Naturally fractured reservoirs have attracted increasing interest in exploration and production geophysics in recent years. The presence of fractures can affect the AVO response of a reservoir. In this paper we analyse this effect of fractures in a porous reservoir on seismic data, and estimate the errors in the estimation of reservoir properties due to ignoring the presence of these fractures. The effect of fractures on elastic properties of a porous rock is studied using the model of fractures as linear-slip interfaces in an isotropic porous background. The effect of fluid on the elastic properties of this medium is modelled using equations of anisotropic poroelasticity. This yields explicit analytical expressions for the elastic properties of the fractured porous medium saturated with a given fluid. We use this theory to model the seismic response of a reservoir with vertical fractures. We then invert this response for reservoir properties (porosity, net-to-gross) using a workflow based on the above theory, and compare the predictions with the results of the standard industry workflow based on the isotropic Gassmann equation. The comparison yields an estimate of the potential error in the prediction of rock properties using an isotropic workflow for a fractured reservoir. These errors can be as large as ±3% in predicted porosity, and ±40% in predicted net-to-gross.

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Naturally fractured reservoirs have attracted increasing interest in exploration and production geophysics in recent years. The presence of fractures can affect the AVO response of a reservoir. In this paper we analyse this effect of fractures in a porous reservoir on seismic data, and estimate the errors in the estimation of reservoir properties due to ignoring the presence of these fractures.The effect of fractures on elastic properties of a porous rock is studied using the model of fractures as linear-slip interfaces in an isotropic porous background. The effect of fluid on the elastic properties of this medium is modelled using equations of anisotropic poroelasticity. This yields explicit analytical expressions for the elastic properties of the fractured porous medium saturated with a given fluid.We use this theory to model the seismic response of a reservoir with vertical fractures. We then invert this response for reservoir properties (porosity, net-to-gross) using a workflow based on the above theory, and compare the predictions with the results of the standard industry workflow based on the isotropic Gassmann equation. The comparison yields an estimate of the potential error in the prediction of rock properties using an isotropic workflow for a fractured reservoir. These errors can be as large as ±3% in predicted porosity, and ±40% in predicted netto-gross.