Aiming at the present situation of seismic exploration in Sulige gas field, Changqing Oilfield Exploration and Development Research Institute further studied the key technologies of 3D multi-component exploration processing and interpretation on the basis of summarizing the 2D multi-component seismic exploration experience in 2003, and summarized a set of processing and interpretation technologies suitable for 3D multi-component exploration technology in Ordos Basin. By studying the distribution of reservoirs and effective reservoirs (gas-bearing sandstone) in Sulige gas field, it provides an important basis for the study of sedimentary facies in this area. At the same time, the relatively rich natural gas areas in this area are optimized, which lays the foundation for the next effective and economic development of gas fields.
The following contents are compiled according to the research report of Changqing Oilfield Exploration and Development Research Institute.
7.3.3. 1 multi-wave seismic test area
The study area is located between Well Su-5 and Well Tao-5 in the north of Sulige gas field, adjacent to Well Su-6 in the south and Well Su-5 in the north. The area is about 185km2, and the total earthquake coverage (more than 120 times) is 100km2. From 2003 to 2004, 324km of 2D multi-wave 12 survey line and multi-wave 3D acquisition 100km2 of Su 5 well area were completed. The surface landform conditions in this area are open sandy land (25%), sandy grassland (20%), black bright zone (15%) and alkali beach (40%).
At present, four wells have been drilled in this area, including two exploratory wells, Tao 5 and Su 13, which were completed in 2009 ~ 2000, and industrial gas flow was obtained through gas testing. The other two wells, Su 3 1- 13 and Su 3 1- 16, were development appraisal wells drilled in 2002. The eighth member of two wells deposited thin interbedded sandstone, and the gas test did not reach the industrial gas flow. The reservoir of He 8 member in Suligemiao area is timely sandstone, and the main electrical characteristics of He 8 gas-bearing sandstone are "three lows, two highs and one big", that is, low natural gamma, low density and low compensated neutron; High resistivity and high time difference; Large-scale spontaneous potential abnormality.
Interpretation of three-dimensional multi-wave data in 7.3.3.2 experimental area
Interpretation of sedimentary facies in (1) He 8 member
The sedimentary background of Sulige gas field is braided river deposition, and the provenance comes from the northern part of the basin, and the rivers are distributed in the north-south direction. Using seismic data to study sedimentary facies, the main research techniques include the following aspects:
1) seismic facies study of coherent body and variance body: coherent body technology was first used to study reservoir fracture system. However, in the process of river deposition, the coherence or variance between the main channel and the beach is often different because of the different hydrodynamic environment. Therefore, the channel can be detected by calculating the difference between seismic traces in the reservoir profile.
2) Using spectrum decomposition technology to study sedimentary facies: In sandy mudstone area, the frequency of low frequency band mainly reflects the change of thick sandstone, while the frequency component of high frequency band mainly reflects the change of thin sandstone. The thickness of sand body can be determined by the reciprocal of frequency. In order to better study the sedimentary characteristics of channel deposits in this area, the tuned volume data and discrete energy volume seismic data calculated by P-wave and converted wave are superimposed and sliced at different times along the layer. The main purpose of superimposed display is to eliminate the influence of background and highlight the sedimentary characteristics of river channels.
3) Analysis of sedimentary evolution characteristics of He-8 member: Based on the study of static sedimentary facies, in order to further study the migration and change of He-8 member in Sulige gas field during the sedimentary period, the sequence change characteristics of He-8 member are divided, and the sequence evolution analysis of sedimentary facies in He-8 member is carried out on the basis of spectral decomposition, coherence volume and main difference volume.
In the process of studying the sequence sedimentary evolution of He 8 member, 20 groups of slices were made for the seismic data volume with longitudinal wave of 20Hz. For converted wave data with the same formation thickness, 40 sets of slices were made. The main reason is that for the same formation thickness, according to the propagation theory of converted wave [12] [13], the velocity of longitudinal wave is about 1.5 ~ 1.8 times that of shear wave. Compared with the formation thickness represented by the time domain corresponding to P-wave and converted wave, the formation thickness reflected by converted wave is about half that of P-wave with the same time difference. That is to say, the stratum thickness represented by one longitudinal wave is approximately equal to that represented by two converted waves. In this way, the sequence research results of P-wave and converted wave can be well compared.
(2) Prediction of reservoir lithology and thickness
Prediction of lithology and reservoir thickness in three-dimensional study area
By analyzing the relationship between the petrophysical properties of He-8 member in Sulige gas field and the multi-wave and multi-component seismic parameters, it is considered that P-wave impedance or gamma inversion and shear wave velocity (shear wave impedance) are mainly considered in lithology and thickness prediction.
The P-wave impedance is preliminarily explained, and the lithology of the whole area is preliminarily identified. Then the gamma area less than 80API is interpreted as sandstone by using the gamma data volume. Combined with the interpretation results of P-wave impedance and gamma impedance, the thickness of sand body in this area can be obtained.
Fig. 7. 12 is the comparison of longitudinal wave impedance and transverse wave impedance profile of INLINE70. P-wave impedance profile shows that the upper and lower sandstone (1815 ~1840 ms, blue-green) of Well Tao 5 and Well 8 is well developed, which is inconsistent with the actual situation. On the shear wave impedance profile, the upper section of Box 8 can be interpreted as mudstone according to its impedance value, which is consistent with the actual drilling results. This further shows that shear wave impedance or velocity is more conducive to lithologic identification in this area.
Fig. 7. 13 is a slice along the average impedance layer of shear wave in box 8. Sandstone shows a large shear wave impedance value. In the picture, sandstone of He 8 member is developed in the area east of Tao 5 well; There is also a sand body development area between Well Tao 5 and Well Su 3 1- 13. The area west of well Su 3 1- 13 shows that the sand bodies in this area are very developed. This map has very similar characteristics to the frequency division attribute map of sedimentary facies research. According to the development degree of sandstone, there are three main rivers in the study area, including two near Well Su 31-Kloc-0/3 and Well Tao 5, and one to the west of Well Su 31-Kloc-0/3.
Identification of lithology and sandstone thickness by rock elastic parameters
The method of identifying sandstone in He 8 member of Sulige gas field by using rock elastic parameters is a lithologic identification method that has only appeared in recent years. Usually, the elastic parameters of rocks are mainly obtained by P-wave prestack inversion, or EI impedances at different angles are obtained by prestack elastic impedance inversion, and then the elastic parameters of rocks are obtained by calculating shear wave velocity. Or the velocity of P-wave and S-wave can be obtained through the joint interpretation technology of P-wave and S-wave, and then the elastic parameters of rock can be calculated.
Attached Figure 7. 14 is the inversion profile of Lame coefficient, shear modulus and density product of Well Guo Tao 5. For sandstone, the product of Lame coefficient, shear modulus and density is greater than that of mudstone. In the figure, the product of three factors on the east side of Tao 5 well is obviously higher, and the sandstone in the lower member of He 8 is more developed vertically than that in the upper member of He 8. Horizontally, the sandstone under He 8 in the east of Tao 5 well is developed. Sandstone under CDP64 1-CDP69 1 section box 8 in the west of well Tao 5 is undeveloped. From CDP64 1 west to Well Su 13 (CDP487), sand bodies are developed in the lower member of Box 8. The sand bodies in the upper member of Box 8 are developed in CDP560-CDP725 and CDP823-CDP892.
Fig. 7. 15 is a bedding slice of the product of Lame coefficient, shear modulus and density of box 8 in the study area, which mainly reflects the distribution law of box 8 sand body on the plane. On the whole, the lateral variation of sand bodies in the north of the study area is greater than that in the south, and the variation of sand bodies in the east is more complicated than that in the west.
(3) Oil (gas) detection
Multi-wave, multi-component and joint gas detection mainly includes three methods [14]: amplitude ratio of P-wave and S-wave, velocity ratio of P-wave and Poisson's ratio of P-wave and S-wave.
In He 8 sandstone gas reservoir in Sulige gas field, it is considered that when the reservoir contains fluid, the amplitude of P-wave in the reservoir profile will decrease slightly, while the amplitude of S-wave in the reservoir profile will remain unchanged. According to this characteristic, the root mean square amplitude of the target layer of P-wave and converted wave is calculated respectively, and then the amplitude ratio of P-wave to P-wave or P-wave is calculated. If the amplitude ratio of P-wave and S-wave is adopted, the smaller the amplitude ratio, the better the gas-bearing property. As for the velocity of P-wave and S-wave, the velocity of P-wave decreases after the reservoir contains gas. Therefore, the low value area of P-wave velocity ratio represents the area with good gas bearing property. When sandstone contains fluid, the decrease of velocity ratio will also lead to the decrease of Poisson's ratio, so low Poisson's ratio can also indicate good gas-bearing property.
Fig. 7. Comparison of P-wave impedance (top) and S-wave impedance (bottom) of Line 70 in12 (Well Tao 5)
Fig. 7. Average impedance slice of 8 sections of13 shear wave box
Fig. 7. 14 Inversion Profile of Lame Coefficient, Shear Modulus and Density Product in Downward Direction of Well Tao 5
Fig. 7. Slicing of the product of average Lame coefficient, shear modulus and density in15 box 8.
Figure 7. 16 shows the plane distribution characteristics of P-wave velocity ratio. In the figure, except Su 13 well, the P-wave and S-wave velocities of high-yield well Tao 5 are relatively low, while the velocities of Su 3 1- 13 and Su 31-6 dry wells are relatively high, and the interpretation results are consistent with the actual drilling results. In addition, from the perspective of the whole region, the gas-bearing property in the east of Well Su 3 1- 13 is better than that in the west, which is consistent with the analysis result of amplitude ratio.
Fig. 7. Gas-bearing test results of P-wave velocity ratio of16
(4) Comprehensive evaluation of reservoir thickness and gas-bearing property.
Comprehensive evaluation of sandstone thickness
In the comprehensive analysis of reservoir characteristics in the study area, it is necessary to comprehensively evaluate the thickness of sand body in order to make the predicted sand body more reliable, facing the difference of thickness prediction of P-wave and converted-wave seismic data and the difference of thickness prediction of various methods. Comprehensive evaluation should fully consider the characteristics of sedimentary facies; Based on the results of longitudinal wave frequency division processing, it is converted into converted wave frequency division results, and multi-attribute classification results are considered at the same time; For the contradiction between various inversions, the maximum principle is adopted instead of the intersection method, and its main purpose is to ensure that all sandstones in He 8 member can be predicted.
Attached Figure 7. 17 shows the sand body thickness distribution in the upper part of Box 8, and attached Figure 7. 18 shows the sand body thickness distribution in the lower part of Box 8. Comparatively speaking, the sandstone in the lower member of He 8 in this area is more developed than that in the upper member. The sandstone thickness of the lower member of Box 8 is generally 10 ~ 30m, and that of the upper member of Box 8 is generally 10 ~ 15m. According to the comprehensive sedimentary facies research results, there are four main river courses in the lower member of He 8, namely Su 13 well, Su 31-Kloc-0/3 well in the west, Su 31-Kloc-0/3 well in the east and Tao 5 well, among which the main river course of Tao 5 well is the most developed. The overall performance of the river is a network intersection, forming a strong heterogeneity.
Fig. 7. 17 Sandstone Thickness Map of Upper Member 8 of Multi-wave 3D Block
Fig. 7. 18 Multi-wave 3D sandstone thickness map under Member 8 with box.
Comprehensive evaluation of gas-bearing property of reservoir
In the comprehensive evaluation of gas-bearing property in three-dimensional multi-wave research area, the weighted coefficient method is used for comprehensive evaluation. Including the results of river sand body prediction and other gas-bearing detection, are given different weight coefficients and then weighted. If the weight coefficient is greater than 7, it is divided into Class I reservoirs; If the weight coefficient is between 5 and 7, it is a Class II reservoir; If the weight coefficient is less than 5, it is a Class III reservoir. The statistical results of seismic effective reservoir thickness and seismic detection results show that the corresponding relationship between seismic gas detection results and their effective reservoir thickness is as follows: Class I reservoir, and the relatively concentrated effective reservoir thickness is generally more than 5 m; Class II effective reservoir is 3 ~ 5mⅲ, and Class III effective reservoir is less than 3m. Then select 10 parameters for evaluation, and the weight coefficient of each parameter is 1.
The selected 10 parameters are: main channel prediction of longitudinal wave and shear wave frequency division; Inversion results of longitudinal wave elastic impedance; λ μ ρ elastic inversion; AVO's Procter & Gamble attribute; Quasi-Poisson ratio of longitudinal wave; Possibility distribution of hydrocarbon polymers detected by AFI: possibility of inversion of oil, gas and water distribution by AFI: application of wavelet transform in oil and gas detection: amplitude ratio of longitudinal wave and shear wave; P-wave velocity ratio (or Poisson ratio).
Attached Figure 7. 19 is the comprehensive evaluation result of gas-bearing property in this area. The area of the first-class favorable gas-bearing area in this area is 17.003km2, accounting for 10.08% of the total area. The secondary gas-bearing favorable area covers an area of 64.47km2, accounting for 38.23% of the total area. The third-grade gas-bearing favorable area covers an area of 9.49km2, accounting for 5.62% of the total area.
Fig. 7. 19 comprehensive evaluation results of 3D multi-wave reservoir