Research On Seismic Rock Physics Modeling And Wavefield Characteristics In Heterogeneous Reservoirs

Seismic rock physics is the bridge between reservoir physical properties and elastic properties, mainly studies the effect of rock physical properties including rock lithology, porosity, stress, pore pressure, fluid saturation, anisotropy, etc., on seismic properties including velocity, attenuation, impendance, and so on. It plays an importan role in the forward modeling of seismic waves and interpretation of seismic inversion results. Seismic Forward modeling mainly studies the relationship between seismic characteristics and reservoir elastic properties. It is a useful tool for understanding the propagation features of seismic waves, also is of theoretical and practical significance for geological interpretation, reservoir prediction and development. Combination of rock physics and seismic wave forward modeling can help establish the quantative relationship between rock physical parameters and seismic characteristics, help interpret and predict the reservoir lithology and fluid types.Rock-physics modeling is one of the most important parts in rock physics research. In this paper, the actual rocks were idealized under certain assumptions, then the elastic properties of saturated rock were related to that of rock matrix, rock frame and pore fluid.The applicability and limitations of Xu-White model were analyzed and variable pore aspect ratio model was proposed for clastic reservoirs. Considering the lithology and pore structure of heterogeneous reservoirs, including low porosity tight sand reservoirs with cracks, carbonate reservoirs with complex pore sturcture, shale reservoirs with organic matter, etc., corresponding rock physics models were built to calcuate the quantitative relationship between the physical parameters and elastic parameters.Study of elastic wave propagation in porous media is one of the key issues to improve the precision of complex reservoir exploration. Seismic wave dispersion and attenuation occur during propagating in oil/gas-bearing reservoirs and relates to the properties of fluid to a large extent. To reasonably describe the propagation of seismic waves in heterogeneous reservoirs, the equivalent anisotropic parameters were calculated based on rock physics theory,the anisotropic porous theoretical model was established combined with anisotropic BISQ theory, then the wave-number equation was derived based on plan wave solution in frequency domain. The phase velocity and attenuation of the four kinds of waves can be calculated by solving this equation. For anisotropic clastic reservoirs induced by horizontal fractures, the changes of wave phase velocities and attenuation with clay content, crack density and shape, porosity, water saturation, viscosity, permeability and different propagation azimuth were analyzed. The theory and method can provide theoretical support for the pore and fracture structure analysis of the reservoirs and for the prediction of oil or gas-bearing reservoirs.Seismic forward modeling combined with the basic theory of seismic waves is the fundation for understanding and interpreting the seismic characteristics. For the first-order velocity and stress equations of Biot theory, use staggered-grid high-order finite difference method to solve the equations and model the wavefield in heterogeneous porous media. The dispersion equations of finite-difference schemes is derived based on plane wave theory and more accurate and stable finite-difference coefficients was calculated using Taylor expansion.Taking into account the coexistence of multi-scale geological bodies, staggered-grid finite difference schemes with variable grid-spacing and time-step for seismic modeling in porous media has been developed. FD operators with small grid-spacing and time-step are adopted for low-velocity and small-scale geological bodies, FD operators with big grid-spacing and time-step are adopted for high-velocity and large-scale regions, The dispersion analysis and modeling results demonstrate that the proposed method can improve the accuracy and computational efficiency.The forward modeling algorithm was applied to models with vuggy/fractured reservoirs, the aim is to analyze the relationship between seismic characteristics and scale of vug/fracture, porosity and fluid type. The results provide theoretical support for interpretion and inversion of multi-scale heterogeneous reservoirs.