Fund Project: National Science and Technology Major Special Project 33 is supported by 001(201kloc-0/zx05033-01).
Author's brief introduction: Li Xuefeng, male, master, engaged in comprehensive research on seismic geology of petroleum and coalbed methane. Mailing address: China Petroleum Email: Lixf20 10@petrochina.com.cn Coalbed Methane Co., Ltd.
(China Petroleum Coalbed Methane Co., Ltd., Beijing 100028)
The development of coalbed methane needs to take the road of reducing costs. In order to deploy well patterns on a large scale and with high efficiency, it is necessary to explore the three-dimensional seismic exploration method of "complex surface coalbed methane in mountainous areas of loess plateau" in Ordos Basin. According to the seismic geological conditions in Hancheng area, combined with economy and technology, this paper points out five key problems to be solved in 3D earthquake, and adopts eight targeted technologies from observation system design to data acquisition, processing, interpretation and reservoir prediction. Then the influence of bin on seismic data is discussed, and the application effect of 3D seismic data is analyzed. Finally, the experience of this 3D seismic application is summarized.
Keywords: Three-dimensional seismic application of coalbed methane Hancheng
Application of 3D seismic technology in Hancheng area
Wen Wen Guihua Li Shuxin
(China Petroleum Coalbed Methane Co., Ltd., Beijing, China 100028)
Because the development of coalbed methane needs low-cost orientation, it is necessary to explore the "complex surface coalbed methane in loess plateau mountainous area" in Ordos basin by using three-dimensional seismic exploration method, so as to effectively promote the deployment of large-scale well pattern. According to the seismic geological conditions of Hancheng and the theme of economic and scientific integration, this paper first points out five key problems that need to be solved in three-dimensional earthquake. Eight specific technologies are applied in observation system design, data acquisition, processing and interpretation, reservoir prediction and so on. Secondly, the influence of bin on seismic data is discussed, and the application of 3D seismic data is analyzed. Finally, the paper summarizes the experience of 3D seismic application.
Keywords: coalbed methane; Three-dimensional earthquake; Hancheng; app; application
1 overview
Necessity of implementing 1. 1 coalbed methane 3D earthquake
Hancheng area is one of the main battlefields for exploration and development of coalbed methane companies. In the structural division, Hancheng is located in the east of Weibei uplift on the southeast edge of Ordos block. The main coal-bearing strata are Permian Taiyuan Formation (1 1# coal) and Shanxi Formation (5# coal). Coalbed methane company completed the 2D earthquake of more than 65,438+000 km in Hancheng area, which contributed to the commercial development of coalbed methane. However, due to the limited control density of 2D seismic network, strong heterogeneity of coal seam and rapid changes in vertical and horizontal directions, 2D seismic network cannot efficiently and accurately deploy well patterns and select well types, thus developing coalbed methane on a large scale (Chang Suoliang, 2008). Three-dimensional earthquake is an effective means to solve this problem. Three-dimensional volume data can provide richer prestack information; Three-dimensional can use more interpretation means (such as three-dimensional visualization, layer slicing, coherent volume, attribute analysis, frequency division, geostatistical inversion, oil and gas detection, etc.). ) to solve more geological problems.
Three-dimensional seismic technology has been very mature in petroleum system, which is mainly used to solve the problem of characterization and prediction of geological bodies with strong heterogeneity (Zhao Zhengzhang, 2005; Li Ming, 2005; A.R. Brown,1998; Qian Rongjun, 2006; Cheng Jianyuan, 20065438+0; Chen Jun, 20065438+0; Xiong Ran, 2008; Chen Qiyuan, 200 1), but the current mainstream petroleum system software system and advanced interpretation methods are mostly aimed at three dimensions. Stereo exploration has also been tried in domestic coal industry. In recent years, a coal mine has carried out many small-area and small-panel three-dimensional earthquakes and achieved success, but the cost is extremely high and there is no reference significance.
Similar to shale gas, coalbed methane is also a large area, low abundance and continuous gas reservoir. Coal seam belongs to low porosity and low permeability reservoir, which is vulnerable and usually needs later reconstruction to produce gas. These determine that the development of coalbed methane needs to take the low-cost road of "multi-well, low production, long period and slow speed".
Therefore, in order to efficiently develop and realize low-cost 3D seismic technology series, it is necessary to carry out 3D seismic test of coalbed methane. In 20 10, CBM Company deployed the first 3D seismic project in Hancheng area, with an area of 100km2. From the practical application, the 3D seismic effect is obvious, and the deployment purpose has been successfully achieved.
Seismic geological conditions in Hancheng area 1.2
Hancheng area is a typical loess mountain landform, with an altitude of 500~ 1300m. The surface structure is complex, and it has been cut into tableland, beam and headland by long-term erosion, and the ravines are vertical and horizontal, with severe ups and downs; The thickness and velocity of underground low deceleration zone change greatly, which leads to the difficulty of earthquake construction in this area and the problem of static correction is prominent.
As the main target layer, the coal seam is shallow buried, thin in thickness, with great lateral change, and the phenomenon of coal seam bifurcation and pinch-out is prominent. Therefore, the seismic reflection interface between coal seam and upper and lower surrounding rocks is clear and easy to distinguish, but it is difficult to identify multiple sets of coal seams, especially the distribution of coal seams by earthquake. At the same time, the observation system of shallow target layer needs enough coverage times.
2. Hancheng 3D seismic technology
Focusing on economic and technological integration, as the first 3D coalbed methane project in China, the 3D seismic project in Hancheng needs to focus on five issues:
(1) Scientific design of observation system integrating economy with science and technology;
(2) Optimize acquisition technology and improve data quality;
(3) Fine data processing to solve the static correction problem;
(4) seismic data interpretation to find out the structural form and fault distribution;
(5) Reservoir prediction, describing the distribution of coal seams and guiding the deployment of development wells.
In view of the above problems, a series of targeted technologies are adopted. Limited by space, the following are some unique technologies that are of great significance to the Hancheng project.
2. 1 Optimal design technology of observation system
When designing the observation system, the following factors were considered:
(1) According to the requirements of geological task: mainly solve the changes of coalbed methane structure, vertical and horizontal direction and thickness of reservoir, and give consideration to fracture prediction and gas-bearing prediction.
(2) According to the buried depth of the main target layer: the offset distribution is uniform, which is beneficial to accurate velocity analysis and accurate imaging; Consider AVO analysis and application.
(3) Considering the surface structure and excitation factors, the relationship between data signal-to-noise ratio and effective coverage times: wide azimuth and moderate coverage times are adopted to ensure the profile signal-to-noise ratio;
(4) Adopt the concept of value engineering: comprehensively analyze the composition and changes of acquisition costs of different seismic acquisition and observation systems.
The comprehensive comparison of various observation systems shows that the coverage times of the finally selected observation system are moderate, the bin 30m×60m meets the technical requirements, the azimuth and offset distribution are reasonable, the shot density is within a reasonable range, and the project cost conforms to the characteristics of coalbed methane exploration. At the same time, according to the needs of scientific research, a 30m×30m bin experiment of 15km2 was deployed to compare the effects of different bins on data quality.
2.2 Multi-information high-precision line selection technology
Through this technology, before the field construction, the shot point and detection point can be selected indoors, which can arrange the construction progress more reasonably and improve the efficiency. At the same time, strengthen the selection of excitation points, excite in the rock area as much as possible, and obtain a single shot with high signal-to-noise ratio; Avoid construction difficulties and dangerous areas in advance, and optimize excitation and reception conditions to the greatest extent.
2.3 Surface Structure Inversion Survey Technology
The near-surface structure of the original 18 two-dimensional survey line in the three-dimensional area is inversed. Combined with the inversion results, surface elevation and obstacle distribution, the surface survey points are arranged and optimized, which also provides basic data for static correction.
2.4 Static correction technology of field chromatography
Due to the complex surface geological conditions in the field, the long-wavelength problem caused by the large lateral change of elevation and the correction of low deceleration zone is reflected in the seismic profile as the same change of strata from top to bottom, forming a structural illusion. In order to solve this problem, we make full use of the ground investigation results, the first arrival information and VSP logging data, choose the correct replacement speed, and apply tomographic static correction technology to solve the static correction problem [9].
2.5 high-precision imaging processing technology
Ensuring high-precision imaging of small faults, low-amplitude structures and thin target layers is the key to the success or failure of processing work. The main measures are: fine resection; Establishing a high-precision migration velocity field; Using prestack time migration technology to improve imaging accuracy.
2.6 3D visual interpretation technology
Three-dimensional visualization interpretation is to directly explain the structure, lithology and sedimentary characteristics of strata in three-dimensional space by using different transparency parameters for seismic reflectivity data volume from underground interface. This three-dimensional scanning and tracking technology can automatically track, quickly, efficiently and accurately explain and visually display geological phenomena from multiple angles, and provide reliable data for the deployment of directional wells and horizontal wells.
2.7 Curvature Body Technology
According to the continuous distribution of curvature attributes, the spatial distribution law of geological bodies can be objectively explained. On the time slice of curvature body, the plane distribution and extension direction of faults can be clearly identified, and the rationality of fault plane combination can be verified, thus improving the accuracy of fault interpretation.
2.8 geostatistics inversion technology
Geostatistical inversion takes seismic inversion as the initial model. Starting from the well point, the well follows the original seismic data, that is, the seismic data is taken as hard data, and the wave impedance quantitative three-dimensional geological model is established to predict the lateral reservoir. It combines the advantages of seismic inversion and reservoir stochastic modeling, and the prediction accuracy of reservoir spatial distribution is high.
Effect analysis of three-dimensional seismic application
3. Influence of1bin on seismic data quality
The development of coalbed methane can accept 30m×60m silo, and the cost of 30m×30m silo is twice that of 30m×60m silo. This time, the comparative experiment of two kinds of silos was carried out. By comparison, it is considered that the CDP spacing between the two survey lines of the main survey line is 30m, which is slightly higher in signal-to-noise ratio and more continuous than that of the small bin profile, but the difference is not significant. The CDP spacing of tie lines is different, but the signal-to-noise ratio of small bin profile is higher and more continuous, and the difference is obvious (Figure 1). However, after offset interpolation, the structure of time slice is basically the same, but the details are slightly different. Therefore, considering the integration of economy and technology and the best cost performance, the processing result of 30m×60m bin can solve the problem.
Figure 1 Comparison of different polygon metadata (30m×30m on the left and 30m×60m on the right)
3.2 Static correction processing effect
Because of the complex surface conditions in the three-dimensional area, the number of micro-logs is not enough to control the whole area, so the first break information of artillery is used more. In the process of processing, the chromatographic static correction method is used to solve the long, medium and short wavelength problems caused by surface elevation change and low deceleration zone, which lays a solid foundation for subsequent processing (Figure 2).
Fig. 2 Comparison of long-wavelength static correction processing effects
3.3 seismic data quality analysis
The quality of three-dimensional earthquake is higher than that of two-dimensional earthquake. It can be seen from the spectrum diagram of the main target interval (Figure 3) that the main frequency of 2D seismic profile is 25Hz and the effective bandwidth reaches 55Hz; the main frequency of 3D seismic profile is 40Hz and the effective bandwidth reaches 75Hz.
Fig. 3D (left) and 3d (right) spectrum comparison of seismic data.
Compared with 2D data, the signal-to-noise ratio of 3D data is obviously improved, eliminating the problem of long-wavelength static correction, with clear wave group characteristics, easy identification of breakpoints, clear reflection insider, richer geological phenomena and more obvious unconformity characteristics of Ordovician top interface reflection (Figure 4), which provides a good data basis for seismic data interpretation and reservoir research and helps to understand the geological structure, fault distribution and fine structural morphology of main target layers. Compared with post-stack, pre-stack time migration profile has more obvious wave group characteristics and clearer faults.
Fig. 4 Comparison of data processing effects between 2D survey line (top) and 3D survey line (bottom)
3.4 Fine Interpretation and Reservoir Prediction
Make a detailed structural explanation. Compared with the two-dimensional structural map, the structural map obtained by variable speed mapping has obvious advantages: the fault combination is more reasonable, the breakpoint position is more reliable, the detail description is clearer and the interpretation accuracy is higher. There is a large amount of three-dimensional data, and the interpretation results can be mapped in three dimensions, which can reflect the underground characteristics more clearly (Figures 5 and 6). In order to check the accuracy of the final structural mapping, the mapping error of the structural mapping of each destination layer is analyzed. Comparing the geological stratification of wells with the interpreted structural depth, the statistical results show that the absolute error between the structural mapping depth of most wells and the logging geological stratification is between 0-3m, and most wells are less than the structural mapping error standard (3‰), which shows that the mapping method is feasible, the mapping accuracy meets the standard requirements, and the mapping results are reliable.
Fig. 5 Structure diagram of top of 3-D 5# coal seam in Hancheng
Fig. 6 Top Buried Depth Map of 3D 5# Coal Seam in Hancheng
Fig. 7 Hancheng 3D 1 1# Coal Seam Thickness Distribution Map
The thickness and spatial distribution of coal seam are described by sparse pulse inversion and geostatistics inversion. According to the inversion results, No.3 coal seam is only developed in a local area, and the thickest coal seam is 3.4m and 2. 1m in the east near Well WLC03 and Well WLC04, respectively, which is No.3.. No.3 coal seam gradually becomes thinner from west to well WLC06, and disappears from south to well WLC05. 5# coal seam is relatively developed in the whole area, and it is thin only in the vicinity of WLC0 1 Well, the north of WLC02 Well and the south of WLC07 Well, and gradually thickens to the west, with the thickest between Han Shi Well 3 and Han Shi Well 4. 1 1# coal seam is thick in the east and thin in the west, and it is thickest near wells WLC0 1, WLC03, WLC05 and WLC06 in the east, and gradually pinches out to well Han Shi 3 in the west and develops near well Han Shi 4 (Figure 7).
Using various seismic attributes to optimize the deployment of development wells. It is considered that amplitude attribute is related to coal seam thickness, while Poisson's ratio attribute is positively related to fracture density. Finally, 28 inefficient wells near the fault were adjusted by comprehensive utilization of 3D seismic results, and the economic benefits were improved.
4 conclusion
Through the development of Hancheng 3D project, the following conclusions are drawn:
(1) Through the three-dimensional practice in Hancheng, a low-cost three-dimensional exploration method suitable for the characteristics of "complex surface coalbed methane in the mountainous area of the Loess Plateau" was found.
(2) It is feasible and effective to solve geological problems such as the structure of coalbed methane, reservoir prediction and well location deployment by using three-dimensional earthquake.
(3) 3D seismic has a broad application prospect in the field of coalbed methane exploration and development, and can be implemented in a large area of development zones to guide the deployment of directional wells and horizontal wells.
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