Method for determining low-permeability difficult-to-produce reserves permeability limit of offshore oilfield

Abstract

The present application relates to offshore oilfield low-permeability difficult-to-produce reserves permeability limit determination method, including: according to the reserves database of offshore oilfield, development atlas system and productivity database statistics low-permeability oilfield reserves producing situation, statistics cumulative production accounts for the proportion of proven geological reserves; according to the cumulative production accounts for the proportion of proven reserves in each section interval, draw statistical histogram and trend line, find the mutation part in trend line; the core of offshore oilfield typical block is carried out water drive oil nuclear magnetic resonance experiment, and the critical water drive producing pore radius of offshore oilfield is obtained, and the permeability corresponding to the pore radius is taken as the lower limit of the limit; select productivity as a measure of dynamic parameter, draw the relationship curve of flow coefficient and productivity, segment fitting is carried out to the relationship curve of flow coefficient and productivity, and the permeability corresponding to the intersection point of fitting line is taken as the upper limit of the limit; on the basis of the above, the low-permeability difficult-to-produce reserves permeability limit of offshore oilfield is determined in the permeability section range corresponding to the mutation part in trend line, and between the lower limit value and the upper limit value.

Description

Technical Field

[0001] This invention relates to the field of dynamic assessment of oilfield development reserves, and in particular to a method for determining the permeability limit of low-permeability, difficult-to-develop reserves in offshore oilfields. Background Technology

[0002] The development of offshore oil and gas resources is constrained by factors such as geological reservoir conditions, marine environment, and engineering facilities, resulting in high extraction costs. This leads to significant difficulties in developing low-quality oil and gas resources (such as low-permeability, low-abundance, and heavy oil), which are currently the main types of untapped geological reserves. Based on reserve quality and the unique development environment and requirements of offshore oilfields, untapped offshore oilfield reserves are defined as: geological reserves that, under current technological and economic conditions, are difficult to extract economically and effectively due to factors such as reserve size, oil quality, reservoir permeability, reserve abundance, production capacity, and technological conditions. Untapped reserves are a relative concept; their definition and outcome will vary under different oil prices, extraction models, management systems, and operational mechanisms.

[0003] Difficult-to-recover reserves mainly include four types: heavy oil difficult-to-recover, low-permeability difficult-to-recover, deepwater difficult-to-recover, and tailings difficult-to-recover. Among them, low-permeability difficult-to-recover reserves are widely distributed and account for a large proportion, making them the most important type of difficult-to-recover reserves in offshore oil and gas fields. The economically effective recovery of low-permeability difficult-to-recover reserves depends not only on improvements in oil and gas extraction technology but also on crucial pre-development assessments. Reasonably determining the lower limit standard is the minimum condition for quickly determining whether an oil (gas) field (reservoir) can be included in a development plan. The main problem with low-permeability difficult-to-recover reserves is the ambiguity of permeability boundaries in reserve classification and the scarcity of relevant assessment data. Generally, only multidimensional seismic data and logging, testing, and production data from some exploration and appraisal wells, as well as limited analytical and chemical data, can be relied upon. Internationally, permeability boundaries for reserve classification are typically assessed by analogy with similar oil fields. In the past, when conducting assessments of difficult-to-recover reserves in domestic oil fields, the classification of each block was usually based on petroleum reserve specifications, using graphical synthesis or analogy methods to arrive at conclusions. Such conclusions are ambiguous, heavily influenced by human factors, and difficult to compare the merits of different blocks, resulting in low reliability and a high risk of erroneous decisions. To overcome these shortcomings and make the evaluation more objective, intuitive, accurate, and scientific, it is necessary to reduce human interference and conduct a scientific evaluation of the permeability limits of untapped reserves from the perspective of static geological characteristics and dynamic data analysis. Summary of the Invention

[0004] To address the aforementioned problems, the purpose of this invention is to provide a method for determining the permeability limit of low-permeability, difficult-to-develop reserves in offshore oilfields. This method fully considers the statistical regularities of oilfield geological data and development dynamic data. It defines the permeability limit of low-permeability, difficult-to-develop reserves in offshore oilfields by analyzing the gradual change in the proportion of cumulative production to proven reserves across different permeability ranges, the permeability range corresponding to the median pore radius used for water-driven production, and the permeability corresponding to the inflection point of the multi-segment linear regression between the flow coefficient and production capacity. The core steps are: using 50mD as the upper limit of low permeability, statistically analyzing the reserve development of low-permeability reservoirs in offshore oilfields, and statistically analyzing the proportion of cumulative production to proven reserves corresponding to different permeability ranges, plotting a histogram, and considering that the permeability limit of difficult-to-develop reserves is contained within the permeability range corresponding to the abrupt change in the trend line. Subsequently, the permeability limit of difficult-to-develop reserves is accurately defined using the median pore radius method for water-driven production and the flow coefficient-production capacity fitting method. This method can effectively define the permeability limit of difficult-to-develop reserves in oilfields, providing technical support for the improvement of oil company reserve technology systems and oilfield development evaluation.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields, comprising:

[0007] Using 50mD as the upper limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields, the utilization of low-permeability oilfield reserves is statistically analyzed based on the offshore oilfield reserve database, development atlas system, and production capacity database, and the cumulative production is calculated as a percentage of proven geological reserves.

[0008] Using 5-10mD as the segmented interval for the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields, the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields within the range of 0-50mD is segmented. Based on the proportion of cumulative production to proven reserves in each segmented interval, statistical histograms and trend lines are drawn. The abrupt changes in the trend lines are identified, and the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields is narrowed down to the permeability segmented intervals corresponding to the abrupt changes in the trend lines.

[0009] Nuclear magnetic resonance experiments were conducted on core samples from typical blocks of offshore oilfields to obtain the critical water-driven pore radius of offshore oilfields. The permeability corresponding to the critical pore radius of porous media was then calculated, and the calculated permeability was used as the lower limit of the permeability limit for low-permeability and difficult-to-access reserves in offshore oilfields.

[0010] Select production capacity as a dynamic parameter, plot the relationship curve between flow coefficient and production capacity, perform piecewise fitting on the relationship curve between flow coefficient and production capacity, calculate the permeability corresponding to the critical pore radius of porous media based on the flow coefficient corresponding to the intersection point of the fitted segment, and use the calculated permeability as the upper limit of the permeability limit of low-permeability and difficult-to-access reserves in offshore oil fields.

[0011] The range of segments corresponding to the abrupt changes in the trend line is determined for the permeability limit of low-permeability and difficult-to-access reserves in offshore oil fields, and is between the lower limit and the upper limit.

[0012] The permeability corresponding to the critical pore radius of porous media can be calculated using the Kozeny-Carman equation, and the expression is as follows:

[0013]

[0014] Where K represents the permeability corresponding to the critical pore radius of the porous medium;

[0015] φ represents the porosity of the porous medium;

[0016] C represents the Kozeny-Carman constant;

[0017] τ represents the tortuosity of the porous medium;

[0018] S represents the specific surface area of ​​the solid phase.

[0019] The permeability corresponding to the critical pore radius of porous media can be calculated by back-calculating the statistical relationship between the median pore throat radius and permeability in oilfields.

[0020] The expression for the flow coefficient is:

[0021]

[0022] Where, N L Indicates the flow coefficient;

[0023] K represents the permeability corresponding to the critical pore radius of the porous medium;

[0024] h represents the effective thickness of the formation;

[0025] μ represents fluid viscosity.

[0026] The permeability range of 0-50mD was divided into eight segments: <1mD, 1-5mD, 5-10mD, 10-15mD, 15-20mD, 20-30mD, 30-40mD, and 40-50mD. The proven geological reserves and cumulative production corresponding to each segment were statistically analyzed, and the proportion of the cumulative production to the proven geological reserves for each segment was calculated.

[0027] The process of drawing statistical histograms and trend lines involves: using each segment interval as the horizontal axis and the ratio of cumulative production to proven geological reserves corresponding to each segment interval as the vertical axis, identifying the inflection point of a sharp change in the trend line. When the permeability is below this inflection point, the ratio of cumulative production to proven geological reserves changes little after increasing the permeability, indicating that the difficulty of reserve extraction does not change significantly. When the permeability gradually increases to above this inflection point, the ratio of cumulative production to proven geological reserves changes drastically, indicating that the difficulty of reserve extraction decreases at this point, and production increases significantly under the same conditions. It is determined that the permeability limit of low-permeability, difficult-to-develop reserves in offshore oil fields is contained within the segment interval corresponding to the inflection point of the trend line.

[0028] Water-drive NMR experiments on core samples from typical blocks of offshore oilfields include: selecting core samples from major oil-bearing blocks of offshore oilfields for water-drive NMR displacement experiments; determining the critical water-drive pore radius of the offshore oilfield using T2 spectra of saturated water, saturated oil, and water-drive oil; and calculating the permeability based on the statistical relationship between the median pore throat radius and permeability of the oilfield or the Kozeny-Carman equation. This calculated permeability is the technical limit permeability of the offshore oilfield and serves as the lower limit of the permeability boundary for low-permeability and difficult-to-access reserves in the offshore oilfield.

[0029] Plotting the relationship curve between flow coefficient and production capacity involves: statistically analyzing the flow coefficient and production capacity of each production well in the offshore oilfield; establishing a two-dimensional rectangular coordinate system with flow coefficient as the abscissa and production capacity as the ordinate; plotting the relationship curve between flow coefficient and production capacity; and performing piecewise linear fitting of the curve according to the rate of change using the least squares method. The intersection point of the fitted curves is then found. When the flow coefficient is less than this intersection point, the fluid flow difficulty within the reservoir is similar and relatively high, resulting in lower production capacity. Conversely, when the flow coefficient is greater than this intersection point, the fluid flow difficulty within the reservoir is similar and relatively low, resulting in higher production capacity.

[0030] The permeability is calculated by back-calculating the flow coefficient corresponding to the intersection point of the fitted curves, and this permeability serves as the upper limit of the permeability corresponding to the difficult-to-access reserves.

[0031] The permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields falls within the range of the permeability limit obtained by the cumulative production-proven reserves ratio method. The lower limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields is higher than the lower limit obtained by the median pore radius method driven by water, while the upper limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields is lower than the upper limit obtained by the flow coefficient-production capacity relationship fitting method.

[0032] The present invention has the following advantages due to the adoption of the above technical solutions:

[0033] 1. The method for determining the permeability limit of low-permeability and difficult-to-access reserves in offshore oil fields disclosed in this invention is the first to concretize the method for determining the permeability limit of difficult-to-access reserves in oil fields, avoiding the ambiguity, subjectivity and uncertainty of analogy in traditional methods, and greatly increasing the accuracy and scientific nature of the decision-making results.

[0034] 2. For the first time, a method for determining the permeability limit of untapped oilfield reserves was established, which considers the combination of static geological characteristics and dynamic data. The permeability limit of untapped oilfield reserves was determined by a three-pronged approach: the cumulative production-proven reserves ratio method, the critical pore radius method for water-driven production, and the flow coefficient-production capacity relationship fitting method.

[0035] 3. The method for determining the permeability limit of low-permeability, difficult-to-develop reserves in offshore oilfields disclosed in this invention combines theoretical methods with standardized diagrams to guide relevant personnel in making scientific and accurate judgments on the permeability limit of difficult-to-develop reserves in target oilfields. The method is simple and can be used immediately even by personnel lacking relevant knowledge, minimizing the impact of research skills and experience. It has high applicability and promotion value. Attached Figure Description

[0036] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings:

[0037] Figure 1 This is a technical roadmap for the permeability limit of low-permeability and difficult-to-access reserves in offshore oil fields in the implementation method of this invention;

[0038] Figure 2 This is a histogram of proven geological reserves and cumulative production corresponding to different permeability ranges in this invention example;

[0039] Figure 3 This is a histogram showing the proportion of cumulative production to proven geological reserves in different permeability ranges in this invention example;

[0040] Figure 4 This is the nuclear magnetic resonance T2 spectrum of the water-driven oil recovery experiment in this invention example;

[0041] Figure 5 This is a graph illustrating the statistical relationship between permeability and median pore throat radius in an example of the present invention.

[0042] Figure 6 This is a schematic diagram of the relationship between flow coefficient and production capacity in an example of the present invention. Detailed Implementation

[0043] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0044] The present invention provides a method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields, comprising the following steps:

[0045] Step 1: Using 50mD as the upper limit of low permeability, statistically analyze the utilization of low-permeability oilfield reserves based on the oilfield's reserve database, development atlas system, and production capacity database. The statistical content includes proven geological reserves and cumulative production.

[0046] Step 2: Divide the 0-50mD range into segments, using 5-10mD as the segment interval. Statistically calculate the proportion of cumulative production to proven reserves corresponding to different permeability ranges, and plot statistical histograms and trend lines. It is assumed that the permeability limit for difficult-to-recover reserves is included in the permeability range corresponding to the abrupt change in the trend line.

[0047] Step 3: Conduct nuclear magnetic resonance experiments on core samples from typical blocks of the target oilfield to obtain the critical water-driven pore radius of the target oilfield. Calculate the permeability based on the statistical relationship chart between the median pore throat radius and permeability of the oilfield or the Kozeny-Carman equation. This permeability can be considered as the technical limit permeability, and its value is the lower limit of permeability corresponding to the difficult-to-access reserves.

[0048] Step 4: Select production capacity as the dynamic parameter for measuring difficult-to-access reserves. Plot the relationship curve between flow coefficient and production capacity, perform piecewise fitting, and calculate the permeability based on the flow coefficient corresponding to the intersection of the fitted segments. This permeability is the upper limit of the permeability corresponding to the difficult-to-access reserves.

[0049] Step 5: By comprehensively comparing and analyzing the results obtained from the three methods in Steps 1-4, the permeability limit of the untapped reserves of the target oilfield is finally determined.

[0050] The expressions for the Kozeny-Carman equation and the flow coefficient in steps 3 and 4 are as follows:

[0051] Kozeny-Carman equations:

[0052]

[0053] Flow coefficient:

[0054]

[0055] In the formula, K represents the permeability of the porous medium (mD); φ represents the porosity of the porous medium; C represents the Kozeny-Carman constant; τ represents the tortuosity of the porous medium; and S represents the specific surface area of ​​the solid phase (m²). 2 / g;N L denoted by , mD*m / (mPa·s); h represents the effective formation thickness, m; μ represents the fluid viscosity, mPa·s.

[0056] This invention provides a method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields, comprising the following steps:

[0057] Guided by the technical roadmap for the permeability limits of low-permeability, difficult-to-access reserves in offshore oil fields, as shown in the attached document. Figure 1 As shown, the target oil field is first identified, with 50mD as the upper limit of low permeability. The utilization of low permeability oil field reserves is statistically analyzed based on the oil field's reserve database, development atlas system, and production capacity database. The statistical content includes proven geological reserves and cumulative production.

[0058] The permeability range of 0-50 mD was divided into eight segments: <1 mD, 1-5 mD, 5-10 mD, 10-15 mD, 15-20 mD, 20-30 mD, 30-40 mD, and 40-50 mD. The proven geological reserves and cumulative production corresponding to each segment were statistically analyzed, and the proportion of cumulative production to proven geological reserves for each segment was calculated.

[0059] The geological statistics include: the relationship between utilized developed reserves and permeability; calculation of cumulative utilized developed geological reserves; and identification of major nodes in permeability value changes. A histogram is constructed with each permeability interval as the horizontal axis and the proportion of cumulative production to proven geological reserves corresponding to each interval as the vertical axis. An overall trend line of the histogram is drawn, and the inflection point of the trend line is identified. The physical meaning of this point is that when the permeability is below this point, increasing the permeability results in a small change in the proportion of cumulative production to proven geological reserves, meaning the difficulty of reserve extraction does not change significantly. When the permeability gradually increases to above this point, the proportion of cumulative production to proven geological reserves changes drastically, indicating that the difficulty of reserve extraction decreases at this point, and production increases significantly under the same conditions. Therefore, it is considered that the permeability limit for difficult-to-utilize reserves is contained within the permeability range corresponding to the inflection point of the trend line.

[0060] The data statistics include a statistical relationship chart of median pore throat radius and permeability. This chart is used to define the permeability value corresponding to the critical water-drive pore throat radius obtained from nuclear magnetic resonance (NMR) experiments. Specifically, core samples from the main oil-bearing blocks of the target oilfield are selected for water-drive NMR displacement experiments. The critical water-drive pore radius of the oilfield is determined by the T2 spectra of saturated water, saturated oil, and the water-drive oil. The permeability is then calculated using the statistical relationship chart of median pore throat radius and permeability or the Kozeny-Carman equation. This permeability can be considered the technical limit permeability of the oilfield, and is generally considered to be the lower limit of permeability corresponding to difficult-to-access reserves.

[0061] The data statistics also include defining the flow coefficient, analyzing the relationship between the flow coefficient and production capacity, performing piecewise linear regression of the flow coefficient and production capacity, and using the intersection points of the piecewise fitted curves to calculate the permeability limit. Specifically, the flow coefficient and production capacity of each production well in the target oilfield are statistically analyzed. A two-dimensional rectangular coordinate system is established with the flow coefficient as the abscissa and the production capacity as the ordinate, and the relationship curve between the flow coefficient and production capacity is plotted. The curve is then piecewise linearly fitted according to the rate of change, usually with two segments as the segment target, and the least squares method is used for fitting. The intersection points of the fitted curves are found. The physical meaning of these intersection points is: when the flow coefficient is less than this point, the fluid flow difficulty in the reservoir is similar but the flow difficulty is relatively high, corresponding to a lower production capacity; when the flow coefficient is greater than this point, the fluid flow difficulty in the reservoir is similar but the flow difficulty is relatively low, corresponding to a higher production capacity.

[0062] The permeability is calculated by back-calculating the flow coefficient corresponding to the intersection point of the fitted segment. This permeability is the upper limit of the permeability corresponding to the difficult-to-access reserves.

[0063] By comprehensively comparing and analyzing the results obtained from the above three methods, the permeability limit of the untapped reserves of the target oilfield was finally determined. The determination was based on the following criteria: the final permeability limit was within the range of the results obtained by the cumulative production-proven reserves ratio method, and the lower limit was higher than the result determined by the water-driven critical pore radius method, while the upper limit was lower than the result determined by the flow coefficient-production capacity relationship fitting method.

[0064] Example:

[0065] Taking a domestic offshore oilfield as an example, as of now, this oilfield has 131 wells in production. Based on the oilfield's reserve database, development atlas system, and production capacity database, statistics on the utilization of low-permeability oilfield reserves are presented in the attached figure. Figure 2 As shown in the figure, the permeability range of 0-50 mD was divided into eight segments: <1 mD, 1-5 mD, 5-10 mD, 10-15 mD, 15-20 mD, 20-30 mD, 30-40 mD, and 40-50 mD. The proven geological reserves and cumulative production corresponding to each segment were statistically analyzed, and the proportion of cumulative production to proven geological reserves for each segment was calculated. The results are shown in the attached figure. Figure 3 As shown. Figure 3 The permeability range corresponding to the abrupt change in the trend line is 10-15 mD. Therefore, it can be considered that the permeability limit of the untapped reserves of this oil field is contained within 10-15 mD.

[0066] Core samples from the main oil-bearing blocks were used for waterflooding nuclear magnetic resonance displacement experiments. The displacement results are attached. Figure 4 As shown in the figure. The critical water-driven pore radius for this oilfield was determined to be 1-1.5 μm based on T2 spectra after saturated water, saturated oil, and water-driven oil recovery. Due to the large range of data such as tortuosity in this oilfield, it was difficult to select representative data. Therefore, the Kozeny-Carman equation was not used to calculate permeability; instead, the statistical relationship between the median pore throat radius and permeability was determined using a chart method. Four lines were plotted on the median pore throat radius and permeability curve: two horizontal lines representing median pore throat radii of 1 μm and 1.5 μm, and two vertical lines representing permeability of 10 mD and 15 mD. (See attached figure.) Figure 5 The data points within the rectangle enclosed by the four curves are counted. In this example, there are few data points within the rectangle, so the value can be directly read, which is 11.7 mD. When there are many data points within the rectangle, statistical analysis is performed, and the penetration rate corresponding to the data points accounting for more than 50% is selected as the final result.

[0067] The main production wells of the oilfield were collected and screened. The average production capacity and flow coefficient of each well to date were statistically calculated. A two-dimensional rectangular coordinate system was established with the flow coefficient as the x-axis and production capacity as the y-axis, and the relationship curve between the flow coefficient and production capacity was plotted. The curve was then piecewise linearly fitted according to the rate of change. The fitting results are shown in the attached figure. Figure 6 As shown. The intersection point of the fitted curves is calculated according to the fitting formula. In this example, the fitted curves intersect at a capacity of 26m³. 3 The corresponding flow coefficient is 35.5 mD*m / (mPa·s), the average viscosity is 3.7 mPa·s, and the average thickness is 10 m. Based on the definition of the flow coefficient, the permeability at this point is calculated to be 13 mD. Therefore, the upper limit of the untapped reserves in this oil field can be considered to be 13 mD.

[0068] Considering the cumulative production-proven reserves ratio method, the water-driven critical pore radius method, and the flow coefficient-production capacity relationship fitting method, the results are as follows: First, the cumulative production-proven reserves ratio method indicates that the permeability limit of the undeveloped reserves in this oil field is between 10-15 mD; the water-driven critical pore radius method indicates that the lower limit permeability of the undeveloped reserves in this oil field is 11.7 mD; and the flow coefficient-production capacity relationship fitting method indicates that the upper limit permeability of the undeveloped reserves in this oil field is 13 mD. Therefore, the permeability limit of the undeveloped reserves in this oil field is 11.7-13 mD.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields, characterized in that, include: Using 50mD as the upper limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oil fields, the utilization of low-permeability oil field reserves is statistically analyzed based on the reserve database, development atlas system and production capacity database of the offshore oil fields, and the cumulative production is calculated as a percentage of the proven geological reserves. Using 5-10mD as the segmented interval for the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields, the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields within the range of 0-50mD is segmented. Based on the proportion of cumulative production to proven reserves in each segmented interval, statistical histograms and trend lines are drawn. The abrupt changes in the trend lines are identified, and the permeability limit of low-permeability and difficult-to-develop reserves in offshore oil fields is narrowed down to the permeability segmented intervals corresponding to the abrupt changes in the trend lines. Nuclear magnetic resonance experiments were conducted on core samples from typical blocks of the offshore oilfield to obtain the critical water-driven pore radius of the offshore oilfield. The permeability corresponding to the critical pore radius of the porous medium was then calculated, and the calculated permeability was used as the lower limit of the permeability limit for the low-permeability and difficult-to-access reserves of the offshore oilfield. Select production capacity as a dynamic parameter, plot the relationship curve between flow coefficient and production capacity, perform piecewise fitting on the relationship curve between flow coefficient and production capacity, calculate the permeability corresponding to the critical pore radius of the porous medium based on the flow coefficient corresponding to the intersection point of the fitted segment, and use the calculated permeability as the upper limit of the permeability limit of the low-permeability and difficult-to-access reserves in the offshore oil field. The permeability limit of the low-permeability and difficult-to-access reserves in the offshore oil field is determined to be within the permeability segment range corresponding to the abrupt change in the slope of the trend line, and is between the lower limit and the upper limit. The process of drawing statistical histograms and trend lines includes: using each segment interval as the horizontal axis and the ratio of cumulative production to proven geological reserves corresponding to each segment interval as the vertical axis, identifying the inflection point of drastic changes in the trend line. When the permeability is below this inflection point, the ratio of cumulative production to proven geological reserves changes little after increasing the permeability, indicating that the difficulty of reserve extraction does not change significantly. When the permeability gradually increases to above this inflection point, the ratio of cumulative production to proven geological reserves changes drastically, indicating that the difficulty of reserve extraction decreases at this point, and the production increases significantly under the same conditions. It is determined that the permeability limit of low-permeability, difficult-to-develop reserves in offshore oil fields is contained within the permeability segment interval corresponding to the inflection point of the trend line.

2. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 1, characterized in that, The permeability corresponding to the critical pore radius of porous media can be calculated using the Kozeny-Carman equation, and the expression is as follows: （1） in, Permeability corresponding to the critical pore radius of a porous medium Indicates the porosity of porous media; C represents the Kozeny-Carman constant; Indicates the tortuosity of porous media; It represents the specific surface area of ​​the solid phase.

3. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 1, characterized in that, The permeability corresponding to the critical pore radius of porous media can be calculated by back-calculating the statistical relationship between the median pore throat radius and permeability in oilfields.

4. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 2, characterized in that, The expression for the flow coefficient is: （2） in, Indicates the flow coefficient; Permeability corresponding to the critical pore radius of a porous medium h represents the effective thickness of the formation; Indicates fluid viscosity.

5. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 1, characterized in that, include: The permeability range of 0-50 mD was divided into eight segments: <1 mD, 1-5 mD, 5-10 mD, 10-15 mD, 15-20 mD, 20-30 mD, 30-40 mD, and 40-50 mD. The proven geological reserves and cumulative production corresponding to each segment were statistically analyzed, and the proportion of the cumulative production to the proven geological reserves for each segment was calculated.

6. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 1, characterized in that, Water-drive NMR experiments were conducted on core samples from typical blocks of the offshore oilfield. This included: selecting core samples from major oil-bearing blocks of the offshore oilfield for water-drive NMR displacement experiments; determining the critical water-drive pore radius of the offshore oilfield using T2 spectra of saturated water, saturated oil, and water-drive oil; calculating the permeability based on the statistical relationship between the median pore throat radius and permeability of the oilfield or the Kozeny-Carman equation; and using the calculated permeability as the technical limit permeability of the offshore oilfield as the lower limit of the permeability boundary for low-permeability and difficult-to-access reserves in the offshore oilfield.

7. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 1, characterized in that, The process of plotting the relationship curve between flow coefficient and production capacity includes: statistically analyzing the flow coefficient and production capacity of each production well in the offshore oilfield; establishing a two-dimensional rectangular coordinate system with flow coefficient as the abscissa and production capacity as the ordinate; plotting the relationship curve between flow coefficient and production capacity; performing piecewise linear fitting on the curve according to the rate of change; using the least squares method for fitting; and finding the intersection point of the fitted curves. When the flow coefficient is less than the intersection point, the fluid flow difficulty within the reservoir is similar and the flow difficulty is relatively high, resulting in a lower production capacity. When the flow coefficient is greater than the intersection point, the fluid flow difficulty within the reservoir is similar and the flow difficulty is relatively low, resulting in a higher production capacity.

8. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 7, characterized in that, include: The permeability is calculated by back-calculating the flow coefficient corresponding to the intersection point of the fitted curves, and this permeability serves as the upper limit of the permeability corresponding to the difficult-to-access reserves.

9. The method for determining the permeability limit of low-permeability, difficult-to-access reserves in offshore oil fields according to claim 7, characterized in that, The permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields is within the range of the permeability limit for low-permeability and difficult-to-develop reserves obtained by the cumulative production-provided reserves ratio method. The lower limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields is higher than the lower limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields obtained by the median pore radius method driven by water. The upper limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields is lower than the upper limit of the permeability limit for low-permeability and difficult-to-develop reserves in offshore oilfields obtained by the flow coefficient-production capacity relationship fitting method.

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