Application of Seismic Physical Forward Modeling in Palaeokarst Reservoir Study
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The type of Palaeokarst reservoir space is complex, which is difficult to be recognized and predicted. Study shows that the pore space of this kind of reservoir is mainly consisted of caves, fracture, fracture and vugs. In connection with 4 types of pore spaces, a physical model is designed to simulate seismic reflection characters and conduct forward simulation. The result of study shows that when the velocity is very low in the cave, obvious diffraction can be observed on single-shot and stack profile, and stronger short axis reflection is represented on 2D stack profile, the fracture growing belt is obvious abnormal on seismic reflection with “chaotic” reflecting property. Strong beaded reflection appears near the cross-over position of multi-group fractures and vugs, vug reservoir represents a stronger discontinuous reflection on seismic profile.Keywords:
Reflection
Physical property
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Fractures are important reservoir spaces in buried hill reservoirs. The production of a buried hill reservoir is directly correlated with the growth of fracture. In this paper we analyzed the factors controlling the growth of fracture and discussed the rules of fracture growth. Although single fracture is hard to be identified in seismic data, fracture zone has its own seismic characteristics. Based on the geophysical characteristics of fractured reservoir, we use logging constrained inversion, absorption coefficient, seismic facies classification, and coherence analysis to predict buried hill reservoir. The effectiveness of the above four technologies in describing buried hill reservoir is confirmed by the case study on the Chegu201 buried hill reservoir.
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Because in tight sandstone, the characteristics of geophysics induced by the changes of low porosity and oil saturation were relatively weak, the prediction of favorable reservoirs was the problem in seismic prediction, better result could not be obtained by using traditional techniques. Seismic rock micro-physics were emphasized in experimental study, forward seismic simulation could be used to reflect the changes of spatial combination, good result could be obtained by combining both. The basic study on seismic rock physics and the study of forward seismic simulation are carried out in the tight sandstone in a work area of Sichuan, in which geological model is combined with a seismic model to study the general laws of seismic response induced by the changes of reservoir thickness, porosity and fluid saturation, thus the characteristics of seismic reflection has the meanings of geology and geophysics, the lithology property is effectively classified and hydrocarbon is effectively inspected, it has an important significance for guiding the seismic exploitation.
Lithology
Seismic to simulation
Saturation (graph theory)
Water saturation
Synthetic seismogram
Seismic attribute
Petroleum reservoir
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Coherent analysis method is adopted to obtain fracture identification marks in fractured reservoir development areas in order to obtain seismic response characteristics of fractured reservoirs on different seismic profiles. Adopted is the forward modeling firstly in this method to design the geological model of the fractured reservoirs consistent with actual conditions in the researched area, establish the seismic model under main frequency conditions in the area accordingly and summarize seismic response characteristics of fracture development areas on the conventional profiles. Conducted is coherent analysis on the seismic model and obtained are fracture identification marks in fracture development areas on relative profiles and it is found through comparison that the identification marks resulting from coherent analysis is consistent with the seismic response characteristics on the conventional profiles and the characteristics are more obvious and the response is more clear. Finally, it is confirmed through drilling that fracture development zones are real in the areas, which shows that the coherence analysis is an effective, reliable and scientific method for identifying and predicting fracture development zones.
Identification
Sichuan basin
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The DX18 reservoir is composed of four mutually superimposed volcanic bodies and is a pore-fracture dual-media igneous reservoir. The fractures inside the volcanic bodies are mainly structural fractures with continuous distribution in space, and the continuity of fracture development positions is poor both vertically and horizontally. For the preparation of reservoir development scheme and further improving production well pattern, the fracture development and distribution characteristics are studied by 3D geological modeling. In view of the complexity of spatial structure of volcanic bodies, a body-controlled structure modeling is applied to describe the entire 3D shape of volcanic bodies. By integrating seismic data and logging interpretation, the relationship between various seismic attributes and fracture development zone is analyzed. Based on the analysis result above, a seismic-constrained modeling method which integrates the well data and seismic data is applied to establish the 3D property model for volcanic bodies. The fracture porosity model is constrained by seismic porosity data volume created by neural network processing, and the fracture density model is constrained by seismic dip volume. The later drilling and production performance proved that the main fracture development zones inside the volcanic bodies are accurately predicted and described by the seismic-constrained geological model and the coincidence ratio reaches 93.8%.
Sichuan basin
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In low porosity, low permeability zones, natural fractures are the primary source of permeability which affect both production and injection of fluids. The open fractures do not contribute much to porosity, but they provide an increased drainage network to any porosity. An important approach to characterizing the fracture orientation and fracture permeability of reservoir formations is one based upon the effects of such conditions on the propagation of acoustic and seismic waves in the rock. We present the feasibility of using seismic measurement techniques to map the fracture zones between wells spaced 2400 ft at depths of about 1000 ft. For this purpose we constructed computer models (which include azimuthal anisotropy) using Lodgepole reservoir parameters to predict seismic signatures recorded at the borehole scale, crosswell scale, and 3 D seismic scale. We have integrated well logs with existing 2D surfaces seismic to produce petrophysical and geological cross sections to determine the reservoir parameters and geometry for the computer models. In particular, the model responses are used to evaluate if surface seismic and crosswell seismic measurements can capture the anisotropy due to vertical fractures. Preliminary results suggested that seismic waves transmitted between two wells will propagate in carbonate fracture reservoirs, andmore » the signal can be received above the noise level at the distance of 2400 ft. In addition, the large velocities contrast between the main fracture zone and the underlying unfractured Boundary Ridge Member, suggested that borehole reflection imaging may be appropriate to map and fracture zone thickness variation and fracture distributions in the reservoir.« less
Petrophysics
Vertical seismic profile
Petroleum reservoir
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For the deep Xujiahe Formation in western Sichuan Basin,its reservoir-forming geological conditions are very complicated.In addition,it is an extra-deep,extra-low porosity and permeability,extra-high pressure,extra tight and a typical fractured gas reservoir.So,fracture prediction becomes the most difficult problem of seismic exploration in this reservoir.Recently,with the wide-azimuth,large-deviation and three-dimensional and three component(3D3C) seismic data as the soul,some researches,which the 3D3C P-wave and converted-wave data was made full use,have been carried out for the reservoir prediction caused by geological history and P-wave seismic attribute and the detection of both P-wave azimuth anisotropy and S-wave splitting fracture by means of multidisciplinary coordination.
Sichuan basin
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It is very difficult to predict carbonate karst reservoir due to serious heterogeneity of the reservoir. The forward modeling can reflect the variation of the reflection events from the carbonate weathering crust and the wavefield properties from the inner caves, such as the seismic reflection and seismic dispersion. The paper applied highorder differential numerical simulation to perform a series of forward numerical simulations for various caves with different sizes and filling material (different velocities) , and comprehensively analyzed the seismic reflection properties and reflection amplitude of the various cave models. The study indicates that the seismic wave from the karst cave reservoir slides back and forth along the cave edges to form multiple diffraction wave which behaves as strong and short reflection segments of shoestring in the seismic section; the horizontal size of the karst cave has large effects on the seismic amplitude, the larger the horizontal size, the stronger the amplitude; and the higher the wave velocity in the cave filling, the weaker the seismic amplitude. The recognition patterns of the karst caves were built by forward numerical simulation, and the study results were applied into the seismic prediction of the carbonate reservoirs in Tahe oil field, good results were gained and the drilling successful rate was increased.
Reflection
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There are various types of fracture-cavity deeply buried reservoir with strong heterogeneity in the Ordovician in Tazhong area.The model is designed in accordance with real condition in Tazhong area.The complex fracture-cavity reservoir has been found to be equal to a geological model composed of staggered micro-network and cavity which can depict the inner fracture-cavity reservoir structure and parameters variation accurately.It applied the staggered-grid high-order finite difference method of wave equation forward modeling technology to simulate seismic wave field,and used the conventional processing methods and wave equation prestack depth migration to get the migration profiles.The interpretation could be evaluated and verified by the migration result.The amplitude,phase and frequency information are used to analysis the seismic wave field characteristics.It studies that the reservoir location and structure variety effect on the seismic wave field.Then it combines the well-to-seismic relation and the seismic simulation response characteristics to establish the recognition mode.It identifies that the seismic response of layered igneous rock,volcanic neck and intrusive igneous rock exist in this area.It illustrates the fracture-cavity reservoir seismic structure in the top surface and the inner of the carbonate layer.
Mode (computer interface)
Synthetic seismogram
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Ordovician fractured vuggy carbonate reservoirs, which are deeply buried in the Tazhong Shunnan area in China, are characterized by high heterogeneity. Meanwhile, there is no significant difference between the geophysical characteristics of the reservoirs and that of the surrounding rocks. We have introduced the multiscale stack random medium theory and built some theoretical seismic-geologic models for the fractured vuggy carbonate reservoirs. Furthermore, we obtained the seismic reflection characteristics corresponding to these models using finite-difference forward modeling. The small random vugs are characterized by weak and chaotic reflections with high frequency, and the large vugs are characterized by strong and chaotic reflections with low frequency. The amplitude of the seismic reflections increases with the increasing vug density, and it decreases with the increasing roughness factor. Combining the synthetic reflection characteristics corresponding to the fractured vuggy carbonate reservoirs and the actual seismic reflections from the drilled reservoirs, we summarized the recognition patterns of the carbonate reservoirs. The predicted results found that the potential fractured vuggy reservoirs at the top of Yijianfang Formation are located in the southwest and northeast, in the vicinity of fault zones. The reservoirs in Peng-Laiba Formation are distributed in the northwest of the block.
Reflection
Fault block
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