CN115656003A - Method for determining permeability limit of low-permeability hard-to-use reserve of offshore oilfield - Google Patents
Method for determining permeability limit of low-permeability hard-to-use reserve of offshore oilfield Download PDFInfo
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Abstract
The invention relates to a method for determining a permeability limit of a hypotonic difficultly-used reserve of an offshore oilfield. The method comprises the following steps: according to a reserve database, a development atlas system and a productivity database of the offshore oil field, counting the reserve utilization condition of the low-permeability oil field, and counting the proportion of the accumulated yield in the ascertained geological reserve; drawing a statistical histogram and a trend line according to the proportion of the accumulated yield in the ascertained reserves in each segmented interval, and finding out a mutation part in the trend line; performing a water flooding nuclear magnetic resonance experiment on a typical block core of the offshore oilfield to obtain a critical water flooding pore radius of the offshore oilfield, and taking the permeability corresponding to the pore radius as a lower limit of the limit; selecting productivity as a dynamic parameter for measurement, drawing a relation curve of the flow coefficient and the productivity, performing segmented fitting on the relation curve of the flow coefficient and the productivity, and taking the permeability corresponding to the intersection point of a fitting line as a limit upper limit; in summary, it is determined that the permeability limit of the offshore oilfield hypotonic refractory reserve is within the permeability segmentation range corresponding to the abrupt change part in the trend line and between the upper limit values of the lower limit values.
Description
Technical Field
The invention relates to the field of dynamic evaluation of oil field development reserves, in particular to a method for determining a permeability limit of a hypotonic difficultly-used reserve of an offshore oil field.
Background
The development of offshore oil and gas resources is limited by factors such as geological reservoir conditions, sea condition environments, engineering facilities and the like, the exploitation cost is high, the development difficulty of low-quality (such as low permeability, low abundance, heavy oil and the like) oil and gas resources is high, and the offshore oil and gas resources are the main development difficulty in geological reserves at present. According to the reserve quality and the special development environment and requirements of the offshore oil field, the hard-to-use reserve of the offshore oil field is defined as follows: under the prior art and economic conditions, the geological reserves are difficult to be economically and effectively exploited by factors such as reserve scale, oil products, reservoir permeability, reserve abundance, productivity, technological conditions and the like. The hard reserve is a relative concept, and under different oil prices, different production modes, different management regimes and different operating regimes, the hard reserve has different definitions and results.
The hard-to-use reserves mainly comprise four types of hard-to-use for thick oil, low-permeability, deep water and tailings, wherein the low-permeability hard-to-use reserves are wide in distribution and large in proportion and are the most main hard-to-use reserves of offshore oil and gas fields. The economic and effective production of the hypotonic difficultly-used reserves depends on the improvement of oil and gas production level technologies, and the evaluation work before development is very important, so that the lower limit standard is reasonably determined, and the method is the minimum condition for quickly judging whether the oil (gas) field (reservoir) can be brought into a development plan. The main problems of the hypotonic hard-to-use reserves are that the permeability limit of the reserve classification is fuzzy and related evaluation data are deficient, and generally, the hypotonic hard-to-use reserves can only depend on multi-dimensional earthquake and data of logging, oil testing, pilot production data, a small amount of analysis and test and the like of partial exploratory wells, evaluation wells. The permeability limit of the reserve classification is generally evaluated and judged by analogy with similar oil fields abroad, and the conclusion of the classification of each block is usually obtained by adopting a chart synthesis method or an analogy method according to the petroleum reserve specification when each oil field in China carries out the difficult-to-use reserve evaluation in the past. The conclusion has ambiguity, is greatly influenced by human factors, is difficult to compare the quality of each block, has low reliability and is easy to generate wrong decisions. In order to overcome the defect, so that the evaluation is more objective, intuitive, accurate and scientific, human interference needs to be reduced, and scientific hard-to-use reserve permeability boundary evaluation is carried out from the aspects of static geological features and dynamic data analysis.
Disclosure of Invention
In view of the above problems, it is an object of the present invention to provide a method for determining a permeability limit for a hypotonic hard-to-use reserve in an offshore oilfield. The method fully considers the geological data statistical rule and development dynamic data of the oil field, and defines the permeability limit of the low-permeability hard-to-use reserve of the offshore oil field through the gradual change rule of the accumulated yield in the interval with different permeability rates in the ratio of the ascertained reserve, the permeability interval corresponding to the median pore radius for water driving and the permeability corresponding to the inflection point of the multi-section linear regression of the flow coefficient and the productivity. The core steps are as follows: and taking 50mD as the upper limit of low permeability, counting the reserve utilization condition of a low permeability reservoir of the offshore oilfield, counting the proportion of the accumulated yield corresponding to different ranges of permeability to the ascertained reserve, drawing a histogram, considering that the hard utilization reserve permeability limit is included in the permeability range corresponding to the trend line mutation part, and then accurately defining the hard utilization reserve permeability limit according to a median pore radius method for water driving and a flow coefficient-capacity relation fitting method. The method can effectively define the permeability limit of the oil field hard-to-use reserves, and provides technical support for the improvement of the oil company reserves technical system and the development and evaluation of the oil field.
In order to achieve the purpose, the invention adopts the following technical scheme:
a method for determining a permeability limit of a hypotonic hard-to-use reserve of an offshore oilfield comprises the following steps:
taking 50mD as the upper limit of the permeability limit of the hypotonic hard-to-use reserve of the offshore oil field, counting the reserve use condition of the hypotonic oil field according to a reserve database, a development atlas system and a productivity database of the offshore oil field, and counting the proportion of the accumulated yield in the ascertained geological reserve;
segmenting the offshore oilfield hypotonic difficulty reserve permeability boundary within the range of 0-50mD by taking 5-10mD as a segmented interval of the offshore oilfield hypotonic difficulty reserve permeability boundary, drawing a statistical histogram and a trend line according to the proportion of the cumulative yield in each segmented interval to the ascertained reserve, finding a mutation part in the trend line, and reducing the offshore oilfield hypotonic difficulty reserve permeability boundary into a permeability segmented interval corresponding to the mutation part in the trend line;
performing a water flooding nuclear magnetic resonance experiment on a typical block core of the offshore oilfield to obtain a critical water flooding pore radius of the offshore oilfield, inversely calculating the permeability corresponding to the critical pore radius of the porous medium, and taking the inversely calculated permeability as a lower limit value of a permeability limit of a low-permeability hard-to-use reserve of the offshore oilfield;
selecting the productivity as a dynamic parameter for measurement, drawing a relation curve of the flow coefficient and the productivity, fitting the relation curve of the flow coefficient and the productivity in a segmented manner, inversely calculating the permeability corresponding to the critical pore radius of the porous medium according to the flow coefficient corresponding to the intersection point of the fitting segments, and taking the inversely calculated permeability as the upper limit value of the permeability limit of the low-permeability difficultly-used reserve of the offshore oilfield;
and determining that the permeability limit of the hypotonic difficultly available reserve of the offshore oilfield is within the range of the section corresponding to the mutation part in the trend line and is between the lower limit value and the upper limit value.
Inversely calculating the permeability corresponding to the critical pore radius of the porous medium according to a Kozeny-Carman equation, wherein the expression is as follows:
wherein K represents the permeability corresponding to the critical pore radius of the porous medium;
phi represents the porosity of the porous medium;
c represents a Kozeny-Carman constant;
τ denotes tortuosity of the porous medium;
s represents the specific surface area of the solid phase.
And inversely calculating the permeability corresponding to the critical pore radius of the porous medium according to the statistical relationship chart of the median pore throat radius and the permeability of the oil field.
The flow coefficient expression is:
wherein, N L Represents the flow coefficient;
k represents the permeability corresponding to the critical pore radius of the porous medium;
h represents the effective thickness of the stratum;
μ denotes the fluid viscosity.
Segmenting the permeability interval 0-50mD, wherein the permeability interval is respectively <1mD, 1-5mD, 5-10mD, 10-15mD, 15-20mD, 20-30mD, 30-40mD and 40-50mD 8, respectively counting the proven geological reserves and the accumulated yield corresponding to each segmented interval, and calculating the proportion of the accumulated yield corresponding to each segmented interval to the proven geological reserves.
Drawing a statistical histogram and a trend line comprises the following steps: when the permeability is lower than the inflection point, the change of the proportion of the accumulated yield to the proven geological reserve is small after the permeability is increased, and the change of the difficulty of reserve exploitation is not obvious; when the permeability is gradually increased to be higher than the inflection point, the ratio of the accumulated yield to the ascertained geological reserve is sharply changed, which shows that the reserve exploitation difficulty is reduced at the moment, the yield is obviously increased under the same condition, and the permeability limit of the reserve for the hypotonic difficult utilization of the offshore oilfield is judged to be included in the subsection interval corresponding to the inflection point part of the trend line.
The water flooding nuclear magnetic resonance experiment for the typical block core of the offshore oilfield comprises the following steps: selecting a rock core of a main oil-containing block of the offshore oil field to perform a water-flooding nuclear magnetic resonance displacement experiment, judging the pore radius for critical water drive of the offshore oil field through saturated water, saturated oil and a T2 map after water flooding, and inversely calculating the permeability according to a statistical relation chart of the median pore throat radius and the permeability or a Kozeny-Carman equation in the oil field, wherein the inversely calculated permeability is the technical limit permeability of the offshore oil field and is used as a lower limit value of a permeability limit of a low-permeability hard-to-use reserve of the offshore oil field.
The step of drawing the relation curve of the flow coefficient and the productivity comprises the following steps: the method comprises the steps of counting the flow coefficient and the productivity of each production well of the offshore oilfield, establishing a two-dimensional rectangular coordinate system by taking the flow coefficient as a horizontal coordinate and the productivity as a vertical coordinate, drawing a relation curve of the flow coefficient and the productivity, and performing piecewise linear fitting on the curve according to the speed of a change trend, wherein the fitting method is a least square method, and finding out a cross point of the fitting curve, and when the flow coefficient is smaller than the cross point, the flow difficulty of fluids in a reservoir is similar, the flow difficulty is larger, and the corresponding productivity is smaller; when the flow coefficient is larger than the intersection point, the flow difficulty of the fluids in the reservoir is similar and smaller, and the corresponding capacity is larger.
And inversely calculating the permeability according to the flow coefficient corresponding to the intersection point of the fitting curve, wherein the permeability is used as the upper limit of the permeability corresponding to the difficulty-for-use reserve.
The reservoir permeability limit for the offshore oilfield hypotonic difficulty is in the range of the reservoir permeability limit for the offshore oilfield hypotonic difficulty obtained by the cumulative yield-proven reservoir proportion method, the lower limit of the reservoir permeability limit for the offshore oilfield hypotonic difficulty is higher than the lower limit of the reservoir permeability limit for the offshore oilfield hypotonic difficulty obtained by the water drive median pore radius method, and the upper limit of the reservoir permeability limit for the offshore oilfield hypotonic difficulty is lower than the upper limit of the reservoir permeability limit for the offshore oilfield hypotonic difficulty obtained by the flow coefficient-capacity relation fitting method.
Due to the adoption of the technical scheme, the invention has the following advantages:
1. the method for determining the permeability limit of the hypotonic difficultly-used reserve of the offshore oil field disclosed by the invention embodies the permeability limit method of the difficultly-used reserve of the oil field for the first time, avoids the uncertainty of ambiguity, subjectivity and analogy in the traditional method, and greatly increases the accuracy and scientificity of a decision result;
2. the method for determining the permeability limit of the oil field difficult-to-use reserve by considering the combination of static geological features and dynamic data is established for the first time, and the permeability limit of the oil field difficult-to-use reserve is determined by a cumulative yield-proven reserve ratio method, a critical pore radius method for water driving and a flow coefficient-capacity relation fitting method.
3. The invention discloses a method for defining the permeability limit of a hypotonic hard-to-use reserve of an offshore oil field, which guides related personnel to scientifically and accurately judge the permeability limit of the hard-to-use reserve of a target oil field in a mode of combining a theoretical method and a standardized graphic representation.
Drawings
Various additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Like parts are designated with like reference numerals throughout the drawings. In the drawings:
FIG. 1 is a technical roadmap for the offshore oilfield hypotonic hard-to-use reserve permeability boundary in a method of practicing the present invention;
FIG. 2 is a histogram of ascertained geological reserves and cumulative production for different permeability intervals in an example of the present invention;
FIG. 3 is a histogram of cumulative yield versus exploratory geological reserves for different permeability intervals in an example of the present invention;
FIG. 4 is a nuclear magnetic resonance T2 spectrum of a water flooding experiment in an example of the present invention;
FIG. 5 is a graphical depiction of the statistical relationship between permeability and throat median radius for an example of the present invention;
FIG. 6 is a graph illustrating flow coefficient versus capacity for an example of the present invention.
Detailed Description
Exemplary embodiments of the present invention will be described in more detail below 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 embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
The invention provides a method for determining a permeability limit of a hypotonic hard-to-use reserve of an offshore oilfield, which comprises the following steps of:
step 1: taking 50mD as the upper limit of low permeability, and counting the reserve utilization condition of the low permeability oil field according to a reserve database, a development atlas system and a productivity database of the oil field, wherein the counting content comprises the exploration of geological reserve and accumulated yield;
step 2: and taking 5-10mD as a segmentation interval, and segmenting 0-50 mD. And counting and calculating the proportion of the cumulative yield corresponding to the permeability in different ranges to the proven reserves, and drawing a statistical histogram and a trend line. The hard-to-use reserve permeability limit is considered to be included in the permeability range corresponding to the abrupt portion of the trend line;
and step 3: performing a water-drive nuclear magnetic resonance experiment on a typical block core of a target oil field to obtain the critical water-drive pore radius of the target oil field, and inversely calculating the permeability according to a statistical relationship chart of the median pore throat radius and the permeability of the oil field or a Kozeny-Carman equation, wherein the permeability can be regarded as the technical limit permeability and is the lower limit value of the permeability corresponding to the difficult-to-use reserve;
and 4, step 4: and selecting the productivity as a dynamic parameter for measuring the hard utilization reserves. Drawing a relation curve of the flow coefficient and the productivity, performing piecewise fitting on the relation curve, and inversely calculating the permeability according to the flow coefficient corresponding to the intersection point of the fitting sections, wherein the permeability is the upper limit of the permeability corresponding to the hard-to-use reserve;
and 5: and (5) comprehensively comparing and analyzing results obtained by the three methods in the steps 1-4, and finally determining the hard-to-use reserve margin permeability of the target oil field.
The expressions of the Kozeny-Carman equation and the flow coefficient in step 3 and step 4, respectively, are:
Kozeny-Carman equation:
flow coefficient:
wherein K represents the permeability of the porous medium, mD; phi represents the porosity of the porous medium; c represents a Kozeny-Carman constant; τ denotes tortuosity of the porous medium; s represents a specific surface area of a solid phase, m 2 /g；N L Represents the flow coefficient, mD m/(mPa · s); h represents the effective thickness of the stratum, m; μ denotes the fluid viscosity, mPas.
The invention provides a method for determining a permeability limit of a hypotonic hard-to-use reserve of an offshore oilfield, which comprises the following steps of:
taking a technical route chart of a low-permeability hard-to-use reserve permeability boundary of an offshore oilfield as a guide, as shown in an attached drawing 1, firstly determining a target oilfield, taking 50mD as a low-permeability upper limit, and counting the reserve use condition of the low-permeability oilfield according to a reserve database, a development atlas system and a capacity database of the oilfield, wherein the statistical content comprises the exploration of geological reserve and cumulative yield;
the permeability intervals of 0-50mD are segmented into 8 intervals, namely <1mD, 1-5mD, 5-10mD, 10-15mD, 15-20mD, 20-30mD, 30-40mD, 40-50mD and the like. Respectively counting the proven geological reserves and the accumulated yield corresponding to each interval, and calculating the proportion of the accumulated yield corresponding to each interval in the proven geological reserves;
wherein the geology statistics include: and calculating the accumulated used developed geological reserves by using the relation between the developed reserves and the permeability, and identifying the main change nodes of the permeability value. Establishing a histogram and drawing an overall change trend line of the histogram by taking each permeability interval as a horizontal axis and the proportion of the cumulative yield corresponding to each interval to the ascertained geological reserve as a vertical axis, and judging an inflection point of the violent change of the drawn trend line, wherein the physical meaning of the point is that when the permeability is lower than the point, the proportion of the cumulative yield to the ascertained geological reserve changes less after the permeability is increased, namely the difficulty of reserve exploitation changes insignificantly; when the permeability is gradually increased to be higher than the point, the ratio of the accumulated yield to the ascertained geological reserve is sharply changed, which shows that the reserve exploitation difficulty is reduced at the moment, and the yield is obviously increased under the same condition. Therefore, the hard-to-use reserve permeability limit is considered to be included in the permeability range corresponding to the inflection point part of the trend line;
the data statistics comprise a chart of statistical relation between the median pore throat radius and the permeability, and permeability values corresponding to the water drive critical pore throat radius obtained by the nuclear magnetic resonance experiment are defined according to the chart. Specifically, a core of a main oil-containing block of a target oil field is selected to perform a water flooding nuclear magnetic resonance displacement experiment, the critical water-driving pore radius of the oil field is judged through saturated water, saturated oil and a T2 spectrum after water flooding, the permeability is inversely calculated according to a statistical relation chart of the median pore throat radius and the permeability of the oil field or a Kozeny-Carman equation, the permeability can be regarded as the technical limit permeability of the oil field, and generally, the value is regarded as the lower limit value of the permeability corresponding to the difficulty-in-use reserve.
The data statistics further comprises flow coefficient definition, the relation between the flow coefficient and the productivity is analyzed, piecewise linear regression is conducted on the flow coefficient and the productivity, the permeability limit is inversely calculated by utilizing the intersection point of piecewise fitting curves, specifically, the flow coefficient and the productivity of each production well of the target oil field are counted, a two-dimensional rectangular coordinate system is established by taking the flow coefficient as a horizontal coordinate and the productivity as a vertical coordinate, and a relation curve of the flow coefficient and the productivity is drawn. The curve is subjected to piecewise linear fitting according to the speed of the change trend, two segments are usually taken as piecewise targets, and the fitting method is a least square method. Finding the intersection point of the fitting curve, wherein the physical meaning of the intersection point is as follows: when the flow coefficient is less than the point, the flow difficulty of the fluids in the reservoir stratum is similar and higher, and the corresponding productivity is lower; when the flow coefficient is larger than the point, the flow difficulty of the fluids in the reservoir is similar and smaller, and the corresponding capacity is larger.
Inversely calculating the permeability according to the flow coefficient corresponding to the intersection point of the fitting sections, wherein the permeability is the upper limit of the permeability corresponding to the difficultly-used reserve;
and comprehensively comparing and analyzing the results obtained by the three methods, and finally determining the permeability of the hard-to-use reserve limit of the target oil field. The basis for the determination is: the finally obtained permeability limit is in the range of the result obtained by the cumulative yield-proven reserve ratio method, the lower limit is higher than the result determined by the water drive critical pore radius method, and the upper limit is lower than the result determined by the flow coefficient-capacity relation fitting method.
The embodiment is as follows:
take a certain offshore oil field in China as an example. At present, the oil field is put into production with 131 wells, and the reserve utilization condition of the hypotonic oil field is counted according to the reserve database, the development atlas system and the productivity database of the oil field, and the result is shown in the attached figure 2. The permeability intervals of 0-50mD are segmented into 8 intervals, namely <1mD, 1-5mD, 5-10mD, 10-15mD, 15-20mD, 20-30mD, 30-40mD, 40-50mD and the like. The exploratory geological reserves and the accumulated yield corresponding to each interval are respectively counted, the proportion of the accumulated yield corresponding to each interval in the exploratory geological reserves is calculated, and the result is shown in the attached figure 3. The permeability interval corresponding to the abrupt change portion of the trend line in fig. 3 is 10-15mD, so the hard-to-use reserve permeability limit of the field can be considered to be comprised between 10-15 mD.
And taking a core of the main oil-containing block to perform a water flooding nuclear magnetic resonance displacement experiment. The displacement results are shown in figure 4. The radius of a critical water driving pore of the oil field is judged to be 1-1.5 mu m through a T2 map of saturated water, saturated oil and water driven oil, and because data spans such as tortuosity of the oil field are large and representative data is difficult to select, the permeability is not considered to be inversely calculated by using a Kozeny-Carman equation, and the limit permeability is determined by using a statistical relationship chart method of the radius of the median pore throat of the oil field and the permeability. Four lines are drawn on the median pore throat radius and permeability curve of the oil field, which are two horizontal transverse lines representing the median pore throat radius of 1 μm and 1.5 μm, and two longitudinal lines representing the permeability of 10mD and 15mD, respectively, see FIG. 5. And counting data points in a rectangle enclosed by the four curves, wherein in the example, the number of data points in the rectangular frame is small, so that the value can be directly read, namely 11.7mD, when the number of data points in the rectangle is large, the data points are subjected to statistical analysis, and the permeability corresponding to the data points accounting for more than 50% is selected as a final result.
Collecting and screening main production wells of the oil field, counting and calculating the average productivity and flow coefficient of each well so far, establishing a two-dimensional rectangular coordinate system by taking the flow coefficient as a horizontal coordinate and the productivity as a vertical coordinate, and drawing a relation curve of the flow coefficient and the productivity. The curve is subjected to piecewise linear fitting according to the speed of the change trend, and the fitting result is shown in figure 6. The intersection point of the fitting curve is calculated according to the fitting formula, in this example, the fitting curve intersects with the production capacity of 26m 3 (ii)/d, corresponding to a flow coefficient of 35.5mD m/(mPas), an average viscosity of 3.7 mPas and an average thickness of 10m. Since the permeability at this point is calculated back from the definition of the flow coefficient to be 13mD, the upper limit permeability of the refractory reserves in this field can be considered to be 13mD.
Comprehensively considering the cumulative yield-proven reserve ratio method and the fitting method of the critical pore radius method for water driving and the relationship between the flow coefficient and the productivity to obtain the results: firstly, an accumulative production-proven reserves proportion method indicates that the permeability limit of the difficult-to-use reserves of the oil field is between 10 and 15 mD; the critical pore radius method for water drive shows that the lower limit permeability of the hard-to-use reserve of the oil field is 11.7mD; the flow coefficient-capacity relation fitting method shows that the upper limit permeability of the difficult-to-use reserves of the oil field is 13mD. Therefore, the difficult-to-use reserve permeability limit of the oil field is 11.7-13mD.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A method for determining a permeability limit of a hypotonic hard-to-use reserve in an offshore oilfield, the method comprising:
taking 50mD as the upper limit of the permeability limit of the hypotonic hard-to-use reserve of the offshore oil field, counting the reserve use condition of the hypotonic oil field according to a reserve database, a development atlas system and a productivity database of the offshore oil field, and counting the proportion of the accumulated yield in the ascertained geological reserve;
segmenting the offshore oilfield hypotonic difficulty reserve permeability limit within the range of 0-50mD by taking 5-10mD as a segmentation interval of the offshore oilfield hypotonic difficulty reserve permeability limit, drawing a statistical histogram and a trend line according to the proportion of the cumulative yield in each segmentation interval to the ascertained reserve, finding a mutation part in the trend line, and reducing the offshore oilfield hypotonic difficulty reserve permeability limit into the permeability segmentation interval corresponding to the mutation part in the trend line;
performing a water flooding nuclear magnetic resonance experiment on a typical block core of the offshore oilfield to obtain a critical water driving pore radius of the offshore oilfield, inversely calculating a permeability corresponding to the critical pore radius of the porous medium, and taking the inversely calculated permeability as a lower limit value of a permeability limit of the offshore oilfield low-permeability hard-to-use reserve;
selecting productivity as a dynamic parameter for measurement, drawing a relation curve of a flow coefficient and the productivity, performing piecewise fitting on the relation curve of the flow coefficient and the productivity, reversely calculating the permeability corresponding to the critical pore radius of the porous medium according to the flow coefficient corresponding to the intersection point of fitting sections, and taking the reversely calculated permeability as the upper limit value of the permeability limit of the hypotonic difficultly-used reserve of the offshore oilfield;
and determining that the permeability limit of the hypotonic difficultly available reserve of the offshore oilfield is in a permeability subsection range corresponding to a gradient abrupt part of a trend line and between the lower limit value and the upper limit value.
2. The method for determining the permeability limit of the hypotonic difficultly available reserve of offshore oilfield according to claim 1, wherein the permeability corresponding to the critical pore radius of the porous medium is inversely calculated according to a Kozeny-Carman equation, and the expression is as follows:
wherein K represents the permeability corresponding to the critical pore radius of the porous medium;
phi represents the porosity of the porous medium;
c represents a Kozeny-Carman constant;
τ denotes tortuosity of the porous medium;
s represents the specific surface area of the solid phase.
3. The method for determining the permeability limit of the hypotonic hard-to-use reserve in the offshore oilfield according to claim 1, wherein the permeability corresponding to the critical pore radius of the porous medium is back-calculated according to a statistical relationship chart of the median pore throat radius and the permeability of the oilfield.
4. The method for determining the permeability limit of the hypotonic hard-to-use reserve of the offshore oilfield according to claim 2, wherein the expression of the flow coefficient is as follows:
wherein, N L Represents the flow coefficient;
k represents the permeability corresponding to the critical pore radius of the porous medium;
h represents the effective thickness of the formation;
μ denotes the fluid viscosity.
5. The method for determining the permeability limit of the hypotonic hard-to-use reserve of the offshore oilfield according to claim 1, comprising:
segmenting the permeability interval 0-50mD, wherein the permeability interval is respectively <1mD, 1-5mD, 5-10mD, 10-15mD, 15-20mD, 20-30mD, 30-40mD and 40-50mD 8, respectively counting the proven geological reserves and the accumulated yield corresponding to each segmented interval, and calculating the proportion of the accumulated yield corresponding to each segmented interval to the proven geological reserves.
6. The method for determining the permeability limit of the hypotonic hard-to-use reserve in the offshore oilfield according to claim 1, wherein the step of drawing the statistical histogram and the trend line comprises: when the permeability is lower than the inflection point, the change of the proportion of the accumulated yield to the proven geological reserve is small after the permeability is increased, and the change of the difficulty of reserve exploitation is not obvious; when the permeability is gradually increased to be higher than the inflection point, the ratio of the accumulated yield to the ascertained geological reserve is changed sharply, which indicates that the reserve exploitation difficulty is reduced at the moment, the yield is obviously increased under the same condition, and the permeability limit of the reserve for the low permeability difficulty of the offshore oilfield is judged to be included in the permeability subsection interval corresponding to the inflection point part of the trend line.
7. The method for determining the permeability limit of the hypotonic hard-to-use reserve of the offshore oilfield according to claim 1, wherein the performing of the water flooding nuclear magnetic resonance experiment on the typical block core of the offshore oilfield comprises: selecting a rock core of a main oil-containing block of the offshore oil field to perform a water-flooding nuclear magnetic resonance displacement experiment, judging the pore radius for critical water drive of the offshore oil field through saturated water, saturated oil and a T2 spectrum after water flooding, and inversely calculating the permeability according to a statistical relation chart of the median pore throat radius and the permeability or a Kozeny-Carman equation of the oil field, wherein the inversely calculated permeability is the technical limit permeability of the offshore oil field and is used as a lower limit value of a permeability limit of the offshore oil field low-permeability hard-to-use reserve.
8. The method for determining the permeability limit of the hypotonic hard-to-use reserve in the offshore oilfield according to claim 1, wherein the plotting the flow coefficient versus the production capacity comprises: counting the flow coefficient and the productivity of each production well of the offshore oilfield, establishing a two-dimensional rectangular coordinate system by taking the flow coefficient as a horizontal coordinate and the productivity as a vertical coordinate, drawing a relation curve of the flow coefficient and the productivity, and performing piecewise linear fitting on the curve according to the speed of a change trend, wherein the fitting method is a least square method, and finding out a cross point of the fitting curve, and when the flow coefficient is less than the cross point, the flow difficulty of fluids in a reservoir is similar, the flow difficulty is higher, and the corresponding productivity is lower; when the flow coefficient is larger than the intersection point, the flow difficulty of the fluids in the reservoir is similar and smaller, and the corresponding capacity is larger.
9. The method for determining the permeability limit of the hypotonic hard-to-use reserve in the offshore oilfield according to claim 8, comprising: and inversely calculating the permeability according to the flow coefficient corresponding to the intersection point of the fitting curve, wherein the permeability is used as the upper limit of the permeability corresponding to the difficulty-for-use reserve.
10. The method for determining the reserve permeability limit for offshore oilfield hypotonic difficulty as recited in claim 8, wherein the reserve permeability limit for offshore oilfield hypotonic difficulty is within the reserve permeability limit for offshore oilfield obtained by the cumulative production-proven reserve ratio method, a lower limit of the reserve permeability limit for offshore oilfield hypotonic difficulty is higher than a lower limit of the reserve permeability limit for offshore oilfield obtained by the median pore radius method for water drive, and an upper limit of the reserve permeability limit for offshore oilfield hypotonic difficulty is lower than an upper limit of the reserve permeability limit for offshore oilfield obtained by the flow coefficient-capacity relationship fitting method.
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