A method and system for fracture log identification under oil-based mud conditions

Abstract

The application provides a fracture logging identification method and system under oil-based mud conditions, which can improve the precision of fracture logging identification under oil-based mud conditions, effectively evaluate the development degree of formation fractures, and judge the filling property of the fractures. The identification method comprises the following steps: S1, based on the conventional logging data of a formation to be analyzed, rock wave impedance values are calculated according to a density curve and a P-wave interval transit time curve; S2, the rock wave impedance values calculated are corrected according to a shale content curve in the conventional logging data, corrected wave impedance values are calculated according to a preset correction formula, and a corrected wave impedance curve graph is drawn; and S3, whether the fractures of the formation to be analyzed are developed is judged based on the corrected wave impedance values calculated, if the corrected wave impedance values are not greater than a preset wave impedance threshold value, it is indicated that the fractures of the corresponding formation are developed fractures, otherwise, the fractures are undeveloped fractures.

Description

Technical Field

[0001] This invention relates to the technical field of well logging evaluation methods for fractures in tight sandstone, specifically a method and system for fracture identification under oil-based mud conditions. Background Technology

[0002] Currently, for tight oil and gas reservoirs, the presence or absence of fractures determines the enrichment and high production of tight oil and gas. However, due to the limited availability of coring and thin section data, utilizing well logging data, especially imaging logging data, to identify fractures is particularly important.

[0003] However, under oil-based drilling mud conditions, micro-resistivity imaging logging makes it difficult to accurately identify and evaluate fractures due to the formation's loss of conductivity caused by the oil-based drilling fluid. Currently, the main problems with fracture logging identification under oil-based drilling mud conditions are as follows:

[0004] 1. Due to accuracy limitations, micro-resistivity imaging logging under oil-based mud conditions is difficult to accurately identify fractures, and fracture trace identification is more difficult compared to water-based mud conditions.

[0005] 2. Due to the influence of oil-based mud, it is difficult to identify the filling properties of fractures in micro-resistivity imaging logging. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a method and system for fracture logging identification under oil-based mud conditions, which can improve the accuracy of fracture logging identification under oil-based mud conditions, effectively evaluate the degree of formation fracture development, and judge the filling properties of fractures.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for fracture logging identification under oil-based mud conditions includes the following steps:

[0009] S1. Based on conventional well logging data of the formation to be analyzed, the rock wave impedance value is calculated according to the density curve and the longitudinal wave sonic transit time curve.

[0010] S2. Correct the calculated rock wave impedance value based on the mud content curve in the conventional well logging data, calculate the corrected wave impedance value according to the preset correction formula, and draw the corrected wave impedance curve.

[0011] S3. Based on the calculated corrected wave impedance value, determine whether the fractures in the formation to be analyzed are developed. If the corrected wave impedance value is not greater than the preset wave impedance threshold, it means that the fractures in the corresponding formation are developed fractures; otherwise, they are undeveloped fractures.

[0012] Preferably, S1 further includes stripping the calculated abnormal rock wave impedance values;

[0013] The criterion for judging anomalies is that the calculated rock wave impedance value deviates from the preset range of rock wave impedance values ​​corresponding to normal rock physical properties.

[0014] Preferably, in S2, the correction includes,

[0015] Multiple undeveloped fractures at locations with different clay contents in the core were selected, and the changes in rock wave impedance corresponding to changes in clay content were calculated.

[0016] By comparing the relationship between changes in clay content and changes in rock acoustic impedance, the correction formula is obtained as follows:

[0017] When the burial depth is ≤5000m, the corrected wave impedance value Z'=Z+1.3VSH, where VSH is the clay content and Z is the rock wave impedance value;

[0018] When the burial depth is greater than 5000m, the corrected wave impedance value Z' = Z - 1.3VSH.

[0019] Preferably, based on multi-well data, a preset acoustic impedance threshold is set to 1300 kg / cm. 3 ·s.

[0020] Preferably, it also includes qualitatively determining the degree of fracture development in the well section to be analyzed based on the corrected wave impedance value;

[0021] The fracture development zones in the well section are divided according to the corrected wave impedance curve.

[0022] The degree of fracture development is the ratio of the thickness of the fractured section to the depth of the entire well section. The larger the ratio, the higher the degree of fracture development in that well section.

[0023] Preferably, it also includes acquiring oil-based mud imaging logging images and comparing them with calibrated wave impedance curves to establish a fracture identification chart;

[0024] For fractures that can be directly identified on oil-based mud imaging logging images, the calculated corrected wave impedance value is compared with the preset wave impedance threshold to determine whether the fractures in the formation to be analyzed are filled. If the corrected wave impedance value is greater than the wave impedance threshold, it indicates that the fractures in the corresponding formation are filled.

[0025] For fractures that cannot be directly identified on oil-based mud imaging logging images, proceed to step S3 to determine whether the fractures are developed.

[0026] Preferably, the formula for calculating the rock wave impedance value in S1 is as follows:

[0027] Rock wave impedance value Z = 105 ×DEN÷3.28DTC

[0028] Where DEN is the density value and DTC is the longitudinal wave travel time.

[0029] A fracture logging identification system under oil-based mud conditions, comprising,

[0030] The calculation module is used to calculate the rock wave impedance value based on conventional well logging data of the formation to be analyzed, according to the density curve and the P-wave sonic transit time curve.

[0031] The calibration module is used to calculate the calibration impedance value based on the mud content curve in conventional well logging data and the preset calibration formula, and to plot the calibration impedance curve.

[0032] The comparison module is used to determine whether fractures are developed in the formation to be analyzed based on the calculated corrected wave impedance value. If the corrected wave impedance value is less than the preset wave impedance threshold, it indicates that the fractures in the corresponding formation are developed fractures.

[0033] Preferably, it also includes an imaging module for acquiring oil-based mud imaging logging images and comparing them with calibrated wave impedance curves to establish a fracture identification chart.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention provides a fracture logging identification method under oil-based mud conditions, comprising calculating the rock wave impedance value based on the density curve and P-wave sonic transit time curve in conventional logging data; correcting the calculated wave impedance value using the formation clay content obtained from logging, according to different formation depths, thereby eliminating the influence of sandstone and mudstone lithology on the wave impedance value; and judging the fracture development status of the formation to be analyzed by comparing the corrected wave impedance value with the wave impedance threshold, which can improve the accuracy of fracture logging identification under oil-based mud conditions and effectively evaluate the degree of formation fracture development in the absence of imaging data.

[0036] This invention enables more accurate well logging identification of fractures in oil-based mud backgrounds. Firstly, it utilizes a combined acoustic-electric method, integrating imaging logging images and core data, to achieve high-precision fracture identification in oil-based mud backgrounds. Secondly, it is also applicable to fracture identification under water-based mud conditions, enabling effective evaluation of formation fracture development even in the absence of imaging data. Thirdly, it can, to a certain extent, identify fracture filling characteristics in oil-based mud backgrounds. Attached Figure Description

[0037] Figure 1 This is a flowchart of the identification method of the present invention;

[0038] Figure 2 This is a core image for identifying fractures in dense sandstone, as described in this embodiment of the invention.

[0039] Figure 3 This is a graph showing the rock wave impedance value of a single well in an embodiment of the present invention;

[0040] Figure 4 This is a corrected wave impedance curve of the corresponding formation fracture in the embodiment of the present invention;

[0041] Figure 5 This is a crack identification diagram against an oil-based mud background, as described in this embodiment of the invention.

[0042] Figure 6 This refers to the crack trace image identified in the imaging image in the embodiments of the present invention;

[0043] Figure 7 This is a diagram showing the results of single-well fracture logging identification using oil-based mud in an embodiment of the present invention.

[0044] In the figure, a) horizontal tension crack; b) low-angle crack; c) high-angle crack; d) multiple sets of high-angle cracks; e) network crack; f) gypsum filling low-angle crack; g) calcite filling high-angle crack; h) crack dissolution; i) calcite filling and crack dissolution. Detailed Implementation

[0045] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0046] This invention discloses a method for fracture logging identification under oil-based mud conditions, such as... Figure 1 As shown, it includes the following steps:

[0047] S1. Based on conventional well logging data of the formation to be analyzed, the rock wave impedance value is calculated according to the density curve and the longitudinal wave sonic transit time curve.

[0048] S2. Correct the calculated rock wave impedance value based on the mud content curve in the conventional well logging data, calculate the corrected wave impedance value according to the preset correction formula, and draw the corrected wave impedance curve.

[0049] S3. Based on the calculated corrected wave impedance value, determine whether the fractures in the formation to be analyzed are developed. If the corrected wave impedance value is less than the preset wave impedance threshold, it means that the fractures in the corresponding formation are developed fractures; otherwise, they are undeveloped fractures.

[0050] This invention provides a fracture logging identification method under oil-based mud conditions, comprising calculating the rock wave impedance value based on the density curve and P-wave sonic transit time curve in conventional logging data; establishing a mud content of the formation calculated using logging as a boundary at 5000m according to different formation depths to correct the calculated wave impedance value, thereby eliminating the influence of sandstone and mudstone lithology on the wave impedance value; and comparing the corrected wave impedance value with the wave impedance threshold to determine the fracture development status of the formation to be analyzed, which can improve the accuracy of fracture logging identification under oil-based mud conditions and effectively evaluate the degree of formation fracture development in the absence of imaging data.

[0051] Furthermore, S1 also includes stripping away the calculated abnormal rock wave impedance values. By stripping away the abnormal segments of the logging values, the judgment results can be made more accurate and the error can be reduced.

[0052] The criterion for judging anomalies is that the calculated rock wave impedance value deviates from the preset range corresponding to the normal rock physical properties, generally by one or two orders of magnitude. In this embodiment, in S2, the correction includes...

[0053] Multiple undeveloped fractures at locations with different clay contents in the core were selected, and the changes in rock wave impedance corresponding to changes in clay content were calculated.

[0054] By comparing the relationship between changes in clay content and changes in rock acoustic impedance, the correction formula is obtained as follows:

[0055] When the burial depth is ≤5000m, the corrected wave impedance value Z'=Z+1.3VSH, where VSH is the clay content and Z is the rock wave impedance value;

[0056] When the burial depth is greater than 5000m, the corrected wave impedance value Z' = Z - 1.3VSH.

[0057] This invention obtains the corrected wave impedance value by measuring the mud content, thus eliminating the influence of sandstone and mudstone lithology on the wave impedance value.

[0058] In this embodiment, based on multi-well data, a preset wave impedance threshold of 1300 kg / cm is set. 3 ·s.

[0059] Furthermore, it also includes qualitatively determining the degree of fracture development in the well section to be analyzed based on the corrected wave impedance value;

[0060] The fracture development zones in the well section are divided according to the corrected wave impedance curve.

[0061] The degree of fracture development is the ratio of the thickness of the fractured section to the depth of the entire well section. The larger the ratio, the higher the degree of fracture development in that well section.

[0062] Furthermore, it also includes acquiring oil-based mud imaging logging images and comparing them with calibrated wave impedance curves to establish a fracture identification chart;

[0063] For fractures that can be directly identified on oil-based mud imaging logging images, the calculated corrected wave impedance value is compared with the preset wave impedance threshold to determine whether the fractures in the formation to be analyzed are filled. If the corrected wave impedance value is greater than the wave impedance threshold, it indicates that the fractures in the corresponding formation are filled.

[0064] For fractures that cannot be directly identified on oil-based mud imaging logging images, proceed to step S3 to determine whether the fractures are developed.

[0065] This invention also provides a method for establishing a fracture identification map against an oil-based mud background by comprehensively utilizing rock impedance calculations and comparing them with oil-based mud imaging logging images. This method, employing a combination of acoustic and electrical methods, enables more accurate fracture identification against the oil-based mud background. Specifically, based on conventional logging data of rock density and P-wave sonic transit time, the rock impedance value (rock impedance = rock density x P-wave velocity) can be calculated. Since the impedance value is affected by lithology, for tight sandstone formations, the influence of clay content on the impedance needs to be corrected based on the clay content value. This results in a corrected impedance value, which is then used to delineate fracture development zones using a corrected impedance curve based on multi-well data. Finally, by comparing conventional logging acoustic-electrical methods—impedance logging—with imaging logging images and core samples, high-precision fracture identification against the oil-based mud background can be achieved. Through these steps, this invention enables more accurate logging identification of fractures against an oil-based mud background. First, this invention utilizes a combination of acoustic and electrical methods, along with imaging logging images and core data, to achieve high-precision fracture logging identification in oil-based mud conditions. Second, this invention is also applicable to fracture identification under water-based mud conditions, enabling effective evaluation of the degree of formation fracture development even in the absence of imaging data. Third, this invention can, to a certain extent, identify fracture filling properties in oil-based mud conditions.

[0066] In this embodiment, the formula for calculating the rock wave impedance value in S1 is as follows:

[0067] Rock wave impedance value Z = 10 5 ×DEN÷3.28DTC

[0068] Where DEN is the density value and DTC is the longitudinal wave travel time.

[0069] A fracture logging identification system under oil-based mud conditions, comprising,

[0070] The calculation module is used to calculate the rock wave impedance value based on conventional well logging data of the formation to be analyzed, according to the density curve and the P-wave sonic transit time curve.

[0071] The calibration module is used to calculate the calibration impedance value based on the mud content curve in conventional well logging data and the preset calibration formula, and to plot the calibration impedance curve.

[0072] The comparison module is used to determine whether fractures are developed in the formation to be analyzed based on the calculated corrected wave impedance value. If the corrected wave impedance value is less than the preset wave impedance threshold, it indicates that the fractures in the corresponding formation are developed fractures.

[0073] It also includes an imaging module, which is used to acquire oil-based mud imaging logging images and compare them with the calibrated wave impedance curve to establish a fracture identification chart.

[0074] Furthermore, the identification method provided by this invention can be implemented in the Techlog or Resform software platform, but is not limited thereto.

[0075] Example

[0076] In this embodiment, it is implemented using Resform, and its specific implementation method is as follows:

[0077] (1) Obtain the characteristics of fracture development in tight sandstone by core observation, including fracture occurrence, fracture morphology, and fracture filling properties.

[0078] ①For example Figure 2 As shown, cracks can be classified into high-angle cracks, oblique cracks, low-angle cracks, and near-horizontal cracks based on their orientation.

[0079] ② Due to the different filling materials, the cracks exhibit different characteristics. According to the filling material, cracks can be divided into unfilled cracks and filled cracks.

[0080] (2) The rock wave impedance value is calculated using the density curve and longitudinal wave sonic time difference curve in conventional well logging data. Rock wave impedance Z = rock density × longitudinal wave velocity.

[0081] The actual calculation formula using conventional well logging data is Z = 10. 5 ×DEN÷3.28DTC;

[0082] Z is the wave impedance, and its unit is kg / cm. 3 ·s, DEN is the density value, the unit is g / cm³ 3 DTC stands for longitudinal wave time difference, and its unit is μs / ft.

[0083] Among them, for abnormal logging values, that is, logging curve values ​​that deviate from the logging values ​​corresponding to normal rock physical properties, usually differing from the normal range by one or two orders of magnitude, they should be stripped away, such as... Figure 3 As shown.

[0084] (3) The calculated wave impedance is corrected based on the mud content curve VSH obtained from conventional logging data, and the corrected wave impedance value is obtained.

[0085] ① Select multiple undeveloped fractures with different clay contents from imaging logging and core samples to obtain the change in wave impedance value under varying clay content.

[0086] ② By comparing the relationship between clay content and wave impedance in unfractured formations across multiple wells, it was found that for every 1% increase in clay content, the wave impedance value changes by approximately 1.3 kg / cm². 3 ·s;

[0087] Due to the influence of burial depth, when the burial depth is <5000m, the wave impedance of sandstone is > that of mudstone. The correction formula is: Corrected wave impedance Z' = Z + 1.3VSH;

[0088] When the burial depth is greater than 5000m, the wave impedance of sandstone is less than that of mudstone. The correction formula is: Corrected wave impedance Z' = Z - 1.3VSH.

[0089] ③ Based on the comparison of wave impedance calculations using multi-well data, it is considered that when no fractures are developed, the formation wave impedance value is >1300Kg / cm. 3 ·s, therefore set at 1300Kg / cm 3 • s is the boundary; when the wave impedance value is <1300Kg / cm 3 When the time is ·s, it is determined that the strata have developed fractures, such as Figure 4 As shown.

[0090] (4) A fracture identification chart under oil-based mud background was established by comparing the corrected wave impedance curve with the obtained oil-based mud imaging logging images and core samples.

[0091] ① Establish a chart by comparing the fracture development zones delineated from the corrected impedance curve with oil-based mud imaging logging images, such as... Figure 5 As shown;

[0092] ② Obvious fracture patterns were found on the oil-based mud imaging logging image. These can be directly picked up on the image, and it can be determined whether the corrected impedance value of the fracture is greater than 1300 kg / cm². 3 If the value is greater than s, it indicates that the crack is filled, which can help identify the filling property of the crack to some extent. Figure 6 As shown.

[0093] ③ The fracture features on oil-based mud imaging logging images are blurry, which may lead to missed detections. Alternatively, for wells lacking imaging data, the ability to detect the fracture can be determined by comparing the corrected impedance curve. If the corrected impedance value is less than 1300 kg / cm², the fracture may be missed. 3·s indicates that it is a fracture development zone. The degree of fracture development in a well section is qualitatively determined by dividing the fracture development zone into fracture development zones using a corrected impedance curve. The larger the proportion of fracture development zones to the entire well section, the more developed the fractures. Figure 7 As shown.

[0094] The identification method provided by this invention compares the wave impedance method with the imaging logging image, reflecting its good identification effect. It can also be seen that the wave impedance method has a good correction and discrimination effect on the missed cracks in the imaging logging image.

Claims

1. A method of fracture log identification under oil-based mud conditions, characterized by, Includes the following steps, S1, based on the conventional logging data of the formation to be analyzed, the rock wave impedance value is calculated according to the density curve and the longitudinal wave sonic time difference curve; the rock wave impedance value Z=10 5 ×DEN÷3.28DTC, Where DEN is the density value and DTC is the longitudinal wave acoustic time difference; S2. Correct the calculated rock impedance value based on the clay content curve in conventional well logging data. Calculate the corrected impedance value according to the preset correction formula and plot the corrected impedance curve. The correction includes... Multiple undeveloped fractures at locations with different clay contents in the core were selected, and the changes in rock wave impedance corresponding to changes in clay content were calculated. By comparing the relationship between changes in clay content and changes in rock acoustic impedance, the correction formula is obtained as follows: When the burial depth is ≤5000m, the corrected wave impedance value Z'=Z+1.3VSH, where VSH is the clay content and Z is the rock wave impedance value; When the burial depth is greater than 5000m, the corrected wave impedance value Z' = Z - 1.3VSH; S3. Based on the calculated corrected wave impedance value, determine whether the fractures in the formation to be analyzed are developed. If the corrected wave impedance value is not greater than the preset wave impedance threshold, it means that the fractures in the corresponding formation are developed fractures; otherwise, they are undeveloped fractures. It also includes acquiring oil-based mud imaging logging images and comparing them with calibrated wave impedance curves to create a fracture identification chart; For fractures that can be directly identified on oil-based mud imaging logging images, the calculated corrected wave impedance value is compared with the preset wave impedance threshold to determine whether the fractures in the formation to be analyzed are filled. If the corrected wave impedance value is greater than the wave impedance threshold, it indicates that the fractures in the corresponding formation are filled. For fractures that cannot be directly identified on oil-based mud imaging logging images, proceed to step S3 to determine whether the fractures are developed.

2. The method of claim 1, wherein, S1 also includes stripping away the calculated abnormal rock wave impedance values; The criterion for judging anomalies is that the calculated rock wave impedance value deviates from the preset range of rock wave impedance values ​​corresponding to normal rock physical properties.

3. The method of claim 1, wherein, Based on multi-well data, the preset wave impedance threshold is set to 1300 Kg / cm 3 ·s.

4. The method of claim 1, wherein, It also includes qualitatively determining the degree of fracture development in the well section to be analyzed based on the corrected wave impedance value; The fracture development zones in the well section are divided according to the corrected wave impedance curve. The degree of fracture development is the ratio of the thickness of the fractured section to the depth of the entire well section. The larger the ratio, the higher the degree of fracture development in that well section.

5. A fracture log identification system under oil-based mud conditions, characterized by, The fracture logging identification method based on any one of claims 1-4 includes, The calculation module is used to calculate the rock wave impedance value based on conventional well logging data of the formation to be analyzed, according to the density curve and the P-wave sonic transit time curve. The calibration module is used to calculate the calibration impedance value based on the mud content curve in conventional well logging data and the preset calibration formula, and to plot the calibration impedance curve. The comparison module is used to determine whether the fractures in the formation to be analyzed are developed based on the calculated corrected wave impedance value. If the corrected wave impedance value is less than the preset wave impedance threshold, it indicates that the fractures in the corresponding formation are developed fractures. The imaging module is used to acquire oil-based mud imaging logging images and compare them with the calibrated wave impedance curve to establish a fracture identification chart.

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