Passive density correction method applied to coal seam measurement

Abstract

The invention discloses a passive density correction method applied to coal seam measurement, and belongs to the technical field of petroleum and natural gas development. The method comprises the following steps: firstly, obtaining the influence of coal seams with different densities on a passive density measurement result through a numerical simulation method; then, calculating the apparent density of the coal seam by utilizing an existing density data processing method, and calculating a passive density error caused by the coal seams with different densities according to the difference between the apparent density and the real density; secondly, establishing a mathematical relationship between the passive apparent density and a passive density error, and forming a passive density coal seam argillaceous correction formula; and finally, substituting the apparent density of the coal seam into a coal seam density correction formula to obtain a density correction value, and superposing the density correction value to a passive density result to obtain relatively accurate coal seam density. According to the method, the measurement precision of passive density logging in the coal seam is improved, and the application scene of the method in the coal seam is expanded.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas development technology, and particularly relates to a passive density correction method for coal seam measurement. Background Technology

[0002] Passive density logging technology uses a DT source instead of a chemical source to measure density. It has advantages such as operational safety, environmental friendliness, and complete controllability. Currently, passive density logging technology has been successfully applied in conventional oil and gas reservoir measurement. However, due to the large difference in elemental composition between coal seams and conventional oil and gas reservoirs, the measurement results deviate significantly from the actual coal seam density.

[0003] Density, as one of the basic parameters for formation evaluation, is widely used in coal-bearing formation exploration. In order to fully leverage the advantages of passive density logging in terms of safety and environmental protection, and to overcome the current problem of inaccurate measurement, it is urgent to develop a passive density correction method for coal seam measurement. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a passive density correction method for coal seam measurement. Compared to traditional gamma-gamma logging techniques that rely on chemical sources, this invention fundamentally eliminates the potential safety risks and environmental hazards associated with radioactive sources. It solves the problem of large measurement errors in passive density logging within coal seams, thus expanding the application scope of passive density logging.

[0005] To achieve the above objectives, the present invention specifically adopts the following technical solution: a passive density correction method applied to coal seam measurement, specifically including the following steps:

[0006] S1. Establish a passive density logging instrument model using numerical simulation methods, set up limestone water layers under different porosity conditions using the simulation model, and calibrate the calculation formula of passive density using the existing publicly available passive density formula.

[0007] S2. Using simulation models, coal seams under different density conditions are set up to obtain passive density logging response simulation data under different density coal seam conditions;

[0008] S3. Based on the passive density calculation formula obtained in S1 and the passive density logging response under different density coal seam conditions obtained in S2, the formation apparent density is calculated, and the apparent density error between the formation apparent density and the formation true density is calculated.

[0009] S4. Establish the mathematical relationship between apparent density error and apparent density to form a coal seam correction formula for passive density logging;

[0010] S5. Substitute the parameters from the actual well logging into the correction formula to calculate the coal seam density correction amount. Then, superimpose the coal seam density correction amount onto the passive density calculation result to obtain the corrected passive density measurement result of the coal seam.

[0011] Preferably, the parameters of the passive density logging instrument model in step S1 are not fixed and need to be adjusted in real time according to the actual passive density logging instrument parameters.

[0012] Preferably, in the numerical simulation method in step S1, Monte Carlo simulation software for simulating neutron-photon transport processes is used, and different software such as Super Monte Carlo, Geant4, and FLUKA are selected as needed.

[0013] Preferably, the existing density data processing method in step S1 refers to the publicly disclosed method of determining the formation density calibration formula by combining inelastic scattering gamma information, ultrathermal neutrons, thermal neutrons, and captured gamma information. Since the existing publicly disclosed passive density calibration formulas use different combinations of neutron and gamma information, their specific forms vary considerably; therefore, they are uniformly represented by the following formula:

[0014] ;

[0015] In the formula, N in For non-elastic gamma counting, N cap To capture gamma counts, N t For thermal neutron counting, N epi For ultrathermal neutron counting, R i R is the non-bullet count ratio. cap To capture the gamma count ratio, R t This represents the thermal neutron count ratio.

[0016] Preferably, in step S2, the coal seams with different densities are set, with a density of 1.05-1.25 g / cm³. 3 , interval 0.05g / cm 3 change.

[0017] Preferably, in step S3, the apparent density of the formation refers to the formation density value calculated using the passive density calibration formula; the true density of the formation refers to the actual density of the formation; and the apparent density error refers to the difference between the true density and the apparent density of the formation, specifically in the following form:

[0018] ;

[0019] In the formula, Δρ is the density error, and ρ b ρ is the true density of the formation. a This represents the apparent density of the formation.

[0020] Preferably, the apparent density error in step S4 is negatively correlated with the calculated apparent density under coal seam conditions; specifically, it is as follows:

[0021] ;

[0022] In the formula, a and b are scale coefficients.

[0023] This invention solves the problem of large calculation errors in coal seam passive density logging, and expands the application scenarios of passive density logging. Specific beneficial effects are as follows:

[0024] 1. Solving core technical challenges and reducing measurement errors: This invention addresses the critical issue of large deviations between measured results and actual coal seam density caused by significant differences in elemental composition between coal seams and conventional oil and gas reservoirs in passive density logging technology. By establishing a mathematical relationship between passive apparent density and passive density error, and a coal seam correction formula, the corrected coal seam density error is controlled within the ±0.025 g / cm³ standard commonly used in the density logging industry, significantly improving the measurement accuracy of passive density logging in coal seam scenarios.

[0025] 2. Retaining the advantages of passive logging technology and expanding its application scenarios: Compared with the traditional gamma-gamma logging technology that relies on chemical sources, this invention retains the core advantages of passive density logging, such as operational safety, environmental friendliness, and complete controllability, fundamentally eliminating the potential safety risks and environmental hazards of radioactive sources; at the same time, it breaks through the previous limitation that this technology could only be applied to conventional oil and gas reservoirs, successfully expanding to coal seam measurement scenarios, providing a safer, more environmentally friendly, and more accurate density measurement solution for coal-bearing strata exploration. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the pulsed neutron instrument-stratum simulation model used in this invention.

[0027] Figure 2 This is a map showing the apparent density of limestone water layers calculated using the calibration formula in this invention.

[0028] Figure 3 This is a comparison diagram of the true density and apparent density of coal seams and water layers in this invention.

[0029] Figure 4 This is a diagram showing the apparent density error under different coal seam conditions in this invention.

[0030] Figure 5 This is a diagram showing the relationship between visual density and visual density error in this invention.

[0031] Figure 6 This is the corrected coal seam density diagram in this invention. Detailed Implementation

[0032] The following are specific embodiments of the present invention described in conjunction with the accompanying drawings, which further illustrate the technical solution of the present invention. However, the present invention is not limited to these embodiments.

[0033] This example uses a pure coal seam as the research object and presents a passive density correction method for coal seam measurement, including the following steps:

[0034] S1. Establish a passive density logging instrument model using numerical simulation methods, set up limestone water layers under different porosity conditions using the simulation model, and calibrate the calculation formula of passive density using the existing publicly available passive density formula.

[0035] Based on the parameters of existing passive density logging instruments, a passive density logging instrument model is established using the Monte Carlo numerical simulation method. Many practical passive density logging instruments exist; this example uses a passive density logging instrument with dual gamma detectors and dual thermal neutron detectors, such as... Figure 1 As shown, the formation was set as limestone, the pore fluid as fresh water, and the porosity as 1%-40%, varying in 5% increments. Following actual well logging, the passive density logging response of the limestone water layer under different porosity conditions was measured by placing the instrument close to the wellbore and measuring the inelastic count ratio and thermal neutron count ratio.

[0036] There are many existing formulas for calculating passive density. In this example, the apparent density of the formation is characterized by a combination of inelastic count ratio and thermal neutron count ratio information. The specific formula is as follows:

[0037] ;

[0038] Figure 2 The purple line represents the apparent density of the limestone water layer calculated using the calibration formula. The purple line indicates the point where the true density equals the apparent density, and the error between the apparent density and the true density is relatively small.

[0039] S2. Using simulation models, coal seams under different density conditions are set up to obtain passive density logging response simulation data under different density coal seam conditions.

[0040] The strata were set as coal seams with a density of 1.05-1.25 g / cm³. 3 , interval 0.05g / cm 3 The porosity was 0%. Based on the Monte Carlo numerical simulation method, the logging responses of inelastic count ratio and thermal neutron count ratio under different coal seam density conditions were obtained.

[0041] S3. Based on the passive density calculation formula obtained in S1 and the passive density logging response under different density coal seam conditions obtained in S2, the formation apparent density is calculated, and the apparent density error between apparent density and true density is also calculated.

[0042] Figure 3This is a comparison chart between the apparent density and true density of a formation calculated using the passive density calibration formula. The horizontal axis represents the true density of the formation, and the vertical axis represents the apparent density. It can be seen that the apparent density calculated for coal seams is too high, and the error is larger compared to that for water seams. Figure 4 The apparent density error of coal seams with different densities is given. The red dashed line represents ±0.025 g / cm³, which is commonly used in density logging. 3 Error standards; the specific values ​​of the true density, apparent density, and apparent density error of the coal seam are shown in Table 1 below.

[0043] Table 1 Apparent density error of coal seams with different densities

[0044] <![CDATA[True density (g / cm 3 )]]> <![CDATA[Apparent density (g / cm 3 )]]> <![CDATA[Apparent density error (g / cm 3 )]]> 1.05 1.79 -0.74 1.10 1.86 -0.76 1.15 1.92 -0.77 1.20 1.97 -0.77 1.25 2.03 -0.78

[0045] S4. Establish the mathematical relationship between apparent density error and apparent density to form a coal seam correction formula for passive density logging;

[0046] Figure 5 This demonstrates the relationship between visual density and visual density error, showing a negative correlation between the two; the specific mathematical relationship is as follows:

[0047] ;

[0048] S5. Substitute the parameters from the actual well logging into the correction formula to calculate the coal seam density correction amount. Then, superimpose the coal seam density correction amount onto the passive density calculation result to obtain a relatively accurate passive density measurement result for the coal seam.

[0049] Substituting the apparent density calculated by the above model into the coal seam correction formula yields a relatively accurate coal seam density. Figure 6 The corrected coal seam density error shows a significant reduction, conforming to the density logging error of ±0.025 g / cm³. 3 The error standard.

[0050] This method utilizes the mathematical relationship between apparent density and apparent density error in coal seams to establish a correction formula for passive density logging in coal seams, solving the problem of inaccurate measurements in passive density logging in coal seams and expanding the application scope of passive density logging.

[0051] The described embodiments are only a portion, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. Therefore, the detailed description of the embodiments of this invention provided is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. This invention is also not limited to the examples described above; changes, modifications, additions, or substitutions made by those skilled in the art within the essential scope of this invention should also be within the scope of protection of this invention.

Claims

1. A passive density correction method for coal seam measurement, characterized in that, Specifically, the following steps are included: S1: Establish a passive density logging instrument model using numerical simulation methods, set up limestone water layers under different porosity conditions using the simulation model, and calibrate the passive density calculation formula using the passive density formula. S2: Using simulation models to set up coal seams under different density conditions, we obtained simulation data of passive density logging response under different density coal seam conditions; S3: Based on the passive density calculation formula obtained in step S1 and the passive density logging response simulation data under different density coal seam conditions obtained in step S2, the formation apparent density is calculated, and the apparent density error between the formation apparent density and the formation true density is calculated. S4: Establish the mathematical relationship between apparent density error and apparent density to obtain the coal seam correction formula for passive density logging; S5: Substitute the parameters from the actual well logging into the correction formula to calculate the coal seam density correction amount. Then, superimpose the coal seam density correction amount onto the passive density calculation result to obtain the corrected passive density measurement result of the coal seam.

2. The passive density correction method for coal seam measurement as described in claim 1, characterized in that, In step S1, the Monte Carlo method is used for numerical simulation.

3. The passive density correction method for coal seam measurement as described in claim 2, characterized in that, In the Monte Carlo method, Monte Carlo simulation software is used to simulate the neutron-photon transport process. The Monte Carlo simulation software is one of Super Monte Carlo, Geant4 or FLUKA software.

4. The passive density correction method for coal seam measurement as described in claim 1, characterized in that, In step S1, the passive density formula is as follows: ; where N in For non-elastic gamma counting, N cap To capture gamma counts, N t For thermal neutron counting, N epi For ultrathermal neutron counting, R in R is the non-bullet count ratio. cap To capture the gamma count ratio, R t This represents the thermal neutron count ratio.

5. The passive density correction method for coal seam measurement as described in claim 1, characterized in that, In step S2, the coal seam parameters under different density conditions are as follows: the density ranges from 1.05 to 1.25 g / cm³, varying in intervals of 0.05 g / cm³.

6. The passive density correction method for coal seam measurement as described in claim 1, characterized in that, In step S3, the formula for calculating apparent density error is as follows: ； In the formula, Δρ is the density error, and ρ b ρ is the true density of the formation. a This represents the apparent density of the formation.

7. The passive density correction method for coal seam measurement as described in claim 1, characterized in that, In step S4, the mathematical relationship between apparent density error and formation apparent density is as follows: ； In the formula, a and b are scale coefficients.

Images