3-D Seismic Fractured Media Forward And Inversion Study

With the economic development and the continuous improvement of living standards, the demand for the oil-gas resources is rising continuously, which has brought opportunities and challenges to the oil and gas resources exploration and development. The original or easy oil-gas resources have been almost completely, which is higher demand to the oil-gas exploration and development.Crack in the crust is the universal phenomenon, and is widely found in various rocks. So far, it has found a large number of industrial gases in the sandstone, the shale, the carbonate, and the igneous. According to statistics, low-permeability reservoir (including all kinds of crack reservoir) for oil-gas proven reserves is about one third of the state's oil-gas reserves, and about three-quarters of low permeability of dense fractured reservoir. In the oil-gas exploration and development field, fractured reserves position will become increasing important.The formation of crack is controlled by many factors, which its physical attributes is complex, horizontal and vertical change is strong, shows a strong anisotropy. The crack generation is in the latter, unlike other reservoirs with the corresponding deposition environmental characteristics. So the fractured reservoir is more difficult than other oil-gas reservoirs in exploration. In the past, using log date to detect the crack, the majority results are effective only in very small well. Because of the complexity of the crack, predicting the direction of crack between wells and density is hard by logging date. When the exploring field is lack of logging date, even when there is no well, it must find other ways. Fortunately, the existence of crack causes the physical nature of the fractured media change with the line, which is called azimuthally anisotropy in seismic exploration. At the same time, because of the overlying load compaction, the horizontal and low-level angles of cracks have almost disappeared, and the high angle and near vertical cracks is easier to be preserved. Because of this reason, it is easy to getting the information by pre-stack seismic data.The S-wave is highly sensitive to crack. When the S-wave polarization direction is not parallel or vertical to crack direction, S-wave will split and occur the fast S-wave which is parallel to crack direction, the slow S-wave which is vertical to crack direction. By observing and measuring the spitting, we can invert fractured media parameter. However, because of the high cost of S-wave acquisition and processing and being risk, it is not conventional application technology.At present, fractured reservoir exploration technology is S-wave, P-S converted wave, multi-component seismic, multi-azimuthally VSP, P-wave AVAZ, and so on. One of the most effective ways should be S-wave, but the S-wave acquiring and processing are the high cost and risk. So it is not conventional technology. Multi-component seismic, multi-azimuthally VSP, P-S converted wave is fine ways, but they are the high cost of exploration, or are non-conventional projects in this stage. Therefore the use of P-wave AVAZ exploration at this stage is economic and practical methods for exploration.The research methods of fractured media can be roughly divided into two categories: the fractured media field simulation, including physical modeling and simulation (wave equation simulation and ray-tracing simulation), the other is the theoretical basis and anisotropy physical model based on observation on the formation of cracks in the various detection methods and techniques of research.The fractured media is the transverse isotropy with a horizontal axis of symmetry (HTI media or EDA media). It is the simplest azimuthally anisotropy model used to describe fractured reservoirs that contain parallel vertical cracks.When seismic wave go though the fractured media, its reflection coefficient, velocity and traveltime should change with the azimuth and angle. Though the wide azimuth exploration of the fractured media, using the seismic dynamics and kinematics character, we can invert the crack direction and the other parameters. Because of the low cost and the simple processing, the azimuth P-wave analysis is the main method of fractured reservoirs at present.In the analysis of the fractured media, the normal moveout velocity in fractured media would change with azimuth and angle, and into elliptical relationship. When the line is parallel the crack direction, the NMO velocity is maximal. When the line is vertical the crack direction, the NMO velocity is minimal. As acquiring the crack direction by the amplitude in the fixed-offset, using the NMO velocity in fractured media to acquire the crack direction. In this paper, when using the two Vavrycuk approximate formula to invert the parameters of fractured media, the direction and the other parameters of fractured media is separate. Firstly using the NMO velocity to the inversion of crack direction, then using the Vavrycuk formula to the inversion of other parameters. In the specific algorithm of inversion, using the super-linear equations of the SVD. The approximate formula is appropriate in small offset.In the inversion of parameter in the fractured media, wide azimuth data is required, and the CMP gathers for inversion should be of high S/N, fidelity and resolution in amplitude, and the NMO velocity is fine-turning analysis data. Vavrycuk approximate formula is based on the weak anisotropy media and the same axis of symmetry crack, so the formula is only suitable big cracks in the inversion.