Effective porosity calculation method and device based on lithologic classification and variable T2 cutoff value

Abstract

The present invention belongs to the field of oil and gas geology and exploration and development engineering technology, and discloses a method and device for calculating effective porosity based on a variable T2 cutoff value for lithologic classification. The method comprises: determining the lithology, mineral composition and content, pore structure, total porosity, and effective porosity of the core through core experiments; using total porosity to calibrate nuclear magnetic resonance logging total porosity to perform a regression correction on the core well depth; using lithology, mineral composition, and content to calibrate element logging to calculate and construct a lithologic indicator curve and identify major lithologic categories; using core effective porosity to calibrate nuclear magnetic resonance logging to calculate the effective porosity T2 cutoff value of the core; performing regression analysis on the core lithologic indicator curve value and the core effective porosity T2 cutoff value to determine a fitting relationship for the variable T2 cutoff value of each lithology; and calculating the effective porosity of the target layer. The present invention can effectively improve the accuracy of effective porosity calculation for unconventional oil and gas reservoirs with complex lithology and strong heterogeneity, such as continental mixed shale.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas geology and exploration and development engineering, and in particular relates to a method and device for calculating effective porosity based on a lithology classification variable T2 cutoff value. Background Art

[0002] The effective porosity of rock is a core parameter for describing rock physical properties. Currently, using the T2 cutoff value of nuclear magnetic resonance logging to calculate effective porosity is a common practice in the industry. However, for unconventional oil and gas reservoirs with variable lithology and complex pore structure, such as mixed shale oil reservoirs in continental basins, the commonly used logging porosity calculation method is often difficult to evaluate effective porosity. There are three main methods for calculating effective porosity using the T2 cutoff value of nuclear magnetic resonance at home and abroad:

[0003] (1) Fixed cutoff value method. The 3ms used in the calculation of effective nuclear magnetic porosity at home and abroad is used as the effective porosity cutoff value of sandstone for calculation. In addition, there is a similar method of classifying reservoirs according to the actual situation, such as sedimentary environment, mud content, etc., and then using different fixed cutoff values ​​for porosity calculation. The advantage of this method is that the calculation is simple and convenient, and it has sufficient calculation accuracy for reservoirs with stable lithology and simple pore structure. The disadvantage is that due to the variable lithology, complex pore structure and strong heterogeneity of reservoirs, there is often no unified T2 cutoff value. In this case, the fixed cutoff value method will lose accuracy.

[0004] (2) Experimental analysis method. Using industry standards (such as the national standard GB / T 29172-2012) or experimental methods developed for rocks with certain characteristics (such as patent documents CN110646331A, CN111175207A), it is often possible to obtain effective porosity experimental values ​​with high accuracy. However, the cost of obtaining cores is high, and it is impossible to obtain a considerable number of cores for each well. Therefore, the experimental analysis method cannot perform vertically continuous effective porosity characterization.

[0005] (3) Pore structure parameter method. This method uses the capillary pressure curve, nuclear magnetic permeability, T2 geometric mean and other parameters that can reflect the pore structure to calibrate the T2 cutoff value, and constructs the conversion relationship between the T2 cutoff value and the pore structure parameters to achieve the calculation of the vertically continuously variable T2 cutoff value. The advantage of this method is that it saves a lot of coring and experimental costs. The disadvantage is that in layers with complex lithology and strong heterogeneity, affected by multiple factors such as diagenesis, oil content, and wettability, the experimental data change pattern is complex, and it is difficult to obtain a high-precision model.

[0006] Chinese patent CN111650108A discloses a method and device for measuring the effective porosity of shale rock. The method for measuring the effective porosity of shale rock includes: obtaining the total volume of the shale rock to be measured, the skeleton volume of the shale rock to be measured, and the volume of particles and / or dust in the shale rock to be measured; determining the volume of a hygroscopic medium; correcting the skeleton volume using the volume of the hygroscopic medium to obtain a skeleton-corrected volume; and obtaining the effective porosity of the shale rock to be measured based on the total volume, the skeleton-corrected volume, and the volume of particles and / or dust. However, the technical solution of this patent solves the problem of inaccurate measurement of effective porosity of mud shale (mud shale has a high clay mineral content and belongs to clay shale. The main controlling factor of effective porosity is the clay mineral content. Montmorillonite among clay minerals is particularly easy to absorb water and swell, which causes core fracture, and thus makes the effective porosity measurement inaccurate). It is not suitable for unconventional oil and gas reservoirs such as continental mixed shale with variable lithology, complex pore structure and strong heterogeneity (continental mixed shale has a low clay mineral content, and the main controlling factor of effective porosity is not clay minerals, but two major types of rock-forming minerals, felsic and carbonate rocks); at the same time, the technical solution of this patent can only obtain discrete core data through experimental measurement, and cannot obtain continuous logging evaluation results. Summary of the Invention

[0007] The present invention aims to solve at least one technical problem existing in the prior art or related art, and provides a method and device for calculating effective porosity based on lithologic classification variable T2 cutoff value, which can effectively solve the problem of low accuracy in effective porosity calculation.

[0008] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0009] A method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithologic classification, the method comprising the following steps:

[0010] Step S1: Obtain a core from the target layer and perform thin section observation, X-ray diffraction, water-saturated nuclear magnetic resonance (NMR) test, and helium injection porosity test on the core to determine the lithology, mineral composition and content, pore structure, total porosity, and effective porosity of the core.

[0011] Step S2: using the core total porosity calibration nuclear magnetic logging total porosity in step S1, the well depth position of the core is calibrated;

[0012] Step S3: Using the core lithology, mineral composition, and content calibration element logging in step S1, calculate and obtain the content curve of each mineral in the target layer; construct a lithology indicator curve based on the content curve of each mineral, and then identify the major lithology categories;

[0013] Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, reversely calculate the core effective porosity T2 cutoff value;

[0014] Step S5: extracting the lithology indicator curve value corresponding to the core of each lithology according to different lithologies; then performing regression analysis on the lithology indicator curve value corresponding to the core and the effective porosity T2 cutoff value of the core to determine the fitting relationship of the variable T2 cutoff value of each lithology;

[0015] Step S6: Based on the lithologic classification of the target layer, the variable T2 cutoff value fitting relationship constructed in step S5 is used to calculate the logging effective porosity T2 cutoff value of each depth point of the target layer according to the lithologic indicator curve value of the corresponding lithology of the target layer; and based on the logging effective porosity T2 cutoff value of each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

[0016] Furthermore, in step S3, the lithology indicator curve can continuously characterize the vertical lithology changes of the formation, and is a function with the content of each mineral in the rock as an independent variable.

[0017] Furthermore, in step S3, the content of each mineral is calculated by element logging.

[0018] Furthermore, the element logging includes ECS logging, FEM logging, PNX logging or lithologic scanning logging.

[0019] Furthermore, in step S3, the calculation formula of the lithology indicator curve is:

[0020] LIC=f(V1,V2,…,V n )

[0021] In the above formula, LIC is the lithologic indicator curve, dimensionless; V1, V2, ..., V n It is the content of the first, second,…, nth mineral, %.

[0022] Furthermore, in step S5, the variable T2 cutoff value fitting relationship is as follows:

[0023] T 2cutoff_i =g i (LIC)

[0024] In the above formula, T 2cutoff_i is the variable T2 cutoff value of the i-th lithology, ms; g i is the T2 cutoff value fitting relationship of the i-th lithology, ms.

[0025] Furthermore, in step S6, according to the logging effective porosity T2 cutoff value of each depth point of the target layer, the calculation formula for the effective porosity corresponding to each depth point of the target layer is calculated as follows:

[0026]

[0027] In the above formula, φ e_k is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the kth depth point of the target layer, a decimal; T 2cutoff_k is the T2 cutoff value of the k-th depth point in the target layer, in ms; T2 is the starting time of the effective porosity of the k-th depth point in the target layer, in ms.

[0028] At the same time, the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, wherein the computer program includes program instructions, and when the program instructions are executed by a computer, the computer executes any of the above methods.

[0029] In addition, the present invention also provides an electronic device, which includes a processor, a memory, and a computer program stored in the memory and runnable on the processor, and when the processor executes the program, it implements any of the methods described above.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) Compared with the fixed cutoff value method, the fixed cutoff value method does not take into account the changes caused by factors such as lithology and mineral changes, and is therefore only applicable to clastic rock formations with single and stable lithology. The rock effective porosity calculation method of the present invention based on the variable T2 cutoff value of lithology classification fully considers the influence of changes in lithology and mineral content on the nuclear magnetic T2 cutoff value, and can be better applied to the calculation of effective porosity in formations with complex lithology, rapid changes in mineral content, etc.

[0032] (2) Compared with the experimental analysis method, the rock effective porosity calculation method based on the variable T2 cutoff value of lithologic classification can be promoted and applied in a certain exploration area after core calibration, without the need to spend a lot of money on core experimental measurement, thus saving measurement costs and measurement cycles;

[0033] (3) Compared with the pore structure parameter method, the pore structure parameter method often cannot obtain a high-precision model in layers with complex lithology and strong heterogeneity. The method of the present invention calculates the effective porosity of rock based on the variable T2 cutoff value of lithology classification, and does not start from the idea of ​​pore structure evaluation, so there is no need to consider the above-mentioned problem.

[0034] (4) Compared with the technical solution of patent CN111650108A, the rock effective porosity calculation method of the present invention based on the variable T2 cutoff value of lithologic classification can be applied to unconventional oil and gas reservoirs such as continental mixed shale with variable lithology, complex pore structure and strong heterogeneity. At the same time, the method of the present invention can calibrate core experimental data and calculate and obtain continuous logging evaluation results. After the core calibration of a certain exploration area, it can be promoted and applied in the exploration area, and has stronger versatility and practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of a method flow in an embodiment of the present invention;

[0036] Figure 2 Figure 1 is a graph showing the relationship between the lithology indicator curve values ​​and the core nuclear magnetic T2 cutoff values ​​of different lithologies in the embodiment of the present invention, wherein Figure (a) is dolomitic siltstone, Figure (b) is limy siltstone, Figure (c) is argillaceous siltstone, Figure (d) is dolomitic mudstone, Figure (e) is limy mudstone, Figure (f) is silty mudstone, and Figure (g) is argillaceous dolomitic rock.

[0037] Figure 3 This is a diagram showing the calculation results of the effective porosity of a certain well in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] Example 1

[0040] The embodiment of the present invention provides a method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithologic classification. Taking a well in a reservoir as an example, combined with Figure 1 As shown, the method includes the following steps:

[0041] Step S1: Obtain a core from the target layer and perform thin section observation, X-ray diffraction (XRD), water-saturated nuclear magnetic resonance (NMR) test, and helium injection porosity test on the core to clarify the core's main properties, including lithology, mineral composition and content, pore structure, total porosity, and effective porosity.

[0042] Step S2: Using the core total porosity calibration nuclear magnetic logging total porosity in step S1, a fine repositioning correction is performed on the core position depth to ensure the accuracy of the subsequently constructed model and the accuracy of the final calculation results.

[0043] Step S3: Use the core lithology, mineral composition and content information in step S1 to calibrate the lithologic scanning logging, calculate and obtain the content curve of each mineral in the target layer; construct the lithologic indicator curve based on the content curve of each mineral, and then identify the major lithologic categories.

[0044] The lithology indicator curve (LIC) is a function of the mineral content. There are many specific formulas for constructing the LIC. In the embodiment of the present invention, the formula for constructing the LIC is:

[0045]

[0046] In the above formula (1), LIC is the lithologic indicator curve, dimensionless; V is the volume fraction of each mineral, %.

[0047] Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, reversely calculate the core effective porosity T2 cutoff value.

[0048] Step S5: Based on the lithologic indicator curve and lithologic classification identification results constructed in step S3, extract the lithologic indicator curve values ​​corresponding to the cores of each lithologic type according to different lithologies; then perform regression analysis on the lithologic indicator curve values ​​corresponding to the cores and the effective porosity T2 cutoff value of the cores to determine the fitting relationship of the variable T2 cutoff value for each lithologic type.

[0049] In the embodiment of the present invention, a total of 7 major lithologies were identified, and the following lithology structures were constructed for each lithology: Figure 2 Figure 2 shows the relationship between the lithologic indicator curve value and the core effective porosity T2 cutoff value, where (a) is dolomitic siltstone, (b) is gray siltstone, (c) is argillaceous siltstone, (d) is dolomitic mudstone, (e) is gray mudstone, (f) is silty mudstone, and (g) is argillaceous dolomite. The horizontal axis in each figure is the lithologic indicator curve value, dimensionless; the vertical axis is the core effective porosity T2 cutoff value, in ms.

[0050] from Figure 2 It can be seen that the correlation between the fitting equations of various lithologies is good. Specifically, the fitting equation of each lithology is:

[0051] Dolomitic siltstone:

[0052] y=34102e -3.851x , R 2 =0.6810(2)

[0053] Gray siltstone:

[0054] y=421.98e -1.928x , R 2 =0.8334(3)

[0055] Silty mudstone:

[0056] y=2.3335e -0.526x , R 2 =0.7963(4)

[0057] Dolomitic mudstone:

[0058] y=10567e -3.643x , R 2 =0.6848(5)

[0059] Limestone:

[0060] y=2765.5e -4.006x , R 2 =0.7906(6)

[0061] Argillaceous siltstone:

[0062] y=7.7251e -0.255x , R 2 =0.6432(7)

[0063] Argillaceous dolomite:

[0064] y=835.95e -4.352x , R 2 =0.6378(8)

[0065] In the above formulas (2)-(8), y is the T2 cutoff value of the core effective porosity, in ms; x is the lithology indicator curve value, dimensionless; and R is the correlation coefficient, dimensionless.

[0066] Step S6: For the seven different lithologies, the variable T2 cutoff value fitting relationship constructed in step S5 is used to calculate the logging effective porosity T2 cutoff value at each depth point of the target layer according to the lithology indicator curve value of the corresponding lithology of the target layer; and based on the logging effective porosity T2 cutoff value at each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

[0067] Among them, according to the logging effective porosity T2 cutoff value of each depth point in the target layer, the calculation formula for calculating the effective porosity corresponding to each depth point in the target layer is:

[0068]

[0069] In the above formula (9), is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the kth depth point of the target layer, a decimal; T 2cutoff_kis the T2 cutoff value of the k-th depth point in the target layer, in ms; T2 is the starting time of the effective porosity of the k-th depth point in the target layer, in ms.

[0070] Figure 3 This is the final effective porosity evaluation result chart for an example of the present invention. From left to right, track 1 is the depth track, tracks 2-4 are conventional curve tracks, track 5 is the mineral content track calculated using lithologic scanning logging, track 6 is the lithologic classification based on mineral content, and track 7 is the lithologic indicator curve calculated from mineral content. As can be seen from this chart, whether it is the mineral content, logging lithologic classification, or lithologic indicator curve, all indicate that the lithologic properties of this well are changing rapidly.

[0071] at the same time, Figure 3 Track 8 shows the variable T2 cutoff value calculated by lithology; Tracks 9, 10, and 11 show the effective porosity calculated using a fixed cutoff of 3ms, a fixed cutoff value for each lithology category, and a variable T2 cutoff value for each lithology in the embodiment of the present invention, respectively. The points in Tracks 9-11 represent the effective porosity obtained from core experimental analysis. As can be seen, the wide range of T2 cutoff values ​​indicates that calculating effective porosity using a fixed cutoff value is inappropriate. The effective porosity calculated using the variable T2 cutoff value in the embodiment of the present invention is most consistent with the effective porosity in cores, with a relative error of only 8%, meeting the accuracy requirements for well logging physical property evaluation in geological research.

[0072] Example 2

[0073] An embodiment of the present invention provides a computer-readable storage medium storing a computer program. The computer program includes program instructions. When the program instructions are executed by a computer, the computer executes the method described in Example 1.

[0074] Example 3

[0075] An embodiment of the present invention provides an electronic device, which includes a processor, a memory, and a computer program stored in the memory and executable on the processor. When the processor executes the program, the method described in Example 1 is implemented.

[0076] The above description is only an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the scope of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for calculating effective porosity of rocks with a variable T2 cutoff value based on lithologic classification, characterized in that: The method comprises the following steps: Step S1: Obtain a core from the target layer and perform thin section observation, X-ray diffraction, water-saturated nuclear magnetic resonance (NMR) test, and helium injection porosity test on the core to determine the lithology, mineral composition and content, pore structure, total porosity, and effective porosity of the core. Step S2: using the core total porosity calibration nuclear magnetic logging total porosity in step S1, the well depth position of the core is calibrated; Step S3: Using the core lithology, mineral composition, and content calibration element logging in step S1, calculate and obtain the content curve of each mineral in the target layer; construct a lithology indicator curve based on the content curve of each mineral, and then identify the major lithology categories; Step S4: using the core effective porosity calibration nuclear magnetic logging in step S1, reversely calculate the core effective porosity T2 cutoff value; Step S5: extracting the lithology indicator curve value corresponding to the core of each lithology according to different lithologies; then performing regression analysis on the lithology indicator curve value corresponding to the core and the effective porosity T2 cutoff value of the core to determine the fitting relationship of the variable T2 cutoff value of each lithology; Step S6: Based on the lithologic classification of the target layer, the variable T2 cutoff value fitting relationship constructed in step S5 is used to calculate the logging effective porosity T2 cutoff value of each depth point of the target layer according to the lithologic indicator curve value of the corresponding lithology of the target layer; and based on the logging effective porosity T2 cutoff value of each depth point of the target layer, the effective porosity corresponding to each depth point of the target layer is calculated.

2. The method according to claim 1, characterized in that In step S3, the lithology indicator curve can continuously characterize the vertical lithology changes of the formation, and is a function with the content of each mineral in the rock as the independent variable.

3. The method according to claim 2, characterized in that In step S3, the content of each mineral is calculated by element logging.

4. The method according to claim 3, characterized in that The element logging includes ECS logging, FEM logging, PNX logging or lithologic scanning logging.

5. The method according to any one of claims 1 to 4, characterized in that In step S3, the calculation formula of the lithology indicator curve is: LIC=f(V1,V2,…,V n ) In the above formula, LIC is the lithologic indicator curve, dimensionless; V1, V2, ..., V n It is the content of the first, second,…, nth mineral, %.

6. The method according to claim 1, wherein In step S5, the variable T2 cutoff value fitting relationship is as follows: T 2cutoff_i =g i (LIC) In the above formula, T 2cutoff_i is the variable T2 cutoff value of the i-th lithology, ms; g i is the T2 cutoff value fitting relationship of the i-th lithology, ms.

7. The method according to claim 6, characterized in that In step S6, based on the logging effective porosity T2 cutoff value of each depth point of the target layer, the calculation formula for the effective porosity corresponding to each depth point of the target layer is calculated as follows: In the above formula, φ e_k is the effective porosity of the kth depth point in the target layer, a decimal; P j is the porosity component with relaxation time j corresponding to the k-th depth point of the target layer, a decimal; T 2cutoff_k is the T2 cutoff value of the k-th depth point in the target layer, in ms; T2 is the starting time of the effective porosity of the k-th depth point in the target layer, in ms.

8. A computer-readable storage medium, characterized in that The computer-readable storage medium stores a computer program, wherein the computer program includes program instructions. When the program instructions are executed by a computer, the computer is caused to perform the method according to any one of claims 1 to 7.

9. An electronic device comprising a processor, a memory, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the method according to any one of claims 1 to 7 is implemented.

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