Characterization of Gas Saturation in Tight-Sandstone Reservoirs with Rock-Physics Templates Based on Seismic Q

Abstract

Tight-sandstone gas reservoirs have low porosity and permeability, dissimilar pore types, and generally high clay content. Partial saturation leads to local fluid flow induced by seismic waves, resulting in velocity dispersion and attenuation, and this is the reason why the dissipation factor (Q−1) (inverse quality factor) is highly sensitive to fluid saturation. A relation between Q and saturation can be based on the self-consistent approximation and poroelasticity theory, to build, in principle, a two-dimensional (2D) rock-physics template. However, there is an ambiguity in the process of estimation because one value of Q may correspond to two saturations. Thus, this paper addresses this limitation. Moreover, a well-log in the Sichuan Basin and reported experimental data show that these reservoirs may have a high clay content, which affects the estimation. To take into account this factor, the hydration effect of clay is considered in the framework of the double double-porosity theory of wave propagation. Ultrasonic measurements were performed on a tight sandstone and the spectral-ratio method was used to estimate Q. Then, three-dimensional (3D) rock-physics templates are built by introducing the phase velocity ratio (VP/VS), clay content, and seismic Q estimated with an improved frequency-shift method. The template is calibrated and tested with ultrasonic, well-log, and seismic data and applied to estimate reservoir porosity, clay content, and gas saturation on 2D and 3D seismic data.