Neutron porosity correction method for gypsum sandstone reservoirs based on Monte Carlo simulation

Abstract

The present invention provides a neutron porosity correction method for a gypsum sandstone reservoir based on Monte Carlo simulation, comprising: step S10: selecting different core samples, performing whole-rock mineral analysis on the core containing anhydrite to determine its anhydrite content; step S20: performing response characteristic analysis on the conventional logging curve of the anhydrite sandstone and selecting a sensitive logging curve reflecting the anhydrite content; step S30: using the sensitive logging curve to construct a curve that can comprehensively reflect the anhydrite content; step S40: using Monte Carlo simulation to qualitatively analyze the influence of different anhydrite contents on the thermal neutron count rate under formation conditions and convert it into the influence on neutron porosity; step S50: establishing a relationship between the neutron porosity correction amount and the anhydrite content; step S60: using the relationship between the neutron porosity correction amount and the anhydrite content to correct the neutron porosity curve. The present invention solves the problem of small neutron porosity in the gypsum sandstone reservoir in the prior art.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum exploration and development, and in particular to a neutron porosity correction method for gypsum sandstone reservoirs based on Monte Carlo simulation. Background Art

[0002] In oil and gas exploration, environmental correction of logging data is an essential link, which is crucial for the later processing and interpretation of logging data. Among them, the neutron porosity curve represents the size of the hydrogen index in the formation, which is directly related to the formation porosity and pore fluid properties, and plays an important role in logging evaluation.

[0003] With the continuous deepening of oil and gas exploration, deep buried reservoirs in some areas have gradually become one of the important areas of risk exploration. The design depth of risk exploration wells is basically above 6000m, and the formation generally contains anhydrite, and the anhydrite content in local reservoirs reaches 25%. Anhydrite has a great impact on the neutrons of sandstone reservoirs, which in turn affects the evaluation accuracy of gypsum sandstone reservoirs.

[0004] Monte Carlo simulation is an important numerical simulation method for radioactive logging. The general program MCNP5 can be used to easily establish a formation model, providing a simple and effective method for basic research on radioactive logging methods. With the help of the Monte Carlo method, calculation models under different conditions are established, logging parameters are obtained through simulation data processing, response relationships with geological parameters are established, and the influence of various influencing factors is studied to form a data processing method.

[0005] The neutron porosity of gypsum sandstone reservoirs obtained by existing analysis and calculation is generally too low and cannot reflect the real porosity of the formation. At present, there is no method to correct the neutron curve of gypsum sandstone reservoirs. Summary of the invention

[0006] The main purpose of the present invention is to provide a neutron porosity correction method for gypsum sandstone reservoirs based on Monte Carlo simulation to solve the problem of small neutron porosity of gypsum sandstone reservoirs in the prior art.

[0007] In order to achieve the above-mentioned purpose, the present invention provides a neutron porosity correction method for gypsum sandstone reservoir based on Monte Carlo simulation, comprising: step S10: selecting different core samples, determining whether the core samples contain anhydrite, performing whole-rock mineral analysis on the core samples containing anhydrite, and determining the anhydrite content; step S20: performing response characteristic analysis on conventional logging curves of anhydrite-containing sandstone, and selecting sensitive logging curves reflecting the anhydrite content; step S30: constructing a curve that can comprehensively reflect the anhydrite content using the sensitive logging curves; step S40: using Monte Carlo simulation to qualitatively analyze the influence of different anhydrite contents on thermal neutron count rate under formation conditions, and converting it into an influence on neutron porosity; step S50: establishing a relationship between the neutron porosity correction amount and the anhydrite content under different neutron porosities; step S60: correcting the neutron porosity curve using the relationship between the neutron porosity correction amount and the anhydrite content.

[0008] Furthermore, in step S10, thin section identification and scanning electron microscope analysis are performed on the selected core sample to determine whether the core sample contains anhydrite.

[0009] Furthermore, in step S20, density and acoustic logging curves are selected as sensitive logging curves.

[0010] Further, in step S20, the natural gamma of the sandstone reservoir containing anhydrite is reduced, the acoustic time difference value is reduced, the neutron value is reduced, the density value is increased, and the resistivity is increased.

[0011] Further, in step S30, the anhydrite content is established Computational model:

[0012] ;

[0013] Among them, a is a constant; b is a constant; Porosity calculated from density, unit: % This is the porosity calculated by sonic wave, in %.

[0014] Further, in step S40, Monte Carlo modeling is performed according to the size of the compensated neutron logging instrument, the wellbore size, and different porosities, and different anhydrite and sandstone ratios in the formation. After simulation calculation, the influence of different anhydrite contents on the long and short source distance count rates under different porosities is established.

[0015] Furthermore, in step S40, a Monte Carlo calculation model is first established; then, the gypsum sandstone formation and the reference formation as a standard are simulated respectively, and simulation calculations are performed on each model, and the counting rate ratio of the far and near thermal neutron detectors under various conditions is statistically calculated and used as the hydrogen content index; the hydrogen content index obtained by simulating different porosities in the reference formation is used as the standard, and the hydrogen content index obtained at each porosity of the gypsum sandstone formation is compared with the standard.

[0016] Furthermore, the reference formation is a pure limestone formation.

[0017] Furthermore, in step S50, a neutron porosity correction calculation model is established:

[0018] ;

[0019] Among them, c is a constant; d is a constant; is the corrected neutron porosity value, unit: % is the anhydrite content, unit: %.

[0020] Furthermore, in step S60, the anhydrite content calculation model and the neutron porosity correction model are combined to obtain the corrected neutron porosity:

[0021] ;

[0022] in, is the corrected neutron porosity, unit: % is the neutron porosity of well logging, unit: % is the corrected neutron porosity value, in %.

[0023] By applying the technical solution of the present invention, the relationship between the anhydrite content and the conventional curve is established by utilizing experimental analysis and conventional logging data, and based on Monte Carlo simulation, the relationship between the neutron porosity correction amount and the anhydrite content is established, and then the neutron porosity of the gypsum-containing reservoir can be corrected, thereby solving the problem that the neutron porosity of the existing gypsum sandstone reservoir is too low and cannot reflect the true porosity of the formation, so that the corrected neutron porosity is more consistent with the actual gypsum formation and rock physical characteristics, and has higher accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0025] Figure 1 A flow chart of the neutron porosity correction method for gypsum sandstone reservoir based on Monte Carlo simulation of the present invention is shown;

[0026] Figure 2 The graph showing the relationship between the anhydrite content, neutrons and density measured in the laboratory in the embodiment of the present invention is shown;

[0027] Figure 3 An XZ-direction view of a Monte Carlo calculation model in an embodiment of the present invention is shown;

[0028] Figure 4 The YX direction view of the Monte Carlo calculation model in the embodiment of the present invention is shown;

[0029] Figure 5 A diagram showing the mineral composition ratio of limestone formations in a Monte Carlo model according to an embodiment of the present invention is shown;

[0030] Figure 6 The diagram of the mineral composition ratio of the gypsum sandstone formation in the Monte Carlo model in the embodiment of the present invention is shown;

[0031] Figure 7 A relationship diagram between the corrected neutron porosity and the anhydrite content in a Monte Carlo simulation in an embodiment of the present invention is shown;

[0032] Figure 8 The application effect diagram of the embodiment of the present invention is shown. DETAILED DESCRIPTION

[0033] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as commonly understood by ordinary technicians in the technical field to which this application belongs.

[0035] In the present invention, unless otherwise specified, the directional words used, such as "up, down, top, bottom", usually refer to the directions shown in the drawings, or to the components themselves in the vertical, perpendicular or gravity directions; similarly, for ease of understanding and description, "inside and outside" refer to the inside and outside relative to the outline of each component itself, but the above-mentioned directional words are not used to limit the present invention.

[0036] In order to solve the problem of low neutron porosity in gypsum sandstone reservoirs in the prior art, the present invention provides a neutron porosity correction method for gypsum sandstone reservoirs based on Monte Carlo simulation.

[0037] like Figure 1The method for correcting neutron porosity of gypsum sandstone reservoir based on Monte Carlo simulation includes: step S10: selecting different core samples to determine whether the core samples contain anhydrite, performing whole-rock mineral analysis on the core samples containing anhydrite to determine the anhydrite content; step S20: performing response characteristic analysis on conventional logging curves of anhydrite-containing sandstone, and selecting sensitive logging curves reflecting the anhydrite content; step S30: constructing a curve that can comprehensively reflect the anhydrite content using the sensitive logging curves; step S40: using Monte Carlo simulation to qualitatively analyze the influence of different anhydrite contents on thermal neutron count rate under formation conditions, and converting it into the influence on neutron porosity; step S50: establishing a relationship between the neutron porosity correction amount and the anhydrite content under different neutron porosities; step S60: correcting the neutron porosity curve using the relationship between the neutron porosity correction amount and the anhydrite content.

[0038] This embodiment uses experimental analysis and conventional logging data to establish the relationship between the anhydrite content and the conventional curve, and based on Monte Carlo simulation, establishes the relationship between the neutron porosity correction amount and the anhydrite content, so that the neutron porosity of the gypsum-containing reservoir can be corrected, thereby solving the problem that the neutron porosity of the existing gypsum sandstone reservoir is low and cannot reflect the true porosity of the formation, so that the corrected neutron porosity is more consistent with the actual gypsum formation and rock physical characteristics, and has higher accuracy.

[0039] The above steps are further described in detail below in combination with actual use. It should be noted that some of the following parameters, formation properties, etc. can be adjusted according to actual conditions.

[0040] In step S10, based on the core and logging data of the gypsum sandstone formation section, a representative core sample of the gypsum sandstone is selected, and thin section identification and scanning electron microscope analysis are performed to determine whether it has been altered and the type of alteration. The porosity of the gypsum sandstone is measured and the whole rock mineral analysis is performed to determine its anhydrite content.

[0041] In this embodiment, a gypsum sandstone formation section in an oil field block is selected as the research target layer, and core data and logging data of the target layer section are collected. Based on these data, 75 representative gypsum sandstone cuttings samples are selected. In this embodiment, the so-called representative core refers to the sample whose lithology is determined to be gypsum sandstone based on the lithology identification plate established in the area and conventional logging, logging, cuttings description and other data; according to the standard procedures of "Rock Thin Section Identification (SY / T5368-2000)", "Rock Sample Scanning Electron Microscope Analysis Method (SY / T5162-2014)" and "Whole Rock Optical Section Microscopic Component Identification and Statistical Method (SY / T6414-2014)", experiments are conducted to determine whether it contains anhydrite. After thin section identification and scanning electron microscope analysis, it is found that all 75 cuttings samples contain anhydrite. Whole rock mineral analysis is performed on the cuttings gypsum sandstone to determine its anhydrite content.

[0042] In step S20, the logging response is a comprehensive reflection of lithology, physical properties, electrical properties and fluid properties. When the sandstone contains anhydrite, some curve characteristics will change significantly: anhydrite will cause the natural gamma to decrease, the acoustic time difference value to decrease, the neutron value to decrease, the density value to increase, and the resistivity to increase. Among them, the neutron value and density value are most affected by anhydrite, while the natural gamma is affected by lithology and the resistivity is affected by oil and gas, which are difficult to quantitatively characterize. Therefore, the density and acoustic logging curves are selected as sensitive curves for the evaluation of gypsum sandstone.

[0043] In step S30, the influence of anhydrite on the well logging curve is analyzed. The neutron value, density value and acoustic wave time difference value are most affected by anhydrite. The anhydrite content is inversely proportional to the neutron value and acoustic wave time difference value, and is proportional to the density value. Combined with the actual geological conditions, the acoustic wave and density data of the corresponding depth points of the gypsum sandstone core sample are extracted to establish the anhydrite content obtained by the whole rock mineral analysis. For the relationship between density and sound wave curve, see Figure 2 , the calculated anhydrite content can be obtained The correlation between the curve and the density and the acoustic wave curve, that is, the anhydrite content Computational model:

[0044] ;

[0045] Among them, a is a constant; b is a constant; Porosity calculated from density, unit: % This is the porosity calculated by sonic wave, in %.

[0046] Combined with the actual situation, the above formula is: .

[0047] In step S40, Monte Carlo modeling is performed according to the size of the compensated neutron logging instrument, the borehole size, and the different porosities, and the different ratios of anhydrite and sandstone in the formation. After simulation calculation, the influence of different anhydrite contents on the long and short source distance count rates under different porosities is established. The specific implementation details are as follows:

[0048] S41. Establish a Monte Carlo calculation model, see Figure 3 and Figure 4 In the model, the formation height is 75cm, the borehole radius is 10cm, the formation thickness is 35cm, and the instrument is placed close to the well wall. The γ source is a 137Cs point source with an energy of 0.662MeV. There is an ideal shield between the source and the detector, and the thickness can be changed with the source distance. In the model, the source strength, source distance, shield material and thickness, detector crystal diameter and length, and the opening shape and angle of the detector and source are designed according to the actual density logging instrument.

[0049] S42, two types of lithology are simulated, one is pure limestone formation, the formation porosity is 0%, 10%, 20%, 30% respectively; the other is gypsum sandstone formation, the formation porosity is 0%, 10%, 20%, 30% respectively. For each porosity, the anhydrite content is 0%, 5%, 10%, 15%, 20%, 25% respectively. The sandstone content is 100% minus the porosity and anhydrite content. For specific formation parameters, see Figure 5 and Figure 6 .

[0050] S43. Simulate and calculate each model, and calculate the counting rate ratio of the two thermal neutron detectors, far and near, under various conditions. This ratio is a direct reflection of the hydrogen content index and can be used as a comparison of the hydrogen content index.

[0051] Since the compensated neutron porosity logging instruments are calibrated in pure limestone calibration wells saturated with fresh water, the measured hydrogen index is recorded as , called compensated neutron porosity. Therefore, taking the pure limestone formation as the reference formation, the hydrogen index obtained by simulating different porosities in the pure limestone formation is used as the standard, and the hydrogen index obtained by various porosities of the gypsum sandstone formation containing anhydrite is compared with the standard.

[0052] In step S50, a correlation equation between the neutron porosity correction amount and the anhydrite content is established for different porosities, that is, a neutron porosity correction amount calculation model:

[0053] ;

[0054] Among them, c is a constant, d is a constant;

[0055] is the corrected neutron porosity value, unit: % is the anhydrite content, unit: %.

[0056] In step S60, the corrected neutron porosity can be obtained by combining the anhydrite content calculation model and the neutron porosity correction model: ;

[0057] in, is the corrected neutron porosity, unit: % is the neutron porosity of well logging, unit: % is the corrected neutron porosity value, in %, such as Figure 7 As shown in the figure, the neutron porosity curve is corrected. The actual application effect diagram is shown in the figure Figure 8 shown.

[0058] It should be noted that, the multiple in the above embodiments refers to at least two.

[0059] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0060] 1. Solve the problem of low neutron porosity in gypsum sandstone reservoir in the prior art;

[0061] 2. Use experimental analysis and conventional logging data to establish the relationship between anhydrite content and conventional curves. Based on Monte Carlo simulation, establish the relationship between neutron porosity correction and anhydrite content, and then correct the neutron porosity of gypsum-containing reservoirs.

[0062] 3. The corrected neutron porosity is more consistent with the actual gypsum formation and rock physical characteristics and has higher accuracy.

[0063] Obviously, the above-described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0064] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0065] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A neutron porosity correction method for gypsum sandstone reservoir based on Monte Carlo simulation, characterized in that: include: Step S10: selecting different core samples to determine whether the core samples contain anhydrite, and performing whole-rock mineral analysis on the core samples containing anhydrite to determine the anhydrite content; Step S20: performing response characteristic analysis on conventional logging curves of anhydrite-containing sandstone, and selecting sensitive logging curves reflecting anhydrite content; Step S30: constructing a curve that can comprehensively reflect the anhydrite content using the sensitive logging curve; Step S40: using Monte Carlo simulation to qualitatively analyze the influence of different anhydrite contents on thermal neutron count rate under formation conditions, and converting it into influence on neutron porosity; Step S50: establishing a relationship between the neutron porosity correction amount and the anhydrite content at different neutron porosities; Step S60: Correcting the neutron porosity curve using the relationship between the neutron porosity correction amount and the anhydrite content; In step S30, the anhydrite content is established. Computational model: ； Among them, a is a constant; b is a constant; Porosity calculated from density, unit: % The porosity calculated by sound wave, unit: % In step S50, a neutron porosity correction calculation model is established: ； Among them, c is a constant; d is a constant; is the corrected neutron porosity value, unit: % is the anhydrite content, unit: %.

2. The neutron porosity correction method for gypsum sandstone reservoir according to claim 1, characterized in that: In step S10, thin section identification and scanning electron microscope analysis are performed on the selected core sample to determine whether the core sample contains anhydrite.

3. The neutron porosity correction method for gypsum sandstone reservoir according to claim 1, characterized in that: In the step S20, density and acoustic logging curves are selected as the sensitive logging curves.

4. The neutron porosity correction method for gypsum sandstone reservoir according to claim 1, characterized in that: In the step S20, the natural gamma of the sandstone reservoir containing anhydrite is reduced, the acoustic time difference value is reduced, the neutron value is reduced, the density value is increased, and the resistivity is increased.

5. The neutron porosity correction method for gypsum sandstone reservoir according to claim 1, characterized in that: In step S40, Monte Carlo modeling is performed according to the size of the compensated neutron logging instrument, the borehole size, and different porosities, and different anhydrite and sandstone ratios in the formation. After simulation calculation, the influence of different anhydrite contents on the long and short source distance count rates under different porosities is established.

6. The neutron porosity correction method for gypsum sandstone reservoir according to claim 5, characterized in that: In step S40, a Monte Carlo calculation model is first established; then, the gypsum sandstone formation and the reference formation as a standard are simulated respectively, and simulation calculations are performed on each model, and the counting rate ratio of the far and near thermal neutron detectors in various cases is statistically calculated and used as the hydrogen content index; the hydrogen content index obtained by simulating different porosities in the reference formation is used as the standard, and the hydrogen content index obtained at each porosity of the gypsum sandstone formation is compared with the standard.

7. The neutron porosity correction method for gypsum sandstone reservoir according to claim 6, characterized in that: The reference stratum is a pure limestone stratum.

8. The neutron porosity correction method for gypsum sandstone reservoir according to claim 1, characterized in that: In the step S60, the anhydrite content calculation model and the neutron porosity correction amount calculation model are combined to obtain the corrected neutron porosity: ； in, is the corrected neutron porosity, unit: % is the neutron porosity of well logging, unit: % is the corrected neutron porosity value, in %.

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