Formation volume density prediction method based on lithology identification
By using cross-plots of well logging data to identify sandstone and mudstone lithology and fitting a bulk density prediction formula, the error problem of the Gardner formula in lithology identification was solved, achieving higher accuracy in density prediction and physical property analysis.
Patent Information
- Application Number
- CN202410545404.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-05-06
- Publication Date
- 2025-11-07
AI Technical Summary
In existing technologies, the Gardner formula tends to overestimate the density of pure sandstone formations and underestimate the density of pure mudstone formations. Furthermore, there are errors in the lithology classification by gamma ray GR, which reduces the accuracy of the calculation and analysis of rock physical properties and mechanical parameters, especially when sandstone and mudstone are interbedded.
By collecting logging data from drilled wells in the target work area, cross plots are drawn using the difference between density porosity and neutron porosity to identify sandstone and mudstone lithology. A volume density prediction formula is then fitted onto the logarithmic cross plot to establish a volume density prediction method applicable to sandstone and mudstone.
It improves the accuracy of formation volume density prediction, reduces errors in rock physical properties and mechanical parameter analysis, and enhances the accuracy of formation property analysis.
Smart Images
Figure CN120906537A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of calculating and predicting the bulk density of sandstone and mudstone formation in oil drilling and completion, and more particularly to a method for predicting the bulk density of formation based on lithology identification. BACKGROUND
[0002] Accurate acquisition of the petrophysical properties and mechanical parameters of the formation is crucial for oil and gas drilling and completion and reservoir development, and the bulk density of the formation is a key basic parameter indispensable for the above-mentioned analysis and research. The bulk density of the formation is not only used for porosity prediction, lithology identification and seismic wave impedance inversion, but also is a core parameter for calculating the elastic modulus of the formation, analyzing the vertical ground stress and predicting the pore pressure of the formation.
[0003] For the prediction of the density of the formation, Gardner (1974) established the Gardner empirical formula through the study of density and P-wave velocity wherein, ρ b is the bulk density of the formation; V p is the P-wave velocity. The Gardner empirical formula is applicable to consolidated water-saturated rocks under significant effective stress, and can be used to calculate the average bulk density of sandstone, mudstone and carbonate rock formations. Although the Gardner empirical formula is simple in form and widely used, the model predicts a higher density for pure sandstone formations and a lower density for pure mudstone formations. If there are interbedded sandstone and mudstone formations in the formation, directly using the Gardner formula to predict the bulk density will result in errors, reducing the accuracy of the calculation and analysis of petrophysical properties and mechanical parameters. Therefore, it is necessary to establish a reasonable bulk density prediction formula for different lithologies.
[0004] For lithology identification, the logging gamma GR is usually used as the basis for dividing sandstone and mudstone, and when the gamma GR is lower than the critical value, it is sandstone, and when the gamma GR is higher than the critical value, it is mudstone. However, there may still be high gamma values for sandstone, and in this case, the sandstone will be incorrectly divided into mudstone based on the gamma GR, resulting in errors in rock physical property analysis. At the same time, the gamma GR logging is missing in some well sections or formations, making it difficult to identify the core. Therefore, a lithology identification method that does not rely on gamma GR needs to be established, and a bulk density prediction method for different lithologies of sandstone and mudstone needs to be established based on this, making full use of existing logging data to improve the accuracy of formation density prediction. SUMMARY
[0005] In order to overcome the defects existing in the prior art, the present application discloses a lithology identification-based formation bulk density prediction method to solve the problems of Gardner formula density prediction and lithology division according to gamma GR in the prior art. The present application collects well logging data of drilled wells in a target work area, and uses the logging data to obtain a porosity difference between density porosity and neutron porosity; then a crossplot of logging formation bulk density and porosity difference is drawn, and the crossplot is used to identify sandstone and mudstone lithology; then a logarithmic crossplot is drawn according to the sandstone and mudstone lithology identification result, and a logarithmic form of sandstone and mudstone bulk density prediction formula is fitted on the logarithmic crossplot, and then the sandstone and mudstone bulk density prediction formula is obtained according to the logarithmic form of sandstone and mudstone bulk density prediction formula; finally, the sandstone and mudstone bulk density of subsequent new drilled wells in the target work area is predicted by using the sandstone and mudstone bulk density prediction formula, which not only improves the formation bulk density prediction precision, but also has important practical significance for formation physical property analysis of subsequent new drilled wells in the target work area. The present application can be used for calculating and predicting sandstone and mudstone formation bulk density in the field of oil drilling and completion technology.
[0006] In order to achieve the above object, the technical scheme adopted by the present application is:
[0007] A lithology identification-based formation bulk density prediction method, comprising the following steps:
[0008] I. Well logging data collection
[0009] S1, collecting well logging data of drilled wells in a target work area;
[0010] Preferably, in the step S1, the well logging data comprises formation bulk density, compensated neutron and compressional slowness, and the formation bulk density, compensated neutron and compressional slowness are corrected by using depth and borehole environment.
[0011] II. Porosity difference calculation
[0012] S2, using the well logging data to obtain a porosity difference between density porosity and neutron porosity;
[0013] Preferably, the step S2 comprises the following steps:
[0014] S21, using the formation bulk density of the well logging data to obtain density porosity;
[0015] Preferably, in the step S21, the density porosity is:
[0016]
[0017] wherein, is the density porosity; ρ b is the logging formation bulk density, g / cm 3 ; ρf ρf is the formation pore fluid density, g / cm 3 ; ρm is the formation matrix density, g / cm ma 3 .
[0018] S22, obtaining neutron porosity by using compensated neutron of logging data;
[0019] Preferably, in the step S22, the neutron porosity is:
[0020]
[0021] wherein, is the neutron porosity; is the compensated neutron value of logging; is the neutron value of formation pore fluid; is the neutron value of formation matrix.
[0022] S23, obtaining the porosity difference between the neutron porosity and the density porosity.
[0023] In the step S23, the porosity difference is:
[0024]
[0025] wherein, is the porosity difference; is the neutron porosity; is the density porosity.
[0026] III. Sandstone and mudstone lithology identification
[0027] S3, drawing crossplot of logging formation bulk density and porosity difference, and identifying sandstone and mudstone lithology by using the crossplot;
[0028] Preferably, the step S3 comprises: drawing crossplot of logging formation bulk density ρ b and porosity difference , determining critical porosity difference according to the inflection point of the trend of the distribution of the scattered points on the crossplot, and taking the critical porosity difference as the basis for identifying sandstone and mudstone lithology, when , it is sandstone, and when , it is mudstone.
[0029] IV. Determination of logarithmic form of bulk density prediction formula
[0030] S4, drawing logarithmic crossplot according to the result of sandstone and mudstone lithology identification, and fitting logarithmic form of sandstone and mudstone bulk density prediction formula on the logarithmic crossplot;
[0031] Preferably, the S4 step comprises: obtaining the P-wave velocity V p Then, the log 10 (ρ b )-log 10 (V p ) crossplot of sandstone and mudstone is drawn respectively, and the log form of Gardner formula is taken as the reference line on the crossplot, the coefficient of the sandstone and mudstone volume density prediction formula is fitted by keeping the Gardner formula index 0.25 unchanged, and the log
[0032] Preferably, in the S4 step, the log
[0033]
[0034] wherein, ρ b，s , ρ b，m are the volume densities of sandstone and mudstone respectively, g / cm 3 ; a, b are the fitting coefficients of the sandstone and mudstone volume density prediction formula respectively; V p is the P-wave velocity calculated from the P-wave time difference, m / s.
[0035] Preferably, in the S4 step, the P-wave velocity V p obtained from the P-wave time difference comprises:
[0036]
[0037] wherein, V p is the P-wave velocity; AC is the P-wave time difference, us / ft.
[0038] Five, volume density prediction formula determination
[0039] S5, obtaining the sandstone and mudstone volume density prediction formula according to the log
[0040] Preferably, in the S5 step, the sandstone and mudstone volume density prediction formula comprises:
[0041]
[0042] wherein, ρ b，s , ρ b，m are the volume densities of sandstone and mudstone respectively, g / cm 3 ; a, b are the fitting coefficients of the sandstone and mudstone volume density prediction formula respectively; V pP-wave velocity for P-wave slowness calculation, m / s.
[0043] Six, density prediction
[0044] S6, the sandstone and mudstone volume density of the target work area subsequent new drilled well is predicted by using the sandstone and mudstone volume density prediction formula.
[0045] The beneficial effects of the present application are:
[0046] The stratum volume density prediction method based on lithology identification provided by the present application only needs to analyze and identify the sandstone and mudstone lithology through the existing volume density, compensated neutron, P-wave slowness and other logging data of the target work area, and then obtain the sandstone and mudstone volume density prediction formula suitable for the whole work area, which not only improves the stratum density prediction precision, but also has important practical significance for the stratum physical property analysis of the target work area subsequent new drilled well. BRIEF DESCRIPTION OF DRAWINGS
[0047] Figure 1 The steps of the stratum volume density prediction based on lithology identification of the present application are:
[0048] Figure 2 The stratum volume density of the present application for sandstone and mudstone lithology identification is: b Difference with porosity Crossplot;
[0049] Figure 3 The log of the present application for sandstone and mudstone is: 10 (ρ b )-log 10 (V p ) crossplot;
[0050] Figure 4 The example result of the stratum volume density prediction based on lithology identification of the present application is:
[0051] Figure 5 The error comparison result of the stratum volume density prediction result based on lithology identification of the present application is: DETAILED DESCRIPTION
[0052] The concept, specific structure and generated technical effects of the present application will be described clearly and completely in combination with the embodiments and the drawings, so as to fully understand the purpose, features and effects of the present application.
[0053] Example 1
[0054] A stratum volume density prediction method based on lithology identification, as shown in the figure, comprises the following steps: Figure 1
[0055] S1, collecting the logging data of the drilled well in the target work area;
[0056] S2, obtaining a porosity difference between the density porosity and the neutron porosity by using the logging data;
[0057] S3, drawing a crossplot of the logging formation bulk density and the porosity difference, and identifying sandstone and mudstone lithology by using the crossplot;
[0058] S4, drawing a logarithmic crossplot according to the sandstone and mudstone lithology identification result, and fitting a logarithmic form of a sandstone and mudstone bulk density prediction formula on the logarithmic crossplot;
[0059] S5, obtaining a sandstone and mudstone bulk density prediction formula according to the logarithmic form of the sandstone and mudstone bulk density prediction formula;
[0060] S6, predicting the sandstone and mudstone bulk density of a subsequent new well in a target work area by using the sandstone and mudstone bulk density prediction formula.
[0061] Embodiment 2
[0062] This embodiment is further illustrated on the basis of Embodiment 1. In the S1 step, the conventional logging data such as formation bulk density, compensated neutron, and compressional wave slowness of the drilled well in the target work area are collected. The collected conventional logging data such as formation bulk density, compensated neutron, and compressional wave slowness are corrected in depth and borehole environment.
[0063] Embodiment 3
[0064] This embodiment is further illustrated on the basis of Embodiment 2. The S2 step includes the following steps:
[0065] S21, calculating the density porosity from the logging bulk density
[0066]
[0067] wherein, is the density porosity; ρ b is the logging formation bulk density, g / cm 3 ; ρ f is the formation pore fluid density, g / cm 3 ; ρ ma is the formation rock skeleton density, g / cm 3 .
[0068] S22, calculating the neutron porosity from the logging compensated neutron
[0069]
[0070] wherein, Neutron porosity; For logging compensation neutron values; The neutron value of the formation pore fluid; The neutron value represents the strata rock framework.
[0071] S23. Calculate neutron porosity and density porosity The difference
[0072]
[0073] in, This represents the porosity difference. Neutron porosity; Density porosity.
[0074] Example 4
[0075] This embodiment further elaborates on step S3 based on embodiment 3. In step S3, the logging volume density ρ is plotted. b Difference with porosity The critical porosity difference is determined based on the inflection points of the scatter plot distribution trend. As a basis for identifying and classifying the lithology of sandstone and mudstone, when It was sandstone at the time, when It was mudstone at the time.
[0076] Figure 2 The stratigraphic bulk density ρ of a certain sandstone and mudstone lithology is shown. b Difference with porosity Example results of cross plot; the figure shows that the difference... The scatter plot on the time-intersection diagram shows inflection points, thus allowing for the identification of the critical porosity difference between sandstone and mudstone. when It was sandstone at the time, when It was mudstone at the time.
[0077] Example 5
[0078] This embodiment further elaborates on step S4 based on embodiment 4. In step S4, the P-wave velocity V is calculated from the P-wave transit time based on the lithological identification results of sandstone and mudstone. p Then, logs for sandstone and mudstone were plotted separately. 10 (ρ b )-log 10 (V p Cross-plot, using Gardner's formula on the cross-plot. (logarithmic form: log) 10 (ρ b ) = log10 (0.31)+0.25log 10 V p ) as the reference line, keeping the Gardner formula index 0.25 unchanged, fitting the coefficients of the sandstone and mudstone volume density prediction formula, and then obtaining the logarithmic form of the sandstone and mudstone volume density prediction formula:
[0079]
[0080] wherein, ρ b，s , ρ b，m are the volume densities of the sandstone and mudstone, g / cm 3 ; a, b are the fitting coefficients of the volume density prediction formula of the sandstone and mudstone; V p is the longitudinal wave velocity calculated from the logging longitudinal wave time difference, m / s. In the S4 step, the longitudinal wave velocity V p is obtained from the longitudinal wave time difference, including:
[0081]
[0082] wherein, V p is the longitudinal wave velocity; AC is the logging longitudinal wave time difference, us / ft.
[0083] Figure 3 shows an example result of the log 10 (ρ b )-log 10 (V p ) crossplot of a certain sandstone and mudstone, taking the Gardner formula (logarithmic form: log 10 (ρ b ) = log 10 (0.31) + 0.25log 10 V p ) as the reference line, keeping the Gardner formula index 0.25 unchanged, fitting the coefficients a = 0.3043, b = 0.3226 of the sandstone and mudstone volume density prediction formula, and then obtaining the logarithmic form of the example sandstone and mudstone volume density prediction formula:
[0084]
[0085] Example 6
[0086] This example is based on Example 5, and further describes the S5 step. In the S5 step, according to the logarithmic form of the sandstone and mudstone volume density prediction formula, the example sandstone and mudstone volume density prediction formula is obtained:
[0087]
[0088] Figure 4 An example result of lithology-identified formation bulk density prediction is shown, compared with the Gardner formula prediction result, the lithology-identified formation bulk density prediction result based on the porosity difference established by the application is consistent with the measured logging bulk density trend, and the consistency is higher. Figure 5 An error comparison result of a certain lithology-identified formation bulk density prediction result is shown, the error in the figure represents the difference between the measured density and the predicted density, and the comparison shows that the density error predicted by the application is smaller than the Gardner formula prediction error.
[0089] The embodiment only needs to analyze and identify the sandstone and mudstone lithology through the existing bulk density, compensated neutron, and longitudinal wave time difference logging data in the target work area, and then obtain the sandstone and mudstone bulk density prediction formula applicable to the whole work area, which not only improves the formation density prediction accuracy, but also can be used for subsequent formation elastic parameter and vertical ground stress calculation and analysis of new drilled wells in the target work area.
[0090] The embodiments of the application are specifically described above, but the application is not limited to the described embodiments, and those skilled in the art can make various equivalent modifications or replacements without departing from the spirit of the application, and these equivalent modifications or replacements are all included in the scope defined by the claims of the application.
Claims
1. A method for predicting formation bulk density based on lithology identification, characterized in that, The method comprises the following steps: S1, collecting well logging data of drilled wells in a target work area; S2, obtaining a porosity difference between density porosity and neutron porosity by using the well logging data; S3, drawing a crossplot of formation bulk density and the porosity difference, and identifying sandstone and mudstone lithology by using the crossplot; S4, drawing a log crossplot according to the identification result of sandstone and mudstone lithology, and fitting a log form of a sandstone and mudstone bulk density prediction formula on the log crossplot; S5, obtaining the sandstone and mudstone bulk density prediction formula according to the log form of the sandstone and mudstone bulk density prediction formula; S6, predicting sandstone and mudstone bulk densities of subsequent new drilled wells in the target work area by using the sandstone and mudstone bulk density prediction formula.
2. The formation bulk density prediction method of claim 1 wherein, In the S1 step, the well logging data comprises formation bulk density, compensated neutron and P-wave interval transit time, and the formation bulk density, compensated neutron and P-wave interval transit time are corrected by using depth and borehole environment.
3. The formation bulk density prediction method of claim 1 wherein, The S2 step comprises the following steps: S21, obtaining density porosity by using formation bulk density of the well logging data; S22, obtaining neutron porosity by using compensated neutron of the well logging data; S23, obtaining a porosity difference between the neutron porosity and the density porosity.
4. The formation bulk density prediction method of claim 3 wherein, In the S21 step, the density porosity is: wherein, is the density porosity; p b is the formation bulk density from well logs, g / cm 3 ; p f is the formation pore fluid density, g / cm 3 ; p ma is the formation rock matrix density, g / cm 3 .
5. The formation bulk density prediction method of claim 3 wherein, In the S22 step, the neutron porosity is: wherein, is the neutron porosity; is the compensated neutron log value; is the formation fluid neutron value; is the formation matrix neutron value.
6. The formation bulk density prediction method of claim 1 wherein, Step S3 includes: plotting the formation bulk density ρ from the well logging. b Difference with porosity The critical porosity difference is determined based on the inflection points of the scatter plot distribution trend. The critical porosity difference As a basis for identifying and classifying the lithology of sandstone and mudstone, when It was sandstone at the time, when It was mudstone at the time.
7. The formation bulk density prediction method of claim 1 wherein, The S4 step comprises: according to the lithology identification results of sandstone and mudstone, obtaining the P-wave velocity Vp from the P-wave time difference, respectively drawing the log 10 (ρ b )-log 10 (V p ) crossplot of sandstone and mudstone, and taking the log form of Gardner formula as the reference line on the crossplot, keeping the Gardner formula index 0.25 unchanged, fitting the coefficients of the sandstone and mudstone volume density prediction formula to obtain the log form of the sandstone and mudstone volume density prediction formula. The S4 step comprises: according to the lithology identification results of sandstone and mudstone, obtaining the P-wave velocity Vp from the P-wave time difference, respectively drawing the log 10 (ρ b )-log 10 (V p ) crossplot of sandstone and mudstone, and taking the log form of Gardner formula as the reference line on the crossplot, keeping the Gardner formula index 0.25 unchanged, fitting the coefficients of the sandstone and mudstone volume density prediction formula to obtain the log form of the sandstone and mudstone volume density prediction formula. The S4 step comprises: according to the lithology identification results of sandstone and mudstone, obtaining the P-wave velocity Vp from 8. The formation bulk density prediction method of claim 7 wherein, In the S4 step, the log form of the sandstone and mudstone bulk density prediction formula is: wherein, ρ b，s , ρ b，m are the bulk densities of sandstone and mudstone, respectively, g / cm 3 ; a, b are the fitting coefficients of the bulk density prediction formula of sandstone and mudstone, respectively; V p is the longitudinal wave velocity calculated from the logging longitudinal wave slowness, m / s.
9. The formation bulk density prediction method of claim 7 wherein, In the S4 step, the longitudinal wave velocity V is obtained from the longitudinal wave time difference p , comprising: where V p is the P-wave velocity; AC is the well log P-wave slowness, us / ft.
10. The formation bulk density prediction method of claim 1 wherein, In the S5 step, the sandstone and mudstone bulk density prediction formula comprises: wherein, p b，s , p b，m are the bulk densities of sandstone and mudstone, respectively, g / cm 3 ; a, b are the fitting coefficients of the bulk density prediction formula of sandstone and mudstone, respectively; V p is the longitudinal wave velocity calculated from the logging longitudinal wave slowness, m / s.