A quantitative evaluation method for the degree of fracture development in carbonate reservoirs.

Abstract

This invention belongs to the field of oil and gas engineering and discloses a quantitative evaluation method for the degree of fracture development in carbonate reservoirs, comprising the following steps: (1) obtaining the fracture angle and fracture width of the evaluation section of the target well; (2) using the fracture angle to calculate the average fracture angle and calculate the fracture network dispersion index; (3) using the fracture width to calculate the average fracture width and calculate the fracture connectivity index; (4) combining the fracture network dispersion index and the fracture connectivity index to calculate the fracture network development index, and evaluating the degree of carbonate reservoir development based on the magnitude of the fracture network development index. This invention is the first to combine the fracture network dispersion index and the fracture connectivity index to calculate the fracture network development index, thereby comprehensively representing the degree of carbonate reservoir development and establishing a fracture development type evaluation standard, overcoming the previous problem of comprehensively evaluating the degree of fracture development by only evaluating the complexity of fractures without considering the connectivity and effectiveness between fracture networks.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas engineering, and in particular relates to a quantitative evaluation method for the degree of fracture development in carbonate reservoirs during exploration and development. Background Technology

[0002] Carbonate rocks are one of China's important strategic resources for ensuring national energy security. Due to their unique geological characteristics, they typically develop fractures and pores, serving as important channels for oil and gas storage and migration. In the process of oil and gas exploration and development, accurately evaluating and predicting the degree of fracture development in carbonate reservoirs is a prerequisite for assessing the resource potential of these reservoirs and optimizing development plans. It provides direct evidence for accurate and precise stimulation of carbonate reservoirs. Various oilfields have invested heavily in research and testing, primarily using methods such as field coring, imaging logging, and laboratory experiments to evaluate the degree of fracture development.

[0003] Patent 201710083194.X discloses a core analysis method for determining the degree of reservoir fracture development. The method consists of the following steps: 1) core splicing; 2) summarizing and identifying the characteristics of natural fractures; 3) imaging logging calibration; 4) final determination of natural fractures; and 5) judging the degree of fracture development, thereby achieving a quantitative characterization of the degree of natural fracture development.

[0004] Patent 202210205635.X discloses a method for judging the degree of development of fractures of different orders in a core. This method first counts the number of natural fractures in the core of the target stratum and divides the number of natural fractures by the total length of the core to obtain the linear density value of natural fractures in the first order. Then, it counts the number of new natural fractures in the core after a certain period of time and divides the number of new natural fractures by the total length of the core to obtain the linear density value of natural fractures in the second order. The linear density values ​​of natural fractures in the first order and the second order are added together to obtain the bus density value. The degree of development of natural fractures of different orders at the core scale is judged based on the bus density value.

[0005] The main problems with the above-mentioned methods are: currently, the evaluation of fracture development is mainly based on local static assessments of downhole core samples, which has the drawback of generalizing from a single point. More importantly, the methods for evaluating fracture development primarily rely on fracture density, failing to consider the core factors influencing fracture development, such as the effectiveness and flow capacity between fractures. These indicators are also key factors affecting reservoir quality and stimulation potential. Therefore, there is an urgent need for an evaluation method that simultaneously considers fracture complexity and fracture network effectiveness to provide a basis for evaluating the resource potential of carbonate reservoirs. Summary of the Invention

[0006] The purpose of this invention is to provide a quantitative evaluation method for the degree of fracture development in carbonate reservoirs. The method combines the fracture network dispersion index and the fracture connectivity index to calculate the fracture network development index, thereby comprehensively representing the degree of development of carbonate reservoirs. Furthermore, it establishes an evaluation standard for fracture development types, performs detailed classification, and achieves the purpose of quantitative evaluation.

[0007] A quantitative evaluation method for the degree of fracture development in carbonate reservoirs includes the following steps:

[0008] (1) Obtain the fracture angle and fracture width of the evaluation section of the target well;

[0009] (2) The average crack angle is obtained by using the crack angle, and the crack network dispersion index is calculated to evaluate the dispersion complexity of the crack region and indicate the degree of crack extension complexity.

[0010] (3) Calculate the average crack width using the crack width, and use it to calculate the crack connectivity index to evaluate the communication ability between crack networks and represent the degree of change in crack width;

[0011] (4) Calculate the fracture network development index by combining the fracture network dispersion index and the fracture connectivity index, and evaluate the degree of carbonate reservoir development based on the magnitude of the fracture network development index; wherein, the calculation formula for the fracture network development index is as follows:

[0012] ,

[0013] in, The crack network development index is dimensionless. The crack network dispersion index is dimensionless. The crack connectivity index is dimensionless. This represents the average crack width, in micrometers. The maximum width of the crack is measured in micrometers.

[0014] Preferably, the evaluation of the development degree of carbonate reservoir based on the fracture network development index is as follows: when 0 < fracture network development index ≤ 0.35, the grade is Class III, with poor fracture development; when 0.35 < fracture network development index ≤ 0.70, the grade is Class II, with moderate fracture development; when 0.70 < fracture network development index ≤ 1.0, the grade is Class I, with extremely well-developed fractures.

[0015] Preferably, the crack network dispersion index is calculated as follows:

[0016] ,

[0017] in, The crack angle is expressed in degrees (°).n The number of cracks, expressed in individual cracks. This represents the average crack angle, in degrees. ;

[0018] This is the theoretical maximum crack network dispersion value, which is dimensionless.

[0019] Preferably, the theoretical maximum crack network dispersion value is calculated as follows:

[0020] .

[0021] Preferably, the crack connectivity index is calculated as follows:

[0022] ,

[0023] in, The crack width is expressed in micrometers.

[0024] K imin To obtain the minimum width in the crack, the unit is micrometer;

[0025] .

[0026] Preferably, the fracture angle and fracture width are obtained through imaging logging.

[0027] Advantages of this invention:

[0028] 1. For the first time, a fracture network development index is calculated by combining the fracture network dispersion index and the fracture connectivity index to comprehensively represent the development degree of carbonate reservoirs. An evaluation standard for fracture development types is established for refined classification. This overcomes the previous difficulty of comprehensively evaluating fracture development degree by only considering the complexity of fractures without taking into account the connectivity and effectiveness between fracture networks.

[0029] 2. Based on actual interpretation data from imaging logging, this method innovatively utilizes the obtained fracture angle parameters to calculate the average fracture angle. By calculating the fracture network dispersion index, it achieves a quantitative evaluation of the dispersion complexity of fracture regions in reservoir sections. This overcomes the inaccuracies caused by previous single-point evaluations of fracture complexity and shifts towards evaluating the overall complexity of fracture regions. Simultaneously, using fracture width parameters, it calculates the fracture connectivity index to achieve a quantitative evaluation of the effectiveness of communication between fracture networks.

[0030] 3. The evaluation method provided by this invention is simple and feasible, and can be extended to the quantitative evaluation of the fracture complexity of similar oil and gas reservoirs, thus having good application value. Attached Figure Description

[0031] Figure 1This is a flowchart illustrating a quantitative evaluation method for the degree of fracture development in carbonate reservoirs, as described in this application.

[0032] Figure 2 Imaging logging image of the evaluation section of the target well;

[0033] Figure 3 Comparison chart of fracture angle data at different locations in the evaluation section of the target well;

[0034] Figure 4 A comparison chart of fracture width data at different locations in the evaluation section of the target well. Detailed Implementation

[0035] A quantitative evaluation method for the degree of fracture development in carbonate reservoirs includes the following steps:

[0036] (1) Obtain the fracture angle and fracture width of the evaluation section of the target well;

[0037] (2) The average crack angle is obtained by using the crack angle, and the crack network dispersion index is calculated to evaluate the dispersion complexity of the crack region and indicate the degree of crack extension complexity.

[0038] (3) Calculate the average crack width using the crack width, and use it to calculate the crack connectivity index to evaluate the communication ability between crack networks and represent the degree of change in crack width;

[0039] (4) Calculate the fracture network development index by combining the fracture network dispersion index and the fracture connectivity index, and evaluate the degree of carbonate reservoir development based on the magnitude of the fracture network development index; wherein, the calculation formula for the fracture network development index is as follows:

[0040] ,

[0041] in, The crack network development index is dimensionless. The crack network dispersion index is dimensionless. The crack connectivity index is dimensionless. This represents the average crack width, in micrometers. The maximum width of the crack is measured in micrometers.

[0042] In a preferred embodiment, the evaluation of the development degree of carbonate reservoir based on the fracture network development index is as follows: when 0 < fracture network development index ≤ 0.35, the grade is Class III, indicating poor fracture development; when 0.35 < fracture network development index ≤ 0.70, the grade is Class II, indicating moderate fracture development; when 0.70 < fracture network development index ≤ 1.0, the grade is Class I, indicating extremely well-developed fractures.

[0043] In a preferred embodiment, the crack network dispersion index is calculated as follows:

[0044] ,

[0045] in, The crack angle is expressed in degrees (°). n The number of cracks, expressed in individual cracks. This represents the average crack angle, in degrees. ;

[0046] This is the theoretical maximum crack network dispersion value, which is dimensionless.

[0047] In a preferred embodiment, the theoretical maximum crack network dispersion value is calculated as follows:

[0048] .

[0049] In a preferred embodiment, the crack connectivity index is calculated as follows:

[0050] ,

[0051] in, The crack width is expressed in micrometers.

[0052] K imin To obtain the minimum width in the crack, the unit is micrometer;

[0053] .

[0054] In a preferred embodiment, the fracture angle and fracture width are obtained through imaging logging.

[0055] Example 1

[0056] The invention will be further illustrated using production data from a certain carbonate reservoir as an example.

[0057] A quantitative evaluation method for the degree of fracture development in carbonate reservoirs includes the following steps:

[0058] (1) Q is a typical carbonate reservoir main development block, and Q1 is a vertical well in this block. The fracture angle and fracture width of the evaluation section of well Q1 were obtained using imaging logging interpretation results, as shown in Table 1 and Figure 3-4 As shown;

[0059] Table 1. Basic Data on Fracture Angle and Fracture Width of the Evaluation Section in the Target Well

[0060]

[0061] (2) The average crack angle is obtained using the crack angle, and the crack network dispersion index is calculated to evaluate the dispersion complexity of the crack region and represent the complexity of crack extension, as follows:

[0062] =16, substitute the values ​​of each crack angle in Table 1 into the formula. The average fracture angle of this well section was calculated. =70.1°;

[0063] because >π / 4, therefore, the theoretical maximum crack network dispersion value is... =5.56;

[0064] Furthermore, the crack network dispersion index =0.581;

[0065] (3) Calculate the average crack width using the crack width, and use this to calculate the crack connectivity index to evaluate the communication ability between crack networks and represent the degree of change in crack width, as follows:

[0066] Substitute the values ​​of each crack width in Table 1 into the formula. The average fracture width of this well section was calculated. =94.5 micrometers;

[0067] As shown in Table 1, the maximum and minimum widths of the cracks were 158 micrometers and 27 micrometers, respectively. Substituting these values ​​into the formula... The crack connectivity index was obtained. =0.668;

[0068] (4) Combining the crack network dispersion index and the crack connectivity index, substitute them into the formula. The crack network development index was calculated. The value is 0.462, indicating that the fracture development level of this carbonate reservoir is classified as Class II, with moderate fracture development.

[0069] Traditionally, fracture linear density is used to evaluate the degree of fracture development. Given that the fracture linear density of well Q1 is 1.8 fractures / meter, the fracture linear density in similar reservoirs typically ranges from 0.7 to 2.7 fractures / meter. The following formula can be used to express the traditional evaluation of fracture development in the same form as the fracture network development index:

[0070] =( - ) / ( - ) = 0.55;

[0071] In the formula: This is a traditional crack network development index, dimensionless; Crack linear density, cracks / meter; This represents the highest fracture linear density for similar reservoirs, in terms of fractures per meter. The minimum fracture linear density for similar reservoirs, in terms of fractures per meter;

[0072] The value is 0.55, indicating moderate crack development.

Claims

1. A quantitative evaluation method for the degree of fracture development in carbonate reservoirs, characterized in that: Includes the following steps: (1) Obtain the fracture angle and fracture width of the evaluation section of the target well; (2) Calculate the average crack angle using the crack angle, and then calculate the crack network dispersion index. (3) Calculate the average crack width using the crack width, and then calculate the crack connectivity index. (4) Calculate the fracture network development index by combining the fracture network dispersion index and the fracture connectivity index, and evaluate the degree of carbonate reservoir development based on the magnitude of the fracture network development index; wherein, the calculation formula for the fracture network development index is as follows: ， in, The crack network development index is dimensionless. The crack network dispersion index is dimensionless. The crack connectivity index is dimensionless. This represents the average crack width, in micrometers. The maximum width of the crack is measured in micrometers.

2. The quantitative evaluation method for the degree of fracture development in carbonate reservoirs according to claim 1, characterized in that: The evaluation of carbonate reservoir development based on the fracture network development index is as follows: when 0 < fracture network development index ≤ 0.35, the grade is Class III; when 0.35 < fracture network development index ≤ 0.70, the grade is Class II; when 0.70 < fracture network development index ≤ 1.0, the grade is Class I.

3. The quantitative evaluation method for the degree of fracture development in carbonate reservoirs according to claim 1, characterized in that: The crack network dispersion index is calculated as follows: ， in, The crack angle is expressed in degrees (°). n The number of cracks, expressed in individual cracks. This represents the average crack angle, in degrees. ; This is the theoretical maximum crack network dispersion value, which is dimensionless.

4. The quantitative evaluation method for the degree of fracture development in carbonate reservoirs according to claim 3, characterized in that: The theoretical maximum crack network dispersion value is calculated as follows: 。 5. The quantitative evaluation method for the degree of fracture development in carbonate reservoirs according to claim 1, characterized in that: The crack connectivity index is calculated as follows: ， in, The crack width is expressed in micrometers. K imin To obtain the minimum width in the crack, the unit is micrometer; 。 6. The quantitative evaluation method for the degree of fracture development in carbonate reservoirs according to claim 1, characterized in that: The fracture angle and fracture width were obtained through imaging logging.

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