DENSITY AND SEISMIC WAVE VELOCITY IN RESERVOIR ROCK

Abstract

The physical properties of reservoir rocks are governed by their porosity and amount and type of fluids filling the pores. Clay content of these rocks also influences these properties. The properties of interest here are formation density and velocities of P- and S-waves. The velocities of compressional and shear waves in rocks are strongly influenced by porosity and saturation conditions. In this study, a density-porosity relation and four important velocity-porosity relationships, namely, the time-average relationship, Raymer's Relationship, the velocity-porosity relationship based on Gardner's relation and Gassmann's relationship have been studied and used to predict variations in density and seismic wave velocities as a function of formation porosity and water saturation. This has been done for the three main types of reservoir rocks: sandstone, limestone and dolomite. This study also compares these variations in seismic wave velocities and density, for reservoir rocks saturated with a single fluid (fresh water, brine, oil and gas) and also for reservoir rocks saturated with two types of fluid (fresh water and oil, brine and oil, fresh water and gas & brine and gas). The present work also comprises of a study of the variation of Vp/VS ratio and clay content with porosity. There are important differences in the variation of physical properties with porosity predicted by different relationships. An understanding and knowledge of these differences is important when these relationships are used to compute formation porosity from sonic log measurements.