A method for predicting sweet spots of shale oil and gas
The method uses frequency attenuation analysis with fracture density constraints to improve the prediction of shale oil and gas sweet spots, accounting for both fracture and sandstone contributions, thereby enhancing prediction accuracy and scope.
Patent Information
- Application Number
- CN202210102625.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-01-27
- Publication Date
- 2025-07-15
- Estimated Expiration
- 2042-01-27
AI Technical Summary
The prior art is difficult to accurately characterize the dessert areas when both the cracks and hydrocarbon-rich sandstone interlayers are developed in mud shale formations, resulting in insufficient accuracy in oil and gas exploration and development.
By predicting the distribution of fractures in mud shale formations, using frequency attenuation attributes combined with the fracture constraint coefficients, the degree of frequency attenuation is calculated, and the prediction range of the oil and gas dessert area is expanded, including setting a small constraint coefficient when the crack density is high, setting an approximate coefficient when the crack density is low, and setting a coefficient between 0 and 1 when it is between the two, reflecting the oil and gas content of the sandstone strips.
Accurate prediction of mud shale oil and gas desserts is achieved, the scope of identification of dessert areas is expanded, and the accuracy and efficiency of mud shale oil and gas exploration is improved.
Smart Images
Figure CN114462237B_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method for predicting sweet spots of shale oil and gas, belonging to the technical field of petroleum exploration. Background Art
[0002] Currently, the successful exploration and development of shale oil in North America has brought great inspiration to the world's shale oil exploration, and there is also increasing attention to the exploration and development of shale oil in China. Continental shale formations in China are widely developed, with high organic matter abundance and large thickness, providing a rich material basis for the formation of shale oil. Exploration breakthroughs of shale oil have been achieved in multiple basins such as the Ordos Basin, Bohai Bay Basin, Nanxiang Basin, and Jianghan Basin. Conducting research on the prediction and evaluation of shale oil "sweet spots" has high economic prospects.
[0003] A "sweet spot" refers to the best areas and horizons for the exploration and development of shale oil and gas. Using geophysical techniques to find sweet spots is an important means for the prediction and evaluation of shale oil and gas resources. Sweet spot areas are often also areas where formation fractures are developed. Therefore, using various geophysical techniques to find fracture-developed areas is the main method commonly used for the prediction and evaluation of shale oil and gas resources at present. These techniques include logging techniques, prestack seismic inversion techniques, and seismic attribute analysis techniques, etc. By conducting anisotropy analysis of shale reservoirs (such as fracture and microfracture evaluation), reservoir elastic parameters, and rock mechanics anisotropy characteristics, the fracture-developed areas are analyzed. Among them, seismic attributes such as coherence, curvature, and maximum likelihood, as well as techniques such as prestack anisotropy prediction, can indicate fractures within the shale formation. In addition, by using elastic attributes obtained from prestack elastic inversion, by predicting TOC content and shale brittleness, the shale sweet spots can also be indirectly predicted.
[0004] In recent years, it has been gradually found in production that even if the degree of fracture development is insufficient, when there are sandstone bands rich in hydrocarbons such as oil and gas in the shale, the standards for the exploration and development of shale oil and gas can still be met. For these situations, it is difficult to completely and accurately depict the shale sweet spots only based on the fracture prediction results. Summary of the Invention
[0005] The purpose of the present invention is to provide a method for predicting sweet spots of shale oil and gas, which is used to solve the problem that it is difficult to accurately depict the sweet spot areas when both fractures and hydrocarbon-rich sandstone interlayers are developed in the shale formation.
[0006] To achieve the above purpose, the present invention provides a method for predicting sweet spots of shale oil and gas, including the following steps:
[0007] 1) Predict the fracture distribution in the shale formation to obtain actual fracture distribution data; the fracture distribution data is reflected by the fracture density;
[0008] 2) Determine the constraint coefficient: At least divide the fracture distribution data into a first interval and a second interval, where the fracture density in the first interval is greater than that in the second interval; if the actual fracture distribution data belongs to the first interval, use the first constraint coefficient as the constraint coefficient; if the actual fracture distribution data belongs to the second interval, use the second constraint coefficient as the constraint coefficient;
[0009] 3) Substitute the constraint coefficient into the frequency attenuation calculation. When the actual fracture distribution data falls into the first interval, the degree of frequency attenuation calculated after substituting the constraint coefficient is greater than that when the actual fracture distribution data falls into the second interval;
[0010] 4) Predict the shale oil and gas sweet spots based on the frequency attenuation calculated after substituting the constraint coefficient. When the degree of frequency attenuation is greater than the set degree, it is an oil and gas sweet spot.
[0011] The beneficial effects of the present invention are as follows: The first interval is used to predict the sweet spot area with better fracture development, and the second interval is used to predict the situation where there are oil and gas in the sandstone bands with poor fracture development, expanding the prediction range of the oil and gas sweet spot area and providing a solution for the oil and gas exploration of shale.
[0012] Further, in the above shale oil and gas sweet spot prediction method, the first constraint coefficient is less than the second constraint coefficient; the frequency attenuation calculation after substituting the constraint coefficient is realized through the following formula:
[0013]
[0014] In the formula, A2 is the second spectral energy, A1 is the first spectral energy, f2 is the frequency corresponding to the second spectral energy, f1 is the frequency corresponding to the first spectral energy, β is the constraint coefficient, and the second spectral energy A2 is less than the first spectral energy A1.
[0015] The beneficial effect of doing this is: In the second interval, the frequency attenuation rate can reflect the situation of the sandstone band containing oil and gas sweet spots, and the fracture distribution data can reflect the fracture development situation. By constraining the calculation process of the frequency attenuation rate with the fracture constraint coefficient, the comprehensive influence of these two on the possibility of the existence of oil and gas sweet spots is manifested, realizing the prediction of oil and gas sweet spots in a comprehensive situation.
[0016] Further, in the above shale oil and gas sweet spot prediction method, divide the fracture distribution data into three intervals. When the actual fracture distribution data belongs to the first interval, set the fracture constraint coefficient to 0; when the actual fracture distribution data belongs to the second interval, the fracture constraint coefficient is between 0 and 1; when the actual fracture distribution data belongs to the third interval, set the fracture distribution coefficient to a constant greater than or equal to 1.
[0017] Further, in the above shale oil and gas sweet spot prediction method, when the actual fracture distribution data belongs to the second interval, the fracture constraint coefficient is set to the reciprocal of the actual fracture data.
[0018] Further, in the above shale oil and gas sweet spot prediction method, the actual fracture distribution data in the first interval is greater than the upper limit value of the fracture density; the actual fracture distribution data in the third interval is less than the lower limit value of the fracture density.
[0019] Further, in the above shale oil and gas sweet spot prediction method, the fracture distribution data in step 1) is the linear density data of the fracture or the bulk density data of the fracture.
[0020] Further, in the above shale oil and gas sweet spot prediction method, the first spectral energy value in step 3) is 60% - 80% of the main frequency energy; the second spectral energy value is 20% - 30% of the main frequency energy. Description of the Drawings
[0021] Figure 1 It is the flowchart of the method of the present invention;
[0022] Figure 2 It is the maximum likelihood attribute prediction fracture density profile;
[0023] Figure 3 It is the frequency attenuation attribute profile under fracture constraint of Well as1;
[0024] Figure 4 It is the comparison diagram of the No. ① layer in Well as1 and the actual logging;
[0025] Figure 5 It is the comparison diagram of the No. ② - ④ layers in Well as1 and the actual logging;
[0026] Figure 6 It is the comparison diagram of the No. ⑤ layer in Well as1 and the actual logging;
[0027] Figure 7 It is the lithofacies distribution map of the shale formation in the embodiment of the method;
[0028] Figure 8 It is the sweet spot prediction map in the embodiment of the method. Detailed Embodiment
[0029] In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments.
[0030] Embodiment:
[0031] When seismic waves propagate in the formation, due to the effects of geometric spreading, scattering, inherent attenuation of the formation, as well as transmission and reflection at the formation interface, etc., the energy of the seismic waves will attenuate, mainly manifested as changes in the dynamic characteristics of seismic waves such as amplitude, frequency, and phase. Characterizing these change characteristics of seismic waves, especially the attenuation change characteristics in frequency, can reflect the degree of absorption of different frequency information of seismic waves by the formation. In this way, the fluid properties in the rock formation can be predicted and evaluated through the frequency absorption attenuation attribute of seismic waves, especially for predicting hydrocarbon-bearing rock formations, which is often referred to as hydrocarbon detection.
[0032] Good results have been achieved for the frequency attenuation attribute in sandstone reservoirs with relatively high porosity. However, there are few examples of its application in predicting the "sweet spot" area of shale. This is because the porosity of shale is relatively low, the attenuation degree of seismic wave frequency is small, and the obtained results have a large degree of ambiguity. However, in shale areas with complex lithofacies, when the degree of fracture development is not high but hydrocarbon-rich sandstone bands are developed, shale sweet spots can also be formed. When the sandstone (or siltstone) bands in shale are rich in hydrocarbons, it can cause a large degree of attenuation of the seismic wave frequency passing through the formation, thus forming an anomaly in the frequency attenuation attribute. Therefore, when there is a large attenuation of the seismic wave frequency of the formation, it indicates that the sandstone bands in the shale are rich in hydrocarbons, that is, there is a sweet spot area.
[0033] The present invention includes the following steps:
[0034] 1) Using the seismic maximum likelihood attribute, predicting the fracture distribution data α in the shale formation; the fracture distribution data α is the linear density of fractures, in units of strips / meter; or the bulk density of fractures, in units of strips / cubic meter.
[0035] 2) Calculating the fracture constraint coefficient β using formula (1).
[0036]
[0037] In formula (1), α high represents the upper limit of fracture density. If the fracture density is higher than this value, then regardless of whether the sandstone band is developed or not, it is a sweet spot; α low represents the lower limit of fracture density. Below this value, even if the sandstone band is well developed, as long as the formation still belongs to the shale category, then due to the lack of fracture communication, it is difficult for hydrocarbons to flow effectively and thus cannot become a sweet spot; when it is between the two, if the sandstone band is developed, a sweet spot can also be formed, but it cannot be identified only by fracture prediction. C is a constant value greater than or equal to 1.
[0038] 3) Frequency attenuation is often used to characterize the attenuation degree of seismic waves by the formation. Since the attenuation degree of seismic wave frequency in fractures is usually small, while that in sandstone bands rich in oil and gas is large, the greater the attenuation degree of frequency, the more likely it is to have an oil and gas sweet spot in the sandstone band.
[0039] By adding a fracture constraint coefficient in the calculation process of the frequency attenuation rate, the degree of fracture development can be used to constrain the frequency attenuation calculation process, so that the constrained frequency attenuation rate can be used to characterize whether there is a shale oil and gas sweet spot in the formation to be measured.
[0040] Substitute the fracture constraint coefficient β into the formula for calculating the frequency attenuation slope to obtain formula (2).
[0041]
[0042] By adding the constraint of the degree of fracture development to the frequency attenuation rate, it represents the frequency attenuation rate γ after fracture constraint. The greater the absolute value of the constrained frequency attenuation rate γ, that is, the greater the degree of frequency attenuation, the more likely it is to have a sweet spot area. A1 selects 60% - 80% of the main frequency energy, A2 selects 20% - 30% of the main frequency energy, f1 and f2 are the corresponding frequency values. Therefore, it can be considered that A1 > A2 and f1 < f2. So γ is negative, and the smaller the value of γ, the more likely it is to have a sweet spot area. Calculate γ for the attenuation spectra at multiple positions to obtain a frequency attenuation attribute map for characterizing the degree of frequency attenuation.
[0043] Combining formula (1) and formula (2), when the fracture density is higher than the upper limit of the fracture density, β is the smallest and always 0, then γ is the smallest, representing the greatest possibility of a sweet spot area, which is in line with the actual situation; when the fracture density is lower than the lower limit of the fracture density, β is the largest and always a constant greater than or equal to 1, then γ is the largest, representing the smallest possibility of a sweet spot area, which is in line with the actual situation; when the fracture density is between the two, γ is inversely proportional to α, and γ is directly proportional to the frequency attenuation rate before constraint, which is in line with the actual situation.
[0044] 4) Overlay and analyze the shale lithofacies distribution map and the frequency attenuation attribute map to predict the shale oil sweet spot area.
[0045] The following is illustrated by the following example:
[0046] In this embodiment, the exploration area is a typical oil-rich sag in the east. A set of lacustrine shale rich in organic matter developed in the third member of the Hetao Formation of the Paleogene, with a cumulative thickness of 200 - 600m, a distribution area of nearly 400 km 2 , a burial depth of 1700 - 4500m, and the shale oil resource is very rich. Optimizing the sweet spots of shale oil reservoirs can accelerate the breakthrough in the exploration of continental shale oil and achieve the goal of large-scale exploration and development of shale oil.
[0047] As shown in Figure 1 the figure, the sweet spot prediction method for shale oil reservoirs of this embodiment is used to identify the sweet spot area of the lower section of H3Ⅲ in the exploration area, and the specific steps are as follows:
[0048] 1) Using seismic maximum likelihood attributes, predict the fracture distribution data α in the shale formation; Figure 2 is the fracture body density profile obtained from the maximum likelihood attribute. It can be seen from the figure that the areas with relatively developed fracture density are often related to the fault distribution.
[0049] 2) Calculate the fracture constraint coefficient β using formula (1).
[0050]
[0051] In formula (1), α high represents the upper limit of fracture density. If the fracture density is higher than this value, then regardless of whether the hydrocarbon-rich interlayer is developed, it is a sweet spot, that is, whether it is a sweet spot area in this case has nothing to do with the fracture density, so the fracture constraint coefficient β is 0; α low represents the lower limit of fracture density. Below this value, even if the hydrocarbon-rich interlayer is highly developed, as long as the formation still belongs to the shale category, then due to the lack of fracture communication, it is difficult for oil and gas to flow effectively and thus cannot become a sweet spot. When it is between the two, if the hydrocarbon-rich interlayer is developed, a sweet spot can also be formed, but it cannot be identified only by fracture prediction. C is a constant value greater than or equal to 1.
[0052] 3) By adding the fracture constraint coefficient during the calculation process of the frequency attenuation rate, the fracture development degree can be used to constrain the frequency attenuation calculation process, so that the constrained frequency attenuation rate can be used to characterize whether there is a shale oil and gas sweet spot in the formation to be measured.
[0053] Substitute the fracture constraint coefficient β into the frequency attenuation slope calculation formula to obtain formula (2),
[0054]
[0055] By adding the constraint of the fracture development degree to the frequency attenuation rate, it represents the frequency attenuation rate γ after fracture constraint. The larger the absolute value of the constrained frequency attenuation rate γ, that is, the greater the degree of frequency attenuation, the more likely it is that there is a sweet spot area. A1 selects 60% - 80% of the main frequency energy, A2 selects 20% - 30% of the main frequency energy, f1 and f2 are the corresponding frequency values, so it can be considered that A1 > A2 and f1 < f2, so γ is negative, and the smaller the value of γ, the more likely it is that there is a sweet spot area. Calculate γ for the attenuation spectra at multiple positions to obtain the frequency attenuation attribute map used to characterize the degree of frequency attenuation.
[0056] Figure 3It is the frequency absorption attenuation attribute profile of Well AS1. According to the comprehensive interpretation results of fractures and lithofacies from the well logging data of Well AS1, through comparative analysis with the frequency attenuation attribute profile, the comparison chart with actual well logging as shown in Figures 4 to 6 is obtained. It can be considered that the results of the frequency absorption attenuation attribute basically coincide with the well logging data. Among them, the shale layer sections with developed fractures from No. ① to No. ⑤ are all indicated. If the conventional method is adopted and the sweet spots are characterized only by the degree of fracture development, then perhaps only the No. ⑤ layer is the sweet spot area because the fractures in the No. ⑤ layer are particularly developed and it is the layer section with the most developed fractures in the whole well. However, through this method, it is considered that the fractures and hydrocarbon-rich sandstone bands in the No. ①, ②, and ④ layers are developed and can also become sweet spot areas. In addition, there are no developed sandstone interlayers in the No. ③ layer section, and the degree of fracture development is insufficient, without obvious high-brightness response in the attribute profile, so it cannot form a sweet spot. In addition, between the No. ② layer and the No. ③ layer, and below the No. ⑤ layer, there are developed sections of sandstone interlayers, but since the fractures in these layer sections are not developed, they cannot become sweet spots either.
[0057] 4) By using the superposition analysis of the shale lithofacies distribution map and the frequency attenuation attribute prediction map as shown in Figure 7 , the prediction of the shale oil sweet spot area is as shown in Figure 8 .
[0058] Figure 3 The frequency attenuation attribute extracted from Figure 7 is consistent with the regional sedimentary characteristics and lithological characteristics in Figure 8 , indicating that the prediction results conform to the regional geological laws. Figure 7 The areas with brighter attributes in the figure in are consistent with the favorable lithofacies belts in Figure 7 . Through comprehensive comparative analysis with the drilled wells, it is considered that the prediction results are accurate and reliable, accurately depicting the shale oil sweet spot area.
Claims
1. A method for predicting sweet spots of shale oil and gas, characterized in that, Including the following steps: 1) Predict the fracture distribution in the shale formation to obtain the actual fracture distribution data; the fracture distribution data is reflected by the fracture density; 2) Determine the constraint coefficient: The calculation formula of the fracture constraint coefficient β is: Among them, α is the fracture distribution data, and α high represents the upper limit of fracture density. If the fracture density is higher than α high , it is identified as an oil and gas sweet spot; α low represents the lower limit of fracture density. If the fracture density is lower than α low , it is identified as not an oil and gas sweet spot; 3) Substitute the constraint coefficient into the frequency attenuation calculation to obtain the calculation formula of the frequency attenuation γ: In the formula, A2 is the second spectral energy, A1 is the first spectral energy, f2 is the frequency corresponding to the second spectral energy, f1 is the frequency corresponding to the first spectral energy, and the second spectral energy A2 is less than the first spectral energy A1; 4) Predict the shale oil and gas sweet spots according to the frequency attenuation calculated after substituting the constraint coefficient. When the degree of frequency attenuation is greater than the set degree, it is an oil and gas sweet spot.
2. The method for predicting shale oil and gas sweet spots according to claim 1, wherein In step 1), the fracture distribution data is the linear density data of fractures or the bulk density data of fractures.
3. The method for predicting shale oil and gas sweet spots according to claim 2, wherein In step 3), the first spectral energy value is 60% - 80% of the main frequency energy; the second spectral energy value is 20% - 30% of the main frequency energy.