Effect of Dilution on Acoustic and Transport Properties of Reservoir Fluid Systems
Authors: Ratnakar Ram R. , Lewis Edward J. , Dindoruk Birol 
Published: March 2018

With a high degree of accuracy, acoustic velocity has been utilized in phase behavior measurements of pure substances and mixtures to supplement other PVT (Pressure-Volume-Temperature) measurements or as a standalone measurement. Several key fluid property parameters are challenging to obtain through traditional measurement techniques, correlations, or equation of state models. In this work, a high-pressure high-temperature (HPHT) tool is utilized to obtain acoustic velocity data, which is correlated with key PVT parameters to bridge gaps in the required data.

The experimental method is based on pulse-echo response utilizing ultrasonic acoustic transducers in a HPHT fluid cell. Compression wave travel times are measured within 0.2% and a special mixing mechanism allows for in-situ homogenization of fluids mixtures. Effect of dilution with methane is studied to capture the impact of GOR/density variations on acoustic velocity and viscosity response. Dilution with CO2 is investigated to interpret CO2-oil interactions for EOR projects. An electromagnetic viscometer is used to measure viscosity which is correlated with acoustic velocity values over a range of pressure, temperature and methane/CO2 concentration levels.

Experimental acoustic velocity and viscosity data for two fluid systems with varying concentrations of methane, and CO2 at pressures from 15 – 62MPa and temperatures from 313K – 344K are presented. Under isothermal conditions, acoustic velocity of a live oil decreases monotonically with decreasing pressure until the saturation point where the trend is reversed. This observation can also be utilized as a technique to determine saturation pressure, at least as a byproduct of the target experiments, or simply to substitute the isothermal classical pressure-volume measurements. In the current work, we have measured the acoustic velocity, viscosity and density of live oil systems diluted with a gas (methane or CO2) at HTHP conditions. Most importantly, acoustic velocity data are correlated with dynamic viscosity to develop an empirical correlation using the extent of dilution that are relates to density and GOR. Such correlation can be utilized in interpreting the sonic velocity responses and calibration of viscosity changes when areal fluid properties vary with GOR, especially in disequilibrium systems.

Novelty of the work includes new acoustic velocity and viscosity data for live reservoir fluids with varying methane/CO2 concentrations. To the author's knowledge, this is the first time an experiment-based empirical correlation is developed to estimate dynamic viscosity from acoustic velocity for fluid mixtures at various temperature, pressure and dilution conditions. When limited viscosity-measurements are available to calibrate fluid viscosities with GOR variations, such correlation can be very useful in establishing the bounds for viscosity, especially in the context of field-scale fluid distributions/reservoir initialization and the interpretation of the acoustic responses.