Modeling Seismic Responses in Heterogeneous Gas Reservoirs: A Theoretical Approach of Rock Physics

In actual gas reservoirs, generally natural gas is heterogeneously distributed in the host matrix of water-saturated rocks, forming countless small "patchy"-like gas packets. This heterogeneity feature (also called the "patchy-saturation") will cause the elastic wave dispersion and attenuation in a seismic exploration band (101~103Hz). The reveal of the relations between seismic responses and reservoir fluid distributions will significantly improve the precision of reservoir characterization and fluid identification in exploration engineering. This paper uses a double-porosity model to describe the gas/water patchysaturated rocks. On the basis of Biot-Rayleigh (B-R) equations, multi-scale rock physics modeling is performed aiming at gas reservoirs. The wave responses in seismic, sonic (103~104Hz) and ultrasonic (>104Hz) bands are predicted and then quantitatively related to the basic properties of lithology and fluids. This approach of modeling is applied in limestone gas reservoirs located on the right bank of Amu Darya river. A multi-scale rock physics template is presented based on the theoretical studies. By combination with the method of the pre-stack inversion of reservoirs' elastic constants, two key parameters of gas saturation and porosity are finally estimated.