Tight gas detection based on the reflectivity dispersion technology

Abstract

The study focuses on the reflectivity dispersion caused by velocity dispersion related to pore fluid and its application to identify tight gas reservoirs. Seismic attenuation in viscoelastic media has two effects: (1) the change in frequency content and amplitude of a pulse propagating through attenuating media; (2) reflection coefficients are frequency dependent. Based on linear viscoelastic theory of velocity dispersion and the linearization of Zoeppritz equations, a new approximate reflection coefficient is presented considering P & S wave velocity dispersion, which adds the effects of frequencies and quality factors in reflection coefficient formula, representing the characteristics of reflection coefficient dependent by frequency, incident angle, quality factor etc. Based on the relationship between P-wave velocity and gas saturation, the reflected energy changes induced by reflectivity dispersion can be calculated and used to directly detect gas reservoirs, combining with seismic velocity, amplitude and energy attenuation mechanism caused by gas, such as squirt flow mechanism. This method has been successfully applied to characterize the distribution of tight gas sands in the GA101 well area of Sichuan Basin, and the results offer reliable foundations for seismic prospecting and well designing. Key words: reflectivity dispersion, seismic attenuation, velocity dispersion, tight gas reservoir, seismic gas detection, prestack