A correction method for the increment of acoustic travel time caused by organic matter in mudstone

Abstract

The present invention belongs to the technical field of oil and gas resource exploration, and particularly relates to a method for correcting the increment of acoustic travel time caused by organic matter in mudstone. This method first uses core samples in the target logging area to obtain the average equivalent matrix modulus and the average equivalent pore aspect ratio for the initialization of the entire well section. Then, the porosity curve of the rock is determined by matching the simulated acoustic travel time and the actual acoustic travel time. Finally, a rock physics model without organic matter is established to reconstruct the acoustic travel time data of mudstone that is not affected by organic matter, effectively removing the abnormal acoustic travel time caused by organic matter in organic-rich mudstone, realizing the correction of the increment of acoustic travel time, providing accurate acoustic travel time data for mudstone compaction research and the determination of the maximum burial depth, and having relatively profound significance for oil and gas resource exploration.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas resource exploration, and particularly relates to a method for correcting the acoustic time difference increment caused by organic matter in mudstone. Background Art

[0002] Acoustic time difference logging data plays an important role in oil and gas accumulation analysis and migration research. The acoustic time difference can reflect porosity information, and the relationship between porosity and mudstone compaction degree can be expressed by the Athy formula. Thus, the compaction degree and the maximum burial depth information of the formation can be determined through the acoustic time difference. However, due to the high acoustic time difference of kerogen in mudstone, abnormal acoustic time difference increments will occur in the organic matter-rich mudstone intervals. For such abnormal acoustic time differences, if not corrected, they will be attributed to porosity or rock pressure during the interpretation of acoustic time difference data, thereby misleading the results of compaction degree and reservoir burial depth.

[0003] The relationship between kerogen content and acoustic time difference cannot be simply obtained because the existence form of kerogen and its mixing method with other minerals will have different effects. The Wyllie formula provides the relationship between the acoustic time difference of rock, rock matrix, and porosity, where the acoustic time differences of the rock matrix and pores are weighted and summed according to volume content to obtain the acoustic time difference of the rock. Similarly, the acoustic time difference of the organic matter-rich rock matrix can be obtained by weighted summing the acoustic time differences of kerogen and other mineral matrices according to volume content. However, the internal structure and components of the rock are very complex, and simply weighted summing the acoustic time differences of different components according to volume does not result in an accurate acoustic time difference.

[0004] Rock physics models can more accurately simulate the influence of internal rock properties on acoustic time difference, thereby accurately analyzing porosity, mudstone compaction degree, and burial depth. Using rock physics models to quantitatively evaluate the influence of organic matter on rock acoustic time difference requires sufficient understanding of rock properties. The data provided by traditional logging is limited, mostly including the content of common minerals such as quartz and clay, as well as acoustic time difference and density properties. It is difficult to obtain accurate rock matrix properties using these data. In addition, an important factor affecting acoustic time difference is porosity. Certain understanding of porosity and pore morphology is necessary to accurately simulate acoustic time difference. How to accurately simulate acoustic time difference using limited logging data and then correct the acoustic time difference increment caused by organic matter has important practical significance. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for correcting the acoustic time difference increment caused by organic matter in mudstone to solve the problem that the existing technical methods cannot accurately achieve acoustic time difference correction.

[0006] To solve the above technical problems, the present invention provides a method for correcting the increment of acoustic time difference caused by organic matter in mudstone, and the method includes the following steps:

[0007] 1) Obtain core samples in the target logging area, and perform the following calculations to obtain the average equivalent matrix modulus, the average equivalent pore aspect ratio, and the porosity value range:

[0008] For a core sample, calculate the equivalent modulus of its rock matrix, which is called the matrix equivalent modulus of the core sample; traverse the value range of the pore aspect ratio of the core sample to calculate the simulated longitudinal wave velocity of the rock corresponding to different pore aspect ratios; take the pore aspect ratio with the simulated longitudinal wave velocity closest to the measured longitudinal wave velocity as the pore aspect ratio of the core sample; calculate the average equivalent matrix modulus based on the matrix equivalent moduli of the core samples, and calculate the average equivalent pore aspect ratio based on the pore aspect ratios of the core samples;

[0009] Statistical data of the porosities of all core samples to obtain the porosity value range;

[0010] 2) Based on the average equivalent matrix modulus, the average equivalent pore aspect ratio, and the porosity value range obtained in step 1), perform the following calculations to invert the porosity curve:

[0011] Take the average equivalent matrix modulus as the rock matrix modulus, take the average equivalent pore aspect ratio as the pore aspect ratio, traverse the porosity value range obtained in step 1), and calculate the simulated longitudinal wave velocity of the rock corresponding to different porosities;

[0012] Invert the porosities at different depth points based on the following objective function to obtain the porosity curve:

[0013]

[0014] In the formula, φ k represents a porosity value in the traversed porosity value range, represents the porosity at the previous depth point of φ k , w represents the smoothing weight, w≥0, V p_model represents the simulated longitudinal wave velocity corresponding to φ k , V p represents the measured longitudinal wave velocity; arg min(·) represents the value of the parameter when the expression in the parentheses is minimized;

[0015] 3) Based on the average equivalent matrix modulus and the average equivalent pore aspect ratio obtained in step 1), and the porosity curve inverted in step 2), perform the following calculations to obtain the acoustic time difference corresponding to the rock after removing organic matter, and achieve acoustic time difference correction:

[0016] Rock after simulated removal of organic matter;

[0017] Taking the average equivalent matrix modulus as the rock matrix modulus and the average equivalent pore aspect ratio as the pore aspect ratio, and combining with the porosity curve, the acoustic travel time corresponding to the rock after simulated removal of organic matter is calculated.

[0018] The beneficial effects are as follows: The present invention first obtains the average equivalent matrix modulus and the average equivalent pore aspect ratio using core samples from the target logging area for the initialization of the entire well section, then determines the porosity curve of the rock by matching the simulated acoustic travel time and the actual acoustic travel time, and finally establishes a rock physics model without organic matter to reconstruct the acoustic travel time data of mudstone unaffected by organic matter, effectively removing the abnormal acoustic travel time caused by organic matter in matrix-rich mudstone, realizing the correction of the acoustic travel time increment, providing accurate acoustic travel time data for mudstone compaction research and determination of the maximum burial depth, and having profound significance for oil and gas resource exploration.

[0019] Furthermore, in step 1), the following method is used to calculate the simulated longitudinal wave velocity of the rock corresponding to different pore aspect ratios:

[0020] 1-1) The components in the mudstone other than clay, organic matter, and pores form the rock matrix. According to the moduli and densities of various minerals in the rock matrix, the equivalent modulus of the rock matrix is calculated.

[0021] 1-2) The rock matrix, clay, and organic matter are mixed to form the rock skeleton. According to the equivalent modulus and aspect ratio of the rock matrix, and the moduli and aspect ratios of clay and organic matter, the equivalent modulus of the rock skeleton is calculated.

[0022] 1-3) The rock matrix, clay, organic matter, and pores are mixed to form the dry rock. According to the equivalent modulus and aspect ratio of the rock matrix, the moduli and aspect ratios of clay and organic matter, and the value range of the modulus and aspect ratio of the pores, the equivalent modulus of the dry rock corresponding to different pore aspect ratios is calculated.

[0023] 1-4) The pores in the dry rock in step 1-3) are replaced with fluid-saturated pores to obtain the saturated rock. According to the relationship between the modulus of the dry rock and the modulus of the saturated rock, and combining with the equivalent modulus of the dry rock corresponding to different pore aspect ratios, the equivalent modulus of the saturated rock corresponding to different pore aspect ratios is obtained.

[0024] 1-5) According to the relationship between the rock modulus and the longitudinal wave velocity, and combining with the equivalent modulus of the saturated rock corresponding to different pore aspect ratios, the longitudinal wave velocity corresponding to different pore aspect ratios is calculated.

[0025] Further, in step 2), the following method is used to calculate the simulated longitudinal wave velocity of rocks corresponding to different porosities:

[0026] 2-1) Mix the rock matrix, clay, and organic matter to form a rock skeleton, and calculate the equivalent modulus of the rock skeleton based on the equivalent modulus and aspect ratio of the rock matrix, and the moduli and aspect ratios of the clay and organic matter; wherein, the equivalent modulus of the rock matrix is the average equivalent matrix modulus;

[0027] 2-2) Mix the rock matrix, clay, organic matter, and pores to form a dry rock, and calculate the equivalent modulus of the dry rock based on the equivalent modulus and aspect ratio of the rock matrix, the moduli and aspect ratios of the clay and organic matter, and the modulus and aspect ratio of the pores; wherein, the aspect ratio of the pores is the average equivalent pore aspect ratio;

[0028] 2-3) Replace the pores in the dry rock in step 2-2) with fluid-saturated pores to obtain a saturated rock, and based on the relationship between the modulus of the dry rock and the modulus of the saturated rock, and in combination with the equivalent modulus of the dry rock obtained in step 2-2) and the porosity value range, obtain the equivalent modulus of the saturated rock corresponding to different porosities;

[0029] 2-4) According to the relationship between the rock modulus and the longitudinal wave velocity, and in combination with the equivalent modulus of the saturated rock corresponding to different porosities, calculate the simulated longitudinal wave velocity corresponding to different porosities.

[0030] Further, in step 3), the following method is used to calculate the acoustic travel time corresponding to the rock after simulated removal of organic matter:

[0031] 3-1) Mix the rock matrix and clay to form a rock skeleton, and calculate the equivalent modulus of the rock skeleton based on the equivalent modulus and aspect ratio of the rock matrix, and the modulus and aspect ratio of the clay; wherein, the equivalent modulus of the rock matrix is the average equivalent matrix modulus;

[0032] 3-2) Mix the rock matrix, clay, and pores to form a dry rock, and calculate the equivalent modulus of the dry rock based on the equivalent modulus and aspect ratio of the rock matrix, the modulus and aspect ratio of the clay, and the modulus and aspect ratio of the pores; wherein, the aspect ratio of the pores is the average equivalent pore aspect ratio;

[0033] 3-3) Replace the pores in the dry rock in step 3-2) with fluid-saturated pores to obtain a saturated rock, and based on the relationship between the modulus of the dry rock and the modulus of the saturated rock, and in combination with the equivalent modulus of the dry rock obtained in step 3-2) and the porosity curve obtained in step 2), obtain the equivalent modulus of the saturated rock;

[0034] 3-4) According to the relationship between the rock modulus and the P-wave velocity, and combining with the equivalent modulus of the saturated rock corresponding to the porosity at different depth points, the P-wave velocity is calculated;

[0035] 3-5) According to the relationship between the P-wave velocity and the acoustic time difference, the acoustic time difference corresponding to the rock after simulating the removal of organic matter is obtained.

[0036] Further, the self-consistent approximation model is used to calculate the equivalent modulus of the rock skeleton and the equivalent modulus of the dry rock.

[0037] The beneficial effect is that the self-consistent approximation model reflecting the relationship between the rock components and the equivalent modulus is used to determine the equivalent modulus of the rock skeleton and the equivalent modulus of the dry rock, improving the accuracy of the calculation of the equivalent modulus of the rock skeleton and the equivalent modulus of the dry rock.

[0038] Further, the relationship between the modulus of the dry rock and the modulus of the saturated rock is obtained through the Gassmann formula.

[0039] Further, in step 1-1), the equivalent modulus of the rock matrix is calculated using the HS limit formula.

[0040] Further, the relationship between the modulus of the rock and the P-wave velocity is:

[0041]

[0042] In the formula, V p represents the P-wave velocity, K represents the bulk modulus of the rock, μ represents the shear modulus of the rock, and ρ represents the density.

[0043] Further, in step 1), the average equivalent matrix modulus is the average of the matrix equivalent moduli of the core samples, and the average equivalent pore aspect ratio is the average of the pore aspect ratios of each core sample.

[0044] Further, in step 3), the following method is used to simulate the rock after removing organic matter: normalize the volume content of the components in the rock other than organic matter while keeping the porosity unchanged to simulate the rock after removing organic matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flowchart of the method for correcting the increment of acoustic time difference caused by organic matter in mudstone of the present invention;

[0046] Figure 2 is the logging curve used in the example of the present invention;

[0047] Figure 3 is the porosity curve calculated in the example of the present invention;

[0048] Figure 4 It is the reconstructed curve of the acoustic time difference of mudstone after removing the influence of organic matter in the example of the present invention. Detailed implementation manners

[0049] Before introducing the present invention, several theoretical knowledge used in the method for implementing the present invention will be introduced first.

[0050] 1. Relationship between acoustic time difference AC and rock bulk modulus K and shear modulus μ.

[0051] 1) The relationships between rock bulk modulus K, shear modulus μ, and density ρ and longitudinal wave velocity V p , transverse wave velocity V s are as follows:

[0052]

[0053] 2) The acoustic time difference is the time for the longitudinal wave to pass through a sampling interval. Therefore, the relationship between acoustic time difference AC and longitudinal wave velocity V p is as follows:

[0054]

[0055] In the formula, d ′ is the depth sampling interval. For logging with a fixed depth sampling interval, the acoustic time difference is proportional to the reciprocal of the longitudinal wave velocity.

[0056] 2. Self-consistent approximation model (SCA).

[0057] This model can establish the connection between rock components and equivalent moduli. The specific connection is:

[0058]

[0059] In the formula, K SC is the equivalent bulk modulus; μ SC is the equivalent shear modulus; f i is the volume fraction of the i-th component; K i and μ i are the corresponding bulk modulus and shear modulus of the i-th component; P i and Q i are geometric factors. The specific formulas are:

[0060]

[0061] Among them:

[0062]

[0063]

[0064] F6 = 1 + A[1 + f - R(f + θ)] + B(1 - θ)(3 - 4R) (11)

[0065]

[0066] F9 = A[(R - 1)f - Rθ] + Bθ(3 - 4R) (14)

[0067] Some of the parameters in the above formula are obtained from the following formula:

[0068]

[0069] In the formula, α is the aspect ratio; v m is the Poisson's ratio, which can be calculated from the bulk modulus and shear modulus.

[0070] In the formulas for solving P and Q, K m and μ m are respectively replaced by K SC and μ SC And the SCA model assumes that the components (including pores) in the rock are ellipsoids. When using this model, the volume fraction, modulus, and aspect ratio (the ratio of the short axis to the long axis of the ellipsoid) of the components are required.

[0071] 3. Gassmann relation.

[0072] The Gassmann relation can estimate the modulus of fluid-saturated rock using the modulus of dry rock:

[0073]

[0074] In the formula, K dry and μ dry are respectively the bulk modulus and shear modulus of dry rock; K0 is the bulk modulus of the rock skeleton; K sat and μ sat are the bulk modulus and shear modulus of saturated rock; K f is the bulk modulus of pore fluid; φ is the porosity.

[0075] That is, the Gassmann relation requires the moduli and porosity of dry rock, rock skeleton, and pore fluid.

[0076] 4. HS limit.

[0077] The HS limit is an equivalent medium theory that can estimate the equivalent modulus of a mixture containing mineral grains:

[0078] K HS+ = Λ(μ max ), K HS- = Λ(μ min ), μHS+ = Γ(ζ(K max , μ max )), μ HS- = Γ(ζ(K min , μ min )) In Equation (17), the superscripts HS+ and HS- represent the upper and lower limits of the equivalent modulus respectively; the subscripts max and min represent the maximum or minimum values of the moduli in the mixed minerals; and

[0079]

[0080] In the equation, <.> represents volume-weighted average; K i and μ i represent the bulk modulus and shear modulus of the i-th mixture respectively.

[0081] That is to say, the HS boundary needs to use the moduli and volume fractions of the mixed minerals.

[0082] Based on the above theoretical knowledge, a method for correcting the acoustic time difference increment caused by organic matter in mudstone according to the present invention can be realized. The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0083] Method Embodiment:

[0084] Implementing the method of the present invention requires the use of three rock physics models, namely the first rock physics model model_1, the second rock physics model model_2, and the third rock physics model model_3. The above-introduced theoretical knowledge is needed in the calculation processes of these three rock physics models. The following introduces these three rock physics models.

[0085] 1. The first rock physics model model_1 is used to calculate the aspect ratio of pores in a rock core sample and the modulus of the rock matrix. Sufficient data of the rock sample can be obtained through rock physics experiments, so as to fully initialize the rock physics model. The construction process of the first rock physics model model_1 is as follows:

[0086] 1) The components in the mudstone except for clay, kerogen (i.e., the organic matter in the mudstone), and pores form the rock matrix, including minerals such as quartz and calcite. Substitute the bulk modulus K i and shear modulus μ i of each mineral contained in the rock matrix into the HS boundary formula, that is, Equation (17) and Equation (18), and calculate the equivalent bulk modulus K m and equivalent shear modulus μ m of the rock matrix, which are called the matrix equivalent moduli. Among them, the bulk modulus, shear modulus, and density of each mineral and fluid are constants and can be obtained by referring to the literature.

[0087] 2) Combine the rock matrix, clay, and kerogen to form a rock skeleton. Substitute the equivalent bulk modulus K m , equivalent shear modulus μ m , volume fraction f m , and aspect ratio α m of the rock matrix, as well as the bulk modulus, shear modulus, volume fraction, and aspect ratio of the clay and kerogen, into the self-consistent approximation model (SCA) to calculate the equivalent bulk modulus K0 and equivalent shear modulus μ0 of the rock skeleton. Among them, the aspect ratio of the rock matrix is set to 1, and the aspect ratios of the clay and kerogen are both set to 0.01.

[0088] 3) Combine the rock matrix, clay, kerogen, and pores to form dry rock. Substitute the equivalent bulk modulus K m , equivalent shear modulus μ m , volume fraction f m , and aspect ratio α m of the rock matrix, as well as the bulk modulus, shear modulus, volume fraction, and aspect ratio of the clay and kerogen, and the bulk modulus, shear modulus, porosity, and aspect ratio of the pores, into the self-consistent approximation model (SCA) to calculate the equivalent bulk modulus K dry and equivalent shear modulus μ dry of the dry rock. Among them, the aspect ratio of the rock matrix is set to 1, the aspect ratios of the clay and kerogen are both set to 0.01, and the aspect ratio of the pores takes values in the range from 0.001 to 1.

[0089] 4) Replace the pores in the dry rock with fluid-saturated pores to obtain saturated rock. Using the results obtained in steps 2) and 3), use the Gassmann formula to calculate the equivalent moduli K sat and μ sat of the saturated rock, which are the equivalent moduli of the real rock. Then, use formula (1) to simulate the P-wave velocity of the rock, i.e., the simulated P-wave velocity. It should be noted that when using the Gassmann formula in this step, the porosity of the core sample in the formula can be directly measured.

[0090] During the use of this rock physics model model_1, the range of the pore aspect ratio from 0.001 to 1 is divided into 1000 parts, and different pore aspect ratios are taken in turn to calculate the simulated P-wave velocity V p_model . When the deviation between the simulated value of the P-wave velocity and the actual measured value of the P-wave velocity is minimized, the value of the pore aspect ratio is determined. That is, solve the following objective function:

[0091] α = arg min(abs(V p_model - V p )) (19)

[0092] In the formula, arg min(·) represents finding the minimum.

[0093] Apply the above model to all core samples, calculate the matrix equivalent modulus and pore aspect ratio of each core sample, and take the average value to obtain the average equivalent matrix modulus and the average equivalent pore aspect ratio Statistical analysis of the porosity data in the core samples yields the porosity value range R φ . These parameters will be used in the subsequent rock physics model.

[0094] 2. The second rock physics model, model_2, is used to calculate the porosity curve in the well. The second rock physics model, model_2, needs to use the results from the first rock physics model, model_1, to initialize the second rock physics model, model_2. The construction process of the second rock physics model, model_2, is as follows:

[0095] 1) Combine the rock matrix with clay and kerogen to form the rock skeleton. Substitute the equivalent bulk modulus, equivalent shear modulus, volume fraction, and aspect ratio of the rock matrix, the bulk modulus, shear modulus, volume fraction, and aspect ratio of clay, and the bulk modulus, shear modulus, volume fraction, and aspect ratio of kerogen into the self-consistent approximation model (SCA) to calculate the equivalent bulk modulus K0 and equivalent shear modulus μ0 of the rock skeleton. Among them, the equivalent bulk modulus and equivalent shear modulus of the rock matrix use the average equivalent bulk modulus and average equivalent shear modulus The aspect ratio of the rock matrix is set to 1, and the aspect ratios of both clay and kerogen are set to 0.01.

[0096] 2) Combine the rock matrix with clay, kerogen, and pores to form the dry rock. Substitute the equivalent bulk modulus, equivalent shear modulus, volume fraction, and aspect ratio of the rock matrix, the bulk modulus, shear modulus, volume fraction, and aspect ratio of clay, the bulk modulus, shear modulus, volume fraction, and aspect ratio of kerogen, and the bulk modulus, shear modulus, porosity, and aspect ratio of the pores into the self-consistent approximation model (SCA) to calculate the equivalent shear modulus K dry and bulk modulus μ dry . Among them, the aspect ratio of the rock matrix is set to 1, the aspect ratios of both clay and kerogen are set to 0.01, and the aspect ratio of the pores uses the average equivalent pore aspect ratio obtained from the core samples

[0097] 3) Replace the pores in the dry rock with fluid-saturated pores to obtain the saturated rock, and use the Gassmann formula to calculate the equivalent moduli K sat and μ sat, that is, the equivalent modulus of real rock. Then, the longitudinal wave velocity of the rock can be simulated using formula (1).

[0098] The porosity in mudstone has a great influence on the acoustic travel time, and it is usually not provided in conventional logging. It is necessary to estimate the porosity of mudstone at different depths in order to accurately simulate the acoustic travel time of mudstone after removing the influence of organic matter. The objective function for inverse calculation of porosity is:

[0099]

[0100] In the formula, φ k represents a porosity value within the range of porosity values, represents the porosity at the previous depth point of φ k , w represents the smoothing weight, and V p_model represents the simulated longitudinal wave velocity corresponding to φ k , and V p represents the measured longitudinal wave velocity; arg min(·) represents finding the minimum.

[0101] During the inverse calculation, the porosity parameter value range R φ is divided into 1000 parts, and different porosities are taken in turn, that is, φ k , and the simulated longitudinal wave velocity V p_model corresponding to different porosities is calculated using the second rock physics model model_2, p and its difference from the measured longitudinal wave velocity V is calculated. The smoothness constraint of porosity is added to this objective function formula to avoid local drastic changes in porosity caused by inaccurate other parameters in the model, which also conforms to the actual rock situation because the property changes of rocks at adjacent depths in actual mudstone are small. In the formula, w is the smoothing weight, and the magnitude of this value can adjust the smoothness of the porosity curve. The smoothing weight is a non - negative number, and when taking 0, it means not using the smoothness constraint.

[0102] 3. The third rock physics model model_3 is used to simulate the acoustic travel time of rocks without organic matter in the well. This third rock physics model model_3 needs to use the porosity curve in the second rock physics model model_2 for initialization. The construction process of the third rock physics model model_3 is as follows:

[0103] 1) Normalize the volume content of components in the rock except kerogen, but keep the porosity unchanged to simulate the rock after removing kerogen.

[0104] 2) Mix the rock matrix and clay to form a rock skeleton. Substitute the equivalent bulk modulus, equivalent shear modulus, volume fraction, and aspect ratio of the rock matrix, and the bulk modulus, shear modulus, volume fraction, and aspect ratio of the clay into the self-consistent approximation model (SCA) to calculate the equivalent bulk modulus K′0 and equivalent shear modulus μ′0 of the rock skeleton. Among them, the equivalent bulk modulus and equivalent shear modulus of the rock matrix are the average equivalent bulk modulus and average equivalent shear modulus obtained from core samples. The aspect ratio of the rock matrix is set to 1, and the aspect ratio of the clay is set to 0.01

[0105] 3) Mix the rock matrix, clay, and pores to form dry rock. Substitute the equivalent bulk modulus, equivalent shear modulus, volume fraction, and aspect ratio of the rock matrix, the bulk modulus, shear modulus, volume fraction, and aspect ratio of the clay, and the bulk modulus, shear modulus, porosity, and aspect ratio of the pores into the self-consistent approximation model (SCA) to calculate the equivalent shear modulus K′ dry and equivalent bulk modulus μ′ dry of the dry rock. Among them, the aspect ratio of the rock matrix is set to 1, the aspect ratio of the clay is set to 0.01, and the aspect ratio of the pores is the average equivalent pore aspect ratio

[0106] 4) Replace the pores in the dry rock with fluid-saturated pores, and use the Gassmann formula to obtain the equivalent bulk modulus K′ sat and equivalent shear modulus μ′ sat of the saturated rock. Then, use formula (1) to simulate the compressional wave velocity of the shale after removing organic matter. Finally, according to the relationship between the compressional wave velocity and the acoustic travel time, the acoustic travel time corresponding to the rock after removing the influence of organic matter can be obtained. Perform operations on each logging depth point to obtain the corrected acoustic travel time curve.

[0107] Combined with the above three rock physics models and Figure 1 , the following is an introduction to the entire method process:

[0108] Step 1: Organize the logging data in the target logging area. Collect outcrop rocks and core samples in the well logging positions, and conduct rock physics experiments to obtain information such as mineral types and volume contents, porosity, and pore fluid properties in the samples.

[0109] Step 2: Use the first rock physics model model_1 to calculate the average equivalent matrix modulus average equivalent pore aspect ratio and statistically obtain the porosity data of the samples to get the porosity value range R φ .

[0110] Step 3: Read the logging data file, which includes depth, acoustic travel time, density, shale content, and kerogen content curve data.

[0111] Step 4: Use the second petrophysical model model_2 and the R φ porosity curve obtained by inversion calculation.

[0112] Step 5: Use the petrophysical model model_3, the porosity curve, and the parameter values provided by the samples to simulate the acoustic travel time of shale after removing the influence of organic matter.

[0113] The effectiveness of the method of the present invention will be illustrated below with specific examples.

[0114] A target logging curve is as Figure 2 shown. From left to right, they are acoustic travel time, density, clay content, kerogen content, and porosity curve (the porosity curve is only used to verify the accuracy of porosity inversion calculation and is not actually applied in the method of the present invention). These curves share the same ordinate.

[0115] Rock samples are taken at outcrops and wells at logging positions. After petrophysical experiments, detailed petrophysical properties of the samples are obtained. The equivalent moduli and pore aspect ratios of multiple core samples are calculated using the first petrophysical model model_1, and the average is taken to obtain the matrix average equivalent bulk modulus matrix average equivalent shear modulus average equivalent pore aspect ratio The porosity value range obtained by statistically analyzing the porosity data of all core samples is R φ = [0.05, 0.22].

[0116] The porosity curve calculated using the logging curve and the parameter values provided by the samples through the second petrophysical model model_2 is as Figure 3 shown. Figure 3 The porosity curve (solid line) calculated therein and the actual curve (dashed line) are generally consistent in trend and have a small deviation, indicating that the accuracy of the porosity curve inverted by the method of the present invention is relatively high.

[0117] Use the third petrophysical model model_3 to simulate the acoustic travel time of the rock after removing the influence of kerogen. The porosity curve obtained by inversion calculation ( Figure 3 ) is used in this process, and the result is as Figure 4 shown. Figure 4The solid line in the left figure represents the acoustic travel time curve (solid line) of mudstone after removing the influence of organic matter and the measured curve (dashed line) of the acoustic travel time of mudstone. It can be seen from the figure that the acoustic travel time generally decreases after reconstruction, eliminating the abnormal phenomenon of the acoustic travel time increment caused by the presence of organic matter. Figure 4 The right figure compares the relationship between the change rate of acoustic travel time (solid line) and the organic matter content (dashed line). The change rate r is calculated by the following formula:

[0118]

[0119] In the formula, Δt represents the measured acoustic travel time of mudstone, and Δt ′ represents the reconstructed acoustic travel time after removing the influence of organic matter.

[0120] From Figure 4 it can be seen that the trends of the organic matter content and the acoustic travel time change rate curve are basically completely consistent. In the intervals with high organic matter content, the change rate of the reconstructed acoustic travel time is also high, indicating that the organic matter content has a great influence on the acoustic travel time.

[0121] In summary, the present invention uses the rock physics modeling method to establish the relationship between rock matrix properties, clay content, kerogen content, porosity, pore aspect ratio and acoustic travel time. For the problem that traditional logging provides insufficient data and it is difficult to initialize the rock physics model, the present invention uses the average values of some rock physics parameters obtained from a small number of core samples in the target logging area for the initialization of the rock physics model of the entire well section, then determines the porosity of the rock by matching the simulated acoustic travel time and the actual acoustic travel time, and finally establishes a rock physics model without kerogen and reconstructs the acoustic travel time data of mudstone that is not affected by organic matter, realizing the correction of the acoustic travel time increment in mudstone.

Claims

1. A method for correcting the increment of acoustic travel time caused by organic matter in mudstone, characterized in that It includes the following steps: 1) Obtain core samples from the target logging area and perform the following calculations to obtain the average equivalent matrix modulus, the average equivalent pore aspect ratio, and the porosity value range: For a core sample, calculate the equivalent modulus of its rock matrix, which is called the matrix equivalent modulus of this core sample; traverse the value range of the pore aspect ratio of the core sample to calculate the simulated longitudinal wave velocity of the rock corresponding to different pore aspect ratios; take the pore aspect ratio with the closest simulated longitudinal wave velocity and measured longitudinal wave velocity as the pore aspect ratio of this core sample; Calculate the average equivalent matrix modulus based on the matrix equivalent moduli of each core sample, and calculate the average equivalent pore aspect ratio based on the pore aspect ratios of each core sample; Statistically analyze the porosity data of all core samples to obtain the porosity value range; 2) Based on the average equivalent matrix modulus, the average equivalent pore aspect ratio, and the porosity value range obtained in step 1), perform the following calculations to invert the porosity curve: Take the average equivalent matrix modulus as the rock matrix modulus, take the average equivalent pore aspect ratio as the pore aspect ratio, traverse the porosity value range obtained in step 1), and calculate the simulated longitudinal wave velocity of the rock corresponding to different porosities; Invert the porosity at different depth points based on the following objective function to obtain the porosity curve: where φ k represents a porosity value within the range of porosity values to be traversed, φ k represents the porosity at the previous depth point, w represents the smoothing weight, w ≥ 0, V p_model represents the simulated P-wave velocity corresponding to φ k , and V p represents the measured P-wave velocity; argmin(·) represents the value of the parameter when the expression within the parentheses is minimized; 3) Based on the average equivalent matrix modulus and the average equivalent pore aspect ratio obtained in step 1), and the porosity curve inverted in step 2), perform the following calculations to obtain the acoustic travel time corresponding to the rock after removing organic matter and achieve acoustic travel time correction: Simulate the rock after removing organic matter; Take the average equivalent matrix modulus as the rock matrix modulus, take the average equivalent pore aspect ratio as the pore aspect ratio, and combine the porosity curve to calculate the acoustic travel time corresponding to the simulated rock after removing organic matter.

2. The method for correcting the increment of acoustic time difference caused by organic matter in mudstone according to claim 1, wherein In step 1), the following method is used to calculate the simulated longitudinal wave velocity of the rock corresponding to different pore aspect ratios: 1-1) The components in the mudstone except clay, organic matter, and pores form the rock matrix. Calculate the equivalent modulus of the rock matrix based on the moduli and densities of various minerals in the rock matrix; 1-2) The rock matrix, clay, and organic matter are mixed to form the rock skeleton. Calculate the equivalent modulus of the rock skeleton based on the equivalent modulus and aspect ratio of the rock matrix, and the moduli and aspect ratios of clay and organic matter; 1-3) The rock matrix, clay, organic matter, and pores are mixed to form the dry rock. Calculate the equivalent modulus of the dry rock corresponding to different pore aspect ratios based on the equivalent modulus and aspect ratio of the rock matrix, the moduli and aspect ratios of clay and organic matter, and the value range of the modulus and aspect ratio of the pores; 1-4) Replace the pores in the dry rock in step 1-3) with fluid-saturated pores to obtain the saturated rock. Based on the relationship between the modulus of the dry rock and the modulus of the saturated rock, and combined with the equivalent modulus of the dry rock corresponding to different pore aspect ratios, obtain the equivalent modulus of the saturated rock corresponding to different pore aspect ratios; 1-5) According to the relationship between the rock modulus and the P-wave velocity, combined with the equivalent modulus of the saturated rock corresponding to different pore aspect ratios, the P-wave velocity corresponding to different pore aspect ratios is calculated.

3. The method for correcting the increment of acoustic travel time caused by organic matter in mudstone according to claim 1, characterized in that, In step 2), the following method is used to calculate the simulated P-wave velocity of the rock corresponding to different porosities: 2-1) The rock matrix is mixed with clay and organic matter to form a rock skeleton. According to the equivalent modulus and aspect ratio of the rock matrix, and the moduli and aspect ratios of clay and organic matter, the equivalent modulus of the rock skeleton is calculated; among them, the equivalent modulus of the rock matrix is the average equivalent matrix modulus. 2-2) The rock matrix, clay, organic matter, and pores are mixed to form a dry rock. According to the equivalent modulus and aspect ratio of the rock matrix, the moduli and aspect ratios of clay and organic matter, and the modulus and aspect ratio of the pores, the equivalent modulus of the dry rock is calculated; among them, the aspect ratio of the pores is the average equivalent pore aspect ratio. 2-3) The pores in the dry rock in step 2-2) are replaced with fluid-saturated pores to obtain a saturated rock. According to the relationship between the modulus of the dry rock and the modulus of the saturated rock, and combined with the equivalent modulus of the dry rock obtained in step 2-2) and the range of porosity values, the equivalent modulus of the saturated rock corresponding to different porosities is obtained. 2-4) According to the relationship between the rock modulus and the P-wave velocity, combined with the equivalent modulus of the saturated rock corresponding to different porosities, the simulated P-wave velocity corresponding to different porosities is calculated.

4. The correction method for the increment of acoustic travel time caused by organic matter in mudstone according to claim 1, wherein In step 3), the following method is used to calculate the acoustic travel time corresponding to the rock after simulating the removal of organic matter: 3-1) The rock matrix is mixed with clay to form a rock skeleton. According to the equivalent modulus and aspect ratio of the rock matrix, and the modulus and aspect ratio of clay, the equivalent modulus of the rock skeleton is calculated; among them, the equivalent modulus of the rock matrix is the average equivalent matrix modulus. 3-2) The rock matrix, clay, and pores are mixed to form a dry rock. According to the equivalent modulus and aspect ratio of the rock matrix, the moduli and aspect ratios of clay and organic matter, and the modulus and aspect ratio of the pores, the equivalent modulus of the dry rock is calculated; among them, the aspect ratio of the pores is the average equivalent pore aspect ratio. 3-3) The pores in the dry rock in step 3-2) are replaced with fluid-saturated pores to obtain a saturated rock. According to the relationship between the modulus of the dry rock and the modulus of the saturated rock, and combined with the equivalent modulus of the dry rock obtained in step 3-2) and the porosity curve obtained in step 2), the equivalent modulus of the saturated rock is obtained. 3-4) According to the relationship between the rock modulus and the P-wave velocity, combined with the equivalent modulus of the saturated rock, the P-wave velocity is calculated. 3-5) According to the relationship between the P-wave velocity and the acoustic travel time, the acoustic travel time corresponding to the rock after removing the influence of organic matter is obtained.

5. The method for correcting the increment of acoustic travel time caused by organic matter in mudstone according to any one of claims 2 to 4, characterized in that, The self-consistent approximation model is used to calculate the equivalent modulus of the rock skeleton and the equivalent modulus of the dry rock.

6. The correction method for the increment of acoustic travel time caused by organic matter in mudstone according to any one of claims 2 to 4, characterized in that The relationship between the modulus of the dry rock and the modulus of the saturated rock is obtained through the Gassmann formula.

7. The correction method for the increment of acoustic travel time caused by organic matter in mudstone according to claim 2, characterized in that, In step 1-1), the HS limit formula is used to calculate the equivalent modulus of the rock matrix.

8. The method for correcting the increment of acoustic travel time caused by organic matter in mudstone according to any one of claims 2 to 4, characterized in that, The relationship between the rock modulus and the P-wave velocity is: Where V p1 represents the longitudinal wave velocity, K represents the rock bulk modulus, μ represents the rock shear modulus, and ρ represents the density.

9. The method for correcting the increment of acoustic time difference caused by organic matter in mudstone according to claim 1, wherein In step 1), the average equivalent matrix modulus is the average of the matrix equivalent moduli of the core samples, and the average equivalent pore aspect ratio is the average of the pore aspect ratios of the core samples.

10. The method for correcting the increment of acoustic time difference caused by organic matter in mudstone according to claim 1, characterized in that In step 3), the following method is used to simulate the rock after removing organic matter: normalize the volume content of the components other than organic matter in the rock while keeping the porosity unchanged to simulate the rock after removing organic matter.

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