A shale core retrieval method

Abstract

The present invention relates to the technical field of core retrieval research, and in particular discloses a shale core retrieval method. The Al element in the core sample is accurately analyzed using an energy dispersive X-ray fluorescence spectrometer. The significant positive correlation between the Al element and the natural gamma log curve is utilized, and the two are combined. The core marking depth is gradually adjusted, and the correlation between the change in Al element content and the natural gamma log curve is utilized to optimize the retrieval result. This process reduces the influence of human subjective factors, ensures the accuracy and consistency of the retrieval, simplifies the operation process, and reduces the cost of the retrieval process. This technical solution effectively overcomes the shortcomings of the existing technology and provides more reliable technical support for the precise exploration of shale resources.

Description

Technical Field

[0001] The invention relates to the technical field of core retrieval research, and in particular discloses a shale core retrieval method. Background Art

[0002] As global energy demand continues to grow, shale oil, as a key component of unconventional oil and gas resources, has garnered widespread attention. Significant progress has been made in shale oil exploration in China's Ordos, Sichuan, and Bohai Bay Basins, with large-scale exploration and development initiatives initiated in several areas. Shale oil is typically found in mudstone shales deposited in semi-deep to deep lake environments. These rocks exhibit significant lithologic variation and heterogeneity due to their complex sedimentary environments, posing significant challenges to resource identification and development.

[0003] In shale oil exploration, cores provide the most direct and reliable data for characterizing subsurface geology. Core analysis allows for formation correlation and the study of key characteristics such as lithology, electrical properties, and physical properties. However, due to differences in drill pipe tension during coring and cable tension in the logging system, core drilling depths and logging depths may differ. Therefore, precise core positioning is essential before detailed analysis can be performed. Accurate core positioning directly impacts subsequent logging data analysis and reservoir evaluation, making it a crucial step in the entire exploration process.

[0004] Existing core retrieval methods typically rely on manual operation, visually observing typical lithologic sections or lithologic combinations and using marker layers as the basis for segmentation control. This method is relatively effective in sandstone and mudstone sections, but in shale, the complex lithology and difficult-to-identify marker layers make the retrieval process not only challenging but also susceptible to operator judgment. In recent years, the introduction of advanced technologies such as high-resolution nuclear magnetic resonance logging and helium injection porosity analysis has improved retrieval accuracy, but high costs and complex operations have limited its application in large-scale exploration.

[0005] Therefore, the main challenges currently faced in core retrieval in shale include: large retrieval errors due to complex lithology, strong subjectivity and poor consistency of results; and the high cost and operational complexity of existing technologies. To address these issues, there is an urgent need to develop a more accurate, simple, and applicable core retrieval method for shale that can reduce the influence of subjective factors, adapt to the complex lithologic characteristics of shale, and thus support the efficient exploration and development of unconventional oil and gas resources. Summary of the Invention

[0006] The purpose of the present invention is to address the deficiencies in the prior art and provide a shale core retrieval method based on natural gamma ray logging response and Al element analysis to improve the accuracy of core retrieval in shale formations, reduce exploration risks, and increase the success rate of exploration of high-yield well locations.

[0007] The specific technical steps of the present invention are as follows:

[0008] S1. Core sample preparation and data collection

[0009] For key continuous coring wells, all core samples were sectioned, polished, and scanned using optical scanning to obtain high-resolution core scan images and perform detailed descriptions down to the centimeter level. After processing, the optical scanning images, detailed descriptions, natural gamma-ray (GR) log data, and other relevant analytical data were imported into Resform geological software to prepare for subsequent core retrieval analysis.

[0010] S2. Al element content data collection

[0011] A portable energy-dispersive X-ray fluorescence spectrometer (pXRF) was used to accurately analyze aluminum in core samples. As an indicator of shale composition, aluminum has a significant positive correlation with natural gamma-ray logs. By collecting aluminum content data at regular intervals, reliable baseline data was provided for subsequent core retrieval.

[0012] S3. Construction of core homing dataset

[0013] Based on the collected Al element data and GR logging data, a core retrieval dataset was compiled and constructed. This involved collecting and compiling GR logging data at the corresponding depth, based on the annotated depth of each Al element sampling point. Given that shale is more continuous and dense than conventional sandstone, and given the limitations of conventional core retrieval depths, additional GR logging data was collected and compiled beyond a certain depth limit (4 meters, adjustable based on actual conditions) for the coring run to ensure the completeness and accuracy of the dataset.

[0014] S4. Core position adjustment and optimization

[0015] During the homing process, the marking depth of the core is gradually adjusted, and the homing results are optimized by analyzing the correlation between the Al element content and the GR logging curve. The specific operation includes gradually adjusting the marking depth within a certain depth range, performing multiple correlation calculations and comparisons, and saving the correlation coefficient R after each adjustment. 2 The adjusted depth corresponding to the maximum correlation coefficient is finally selected as the final homing depth of the core section.

[0016] S5. Repositioning of the entire well section

[0017] The core homing process of the entire well section is carried out section by section in the same way, and the core homing work of the entire well section is finally completed.

[0018] Beneficial effects of the present invention:

[0019] (1) This invention makes full use of the Al element analysis data and natural gamma ray logging curves of shale core samples. Through a systematic relocation adjustment method, the accuracy of shale core relocation is significantly improved. By utilizing the positive correlation between Al and mud composition and combining it with natural gamma ray logging data, the depth calibration of the core is more accurate, while reducing the influence of human subjective factors and ensuring the consistency of the relocation results.

[0020] (2) The elemental analysis was performed using a portable energy dispersive X-ray fluorescence spectrometer (pXRF), which is easy to operate, low cost, suitable for large-scale application, and has good economic benefits.

[0021] (3) The present invention effectively overcomes the shortcomings of the existing technology, has strong operability, and produces accurate results. It can be widely applied to the exploration and development of mud shale and other fine-grained sedimentary rocks, has good prospects for promotion and application, and provides more reliable technical support for the precise exploration of mud shale resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described below with reference to the accompanying drawings.

[0023] Figure 1 This is a flow chart of an embodiment of a method for shale core retrieval according to the present invention;

[0024] Figure 2 This is a graph showing the correlation coefficient change trend during the core depth adjustment process of a specific embodiment;

[0025] Figure 3 This is a comparison chart of the matching effect between the Al element curve and the natural gamma ray (GR) logging curve at different depths before and after the core is returned to the specific embodiment;

[0026] Figure 4 This is a comparison diagram of the Al element and GR logging values ​​before and after the core is returned to the specific embodiment. DETAILED DESCRIPTION

[0027] To clarify the technical solution of the present invention, the following detailed description of the shale core retrieval method is provided, using core samples from the upper Sha 4th sub-member of Well NY1 in the Dongying Depression as a specific example. The described embodiments are only a subset of the present invention, not the complete set. All other embodiments derived by persons of ordinary skill in the art based on the embodiments described herein without inventive effort are considered within the scope of protection of the present invention.

[0028] Figure 1 As shown, the method for lacustrine shale sequence stratigraphic division based on principal component analysis of the present invention has the following specific implementation steps:

[0029] S1. Core sample preparation and data collection

[0030] Core sample processing is particularly important. First, continuous coring is performed within the selected well section. Each core section is sectioned and polished to obtain a clear core surface, ensuring that all surface details are captured. Subsequently, high-resolution scanning equipment is used to perform general optical scanning on the processed core samples, and detailed geological descriptions are performed to record every subtle feature of the core. These images and descriptions, along with natural gamma-ray log data, are input into Resform geological analysis software to provide data support for retrieval.

[0031] S2. Al element content data collection

[0032] During core repositioning, Al concentration is a key indicator. High-precision elemental analysis was performed on processed core samples using a portable X-ray fluorescence spectrometer (pXRF). Al concentrations were collected at 0.125 m intervals, and the results showed a high correlation between changes and the GR log. These measurements will serve as a crucial data foundation for subsequent core depth repositioning, providing more accurate support.

[0033] S3. Construction of core homing dataset

[0034] When constructing the homing dataset, the annotated depth of each Al sampling point was used as a benchmark to collate the GR logging values ​​at the corresponding depth. To ensure the accuracy of shale homing, in addition to collating the GR logging curves for the coring section, additional GR logging data was collected within a 2-meter range exceeding the upper and lower limits of the coring. This extended data processing helps reduce potential errors in the homing process and improve the accuracy of the final results. For example, the mth coring of the upper Sha 4 sub-section of Well Niuye 1 was performed, with a sampling interval of 0.125 meters and a total of 140 sample points. The Al concentration data and the corresponding GR logging response values ​​for each sample point were compiled into the dataset. The Al concentration reflects the changes in the shale composition, while the GR logging curve provides information on the physical properties of the formation. The correlation between the two is the key basis for core homing. For example, data from some sample points showed that at depths of 3359.72 meters, 3362.72 meters, and 3365.72 meters, Al concentrations were 5178 ppm, 4954 ppm, and 3462 ppm, respectively, while GR logging response values ​​were 67.963, 83.015, and 69.196, respectively. This clearly demonstrates a positive correlation between the two, with fluctuations in Al concentration accompanied by corresponding changes in GR logging response values. This detailed data compilation formed a complete homing dataset, providing a solid foundation for subsequent core depth homing adjustments and ensuring improved core homing accuracy.

[0035] Table 1 is a comparison table of Al element concentrations and natural gamma ray (GR) logging response values ​​collected from some sample points of the mth coring in the upper Sha 4 sub-member of Well Niuye 1, showing the changes in Al element and GR logging values ​​at different depths, providing a reliable data basis for repositioning adjustments.

[0036]

[0037] S4. Core position adjustment and optimization

[0038] During the repositioning adjustment process, the adjustment interval was set from -2m to +2m with a step length of 0.1m. The marked depth of the core section was gradually adjusted within this interval, and multiple adjustment operations were performed. Specifically, during each adjustment, the concentration of the Al element was compared with the GR logging curve value corresponding to the newly marked core depth, and the correlation coefficient R was calculated. 2 Each adjusted R 2 The values ​​will be saved, and the adjustment depth corresponding to the maximum correlation coefficient will be selected as the final homing adjustment depth of the coring section.

[0039] Figure 2 The following is a trend chart of the correlation coefficient change during the core depth adjustment process. When the core depth is gradually adjusted in steps of 0.1m in the coring section depth range of 3359.72m-3377.33m, the correlation coefficient of the Al element concentration and the natural gamma GR logging curve value corresponding to each adjusted depth changes. In this example, the coring section depth range is 3359.72m-3377.33m. By gradually adjusting the core depth, it is found that when the depth is adjusted by 1.2m, the correlation coefficient reaches a maximum value of 0.6190, which is 0.3805 higher than the 0.2385 before adjustment. Figure 4 As shown, the effectiveness and accuracy of the homing method of the present invention are significantly verified.

[0040] Figure 3 The following figure compares the matching effects of the Al element curve and the natural gamma ray (GR) logging curve at different depths before and after the core is returned. After the core marked depth is lowered by 1.2m, the change trend of the Al element curve and the natural gamma ray GR logging curve has the highest consistency.

[0041] S5. Repositioning of the entire well section

[0042] After adjusting a single coring section and achieving optimal placement results, the same technical steps are used to position the cores for the entire well section, section by section. The depth markings are adjusted and optimized for each coring section to ensure optimal placement accuracy for each core. By sequentially processing and precisely adjusting the core samples from the entire well section, consistent and accurate core placement results are ensured throughout the entire section. Finally, core placement for the entire coring well is complete, providing a comprehensive and reliable data foundation for subsequent geological analysis.

[0043] Through the above-mentioned specific implementation scheme, the present invention achieves precise homing of shale cores, reduces the influence of subjective factors, and significantly improves the accuracy and consistency of homing. This method has an optimized operational process and low cost, making it particularly suitable for the exploration and development of shale resources and has good prospects for widespread application.

[0044] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0045] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in similar ways. As long as they do not deviate from the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A shale core retrieval method, characterized in that: The method comprises the following steps: S1. Core sample preparation and data collection For key continuous coring wells, all core samples are sectioned, polished, and scanned using optical scanning to obtain high-resolution core scan images and perform centimeter-level detailed descriptions. After processing, the optical scanning images, detailed description data, natural gamma-ray logging curve data, and other relevant analytical data are imported into geological analysis software to prepare for subsequent core retrieval analysis. S2. Al element content data collection Portable energy-dispersive X-ray fluorescence spectroscopy is used to accurately analyze the Al element in core samples. Al, as an indicator of shale composition, has a significant positive correlation with natural gamma-ray logs. By collecting Al element content data at fixed intervals, reliable basic data is provided for subsequent core retrieval. S3. Construction of core homing dataset Based on the collected Al element data and GR logging curve data, the core homing data set was organized and constructed; S4. Core position adjustment and optimization During the retrieval process, the core marking depth was gradually adjusted, and the retrieval results were optimized by analyzing the correlation between the Al element content and the GR logging curve; S5. Repositioning of the entire well section The core retrieval of the entire well section is processed section by section, and finally the core retrieval work of the entire well section is completed.

2. A shale core retrieval method according to claim 1, characterized in that: The fixed interval in step S2 is 0.125 m.

3. The shale core retrieval method according to claim 1, characterized in that: The specific method of step S3 is to collect and organize the GR logging curve data of the corresponding depth based on the marked depth of each Al element sampling point. Considering that mud shale is more continuous and dense than conventional sandstone and the conventional core retrieval depth is limited, additional GR logging curve data exceeding the upper and lower limits of 4m of coring are collected and organized to ensure the integrity and accuracy of the data set.

4. The shale core retrieval method according to claim 1, characterized in that: The specific method of step S4 is to gradually adjust the annotation depth, perform multiple correlation calculations and comparisons, and save the correlation coefficient R after each adjustment. 2 The adjusted depth corresponding to the maximum correlation coefficient is finally selected as the final homing depth of the core section.

5. The shale core retrieval method according to claim 1, characterized in that: The specific method of step S5 is to adjust and optimize the marked depth of each coring section to ensure that each core section can achieve the best homing accuracy. By sequentially processing and accurately adjusting the coring samples of the entire well section, the core homing results of the entire well section are ensured to be consistent and accurate.

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