Zhijing Wang

Dispersion analysis of acoustic velocities in rocks

Acoustic velocity dispersions in three rocks saturated with water, normal decane (n‐decane), and a heavy oil (in petroleum engineering, heavy oil is referred to as crude oil with viscosity higher than 0.5 Pa s), respectively, are calculated in this paper. The results show that the apparent velocity dispersion in light fluid (low‐viscosity)‐saturated rocks is relatively small, usually less than 3% to 5%, whereas that in the same rocks saturated with heavy oil is much larger. Such apparent velocity dispersion can be well explained by the ‘‘local flow’’ mechanism, which relates dispersion to the viscosity of the pore fluid, the pore geometry of the rock, and the effective pressure and temperature. The Biot velocity dispersion calculated using the Gassmann equation and the Biot high‐frequency limit of the velocities in the rocks is very small, typically less than 2%. Such Biot dispersion can be explained in terms of the Biot theory, which relates dispersion to the viscosity of the pore fluid and the permeabil...

Effects of CO2 Flooding on Wave Velocities in Rocks With Hydrocarbons

Compressional and shear-wave velocities were measured in the laboratory in seven sandstones (porosities ranging from 6 to 29%) and one unconsolidated sand (37% porosity) saturated with n-hexadecane (C/sub 16/H/sub 34/) both before and after CO/sub 2/ flooding. CO/sub 2/ flooding decreased compressional-wave velocities significantly, while shear-wave velocities were less affected. The magnitude of these effects was found to depend on confining and pore pressures, temperature, and porosities of the rocks. The experimental results and theoretical analysis show that the decreases in compressional-wave velocities caused by CO/sub 2/ flooding may be seismically resolvable in situ. Therefore, seismic--especially high-frequency, high-resolution seismic--methods may be useful in mapping and locating CO/sub 2/ zones, tracking movements of CO/sub 2/ fronts, and monitoring flooding processes in reservoirs undergoing CO/sub 2/ flooding.

ULTRASONIC VELOCITIES IN PURE HYDROCARBONS AND MIXTURES

Ultrasonic velocities were measured in hydrocarbons of n-alkanes, 1-alkenes, and naphthenes as a function of temperature, using the ultrasonic pulse transmission method. The velocities of all the hydrocarbons measured decrease with increasing temperature approximately linearly, although the rate of decrease is different for different hydrocarbons. The data also show that hydrocarbons of the same homologous series with higher carbon content have higher bulk moduli (or lower compressibilities) and the measured velocities V in mixtures of these pure hydrocarbons obey a simple physical mixing law V=Σi=1nXiVi, where Xi is the volume fraction and Vi is the velocity, respectively, of the ith component.

Effect of Temperature on Wave Velocities in Sands and Sandstones With Heavy Hydrocarbons

A laboratory investigation was made of the effects of temperature on wave velocities in well cemented Massillon and Boise sandstones and unconsolidated Ottawa sand saturated with heavy hydrocarbons, as well as the dependence of compressional velocities in the hydrocarbons themselves as a function of temperature. The hydrocarbons selected as pore saturants were a commercial paraffin wax, 1-Eicosene, natural heavy crude, and natural tar. The experimental results show that the compressional wave velocities in the hydrocarbons decrease markedly with increasing temperature. In contrast wave velocities in the Massillon and Boise sandstones and unconsolidated Ottawa sand saturated with air or water decrease only little with increasing temperatures. The main reason for the large decreases in rocks with hydrocarbons is the melting of solid hydrocarbons, and high pore pressure. Thermal expansion of the saturants, and possibly thermal cracking of the heavy fractions and vaporization of the light fractions of the hydrocarbons may also contribute. The large decreases of the compressional and shear wave velocities in the hydrocarbon-saturated rocks and sands with temperature, suggest that seismic measurements such as used in seismology or borehole tomography may be very useful in detecting steam fronts in heavy hydrocarbon reservoirs undergoing steam flooding.

Wave velocities in hydrocarbon-saturated rocks: Experimental results

This paper contains laboratory measurements of the temperature dependence of velocities, determined by ultrasonic pulse transmission methods, in hydrocarbon liquids and rock samples saturated by the liquids. The samples discussed include 26 hydrocarbons of varying molecular weight, nine mixtures of these hydrocarbons, four heavy oils (tar), three saturated sandstones, and a saturated sand. The data provide encouragement that high‐frequency, high‐resolution seismic techniques may infer formation temperature changes, thereby detecting the progress of thermal enhanced oil recovery processes.

Velocities in hydrocarbons and hydrocarbon‐saturated rocks and sands

Compressional ultrasonic velocities were measured in pure alkanee, alkenes, and naphthenes (cycloparaffins) as a function of temperature. It was found that the velocities in all the hydrocarbons decreased with increasing temperature approximately linearly. Furthermore, the velocities show also a linear relationship with the inverse of the carbon number (or equivalently the molecular weight). Measured wave velocities in mixtures of the above hydrocarbons were found to obey very closely values calculated by a simple mixing model.