A method for evaluating wellbore fractures using photoelectric absorption cross-section index

Abstract

The invention discloses a method for evaluating wellbore cracks by using a photoelectric absorption cross-section index, comprising the following steps: a. qualitatively identifying cracks by using a Pe curve: obtaining the lithology density logging information of the wellbore to be tested, extracting the photoelectric absorption cross-section index Pe from the lithology density logging information, and then obtaining the Pe curve response characteristics to identify whether the wellbore to be tested has cracks; b. constructing a mathematical model of the crack development index of the wellbore to be tested: according to the Pe curve of the wellbore to be tested obtained in step a, the density of the wellbore drilling fluid and the Pe values ​​of different rocks / minerals are known, and a mathematical model between the crack development index of the wellbore to be tested and the above factors is constructed; c. evaluating the effectiveness and development scale of cracks: according to step b, the crack development index of the wellbore to be tested is obtained, and the effectiveness and development scale of the cracks in the wellbore to be tested are evaluated, thereby solving the problems of poor oil and gas exploration and development effects and high costs of oil and gas evaluation wells and development wells in the prior art.

Description

Technical Field

[0001] The invention belongs to the technical field of well logging data evaluation, relates to a wellbore crack evaluation technology, and specifically relates to a method for evaluating wellbore cracks with a photoelectric absorption cross-section index. Background Art

[0002] Fractures are discontinuous surfaces formed by the fracturing of rocks. The causes of underground fractures are complex, the controlling and influencing factors are diverse, the formation and development are random, and the distribution is highly heterogeneous. Fracture evaluation is still a world-class problem in the current petroleum geology community, but fractures are both storage spaces for oil and gas and infiltration channels for oil and gas, so it is of great significance to correctly evaluate fractures.

[0003] At present, the commonly used logging fracture evaluation methods are electrical imaging logging and deep and shallow dual lateral logging technology. For example, Qu Haizhou, Zhang Fuxiang, et al. published an article entitled "Quantitative characterization method of fractures based on core-electrical imaging logging - taking the Cretaceous Bashkiqik Formation sandstone in the KS2 block of the Kuqa Depression as an example" in the journal Petroleum Exploration and Development, 2016, 43 (3); Huang Jixin, Peng Shimi, et al. published an article entitled "Application of imaging logging data in fracture and geostress research" in the journal Acta Petrolei Sinica, 2006, 27 (6).

[0004] However, for oil and gas evaluation wells and development wells, electrical imaging logging data is generally scarce and expensive. Due to its shallow radial detection depth (only 1 inch), it is difficult to meet the needs of oil and gas exploration and development. Therefore, it is necessary to find a logging technology with more common data to replace the existing electrical imaging logging technology, and to achieve the effect of electrical imaging logging fracture evaluation. A low-cost technical solution is better. Summary of the invention

[0005] The purpose of the present invention is to provide a method for evaluating wellbore fractures using a photoelectric absorption cross-section index, which is used to evaluate wellbore fractures in oil and gas evaluation wells and development wells to meet the needs of oil and gas exploration and development in oil and gas evaluation wells and development wells in the prior art.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for evaluating wellbore fractures using a photoelectric absorption cross-sectional index comprises the following steps:

[0008] a. Qualitative identification of cracks using Pe curve

[0009] Obtain the lithology density logging information of the wellbore to be tested, and extract the photoelectric absorption cross-section index Pe from the lithology density logging information, analyze and extract the response characteristics of the Pe curve, and identify whether the wellbore to be tested has cracks;

[0010] b. Construct a mathematical model of the fracture development index of the wellbore to be tested

[0011] According to the Pe curve of the wellbore to be tested obtained in step a, the density of the wellbore drilling fluid and the Pe values ​​of different rocks / minerals are known, and a mathematical model is constructed between the fracture development index of the wellbore to be tested and the density of the wellbore drilling fluid, the Pe values ​​of different rocks / minerals, the Pe value of barite and the Pe curve of the wellbore to be tested;

[0012] c. Evaluation of fracture effectiveness and development scale

[0013] According to step b, the fracture development index of the wellbore to be tested is obtained to evaluate the fracture effectiveness and fracture development scale of the wellbore to be tested.

[0014] Furthermore, in step a, the method for identifying whether the wellbore to be tested has cracks is:

[0015] a1. When barite is added to the drilling fluid, the photoelectric absorption cross-section index Pe0 value of the well section with the same lithology is analyzed;

[0016] b1. Reanalyze the abnormally increased photoelectric absorption cross-section index Pe1 value in the well section with the same lithology;

[0017] c1. Compare Pe1 with Pe0. If the Pe1 value is significantly greater than the Pe0 value, it indicates that there are cracks in the wellbore to be tested.

[0018] Furthermore, in step b, the mathematical model of the crack development index is:

[0019] a2. First, obtain the photoelectric absorption cross-section index Pe curve of the wellbore section with the same lithology and relatively dense to be measured, and calculate the cumulative average Pe value:

[0020] , ,

[0021] In the formula, It represents the average value of the photoelectric absorption cross-section index of the same lithology and denser well section. is the photoelectric absorption cross section index at each depth, To end the depth, is the starting depth, is the depth sampling interval, n Represents the number of depth sampling points;

[0022] b2. Then through actual measurement Values ​​and known rock theory Value, barite theory Value, drilling fluid density , construct the fracture development index and , , , The data model between:

[0023] ,

[0024] In the formula, is the crack development index, It is the lowest value of the photoelectric absorption cross-section index in the same lithology section.

[0025] Furthermore, in step b2, the measured The value is obtained by extracting the photoelectric absorption cross-section index from the lithology density logging information.

[0026] Furthermore, in step c, the evaluation method for the fracture effectiveness and fracture development scale of the wellbore to be tested is: the fracture development index obtained in step b2 To evaluate the effectiveness of the wellbore to be tested, the larger the fracture development index, the better the effectiveness of the fracture.

[0027] Furthermore, in step a2, the depth sampling interval is 0.125 m.

[0028] Furthermore, the method combines the drilling coring technology, the imaging logging calibration technology and the oil and gas production to establish the evaluation criteria of the three levels of fracture development scale: large, medium and small. When in step b2, ∈(0,3], the scale of fracture development in the wellbore to be measured is relatively small; ∈(3,8], the scale of fracture development is medium; When >8, the scale of crack development is larger.

[0029] Furthermore, in the step a, lithology density logging technology is used to obtain lithology density logging information of the wellbore to be tested.

[0030] Further, in step b2, the Pe value of dolomite is 3.142 b / ev, the Pe value of limestone is 5.084 b / ev, and the Pe value of barite is 266.8 b / ev.

[0031] Furthermore, the lithology density logging information is obtained using a lithology density logging instrument.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] 1. In the present invention, the method of this scheme is used, the principle of photoelectric effect is applied, and the photoelectric absorption cross-section index (Pe) in the lithology density logging information is used to indicate the development of wellbore fractures. Then, by analyzing the wellbore drilling fluid density, the Pe values ​​of different rocks / minerals, the Pe value of barite and other information, and constructing a mathematical model of fracture development index, the constructed mathematical model is then used to quantitatively calculate and evaluate the degree of fracture development in the wellbore to be measured. The lithology density acquisition technology used in this method is already a relatively common conventional logging technology for various oil and gas wells, and has the advantages of being inexpensive and good quality.

[0034] 2. In the present invention, the scheme provided can effectively and accurately evaluate the development of wellbore cracks, overcoming the shortcomings of using electrical imaging logging technology to evaluate the crack information of the wellbore to be tested, such as high price, shallow radial detection depth, and difficulty in evaluating the effectiveness of cracks. For example, when barite is added to the drilling fluid, the photoelectric absorption cross-section index Pe can indicate the opening of wellbore cracks, solving the problem that electrical imaging logging is difficult to evaluate the effectiveness of wellbore cracks and the problem that wellbore cracks cannot be evaluated without electrical imaging logging. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is the photoelectric absorption cross-section index crack evaluation diagram of Well L2 in Example 1. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0037] Example 1

[0038] A method for evaluating wellbore cracks by photoelectric absorption cross-sectional index belongs to the technical field of well logging data evaluation, and comprises the following steps:

[0039] a. Qualitative identification of cracks using Pe curve

[0040] Obtaining the lithology density logging information of the wellbore to be tested, and extracting the photoelectric absorption cross-section index Pe from the lithology density logging information, analyzing and extracting the Pe curve response characteristics, and identifying whether the wellbore to be tested has cracks;

[0041] b. Construct a mathematical model of the fracture development index of the wellbore to be tested

[0042] If it is known that the wellbore to be tested has cracks according to the Pe curve of the wellbore to be tested obtained in step a, then continue to perform the following steps, knowing the density of the wellbore drilling fluid and the Pe values ​​of different rocks / minerals, and constructing a mathematical model between the fracture development index of the wellbore to be tested and the density of the wellbore drilling fluid, the Pe values ​​of different rocks / minerals, the Pe value of barite, and the Pe curve of the wellbore to be tested;

[0043] c. Evaluation of fracture effectiveness and development scale

[0044] According to step b, the fracture development index of the wellbore to be tested is obtained to evaluate the fracture effectiveness and fracture development scale of the wellbore to be tested.

[0045] Example 2

[0046] A method for evaluating wellbore cracks using a photoelectric absorption cross-sectional index, which relates to a wellbore crack evaluation technology, comprises the following steps:

[0047] a. Qualitative identification of cracks using Pe curve

[0048] Obtain the lithology density logging information of the wellbore to be tested, and extract the photoelectric absorption cross-section index Pe from the lithology density logging information, analyze and extract the response characteristics of the Pe curve, and identify whether the wellbore to be tested has cracks;

[0049] b. Construct a mathematical model of the fracture development index of the wellbore to be tested

[0050] According to the Pe curve of the wellbore to be tested obtained in step a, the density of the wellbore drilling fluid and the Pe values ​​of different rocks / minerals are known, and a mathematical model is constructed between the fracture development index of the wellbore to be tested and the density of the wellbore drilling fluid, the Pe values ​​of different rocks / minerals, the Pe value of barite and the Pe curve of the wellbore to be tested;

[0051] c. Evaluation of fracture effectiveness and development scale

[0052] According to step b, the fracture development index of the wellbore to be tested is obtained to evaluate the fracture effectiveness and fracture development scale of the wellbore to be tested.

[0053] Furthermore, in step a, the method for identifying whether the wellbore to be tested has cracks is:

[0054] a1. When barite is added to the drilling fluid, the photoelectric absorption cross-section index Pe0 value of the well section with the same lithology is analyzed;

[0055] b1. Reanalyze the abnormally increased photoelectric absorption cross-section index Pe1 in the well section with the same lithology;

[0056] c1. Compare Pe1 with Pe0. If the Pe1 value is significantly greater than the Pe0 value, it indicates that there are cracks in the wellbore to be tested.

[0057] Furthermore, in step b, the mathematical model of the crack development index is:

[0058] a2. First, obtain the photoelectric absorption cross-section index Pe curve of the wellbore section with the same lithology and relatively dense to be measured, and calculate the cumulative average Pe value:

[0059] , ,

[0060] In the formula, It represents the average value of the photoelectric absorption cross-section index of the same lithology and denser well section. is the photoelectric absorption cross section index at each depth, To end the depth, is the starting depth, is the depth sampling interval, n represents the number of depth sampling points, where the depth sampling interval is preferably 0.125m;

[0061] b2. Then through actual measurement Values ​​and known rock theory Value, barite theory Value, drilling fluid density , construct the fracture development index and , , , The data model between:

[0062] ,

[0063] In the formula, is the crack development index, It is the lowest value of the photoelectric absorption cross-section index in the same lithology section.

[0064] Preferably, in step b2, the measured The value is obtained by extracting the photoelectric absorption cross-section index from the lithology density logging information.

[0065] Furthermore, in step c, the evaluation method for the fracture effectiveness and fracture development scale of the wellbore to be tested is: the fracture development index obtained in step b2 To evaluate the effectiveness of the wellbore to be tested, the larger the fracture development index, the better the effectiveness of the fracture.

[0066] Furthermore, in the step a, lithology density logging technology is used to obtain lithology density logging information of the wellbore to be tested.

[0067] Now, taking a L2 well (known well) in the XX structure of the Sichuan Basin as an example, fracture identification and fracture development index calculation are performed to illustrate this technical solution.

[0068] Step 1: Use Pe curve to qualitatively identify cracks

[0069] Using the aforementioned method of qualitatively identifying fractures using the Pe curve, it can be determined that the well has fractures.

[0070] The coring data and logging data show that the L2 well section 6111-6150m is dolomite. 3 ), see Figure 1 , electrical imaging near 6133.5m ( Figure 1 Electrophysiological imaging) and deep and shallow bilateral lateral (see Figure 1 The cracks are not obvious on the well logging diagram (curves 5 and 6 in the middle label), but the photoelectric absorption cross-section index increases significantly (see Figure 1 The curve with the number 3 in the middle indicates the development of fractures. Cracks are also found in the core samples (see Figure 1 (Figure 2(a) shows that the Pe curve is effective in qualitatively identifying cracks.

[0071] Further, it is known that the Pe value of dolomite is 3.142 b / ev, the Pe value of limestone is 5.084 b / ev, and the Pe value of barite is 266.8 b / ev.

[0072] Step 2: Calculate the crack development index

[0073] Through the mathematical model of fracture development index Calculate the fracture development index. The fracture development index near 6133.5m is Fi 1 is 4.5, and the fracture development index near 6112 m Fi 2 is 0.

[0074] Furthermore, this solution combines the drilling coring technology, imaging logging calibration technology and oil and gas production to establish three levels of evaluation criteria for fracture development: large, medium and small. In step b3, ∈(0,3], the scale of fracture development in the wellbore to be measured is relatively small; ∈(3,8], the scale of fracture development is medium; When >8, the scale of crack development is larger.

[0075] Of course, according to different geological environments, the fracture development index can be combined with local oil and gas production and local discrimination criteria to establish evaluation criteria for fracture development scales at different levels, and finally divide the fracture development index into The value range of .

[0076] Step 3: Evaluation of fracture effectiveness and development scale

[0077] According to the fracture development index calculated in the second step Fi 1. According to the evaluation method of this scheme, the scale of fracture development near 6133.5m is small, and the scale of fracture development near 6112m is medium. Finally, the test oil production of 6112-6132m section is 71.2×10 4 m3 / d, which is consistent with the medium-scale development of fractures. The well section 6143-6150m oil test produced 12.5×10 4 m 3 / d, which is consistent with the small scale of fracture development.

[0078] Preferably, in step a, lithology density logging technology is used to obtain lithology density logging information of the wellbore to be tested.

[0079] Furthermore, the lithology density logging information is obtained using a lithology density logging instrument.

[0080] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for evaluating wellbore fractures using photoelectric absorption cross-sectional index, characterized in that: The following steps are involved: a. use P e Qualitative identification of cracks using curves Obtain the lithology density logging information of the wellbore to be tested, and extract the photoelectric absorption cross-section index from the lithology density logging information P e , analyzed and extracted P e Curve response characteristics to identify whether the wellbore to be tested has fractures; The method for identifying whether the wellbore to be tested has cracks is: a1. Analyze the photoelectric absorption cross-section index of the same lithology section when barite is added to the drilling fluid P e0 value; b1. Reanalyze the abnormally increased photoelectric absorption cross-section index in the well section with the same lithology P e1 value; c1. P e1 and P e0 For comparison, P e1 The value is significantly greater than P e0 If the value is too low, it indicates that there are cracks in the wellbore; b. Construct a mathematical model of the fracture development index of the wellbore to be tested The wellbore to be tested obtained according to step a P e Curve, the density of the drilling fluid in the wellbore, the P e The fracture development index of the wellbore to be tested is constructed based on the wellbore drilling fluid density, different rocks / minerals P e Barite P e Value and wellbore to be tested P e Mathematical models between curves; The mathematical model of the fracture development index is: a2. First, obtain the photoelectric absorption cross-section index of the wellbore section with the same lithology and relatively dense to be measured P e curve and calculate the cumulative average P e value: ， ， In the formula, It represents the average value of the photoelectric absorption cross-section index of the same lithology and denser well section. P e i is the photoelectric absorption cross section index at each depth, DEP en To end the depth, DEP st is the starting depth, Δ si is the depth sampling interval, n Represents the number of depth sampling points; b2. Then through actual measurement P e Values ​​and known rock theory P e m Value, barite theory P e b Value, drilling fluid density ρ m , construct the fracture development index F i and F e , P e m , P e b , ρ m The data model between: ， In the formula, F i is the crack development index, P e min It is the minimum value of the photoelectric absorption cross-section index in the well section with the same lithology; c. Evaluation of fracture effectiveness and development scale According to step b, the fracture development index of the wellbore to be tested is obtained, and the fracture effectiveness and fracture development scale of the wellbore to be tested are evaluated; The evaluation method of the fracture effectiveness and fracture development scale of the wellbore to be tested is: the fracture development index obtained in step b2 F i To evaluate the effectiveness of the wellbore to be tested, the larger the fracture development index, the better the effectiveness of the fracture.

2. The method for evaluating wellbore fractures using photoelectric absorption cross-section index according to claim 1, characterized in that: In step b2, the measured P e The value is obtained by extracting the photoelectric absorption cross-section index from the lithology density logging information.

3. The method for evaluating wellbore fractures by using photoelectric absorption cross-section index according to claim 2, characterized in that: In step a2, the depth sampling interval is 0.125 m.

4. The method for evaluating wellbore fractures using photoelectric absorption cross-sectional index according to claim 3, characterized in that: This method combines drilling coring technology, imaging logging calibration technology, and oil and gas production to establish three evaluation criteria for fracture development scale: large, medium, and small. When in step b2, F i ∈(0,3], the scale of fracture development in the wellbore to be measured is relatively small; F i ∈(3,8], the scale of fracture development is medium; F i When >8, the scale of crack development is larger.

5. The method for evaluating wellbore fractures using photoelectric absorption cross-sectional index according to claim 4, characterized in that: In the step a, the lithology density logging technology is used to obtain the lithology density logging information of the wellbore to be tested.

6. The method for evaluating wellbore fractures by using photoelectric absorption cross-section index according to claim 1, characterized in that: In step b2, the dolomite P e The value is 3.142b / ev, the limestone P e The value is 5.084b / ev, barite P e The value is 266.8 b / ev.

7. The method for evaluating wellbore fractures using photoelectric absorption cross-sectional index according to claim 5, characterized in that: The lithology density logging information is obtained by using a lithology density logging instrument.

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