Shale reservoir stratum pore pressure logging prediction method based on improved Bowers equation

Abstract

The invention relates to a shale reservoir stratum pore pressure logging prediction method based on an improved Bowers equation, and the method comprises the steps: building an acoustic wave velocity response equation considering the influence of density and organic matter content in combination with the Bowers equation; carrying out a soaking experiment on the rock sample, and obtaining formation acoustic velocity and density subjected to hydration correction based on interval transit time, density and depth resistivity in logging information: obtaining a Bowers equation of a research formation based on the acoustic velocity and density of an undisturbed formation subjected to hydration correction; the Bowers equation of the research stratum is corrected, and an improved Bowers equation of the research stratum is obtained; based on an improved Bowers equation, effective stress and vertical stress of the stratum are calculated; and based on the effective stress and the vertical stress of the stratum, calculating to obtain the stratum pore pressure of the research stratum. According to the corrected Bowers formation pore pressure prediction model established by the invention, the prediction precision is obviously improved, and the development cost is reduced while the occurrence of construction accidents is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pore pressure prediction, and particularly relates to a shale reservoir formation pore pressure logging prediction method based on an improved Bowers equation. BACKGROUND

[0002] Continental shale oil is increasingly concerned due to its large resource volume. The formation pressure mechanism of continental shale oil reservoir is complex, and is affected by undercompaction, hydrocarbon expansion, tectonic extrusion and the like. Hydrocarbon generation and tectonic extrusion affect the logging response of abnormal high pressure formation, and make it difficult to accurately predict the formation pore pressure of continental shale oil based on the undercompaction theory. Therefore, the hydrocarbon generation of organic matter needs to be corrected for the prediction of the formation pore pressure of continental shale oil, so as to avoid the influence of the factor in the calculation of the formation pressure.

[0003] Existing researches show that the formation pressure prediction method based on effective stress is suitable for the prediction of pore pressure of complex formation, such as the Bowers method, but the method does not consider the influence of hydrocarbon generation correction of organic matter. Meanwhile, during the drilling of shale oil reservoir, after the drilling fluid invades the formation, a series of physical and chemical reactions occur between the drilling fluid and the shale, which will cause the change of the physical properties of the shale formation, and further cause the logging physical parameters of the formation to be unable to reflect the true physical properties of the original formation, such as the decrease of resistivity, the decrease of P-wave velocity and the decrease of density after the drilling fluid invades the formation. Therefore, when calculating the formation pore pressure of shale oil reservoir, the logging information of the shale oil formation after contacting with the drilling fluid needs to be corrected by hydration to obtain the logging information of the original formation. SUMMARY

[0004] The present application provides a shale reservoir formation pore pressure logging prediction method based on an improved Bowers equation to solve the above technical problems.

[0005] The present application is realized by the following technical scheme:

[0006] The shale reservoir formation pore pressure logging prediction method based on the improved Bowers equation comprises the following steps:

[0007] The Bowers equation is combined to establish a sound wave velocity response equation considering the influence of density and organic matter content;

[0008] The collected rock samples are subjected to soaking experiments, and the resistivity, sound wave velocity and density of the soaked rock samples are measured to obtain the relationship between the change amplitudes of density, P-wave velocity and resistivity after the drilling fluid invades;

[0009] Based on the relationship between the density variation range, the longitudinal wave velocity variation range and the resistivity variation range after the drilling fluid invasion, the corrected acoustic wave velocity and the density are obtained according to the acoustic time difference, the density and the deep-shallow resistivity in the logging data:

[0010] Based on the corrected acoustic wave velocity and the density of the original formation, the Bowers equation of the research formation is obtained by bringing the measured formation acoustic wave velocity and the effective stress and the maximum vertical effective stress at the beginning of unloading of the overlying layer into the loading equation and the unloading equation of Bowers;

[0011] Based on the acoustic wave velocity response equation considering the influence of the density and the organic matter content, the unloading equation of the Bowers equation of the research formation is modified by reading the density and the organic carbon content corresponding to the measured point depth of the formation pressure, and the improved Bowers equation of the research formation is obtained;

[0012] Based on the improved Bowers equation, the acoustic wave velocity before hydration of the formation is obtained by inversely calculating the measured acoustic wave velocity logging data of the formation, and the effective stress of the formation is inversely calculated; the density of the formation before hydration is obtained by inversely calculating the density logging data in the logging data, and the vertical stress is calculated;

[0013] Based on the effective stress and the vertical stress of the formation, the formation pore pressure of the research formation is calculated.

[0014] The loading equation and the unloading equation of Bowers are as follows:

[0015] The loading curve is as follows:

[0016] The unloading curve is as follows:

[0017] In the above formula, V is the acoustic wave velocity, m / s; σ m is the effective stress of the formation; σ max is the maximum vertical effective stress at the beginning of unloading, MPa; U is the elastic-plastic coefficient of the formation shale, dimensionless; A and B are coefficients in the relationship, dimensionless.

[0018] The acoustic wave velocity response equation considering the influence of the density and the organic matter content is as follows:

[0019] The loading curve is as follows:

[0020] The unloading curve is as follows:

[0021] In the above formula, V is the acoustic wave velocity, m / s; ρ b is the density, g / cm 3; w(TOC) is the organic carbon content, %; s max is the maximum vertical effective stress at the beginning of unloading, MPa; U is the shale elastic-plastic coefficient of the formation, dimensionless; A, B, C, D are coefficients in the relational expression, dimensionless.

[0022] Further, the relationship between the density variation amplitude, the longitudinal wave velocity variation amplitude and the resistivity variation amplitude is:

[0023] D d = 0.0695D R - 0.1053

[0024] D p = 0.4431D R + 1.6891

[0025] D R = d x (R d - R s ) / R d x 100

[0026] In the above formula, D d is the density variation amplitude of the rock sample, D p is the longitudinal wave velocity variation amplitude, D R is the resistivity variation amplitude, d is the conversion coefficient between the laboratory resistivity of the rock sample and the logging resistivity, R d is the deep lateral resistivity value, Ω·m, and R s is the shallow lateral resistivity value, Ω·m.

[0027] Further, the calculation formula of the hydration-corrected longitudinal wave velocity and density is:

[0028] V p水化前 = V p / (1-D p )

[0029] DEN 水化前 = DEN / (1-D d )

[0030] In the above formula, V p水化前 is the hydration-corrected formation longitudinal wave velocity, m / s; DEN 水化前 is the hydration-corrected formation density, g / cm 3 ; V p is the longitudinal wave velocity of the formation in the logging data, m / s; and DEN is the formation density in the logging data, g / cm 3 .

[0031] Further, the Bowers equation of the studied formation is:

[0032] Loading mechanism:

[0033] Unloading mechanism:

[0034] In the above equation, σ p水化前 is the formation effective stress; V m is the formation P-wave velocity corrected for hydration, m / s. max is the maximum vertical effective stress at the beginning of unloading, MPa.

[0035] Further, the improved Bowers equation for the formation under study is:

[0036] Loading mechanism:

[0037] Unloading mechanism:

[0038] In the above equation, σ m is the formation effective stress; V p水化前 is the formation P-wave velocity corrected for hydration, m / s; DEN 水化前 is the formation density corrected for hydration, g / cm 3 ; w(TOC) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

[0039] Further, the formation effective stress is calculated using the following equation:

[0040] Loading mechanism:

[0041] Unloading mechanism:

[0042] In the above equation, σ m is the formation effective stress, MPa; V p水化前 is the formation P-wave velocity corrected for hydration, m / s; DEN 水化前 is the formation density corrected for hydration, g / cm 3 ; w(TOC) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

[0043] Further, the vertical stress is calculated using the following equation:

[0044] σ v = ∫ ρg dh

[0045] In the above equation, σ v is the vertical stress, MPa; ρ is the density log value, g / cm 3 ; g is the acceleration of gravity, with a value of 9.8 m / s2 ; d is the integral algorithm; h is the density logging value corresponding to the measuring depth, m.

[0046] Further, the formation pore pressure of the research formation is calculated by the following formula:

[0047] P p = σ v - σ m

[0048] In the formula, P p is the formation pore pressure, MPa; σ m is the effective stress of the formation, MPa; σ v is the vertical stress, MPa.

[0049] Compared with the prior art, the present application has at least the following beneficial effects:

[0050] The present application establishes a recovery method of real formation data by establishing a hydration correction equation, and establishes a modified Bowers formation pore pressure prediction model based on formation density and formation organic carbon content based on experimental data analysis and analysis results of the formation pore pressure formation mechanism of the research formation, and the prediction accuracy of the modified model is obviously improved; the modified model can provide accurate formation pore pressure data for drilling engineering, fracturing optimization, well trajectory optimization and the like, reduce the occurrence of construction accidents and reduce the development cost. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments, and it should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation to the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0052] Figure 1 It is a relationship diagram between effective stress and longitudinal wave velocity in the loading process and unloading process in the embodiment;

[0053] Figure 2 It is a relationship diagram between rock acoustic wave velocity and organic carbon content in the embodiment;

[0054] Figure 3 It is a relationship diagram between rock acoustic wave velocity and density in the embodiment;

[0055] Figure 4 It is a comparison of acoustic wave velocity calculation results of the improved Bowers equation and the Bowers equation in the embodiment;

[0056] Figure 5 It is a correlation diagram of rock density variation amplitude and resistivity variation amplitude in the embodiment

[0057] Figure 6 Correlation of amplitude of P-wave velocity change and amplitude of resistivity change in the example

[0058] Figure 7 Analysis chart for studying formation pore pressure formation mechanism of the formation

[0059] Figure 8 Fitting result of Bowers' loading equation in the example

[0060] Figure 9 Fitting result of Bowers' unloading equation in the example

[0061] Figure 10 Calculation result of formation pore pressure in the example

[0062] Figure 11 Prediction profile of formation pore pressure in the example DETAILED DESCRIPTION

[0063] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in conjunction with the drawings of the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application.

[0064] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. It should be noted that each embodiment in the present specification is described in a progressive manner, and each embodiment mainly describes the difference from other embodiments, and the same and similar parts between each embodiment can be referred to each other.

[0065] The shale reservoir formation pore pressure logging prediction method based on the improved Bowers equation disclosed in the embodiment comprises the following steps:

[0066] Step 1, preparing experimental core data, geological data, drilling fluid for field drilling, logging data and drilling and completion report of the study area; describing the obtained downhole experimental sample in the study area, and obtaining the porosity and permeability, mineral composition, total organic carbon content (TOC), density, resistivity and acoustic velocity of the original rock sample.

[0067] Specifically, 20-30 standard rock samples are drilled, and after drying treatment of the experimental samples, first, core description is performed, and the main description contents include core appearance characteristics and lithological characteristics; second, the dry rock samples are tested for porosity, permeability, mineral composition, organic carbon content, density, resistivity and acoustic velocity, so as to obtain the porosity and permeability, mineral composition, organic carbon content (TOC), density, resistivity and acoustic velocity of the original rock sample;

[0068] Step 2, according to the test results of the organic carbon content of the rock sample, rock samples with different organic carbon content distribution ranges are selected from the rock sample, and the formation loading process is simulated by increasing the confining pressure and constant pore pressure, and the formation unloading process is simulated by increasing the pore pressure under constant confining pressure, and the acoustic velocity under different pressure points is obtained by first being constant for 4h and then being tested, and the test results are shown in Figure 1 From Figure 1 , it can be seen that in the loading process and the unloading process, the acoustic velocity of the rock increases with the increase of the effective stress. In combination with the test results in step 1, the correlation between the acoustic velocity of the rock and the parameters such as porosity, permeability, density, organic carbon content, quartz content, feldspar content, carbonate rock content and clay mineral content under the same pressure point in the loading process and the unloading process is further analyzed, and it is found that the organic matter content and the density of the rock show a high correlation with the acoustic velocity of the rock, as shown in Figure 2 , Figure 3 .

[0069] Step 3, on the basis of step 2, the acoustic velocity response equation based on the influence of density and organic matter content is established in combination with the Bowers equation (as shown in formula (1)), that is, the improved Bowers equation considering the organic matter content and the density of the rock, as shown in formula (2):

[0070]

[0071] Loading curve:

[0072] Unloading curve:

[0073] In the above formula, V is the acoustic velocity, m / s; p b is the density, g / cm 3 ; w(TOC) is the organic carbon content, %; s max is the maximum vertical effective stress at the beginning of unloading, MPa; U is the elastic-plastic coefficient of the formation mud shale, dimensionless; A, B, C and D are coefficients in the relationship, dimensionless.

[0074] Based on the experimental data, the coefficients obtained by fitting are shown in Table 1, and the acoustic velocity calculation results are shown in Figure 4The results show that the rock acoustic wave velocity difference calculated by the improved Bowers equation is smaller, and the prediction result is more accurate.

[0075] Table 1: Correction coefficient

[0076]

[0077] Step 4: The rock sample is soaked in the drilling fluid used in the field drilling. During the soaking process, the rock sample and the drilling fluid will undergo hydration, thereby changing the physical properties of the rock. The resistivity, acoustic velocity, and density of the soaked rock sample are measured to obtain the density, resistivity, and acoustic velocity of the soaked rock sample. It is found that the density, resistivity, and acoustic velocity of the soaked rock sample decrease. Taking the resistivity change amplitude as the independent variable and the density change amplitude and the longitudinal wave velocity change amplitude as the dependent variable, statistical analysis shows that the density change amplitude and the longitudinal wave velocity change amplitude after hydration of the drilling fluid invasion have a good relationship with the resistivity change amplitude, as shown in Figure 5 、 Figure 6

[0078] V p = V p水化前 × (1-D p ) (3)

[0079] DEN = DEN 水化前 × (1-D d ) (4)

[0080] R = R0(1-D R ) (5)

[0081] D d = 0.0695D R - 0.1053 (6)

[0082] D p = 0.4431D R + 1.6891 (7)

[0083] In the above formula, V p is the longitudinal wave velocity of the formation in the logging data, m / s; V p水化前 is the longitudinal wave velocity of the formation after hydration correction, m / s; DEN is the formation density in the logging data, g / cm 3 ; DEN 水化前 is the formation density after hydration correction, g / cm 3 V'0; R0 is the resistivity of the dry rock sample, Ω·m; R is the resistivity of the rock sample after hydration, Ω·m; D d is the density change amplitude of the rock sample after hydration, D p is the longitudinal wave velocity change amplitude after hydration.​R D is the change range of resistivity after hydration, δ is the conversion coefficient between indoor resistivity and logging resistivity; R is the deep lateral resistivity value, Ω·m; R is the shallow lateral resistivity value, Ω·m. R The change range of resistivity after hydration can be calculated by formula (8).

[0084] Step 5, during the drilling process, drilling fluid invades the formation, and the petrophysical properties of the formation will change. The dual laterolog resistivity logging can reflect the difference between the invaded zone of the formation by the drilling fluid and the resistivity of the undisturbed formation, wherein the deep dual laterolog resistivity R d reflects the resistivity of the undisturbed formation, and the shallow dual laterolog resistivity R s reflects the resistivity of the invaded zone of the formation, and the change range of the resistivity of the formation after the invasion of the drilling fluid can be obtained:

[0085] D R = δ × (R d -R s ) / R d × 100 (8)

[0086] In the above formula, D R is the change range of the resistivity of the formation after the invasion of the drilling fluid, and δ is the conversion coefficient between the indoor resistivity and the logging resistivity; R d is the deep lateral resistivity value, Ω·m; and R s is the shallow lateral resistivity value, Ω·m.

[0087] Step 6, based on the information of the acoustic time, density, deep and shallow resistivity in the logging data, and according to the inverse operation of formula (3) and formula (4), the corrected formation P-wave velocity and density after hydration can be obtained as follows:

[0088] V p水化前 = V p / (1-D p ) (9)

[0089] DEN 水化前 = DEN / (1-D d ) (10)

[0090] In the above formula, V p is the P-wave velocity of the formation in the logging data, m / s; V p水化前 is the corrected P-wave velocity of the formation after hydration, m / s; DEN is the formation density in the logging data, g / cm 3 ; and DEN 水化前 is the corrected formation density after hydration, g / cm 3 .

[0091] Step 7, based on the geological data collected in step 1, well logging data, the formation abnormal pressure mechanism analysis of the research stratum is carried out, through the geological data collection research stratum measured formation pore pressure data and measured depth, combined with logging data, the measured formation pore pressure depth point corresponding acoustic velocity, compensated density and other data are obtained, based on the above data, the acoustic velocity-density crossplot is drawn, as shown in Figure 7 From Figure 7 It can be seen that the formation pore pressure of the research stratum is mainly caused by tectonic extrusion, hydrocarbon generation and undercompaction.

[0092] Step 8, based on the water correction of acoustic velocity and density data, the formation pressure mechanism analysis chart is drawn, as shown in Figure 7 If the formation is normal pressure, the data points should be concentrated around the black fitting line, and from the figure it can be seen that the data points of the target stratum Lucaogou group are relatively discrete, and part of the data points are far away from the standard fitting line, combined with the loading and unloading mechanism research results of the abnormal pressure of the research stratum, it can be analyzed that the target stratum is affected by tectonic extrusion, hydrocarbon generation and undercompaction, and the formation pressure shows abnormal high pressure, therefore, the Bowers method suitable for abnormal pressure formation is used for prediction, based on the measured pressure data of the research stratum, according to the loading equation and unloading equation of Bowers, V p The V p水化前 , effective stress σ m and the maximum vertical effective stress σ max of the overlying starting unloading are brought into the loading equation and unloading equation of Bowers in formula (1), and the coefficients A, B and U in Bowers equation are fitted respectively, A=274.52, B=0.6478, U=1.2, as shown in Figure 8 and Figure 9 .

[0093] Further, the Bowers equation of the research stratum is obtained, as shown in formula (11):

[0094] Loading mechanism:

[0095] Unloading mechanism:

[0096] In the above formula, V p水化前 is the water corrected formation P-wave velocity, m / s; σ m is the effective stress of the formation; σ max is the maximum vertical effective stress when starting unloading, MPa.

[0097] The formula (11) can be used to calculate the pore pressure of the research stratum, and the calculation result is as shown in Figure 10As shown from Figure 10 It can be seen from the above that the relative error of the formation pore pressure prediction result based on the Bowers equation is 85.68%.

[0098] Step 9, based on the improved Bowers equation form constructed in step 3 (as shown in formula (2)), combined with the density logging information and the organic carbon content logging calculation result, the density and organic carbon content corresponding to the measured point depth of the formation pore pressure are read, the influence of the density and the organic carbon content is considered, the Bowers unloading equation shown in formula (11) is modified, and then the improved Bowers equation of the research formation is obtained, as shown in formula (12):

[0099] Loading mechanism:

[0100] Unloading mechanism:

[0101] In the above formula, σ m is the effective stress of the formation; V p水化前 is the water-corrected formation P-wave velocity, m / s; DEN 水化前 is the water-corrected formation density, g / cm 3 ; w(TOC) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

[0102] Step 10, based on the Bowers modified model established in step 9, the measured acoustic wave velocity logging data of the formation obtained from the logging data are used to inversely calculate the effective stress of the formation, and the calculation formula is shown in formula (13), and based on the density logging data in the logging data, the vertical stress, i.e. the overburden pressure, is calculated by using formula (13).

[0103] Loading mechanism:

[0104] Unloading mechanism:

[0105] σ v =∫ρgdh (14)

[0106] In the above formula, σ m is the effective stress of the formation, MPa; V p水化前 is the water-corrected formation P-wave velocity, m / s; DEN 水化前 is the water-corrected formation density, g / cm 3 ; w(TOC) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa; σ v is the vertical stress, MPa; and ρ is the density logging value, g / cm3 ; g is the acceleration of gravity, and the value is 9.8 m / s 2 ; h is the density logging value corresponding to the depth, m; d is the integral algorithm.

[0107] Step 11, based on the vertical stress and the effective stress of the formation obtained in step 10, the formation pore pressure of the research formation can be obtained by using formula (15).

[0108] P p = σ v - σ m (15)

[0109] In the above formula, P p is the formation pore pressure, MPa; σ m is the effective stress of the formation, MPa; σ v is the vertical stress, MPa;

[0110] The results of the formation pore pressure calculated by formula (15) are shown in Figure 10 From the figure, it can be seen that the relative error of the formation pore pressure prediction result based on the improved Bowers equation is 92.13%, and the prediction accuracy is improved by 6.45%. The logging prediction profile is shown in Figure 11 From the figure, it can be seen that the formation pore pressure prediction result based on the improved Bowers equation is more consistent with the measured points.

[0111] It should be noted that when the hydration in the shale formation, the hydration correction of the formation density and the longitudinal wave velocity and other logging information can not be carried out.

[0112] The present application establishes a recovery method of real formation data by establishing a hydration correction equation, based on experimental data analysis and the analysis results of the formation pore pressure formation mechanism of the research formation, a modified Bowers formation pore pressure prediction model based on the formation density and the formation organic carbon content is established, and the prediction accuracy of the modified model is obviously improved. The modified model can provide accurate formation pore pressure data for drilling engineering, fracturing optimization, well trajectory optimization and the like, reduce the occurrence of construction accidents and reduce the development cost.

[0113] The above is only a preferred embodiment of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for shale reservoir formation pore pressure log prediction based on modified Bowers equation, characterized in that, The method comprises the following steps: The Bowers equation is combined to establish a sound wave velocity response equation considering the influence of density and organic matter content; The collected rock samples are subjected to soaking experiments, and the resistivity, sound wave velocity and density of the soaked rock samples are measured to obtain the relationship between the density variation amplitude, the longitudinal wave velocity variation amplitude and the resistivity variation amplitude after the hydration of the drilling fluid invasion; Based on the relationship between the density variation amplitude, the longitudinal wave velocity variation amplitude and the resistivity variation amplitude after the hydration of the drilling fluid invasion, the hydration-corrected sound wave velocity and density are obtained according to the sound wave velocity, density and deep-shallow resistivity in the logging data; Based on the hydration-corrected sound wave velocity and density of the original formation, the measured formation sound wave velocity and effective stress and the maximum vertical effective stress at the beginning of unloading of the overlying layer are brought into the loading equation and the unloading equation of Bowers to obtain the Bowers equation of the research formation; Based on the sound wave velocity response equation considering the influence of density and organic matter content, the density and organic carbon content corresponding to the measured depth of the formation pressure are read out by combining the density logging information and the logging calculation result of the organic carbon content, the unloading equation of the Bowers equation of the research formation is modified, and the improved Bowers equation of the research formation is obtained; Based on the improved Bowers equation, the formation sound wave velocity before hydration is inversely calculated by using the measured formation sound wave velocity in the logging data, and the formation effective stress is inversely calculated; the formation density before hydration is inversely calculated based on the density logging data in the logging data, and the vertical stress is calculated; Based on the formation effective stress and the vertical stress, the formation pore pressure of the research formation is calculated.

2. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The loading equation and the unloading equation of Bowers are as follows: Loading curve: Unloading curve: In the formula, V is the acoustic wave velocity, m / s; σ m is the effective stress of the formation; σ max is the maximum vertical effective stress at the beginning of unloading, MPa; U is the shale elastic-plastic coefficient of the formation, dimensionless; A and B are coefficients in the formula, dimensionless.

3. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The sound wave velocity response equation considering the influence of density and organic matter content is as follows: Loading curve: Unloading curve: In the above formula, V is the velocity of sound, m / s; ρ b Density, g / cm³ 3 w(TOC) represents the organic carbon content, in percent; σ max is the maximum vertical effective stress at the start of unloading, MPa; U is the elastic-plastic coefficient of the formation mudstone and shale, dimensionless; A, B, C, and D are coefficients in the formula, dimensionless.

4. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The relationship between the density variation amplitude, the longitudinal wave velocity variation amplitude and the resistivity variation amplitude is as follows: R = R0(1 - D R ) D d = 0.0695D R -0.1053 D p = 0.4431D R + 1.6891 In the above formula, R0 is the resistivity of the dry rock sample, Ω m; R is the resistivity of the rock sample after hydration, Ω m; D d is the change amplitude of the density of the rock sample after hydration, D p is the change amplitude of the P-wave velocity after hydration, D R is the change amplitude of the resistivity after hydration.

5. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1 or 4, wherein, The calculation formula of the hydration-corrected sound wave velocity and density is as follows: V p水化前 = V p (1 - D p ) DEN 水化前 = DEN / (1-D d ) In the above equation, V p水化前 is the water- corrected formation P-wave velocity, m / s; DEN 水化前 is the water- corrected formation density, g / cm 3 ; V p is the P-wave velocity of the formation from the log, m / s; DEN is the formation density from the log, g / cm 3 ; D d is the amplitude of the rock sample density variation, D p is the amplitude of the P-wave velocity variation.

6. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The Bowers equation of the research formation is as follows: Loading mechanism: Offload mechanism: In the above equation, V p水化前 is the formation P-wave velocity corrected for hydration, m / s; σ m is the effective stress of the formation; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

7. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The improved Bowers equation of the research formation is as follows: Loading mechanism: Offload mechanism: In the above equation, V p水化前 is the formation P-wave velocity corrected for hydration, m / s; σm is the formation effective stress; DEN 水化前 is the formation density corrected for hydration, g / cm 3 ; w(TOC) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

8. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1 or 7, wherein, The formation effective stress is calculated by using the following formula: Loading mechanism: Offload mechanism: In the above formula, σm is the formation effective stress, MPa; V p水化前 is the formation longitudinal wave velocity after hydration correction, m / s; DEN 水化前 is the formation density after hydration correction, g / cm 3 ; w ( TOC ) is the organic carbon content, %; σ max is the maximum vertical effective stress at the beginning of unloading, MPa.

9. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The vertical stress is calculated by using the following formula: σ v = ∫ρgdh In the above formula, σ v is the vertical stress, MPa; p is the density log value, g / cm 3 ; g is the acceleration of gravity, with a value of 9.8 m / s 2 ; h is the depth of the density log value, m.

10. The method for shale reservoir formation pore pressure log prediction based on modified Bowers equation of claim 1, wherein, The formation pore pressure of the research formation is calculated by using the following formula: P p = σ v - σ m In the above formula, P p is the formation pore pressure, MPa; σ m is the formation effective stress, MPa; σ v is the vertical stress, MPa.

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