Acoustic Velocities in Petroleum Oils
Authors: Wang Zhijing , Nur Amos M. , Batzle Michael L. 
Published: December 2007

Summary
Results of laboratory experiments on acoustic-wave velocities in oils showthat the measured acoustic velocities are strong functions of both temperatureand pressure. The experimental results are discussed in light of existingliquid-state theories and models to interpret and understand theacoustic-velocity behaviors of oils. Correlations are made between acousticvelocity and temperature, pressure, API gravity, and molecular weight. Empirical equations are established to calculate acoustic velocities in oilswith known API gravities. Various applications or potential applications of theexperimental results are also discussed.

Introduction
With the rapid developments in seismic and well logging technologies, detailed studies on hydrocarbon resrvoirs are getting more attention. In recentyears, seismic borehole-to-borehole tomography, 3D seismic reservoir imaging, seismic delineation of reservoir fluid saturations, seismic evaluation andcharacterization of hydrocarbon reservoirs, seismic monitoring of productionand EOR processes in time, and detailed borehole sonic logging have all begunto emerge. These technologies will become routine in the near future. Usingthese new methods and interpreting their results, however, require that weunderstand the acoustic properties of reservoir fluids and rocks. Unfortunately, the importance of the acoustic properties of reservoir liquidshas not yet been recognized; i.e., there are very few experimental data onacoustic velocities in crude oils and no systematic studies. Therefore, themain purpose of this paper is to provide acoustic-wave-velocity information ondifferent oils.
We measured acoustic velocities at the ultrasonic frequency of 800,000cycles/sec [800 kHz] in eight dead oils and two refined petroleum hydrocarbonswith gravity ranging from 5 to 62°API [1.037 to 0.73 g/cm3]. We alsomeasured acoustic velocities in a live or gas-saturated oil. The experimentalresults show that all velocities in oils strongly depend on temperature andpressure and that dissolved gases substantially reduce the acoustic velocitiesin live oils.
In this paper, we briefly describe the experimental method and theproperties of the oil samples used in the experiments, and we show theexperimental results of acoustic velocities in dead and live oils. Tounderstand and interpret the acoustic-velocity behaviors of the oils better, wediscuss various liquid-state theories and models and the relationship betweenacoustic velocities and PVT measurements. We correlate acoustic velocities indead oils with temperature, pressure, API gravities, and molecular weights ofthe oils and establish empirical equations to calculate or to estimate acousticvelocities in oils if the API gravities are known. Finally, we discusspotential applications of our velocity results in both geophysical andpetroleum engineering aspects and present conclusions from this study.

Experiments
For the experiments, we used the pulse transmission method with an apparatusconsisting of a pulse generator/receiver, a digital oscilloscope, two acoustictransducers, and a pressure vessel. Electrical pulses generated by the pulsegenerator/receiver were sent to the emitting piezoelectric transducer, whichconverts electrical energy into mechanical vibrations. After traveling throughthe fluid sample, the ultrasonic waves were converted back to electricalsignals by the receiving transducer and recorded by the digital oscilloscope. Travel times of the ultrasonic waves through the oil sample were measured witha precision of 0.05 µsec. Ultrasonic velocities were calculated by v(p, T)=L(p, T)/?t(p, T), whereL(p, T)=distance between the two transducers corrected forpressure and temperature and ?t(p, T)=travel time of theultrasonic waves, which is a function of pressure and temperature.
Temperatures were controlled by a heating coil and a temperature controllerand were measured by two thermocouples connected to a digital meter. Theaccuracy of the temperature measurements was about 1°F [0.5°C]. Pressures inthe experiments were controlled by an automatic fluid pump and were readthrough a digital pressure gauge with an accuracy of about 10 psi [0.07Mpa].