| With the development of hydroccarbon exploration and production, it is proven that fractured reservoirs contain significant portions of global hydrocarbon reserves. Natural occurring fractures in petroleum reservoirs play an extremely significant role in hydrocarbon production, as they contribute to the porosity and permeability of the reservoir. In the mean time, fractures in carbonate reservoirs may affect the dissolution pore and Karst cave. Given the considertation that fractures can control the distribution of petroleum and act as primary conduits for fluid flow, knowledge of the orientation and density of fractures is important for optimal production. We usually use P-wave azmiuthal attributes may provide an way to predict fracture orientation and density. However, the fracture density obtained form these methods are just semi-quantitative description of the degree of anisotropy, which are different from the fracture density in geology. We propose to choose proper rock physics model for the fractured reservoir, investigate the physical significance of the fracture parameters, and estimate the fracture parameters and fluid indicater using prestack seismic data. Inversion of fracture weaknesses from P-wave AVOA data is an quantitative way to estimate the fracture density and predict the development of fractures, which may contribute to the fractured reservoir prediction and interpretation.We treat the fractured reservoirs as linear-slip model and quasi-static porous fractured model, invert fracture weaknesses and general fracture weaknesses, thus to characterize the distribution of fractures and fluid type.The fluid indicator is the ratio of normal and shear excess compliance caused by the presence of fractures. Fluid indicator is close to unity for dry and gas filled cracks and almost vanishes for fluid-filled ones. This experiment is based on Hudson’s theory, which is appropriate for high-frequency laboratory conditions, and there is no exchange of fluids neither between fractures themselves nor between fractures and surrounded equant pores. When surrounded by equant pores, the fluid in the fracture has enough time to escape to the pore when compressed, the fracture becomes compliant, and therefore the fluid indicator does not vanish even for fluid-filled fractures. We propose to use fluid influence factor to characterize the effect of fluid on the host rock and the effctive bulk modulus of the fluid for particial fluid saturation condition. The advantages of fluid influence factor and the relationship between fluid influence factor and fluid indicator are analyzed. The advantages of using fluid influence factor as an indicator of fluid content are as follows:(1) The fluid influence factor is fairly intuitive to characterize the effect of fluid content on the properties of the fractures, as it increase with the increase of effective bulk modulus of the fluid or fluid saturation and decrsase with the decrease of effective bulk modulus of the fluid or fluid saturation. On the contrary, the fluid indicator approaches unity for dry or gas-filled fractures. (2)The fluid indicator varies between 0 and a parameter that is related to the Poisson’s ratio of the host rock. The fluid influence factor varies between 0 and 1, which removes the effects of the host rock. (3)The fluid influence factor is based on the Hudson’s model for dry cracks. However, it can be used for all the fractured model with stiffness matrix having the same structure with linear-slip model. The fluid influence factor is valid for efftive fractured model for any frequency band, and can reflect the effective bulk modulus of the fluid contents and hydrollically conection between the fractures and pores.We proposed a method of deriving fracture parameters form vertical-well-log data with the assumption that the fractured medium is transversely isotropic with a horizontal axis of symmetry. Ellipse-fitting of P-wave attributes, such as amplitude, can be used to invert for fracture orientation and fracture density. We use four models to simulate class 1-class 4 AVO response, and take these four models as the isotropic background medium, which are embedded with an array of parallel fractures. We investigate the effect of fracture parameters and incidence angle on the flattening of the ellipse. However, the fracture density estimated from ellipse-fitting of the P-wave azimuthal amplitude can only indicate the fracture intensity indirectly, and is different from the fracture density in geology. We proposed four equations to characterize the flattening of the ellipse, and derived the relationship between flattening and anisotropy parameters or fracture weaknesses, which provided theoretical support for conventional fracture density estimation method.We expressed the reflection coefficient as a funtion of contrasts of the fracture weaknesses, based on the relationship between anisotropy parameters and fracture weaknesses. Conventionally, we assume the medium above the reflecting interface is isotropy, the medium below the reflecting interface is anisotropic medium, and treat the contrasts of the fracture weaknesses as fracture weaknesses themselves. The other way we use to invert for fracture weaknesses is estimate the contrasts of the fracture weaknesses, and then use layer stripping to estimate the fracture weaknesses for each layer. One problem with layer stripping for fracture weaknesses estimation is that large errors will be introduced. We proposed to express the contrast of the fracture weaknesses as multipication of the first order difference matrix and the fracture weaknesses, and express the reflected amplitude as a funtion of the fracture weaknesses, investigated the sensitivity of the linearized reflection coefficients to the nornal and tangential weaknesses. The resolution and accuracy of the inverted fracture weaknesses using the improved method are higher than that of the results estimated using conventional layer stripping method. Apply the improved fracture weaknesses inversion method in Tarium basin, estimate fracture density and fluid influence factor to predict the distribution of fractured reservoirs. It is proven that the inversion results agree with the well and log data.The fractures can be treated as infinitely thin and highly compliant layers or planes of weakness with linear slip boundary conditions, ignoring the shape and structure of the fractures. Combining linear-slip model and anisotropic Gassmann theory provides a straightforward way to derive explicite analytical expressions for low-frequency elastic constants of a fractured porous medium saturated with fluids. The compliant matrix of the quasi-static fractured porous model can not be represented by the sum of compliant matrix of an siotropic matrix and an excess compliance matrix with just normal and tangential components. We proposed to use general fracture weaknesses to characterize fractures in saturated porous rocks. General fracture weaknesses are functions of physical parameters of the host rock, fracture weaknesses for dry cracks and fluids. We analyze the effect of fracture density, bulk modulus of the fluid and equant porosity on the general fracture weaknesses, and analyze the difference of general fracture weaknesses components caused by fluid type, which will provide an theoretical support for fluid prediction using difference of the general fracture weaknesses. The equation expressing the relationship between reflection amplitude and general fracture weaknesses is derived. Apply general fracture weaknesses inversion in Tarim basin, and estimate fracture density. The inversion result shows quasi-static fractured porous model is appropriate to characterize the fractured reservoir, and general fracture weaknesses inversion can be used to for fractured reservoir exploration and prediction.Asignificant percentage of oil and gas reserves is found in the Makit carbonate in Tarim basin. Well and log data shows that significant amount of hydrocarbons are trapped in fractured reservoirs. According to the drilling core and FMI data, most of the reservoirs are fractured reservoirs with large amount of vertical or near-vertical fractures, and small portions of small cracks, randomly oriented fractures and fractures with small dip angle. Based on linear-slip model and quasi-static fractured porous model, we aim to estimate the fracture density and fluid influence factor, combing geological information, well, log and rock-physics information.The innovations of this research are as follows:1) We propose to use the so-called fluid influence factor as an indicator of fluid content. The expressions of fluid influence factor for certain static fracture models are derived, and the relationship between conventional fluid indicator and fluid influence factor is analyzed.2) We propose to express the constrast of fracture weakness as mutiplication of an first order derivative matrix and fracture weakness. The reflection amplitude can be expressed as a function of fracture weaknesses, which can be used in fracture weaknesses inversion.Both resolution and accuracy of the inverted fracture weaknesses are higher than that of the fracture parameters estimated using conventional layer stripping method.3) We introduce the dimensionless quantities general fracture weaknesses for Gurevich’s quasi-static fractured-porous model, as normal weakness can not be used in quasi-static model to characterize the fractures. The effects of fracture density, fluid content and equant porosity on general fracture weaknesses are investigated. The general fracture weaknesses inversion result s show that quasi-static fractured-porous model is appropriate to characterize the fractured resertvoir, and fracture weaknesses inversion can be used in petroleum exploration and fractured reservoir prediction. |
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