Rock acoustic velocities and temperature

This topic describes the effect of temperature on rock acoustic velocity.

Relationship of velocity and temperature

For consolidated rocks (Classes I, II, and V as defined in Rock acoustic velocities and porosity ), the elastic mineral frame properties are usually only weakly dependent on temperature. This is true for most reservoir operations with the exception of some thermal recovery procedures. In the case of poorly consolidated sands containing heavy oils, velocities show that a strong temperature dependence is observed (Fig. 1). Several factors can combine to produce such large effects. First, in heavy oil sands, the material may actually be a suspension of minerals in tar. The framework is basically a fluid, not solid. In addition, during many measurements, pore pressure cannot reach equilibrium. The large coefficient of thermal expansion of oils combined with the high viscosity often results in high pore pressures within the rock samples. Thus, effective pressures can drop substantially (Eq. 1). Care needs to be taken during such measurements that equilibrium pressures are reached.

....................(1)

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  Fig. 1 – Compressional velocity as a function of temperature for a sandstone sample testing the effects of thermal flooding. The large drop in velocity is a combined effect of grain separation, pore pressure, and fluid modulus changes.

The primary influence of temperature is through the pore fluid properties. Fig. 2 demonstrates this general temperature dependence. For dry (gas-saturated) rock, or rock saturated with brine, almost no change in velocity is observed, even for changes of almost 150°C. At elevated pore pressures, both gas and brine have only weak temperature dependence. Mineral properties are almost unchanged. However, when the rocks are even partially saturated with oil, dramatic temperature dependence is observed. Such changes can be understood by first calculating fluid properties with temperature, then using a Gassmann substitution to calculate the bulk rock properties. Note that for heavy viscous oils, velocity dispersion (velocity dependence on frequency) can be significant, and measured ultrasonic data may not agree with seismic results.

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  Fig. 2 – Compressional velocity as a function of temperature for Venezuelan (a) and Kern River California (b) oil sand samples. When either dry (gas-saturated) or brine-saturated, there is little temperature dependence. With increasing oil saturation, temperature dependence becomes strong. The effects of fluid phase changes can be seen by the drop in velocity in (b) with increasing gas saturation (from Tosaya).

Fluid phase changes may also occur as temperature is raised. These phase changes can have a strong influence, particularly for high-porosity rocks at low pressures. The effect can be seen in Fig. 2b, where exsolving a gas phase could reduce the velocity from nearly 3.2 km/s to around 2.1 km/s. In several thermal recovery monitoring projects, the strongest seismic expression was a result of gas coming out of solution to form a separate phase, rather than the thermal effects themselves.

Nomenclature

P	=	pressure, MPa
Pc	=	confining pressure, MPa
S, S′	=	general rock property

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Timur, A. 1977. Temperature dependence of compressional and shear wave velocities in rocks. Geophysics 42 (5): 950-956. http://dx.doi.org/10.1190/1.1440774

See also

Compressional and shear velocities

Rock acoustic velocities and porosity

Rock acoustic velocities and pressure

Rock acoustic velocities and in-situ stress

Seismic attributes for reservoir studies

Seismic time-lapse reservoir monitoring

PEH:Rock_Properties