Study On High-precision Elastic Impedance For Fractured Reservoir And Coherence Properties Enhancement

Fractures in the reservoir are the storage space of petroleum and the main passage of itstransportation. Therefore, the exploration of the fractured reservoir is an important field ofoil and gas exploration at present. The presence of fractures induces the azimuthal anisot-ropy of the media, which makes that the seismic data recorded in ground contain an abun-dance of Wave field information closely related with the orientation of fracture and frac-ture density during the seismic wave propagation in the fractured media. Consequently,It has important meaning to detect the spatial distribution of fractures in reservoir andexplore fracture strike and fracture density in reservoir. Fractured reservoir is an importantremaining petroleum resource, whose oil and gas reserves respectively account for60%and40%of proven reserves all over the world. The network of natural fractures is used tothe migration pathway of hydrocarbons and other fluid. Some important parameters offracture pretrophysical properties such as fracture density, joints opening, oil-gas bearingproper, orientation of fracture et al, are closely related to porosity and permeability infractured reservoir, and the characteristics of oil and gas distribution in corresponding res-ervoir are different with conventional oil and gas reservoirs. The presence of fractures is adouble-edged sword for reservoir, especially tight reservoir. It may provide requisitepermeability for the form of reserving space, and could provide passageway for the migra-tion of oil and gas. However, it is a risk factor in reservoir leakage. Therefore, that we car-ry out study on the theory and method of fracture prediction in reservoir has great signifi-cance to exploration and development of hydrocarbon reservoir.This article mainly studies the prediction of reservoir fractures from two faces, one is Pwave and P-SV wave elastic impedance inversionfor fractured reservoirs, following theanalysis of the fluid factor based on elastic impedance for these reservoirs, the other is thestudy on seismic coherent techniques and the method of fracture enhancement.The elastic impedance technique is an importan tool of pre-stack reservoir prediction.Elastic impedance previously proposed in fractured media didn’t provide directly function-al relations with crack parameters. In this article, we review the Hudson cracks model andlinear slip model, and introduce crack fillings, distribution density and the square of Vs/Vp of host rock into HTI elastic impedance equation, and then obtain new equation and nor-malize it. The numerical simulation results show that, elastic impedance is azimuthally an-isotropic and increase with crack density. PP wave and P-SV wave elastic impedancechanges in obviously quite different ways; For typical sandstone, elastic impedance dif-ference provide a possible fluid factor identification of crack fillings, which elastic im-pedance of cracked gas-filled media changes more with the variation of crack density,while the cracked fluid-filled case are more sensitive to the variations of Vs/Vp. The au-thor also manages to introduce crack parameters, incidence and azimuth into conventionalfluid factor, forms new factors and simulate the influence.Otherwise, considering the dual-porosity of cracked media, we compare Thomsen frac-tures model with linear slip model, and introduce crack fillings, fracture density, aspectration, equant porosity and Vs/Vp in Thomsen model into HTI elastic impedance equationof PP wave and P-SV wave, yields the elastic impedance in equivalent fractured porousmedia of Thomsen model. The new equation establishes the functional relationship amongfracture parameters, lithological parameters, physical parameters and the elastic impedance.Simulation results show that the new equations are extremely sensitive to material filled infracture. Besides the similar rules with Hudson model, we reach another conclusion relatedto equant porosity, which is the elastic impedance changes obviously with the increase ofequant porosity both for gas-and water-saturation. This indicates that equant porosity has agreater effect on anisotropy. Furthermore, the optimal fluid factor may be a good fluididentification of Thomsen fractured porous media. This result provide the possibility ofbuilding general fluid factor of cracked media base on conventional fluid factor and pro-vide new theory evidence for fluid prediction.Research on elastic impedance and generalfluid factor for these two kinds of fractures models will provide a more straightforwarddistinguish basis for fracture prediction and the fluid identification work on fractured res-ervoirs.Research on elastic impedance and general fluid factor for these two kinds of crackedmodels may provide a more straightforward theory evidence for fracture prediction and thefluid identification.One of the most common fractures prediction techniques is the Eigenstructure-basedcoherent property technology that is an efficient and ripe technique for large-scale frac-tures detection. However, even though in horizontally bedded and continuous strata, frac-tures buried in close high gray values zones are difficult to identify; Furthermore, the mid-dle and small-scale fractures in crushed zone where migration image energies are usually not concentrated perfectly, are also hard to distinguish because of the fuzzy, cloudedshadow composed of close low gray values. A new fractures enhancement method withhistogram equalization as the core is designed to solve the above problems in this paper.With this method, the contrast between discontinuities and background of coherence imageis obviously increased, and linear structures are highlighted; by adjusting thresholds ofcoherence image in steps, fractures in different scales are detected. Application examplesshow that this method also improves explicit and implicit fractures cognition capabilityand accuracy in the same time.