2i is the decay time of the i-th transverse relaxation component. It should be noted that M i (0) is proportional to the volume of the pore fluid. A system composed of pores of different sizes corresponds to a T2 distribution, and the signal amplitude is the sum of the signal amplitudes generated in a single pore fluid.
[0069] The immersion liquid includes clean water, oil-based solution and water-based solution, among which the oil-based solution is a liquid containing colored flammable oily liquid minerals, that is, a liquid whose continuous phase is oil; the water-based solution refers to a solution with water as the solvent, which is mainly composed of solute and solvent, and the solvent is water, that is, a liquid whose continuous phase is water; the clean water is distilled water.
[0070] The NMR detection collects spin echo trains. According to the NMR logging theory, the relaxation response T of different pore components 2i The superposition constitutes the observed spin echo train, and the T2 distribution spectrum is obtained by fitting the spin echo train by selecting an appropriate mathematical fitting method.
[0071] The CPMG sequence is a multi-pulse sequence consisting of a series of 180° and 90° pulses. In this sequence, each 90° pulse is followed by a series of equally spaced 180° pulses, which cause the spins to accumulate phases at different time points, thereby reducing the influence of the spin-spin coupling effect. This can effectively reduce the spin-spin coupling effect, improve the signal-to-noise ratio, and thus obtain a more accurate signal.
[0072] The present invention does not limit the soaking time and can be set according to specific circumstances.
[0073] S3: Invert the acquired spin echo train attenuation signal through a step-by-step iterative algorithm to obtain the distribution of the T2 inversion spectrum;
[0074] The distribution of T2 spectrum is a continuous function. By simplifying the T2 spectrum into a multi-exponential model, it is assumed that the T2 distribution contains n independent relaxation times T 2i , and the corresponding porosity component is Preset T 2i The values are 0.5ms, 1ms, 2ms, 4ms, etc., and the porosity components of each distribution in the inversion process are obtained as follows:
[0075]
[0076] In the above formula, t(1) = iTE, i = 1, ..., k is the time when the i-th echo is collected, where TE represents the echo time, echo represents the echo string, and the right side of the equation is the inversion result. A multi-exponential model is used to represent the equation system for each echo, and noise is the noise. The system of equations consisting of the above k linear equations and n unknowns, k ≥ n, n represents the number of components, ranging from 2 to 100, and the number of echoes k ranges from 10 to 20000. The algorithm for finding the optimal solution is gradually iterated, and the number of iterations is set to control the inversion accuracy. The final inversion curve is f(T2), that is, the T2 inversion spectrum is f(T2).
[0077] S4: Determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, where the fluid content is represented by the value of the total cumulative porosity component, for example, it can be the percentage of the total cumulative porosity component. Since the T2 spectrum is proportional to the pore size, and the porosity component represents the signal amplitude generated in the pore fluid, the fluid content in different pore sizes can be obtained by the position of the T2 inversion spectrum peak.
[0078]
[0079] Where C is the cumulative porosity component; f(T2) is the final inversion curve, T2 is the transverse relaxation time, and ΔT2 is the infinitesimal quantity in the integral.
[0080] S5: Adjust the immersion time of the core to obtain the T2 inversion spectrum under different immersion times, and establish a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time; reflect the change of pore size according to the fluid content represented by different spectral peaks.
[0081] S6: The core microscopic damage is quantitatively characterized according to the slope in the linear relationship, and the linear relationship is as follows:
[0082] lg(C)=blg(h)+c
[0083] Among them, h is the immersion time, C is the cumulative porosity component, b is the slope, and the absolute value of b is used to quantitatively characterize the degree of core damage. The larger the absolute value of b, the greater the degree of damage. c is the intercept, which is used to quantitatively characterize the increase of the cumulative porosity component with time, so as to evaluate the degree of damage of the drilling fluid to the core.
[0084] In order to explain the above method more clearly, the above method is described in detail below in conjunction with embodiments.
[0085] The specific operations are as follows:
[0086] S1: Determine the basic parameters of shale, dry the core until the weight does not change, and measure the porosity and permeability of the core.
[0087] S2: The cores were immersed in three kinds of liquids: clean water, oil-based solution, and water-based solution. The cores were taken out after immersion for 0.5 h, 30 h, and 120 h, respectively. After being wiped dry, they were tested by nuclear magnetic resonance. The spin echo train attenuation signal was recorded using the CPMG sequence to obtain the attenuation distribution of T2. Figure 2-4 As shown, Figure 2 , Figure 3 and Figure 4 These are the T2 attenuation spectra of shale when immersed in water-based solution, oil-based solution and clean water for 0.5h, 30h and 120h respectively.
[0088] S3: Invert the spin echo train decay signal collected in step S2 through a stepwise iterative algorithm to obtain the distribution of the T2 inversion spectrum, as shown in the following example: Figure 5-7 As shown, Figure 5 , Figure 6 and Figure 7 The T2 inversion spectra of shale after immersion in water-based solution, oil-based solution and clean water for 0.5h, 30h and 120h respectively. Figure 2-4 It can be seen that in the initial state of immersion, the T2 inversion spectrum is dominated by one peak, indicating that the micropores in the shale occupy a major position. However, with the increase of immersion time, the peak shape changes significantly, and the T2 inversion spectrum tends to move to the right. At the same time, one main peak gradually evolves into two main peaks, and new peaks appear in the long relaxation time area. With the immersion of liquid, large pore areas appear, which indicates that cracks have occurred. The hydration of the liquid on the core causes cracks in the core, and as time goes by, the cracks continue to expand.
[0089] S4: Based on the fact that T2 is proportional to the pore size and the porosity component represents the signal amplitude generated in the pore fluid, the percentage of the changing spectral peak is calculated, and the number of pores with corresponding pore sizes is related to the spectral peak percentage.
[0090] S5: Establish the relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time. In the logarithmic coordinate, a linear correlation is obtained. The slope is used to quantitatively characterize the increase of the cumulative porosity component over time, so as to evaluate the degree of damage to the core caused by the drilling fluid. The obtained linear relationship is as follows: Figure 8 As shown in the figure, the linear relationship between the cumulative porosity component and the immersion time obtained by immersing the core in clean water, oil-based solution and water-based solution is:
[0091] In oil-based solution: lg(C)=0.126lg(h)+1.937
[0092] In water-based solution: lg(C)=0.171lg(h)+1.916
[0093] In clean water: lg(C)=0.175lg(h)+1.926
[0094] It can be seen that in the oil-based solution, the percentage of the main peak cumulative pore component decreases the most slowly over time, with an absolute slope of 0.126, while in the water-based solution and clean water, the percentage of the main peak cumulative pore component decreases to a similar degree over time, with absolute slopes of 0.175 and 0.171, respectively. This indicates that the oil-based solution has a smaller degree of microscopic damage to the shale, and can effectively slow down the trend of microscopic damage compared to clean water; while the water-based solution has a similar degree of microscopic damage to the shale as clean water, which is related to the specific components and content in the liquid. This method can more intuitively and effectively optimize drilling fluids with different degrees of core damage, and there are no strict requirements for the core size, so the damage to the core can be intuitively and quickly quantified. This invention can provide a theoretical basis for evaluating the microscopic damage of the core and evaluating the optimization of drilling fluids.
[0095] Based on the same concept of the present invention, another exemplary embodiment of the present invention provides a system for evaluating core microscopic damage based on low-field nuclear magnetic resonance, such as Fig. 9 As shown, including:
[0096] The pre-processing unit 901 is used to determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core; and place the dried core in three liquids, namely, clean water, oil-based solution and water-based solution, for immersion experiments;
[0097] The nuclear magnetic resonance detection unit 902 is used to perform nuclear magnetic resonance detection on the soaked core using a CPMG sequence, and record the spin echo train attenuation signal to obtain the attenuation distribution of T2;
[0098] An inversion unit 903 is used to perform fitting inversion on the spin echo train to obtain a porosity component of the T2 attenuation distribution and a T2 inversion spectrum during the inversion process;
[0099] The cumulative porosity component determination unit 904 is used to determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, and the fluid content is represented by the value of the total cumulative porosity component occupied;
[0100] Establishing unit 905, for adjusting the immersion time of the core to obtain T2 inversion spectra under different immersion times, and establishing a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time;
[0101] The characterization unit 906 is used to quantitatively characterize the core microscopic damage according to the slope in the linear relationship.
[0102] Based on the same inventive concept, another exemplary embodiment of the present invention provides an electronic device for evaluating core microscopic damage based on low-field nuclear magnetic resonance. Fig.10 As shown, the electronic device includes at least one processor 1001, at least one communication interface 1002, at least one memory 1003 and at least one communication bus 1004; wherein the processor 1001, the communication interface 1002 and the memory 1003 communicate with each other through the communication bus 1004;
[0103] Memory 1003, storing computer programs;
[0104] The processor 1001 is used to implement the method for evaluating core microscopic damage based on low-field nuclear magnetic resonance when executing the program stored in the memory 1003.
[0105] Optionally, the communication interface may be an interface of a communication module, such as an interface of a GSM module; the processor may be a processor CPU, or an application specific integrated circuit ASIC (Application Specific Integrated Circuit), or one or more integrated circuits configured to implement the embodiments of the present invention. The memory may include a high-speed RAM memory, and may also include a non-volatile memory (non-volatile memory), such as at least one disk memory. The memory stores a program, and the processor calls the program stored in the memory to execute some or all of the above method embodiments.
[0106] Based on the same inventive concept, the embodiment of the present application further provides a computer-readable storage medium storing a computer program, which implements part or all of the above method embodiments when the computer program is executed. Optionally, the storage medium may be a non-temporary computer-readable storage medium, for example, the non-temporary computer-readable storage medium may be a ROM, a random access memory (RAM), a CD-ROM, a magnetic tape, a floppy disk, an optical data storage device, etc.
[0107] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims
1. A method for evaluating core microscopic damage based on low-field nuclear magnetic resonance, characterized in that: The following steps are involved: The cores after immersion were tested by nuclear magnetic resonance using a spin echo magnetic resonance pulse sequence, and the spin echo train attenuation signal was recorded to obtain the attenuation distribution of T2. Performing fitting inversion on the spin echo train to obtain a porosity component of T2 attenuation distribution and a T2 inversion spectrum during the inversion process; Determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, wherein the fluid content is expressed as a value of the total cumulative porosity component occupied; The immersion time of the core was adjusted to obtain the T2 inversion spectrum under different immersion times, and a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time was established; The core microscopic damage is quantitatively characterized according to the slope of the linear relationship.
2. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The steps of performing nuclear magnetic resonance testing on the soaked cores include: Determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core; The dried cores were placed in three liquids: clean water, oil-based solution and water-based solution for immersion experiments.
3. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The attenuation distribution of T2 can be expressed by the following formula: Where M(t) is the magnetization vector at time t, M i (0) is the initial value of the magnetization vector of the i-th relaxation component, T 2i is the decay time of the i-th transverse relaxation component.
4. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The porosity component of the attenuation distribution of each T2 in the inversion process is obtained by the following steps: Assume that each T2 decay distribution contains n independent relaxation times T 2i And pre-set T 2i The value of An expression for the porosity component of the decay distribution of T2 during the inversion process is established based on the relaxation time.
5. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 4, characterized in that: The expression of the porosity component of the attenuation distribution of T2 in the inversion process is as follows: In the formula, t(k)=iTE, i=1,…,k, which indicates the time when the i-th echo is collected, where TE indicates the echo time, echo(k) indicates the echo string of the i-th echo, and noise indicates the noise; n indicates the number of components, ranging from 2 to 100; the number of echoes k ranges from 10 to 20000; are relaxation time T 21 、T 22 …T 2n The corresponding porosity component.
6. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The inversion curve is obtained according to the following method: An algorithm is used to iterate the expression of each porosity component and determine the optimal solution; Set the number of iterations to control the inversion accuracy; The optimal solution algorithm is used to iterate to obtain a final inversion curve.
7. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The expression of the fluid content is as follows: Where C is the cumulative porosity component; f(T2) is the final inversion curve, T2 is the transverse relaxation time, and ΔT2 is the infinitesimal quantity in the integral.
8. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 1, characterized in that: The step of establishing a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time comprises the following steps: Establish the relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time; The relationship is processed in logarithmic coordinates to obtain a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time.
9. The method for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 8, characterized in that: The linear relationship is as follows: lg(C)=blg(h)+c Among them, h is the immersion time, C is the cumulative porosity component, b is the slope, and the absolute value of b is used to quantitatively characterize the degree of core damage, and c is the intercept.
10. A system for evaluating core microscopic damage based on low-field nuclear magnetic resonance, characterized in that: include: A nuclear magnetic resonance detection unit is used to perform nuclear magnetic resonance detection on the soaked core using a spin echo magnetic resonance pulse sequence, and record the spin echo train attenuation signal to obtain the T2 attenuation distribution; An inversion unit, used for performing fitting inversion on the spin echo train to obtain a porosity component of the T2 attenuation distribution and a T2 inversion spectrum during the inversion process; A cumulative porosity component determination unit, used to determine the fluid content in different pore sizes according to the position of the peak in the T2 inversion spectrum, wherein the fluid content is represented by the value of the total cumulative porosity component occupied; Establishing a unit for adjusting the immersion time of the core to obtain the T2 inversion spectrum under different immersion times, and establishing a linear relationship between the cumulative porosity component of the T2 inversion spectrum change peak and the immersion time; The characterization unit is used to quantitatively characterize the microscopic damage of the core according to the slope in the linear relationship.
11. The system for evaluating core microscopic damage based on low-field nuclear magnetic resonance according to claim 10, characterized in that: The system further comprises a pre-processing unit for performing the following steps before performing nuclear magnetic resonance testing on the soaked core: Determine the basic parameters of the core, dry the core until the weight does not change, and measure the porosity and permeability of the core; The dried cores were placed in three liquids: clean water, oil-based solution and water-based solution for immersion experiments.
12. An electronic device, characterized in that: It includes a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other via the communication bus; a memory storing a computer program; The processor is used to implement the method for evaluating core microscopic damage based on low-field nuclear magnetic resonance as described in any one of claims 1 to 9 when executing the program stored in the memory.
13. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed, the method for evaluating core microscopic damage based on low-field nuclear magnetic resonance as described in any one of claims 1 to 9 is executed.