A method for predicting sandstone reservoirs in coal-bearing strata

Abstract

The present invention relates to a method for predicting sandstone reservoirs in coal-bearing strata, belonging to the technical field of predicting sandstone reservoirs in coal-bearing strata. The present invention utilizes the difference in frequency attenuation of seismic waves propagating in sandstone and mudstone in coal-bearing strata. First, forward modeling is performed on the well logging data of the wells with measured sandstone reservoirs obtained to obtain the corresponding pre-stack gathers; then, the well logging data of the sandstone section in the well is replaced with mudstone well logging data, and forward modeling is performed again using the replaced data to obtain the corresponding pre-stack gathers; frequency analysis is performed on the pre-stack gathers obtained before and after replacement to determine the attenuation of the reflected waves in the sandstone section; the pre-stack gathers of the target area to be predicted are obtained, and sandstone prediction is performed on the target area according to the attenuation of the reflected waves in the sandstone section. The present invention does not require complex post-stack data processing, reduces the errors caused by data processing, and improves the prediction accuracy.

Description

Technical Field

[0001] The present invention relates to a method for predicting sandstone reservoirs in coal-bearing strata, belonging to the technical field of sandstone reservoir prediction in coal-bearing strata. Background Art

[0002] Coal-bearing strata refer to underground strata containing two or more coal seams, usually with a thickness between 2 and 10 meters. The sandstone reservoir in coal-bearing strata refers to the sandstone section sandwiched between two or more coal seams, and the non-reservoir refers to the mudstone section between coal seams.

[0003] The commonly used sandstone reservoir prediction method in the prior art is generally the post-stack attribute inversion method. The specific process is to load various lithologic logging curves on the post-stack data volume, and interpolate the information of the lithologic logging curves to the horizons of the post-stack section through various calculations, so as to achieve reservoir prediction. This method has two major limitations: one is that coal seams have a strong shielding effect on seismic waves, and the reflection of sandstone below the coal seam is weak. In many cases, it is difficult to observe the reflection layer in the post-stack volume; the other is that the essence of the post-stack inversion method is to interpolate the sandstone logging values of known wells to the horizons of the post-stack volume. The prediction calibration is good near the wells, and the prediction effect will be worse in places far from the wells and with few wells.

[0004] Conventional reservoir prediction technologies in coal seams mainly rely on post-stack wavelet decomposition and reconstruction, seismic phase rotation, the relationship between drilling attributes and waveforms, etc. Since these technologies all perform secondary processing on seismic waves after stacking and use well data in other fields, secondary errors will be generated, affecting the accuracy of prediction calculations. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for predicting sandstone reservoirs in coal-bearing strata to solve the problem of low accuracy of sandstone reservoirs in coal-bearing strata at present.

[0006] The present invention provides a method for predicting sandstone reservoirs in coal-bearing strata to solve the above technical problems. The prediction method includes the following steps:

[0007] 1) Obtain the logging data of wells with measured sandstone reservoirs in the target coal-bearing strata, and perform forward modeling according to the obtained logging data to obtain the pre-stack gather corresponding to the well. The logging data includes acoustic logging data, longitudinal wave logging data, density logging data, and shear wave logging data;

[0008] 2) Replace the data of the sandstone section in the logging data obtained in step 1) with mudstone data, and perform forward modeling on the replaced logging data to obtain the pre-stack gather after mudstone replacement of the well;

[0009] 3) Perform frequency analysis on the pre-stack gathers in steps 1) and 2) to determine the frequency attenuation variation law of the sandstone section;

[0010] 4) Obtain the pre-stack gather data to be predicted in the target coal-bearing formation, conduct frequency analysis on it, and predict the sandstone reservoir according to the frequency attenuation variation law in the sandstone section.

[0011] The present invention utilizes the frequency attenuation difference of seismic waves propagating in sandstone and mudstone in the coal-bearing formation. First, forward modeling is performed on the logging data of the wells with measured sandstone reservoirs obtained, and the corresponding pre-stack gathers are obtained; then, the logging data of the sandstone section in the well is replaced with mudstone logging data, and forward modeling is performed again using the replaced data to obtain the corresponding pre-stack gathers; frequency analysis is performed on the pre-stack gathers obtained before and after replacement to determine the attenuation of the reflected waves in the sandstone section; the pre-stack gathers of the target area to be predicted are obtained, and sandstone prediction is performed on the target area according to the attenuation of the reflected waves in the sandstone section. The present invention does not require complex post-stack data processing, reduces the errors caused by data processing, and improves the prediction accuracy.

[0012] Further, the frequency attenuation variation law determined in step 3) is:

[0013] f = k × m + n

[0014] where f is the frequency, m is the offset, k is the coefficient of the linear relationship, and n is a constant.

[0015] Further, to quickly and accurately perform frequency analysis on the pre-stack gathers, the frequency analysis is implemented using AVO software.

[0016] Further, to ensure the accuracy of the pre-stack gathers and improve the accuracy of frequency analysis, step 4) also includes a step of amplitude preservation processing on the obtained pre-stack gather data. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a flow chart of the method for predicting sandstone reservoirs in the coal-bearing formation of the present invention;

[0018] Figure 2 is a schematic diagram of the original logging data of Well A1 in the embodiment of the present invention;

[0019] Figure 3-a is a schematic diagram of the pre-stack gather obtained by forward modeling of the original logging data of Well A1 in the embodiment of the present invention;

[0020] Figure 3-b is a schematic diagram of the pre-stack gather obtained by forward modeling of the logging data after mudstone replacement in Well A1 in the embodiment of the present invention;

[0021] Figure 4 is a schematic diagram of the frequency analysis results of the pre-stack gathers before and after replacement in Well A1 in the embodiment of the present invention;

[0022] Figure 5 It is a schematic diagram of the prediction result of the sandstone reservoir in 38 wells in Block A in the embodiment of the present invention. Specific Embodiments

[0023] The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0024] In the coal-bearing strata of the Honghe Yan'an Formation in southern Hubei, the relationship between the densities of sandstone, mudstone, and coal seams is as follows: the density of sandstone is greater than that of mudstone, and the density of mudstone is greater than that of coal seams. The relationship between the acoustic wave time differences of the three is that the time difference of coal seams is greater than that of mudstone, and the acoustic wave time difference of mudstone is greater than that of sandstone. According to this relationship, it can be known that when the seismic wave propagates downward from the coal seam, a new wave source will be generated when it encounters the sandstone with a greater density, and the frequency attenuation slows down, while the propagation frequency of the seismic wave in the formation with only mudstone without sandstone attenuates faster. Based on this discovery, the present invention proposes a method for predicting sandstone reservoirs in coal-bearing strata.

[0025] The method for predicting sandstone reservoirs in coal-bearing strata of the present invention is based on the characteristics that the frequency attenuation of seismic waves is slower when propagating in low-density sandstone and faster when propagating in high-density mudstone, and predicts sandstone reservoirs through the frequency attenuation characteristics of prestack seismic. First, based on the longitudinal wave logging data, density logging data, and shear wave velocity data of the wells with measured sandstone reservoirs, a prestack gather is forward modeled; then, the longitudinal wave logging data, density logging data, and shear wave velocity data of the sandstone section in the coal seam of the well are replaced with the corresponding data of mudstone, and another prestack gather is forward modeled using the replaced data; then, the two prestack gathers are compared to determine the attenuation of seismic waves at different frequencies in the sandstone section; the prestack seismic data of the formation to be predicted is obtained, and the frequency attenuation product of the prestack seismic data of the formation to be predicted is analyzed according to the attenuation of seismic waves at different frequencies in the sandstone section, and the judgment of the sandstone reservoir is realized according to the analysis result.

[0026] The following takes the coal-bearing strata of the Honghe Yan'an Formation in southern Hubei as an example to describe the specific implementation process of the present invention in detail. The implementation process of this method is as Figure 1 shown, and the specific implementation process is as follows.

[0027] 1. Obtain the acoustic wave logging data, longitudinal wave logging data, density logging data, and shear wave logging data of the wells with measured sandstone reservoirs, and forward model a prestack gather.

[0028] For this embodiment, assume that Well A1 in the coal-bearing strata of the Honghe Yan'an Formation in southern Hubei is a well containing a sandstone reservoir, then the relevant logging data of Well A1 is obtained, such as Figure 2As shown, the logging data includes compressional wave logging data, acoustic logging data, density logging data, and shear wave logging data, as shown in the left half of Table 1, where the sandstone section is the formation section with a depth ranging from 1923.63 m to 1956.88 m. Forward modeling is performed based on the original logging data of Well A1 in Table 1, and the obtained forward model is as Figure 3-a shown.

[0029] 2. Replace the sandstone section in the above logging data with mudstone, and forward model a prestack gather using the acoustic logging data, density logging data, and shear wave logging data of the replaced mudstone.

[0030] For this embodiment, replace the logging data (including compressional wave logging data, density logging data, and shear wave logging data) of the sandstone reservoir in Well A1 with the corresponding mudstone data, and replace the logging data in the formation with a depth ranging from 1923.63 m to 1956.88 m with mudstone data. Among them, the acoustic logging data is 247.22, the density logging data is 2.19, the compressional wave logging data is 4045.01, and the shear wave logging data is 2314.07. The result is shown in the right half of Table 1. Forward modeling is performed based on the replaced data, and the obtained forward model is as Figure 3-b shown.

[0031] Table 1

[0032]

[0033] 3. Compare the different attenuation situations of the seismic wave frequencies in the sandstone section of the two forward models,

[0034] For this embodiment, compare the forward models before and after replacement, that is, perform frequency analysis on the results of Figure 3-a and Figure 3-b (which can be implemented using software AVO). The result is as Figure 4 shown. It can be seen from this that the frequencies of both sandstone and mudstone decrease and attenuate with the increase of the offset, but the frequency distribution of sandstone is lower and its attenuation speed is slower than that of mudstone. At the same time, according to the frequency analysis results, the following relationship exists between its frequency and offset:

[0035] f = k × m + n

[0036] where f is the frequency, m is the offset, k is the coefficient of the linear relationship (also called the attenuation coefficient), and n is a constant.

[0037] 4. Obtain the prestack gather data of the coal measure strata to be measured, and perform frequency attenuation analysis on the obtained prestack gather to judge the distribution of the sandstone reservoir.

[0038] For this embodiment, all pre-stack gather data of the coal-bearing strata in the Honghe Yan'an Formation in southern Hubei are acquired, calibrated using actual wells, the well-seismic relationship and the wave group correspondence are determined to define the time range of the pre-stack horizons. Meanwhile, amplitude-preserving processing is performed on the acquired pre-stack gather data (which can be achieved by using radon transform, median filtering, band-pass filtering, etc.). Frequency attenuation analysis is carried out on all the pre-stack gather data after the above processing, and the corresponding attenuation coefficients are calculated. The distribution of sandstone is determined based on the attenuation coefficient values at each pre-stack gather (CMP) point.

[0039] There are different attenuation situations in different regions. In one region, the faster the attenuation, the greater the possibility of sandstone, that is, the larger the attenuation coefficient k, the greater the possibility of it being sandstone. There will also be certain differences in the attenuation coefficients of sandstone in different regions, and it is impossible to use a fixed value to judge. It can only be analyzed according to the attenuation situation of the specific work area, and judged according to the principle that the larger the attenuation coefficient k, the greater the possibility of sandstone.

[0040] The Yan'an Formation in Block A is tested, and the gathers of all 38 wells are analyzed. As Figure 5 shown, 36 wells meet the requirements, and the success rate is 94.73%. During the analysis of the small block of AH60, the sandstone distribution of the small block can be clearly depicted, which is basically consistent with the analysis results of geological peers.

[0041] Therefore, the present invention utilizes the frequency attenuation difference of seismic waves propagating in sandstone and mudstone in the coal-bearing strata to obtain the attenuation of the reflected waves in the sandstone section, performs frequency analysis on the pre-stack gathers in the target area to obtain the attenuation of the reflected waves in the target area, and realizes the prediction of sandstone in the target area by using the attenuation of the reflected waves in the sandstone section. The present invention does not require complex post-stack data processing, reduces the errors caused by data processing, and greatly improves the prediction accuracy.

Claims

1. A method for predicting sandstone reservoirs in coal-bearing strata, characterized in that, The prediction method includes the following steps: 1) Obtain the logging data of the wells with measured sandstone reservoirs in the target coal-bearing formation, and perform forward modeling based on the obtained logging data to obtain the prestack gather corresponding to the well. The logging data includes acoustic logging data, longitudinal wave logging data, density logging data, and shear wave logging data; 2) Replace the data of the sandstone section in the logging data obtained in step 1) with shale data, and perform forward modeling on the replaced logging data to obtain the prestack gather after shale replacement for the well; 3) Perform frequency analysis on the prestack gathers in steps 1) and 2) to determine the frequency attenuation variation law of the sandstone section; 4) Obtain the prestack gather data to be predicted in the target coal-bearing formation, perform frequency analysis on it, and predict the sandstone reservoir according to the frequency attenuation variation law of the sandstone section.

2. The sandstone reservoir prediction method for coal measures strata according to claim 1, characterized in that The frequency attenuation variation law of the sandstone section determined in step 3) is: f = k × m + n where f is the frequency, m is the offset, k is the coefficient of the linear relationship, and n is a constant.

3. The sandstone reservoir prediction method for coal measures strata according to claim 1, characterized in that, The frequency analysis is implemented using AVO software.

4. The method for predicting sandstone reservoirs in coal measures strata according to any one of claims 1-3, characterized in that Step 4) also includes a step of amplitude preservation processing on the obtained prestack gather data.

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