Studies on how the velocity of [Formula: see text] is affected by temperature and pressure are important for understanding seismic properties of fluid and rock systems with a [Formula: see text] component. We carried out laboratory experiments to investigate velocity of [Formula: see text] in temperatures ranging from [Formula: see text] and pressures ranging from [Formula: see text], in which [Formula: see text] is in a liquid phase. The results show that under the above conditions, in general, the velocity of [Formula: see text] increases as pressure increases and temperature decreases. Near the critical point ([Formula: see text] and [Formula: see text]), the velocity of [Formula: see text] reaches a minimum and has a complicated behavior with temperature and pressure conditions due to the [Formula: see text] transition between gas and liquid phases. We also developed preliminary empirical models to calculate the velocity of [Formula: see text] based on newly measured data.
We have investigated the elastic properties of heavy oil sands influenced by the multiphase properties of heavy oil itself and the solid matrix with regard to temperature, pressure, and microstructure. To separately identify the role of the heavy oil and solid matrix under specific conditions, we have designed and performed special ultrasonic measurements for the heavy oil and heavy oil-saturated solids artificial samples. The measured data indicate that the viscosity of heavy oil reaches [Formula: see text] at the temperature of glass point, leading the heavy oil to act as a part of a solid frame of the heavy oil sand sample. The heavy oil is likely movable pore fluid accordingly once its viscosity dramatically drops to approximately [Formula: see text] at the temperature of liquid point. The viscosity-induced elastic modulus of heavy oil in turn makes the elastic properties of heavy oil-saturated grain solid sample to be temperature dependent. In addition, the rock physics model suggests that the microstructure of heavy oil sand is transitional; consequently, the solid Gassmann equation underestimates the measured velocities at the low temperature range of the quasisolid phase of heavy oil, whereas overestimates when the temperature exceeds the liquid point. The heavy oil sand sample has a higher modulus and approaches the upper bound due to the stiffer heavy oil itself acting as a rock frame as the temperature decreases. In contrary, heavy oil sand displays a lower modulus and approaches the lower bound when the heavy oil becomes softer as the temperature goes up.
Within a wide range of temperatures (25°C up to 200°C) and pressures (10,000 psi to 20,000 psi), the velocities of CO 2-saturated water and methane-saturated water were investigated and compared. Experimental results show that the effects of temperature and pressure on the velocities of CO 2-saturated water and methane-saturated water are the same trends as on that of pure water. The effect of solubility generally decreases the velocity of saturated water, but only 1~3 % for the velocity of methane-saturated water and the difference is ignorable. A special temperature around 60°C reverses the velocity of CO 2-saturated water from higher to lower than that of pure water with temperature increases. The different velocity slops and solubility effects may separate velocity properties of CO 2saturated water from that of methane-saturated water.
We have investigated velocity and density of degassed water (H 2 O) with dissolved CH 4-CO 2 in temperature ranged from 25°C to 200°C and pressure ranged from 20MPa to 138MPa. Pressure effect shows the same trends as that on degassed water. Temperature effect on velocity of water with dissolved CO 2 shows a minor modification: dissolved CO 2 causes an increase of velocity when temperature is lower than 60°C, but a decrease of velocity with higher temperature. With more dissolved CO 2 , the temperature effect on velocity appears to increase. Higher GWR also causes a higher density. The effect of GWR on density tends to decrease with increasing temperature. Velocity of degassed water can be decreased by saturated CH 4 about 1~2 %, which can be ignorable, and by saturated CO 2 about 5%, which may be resolvable seismically. Preliminary models have been developed to predict velocity and density of H 2 O-CH 4-CO 2 miscible mixtures.
Knowledge of dispersion and attenuation is essential for better reservoir characterization and hydrocarbon identification. However, limited by reliable laboratory data at seismic frequency bands, the roles of rock and fluid properties in inducing dispersion and attenuation are still poorly understood. Here we perform a series of laboratory measurements on two sandstones under both dry and partially water-saturated conditions at frequencies ranging from 2 to 600 Hz. Two samples, Bentheimer and Bandera sandstones, have similar porosity of ∼20% but different permeability of 1830 mD and 33 mD. At vacuum-dry conditions, the bulk dispersion and attenuation in Bandera sandstone with more clay contents are distinctly larger than those in Bentheimer sandstone, suggesting clay contents might contribute to the inelasticity of the rock frame. The partially water-saturated results show the combined effects of rock permeability and fluid saturation on bulk dispersion and attenuation. Because of the high compressibility of gas, even a few percent of gas (∼5%) can substantially dominate the pore-fluid relaxation by providing a quick and short communication path for pore pressure gradients. The consequent bulk dispersion and attenuation are negligible. However, when the sample is approaching a fully water-saturated condition (gas saturation <5%), the gas effect gradually decreases. Instead, the rock permeability begins to play an essential role in the pore-fluid relaxation. For Bandera sandstone with lower permeability, a partially relaxed status of pore fluids is achieved when the gas saturation is lower than 5%, accompanied by significant attenuation and dispersion.
Figure 1: Geography around Lake Kivu. Figure 3: Density data in comparison with the model by Schmid et al. (2004), and the FLAG model (2010) of temperature, salinity and pressure versus depth in the Lake Kivu.
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