Variations in Elastic and Electrical Properties of Crustal Rocks With Varying Degree of Microfracturation

Abstract

This work aims at investigating the effect of varying degrees of microfracturing on the joint elastic and electrical transport properties of rocks. Different crustal rocks were subjected to thermal treatment, expected to induce microfracturing, up to different target temperatures from 100 up to 900°C. The rocks bulk and pore volumes, P and S wave velocities, and frequency-dependent electrical impedance were measured before and after the treatment. As the temperature of treatment increased, P and S wave velocities and the electrical formation factor drop and pore and bulk volumes increase. As indicated by the microscopic images, this is consistent with the creation of microcracks. These created microcracks do not affect the properties in the exact same manner, and a strong effect of the initial porosity is observed for electrical formation factor. Extrapolating to low-frequency field-scale measurements in water-saturated reservoir rocks highlights an interesting observation: Both seismic and electrical properties at the field scale might highlight similar variations with increasing damage in all rocks. The maximum amount of decrease expected for all rocks is about 1 order of magnitude in electrical formation factor and 40% decrease in P and S wave velocities. Finally, temperature proved to affect very differently the rocks of varying porosity and mineral content, in ways not fully covered by existing works. Porosity, by increasing the matrix compressibility, damps the degree of microfracturing and crack opening. The distinct behavior in calcite-pure marble evidences a dominance of anisotropic of thermal expansion over isotropic expansion mismatch.

Temperature Effect on the Velocity‐Porosity Relationship in Rocks

“…Although the total porosity does not seem to change significantly below 300°C (Pimienta et al, 2019;Tian et al, 2012;Yavuz et al, 2010), the change in microcrack porosity with temperature and pressure can be QI ET AL.…”

“…However, the microcrack (compliant) porosity cannot be directly measured. It is sometimes estimated by using scanning electron microscopy (SEM) images (Fredrich & Wong, 1986; Pimienta et al., 2019), or by adjusting relevant parameters of idealized microcracks to fit observed changes in measured rock properties. Here, we use the DZ model (David & Zimmerman, 2012) to calculate the microcrack porosity.…”

“…Figure 7 illustrates the process of microcrack opening during the temperature increase. Spatial variations of mineral thermal expansion are known to cause microcracking in heat treated rock (Fredrich & Wong, 1986; Pimienta et al., 2019). A mismatch of thermal expansion between minerals or between a mineral and the rock matrix must obviously exist in polymineralic rocks.…”

“…In addition to mineral thermoelasticity, the thermal expansion of the fluid can also have an important effect on microcracks (Lin, 2002;Mctigue, 1986). Although the total porosity does not seem to change significantly below 300°C (Pimienta et al, 2019;Tian et al, 2012;Yavuz et al, 2010), the change in microcrack porosity with temperature and pressure can be observed in the SEM or thin section image. Tran et al (2008) observed some minor microcracking and grain separation after heat-treating to 300°C with the thin section image.…”

“…A mismatch of thermal expansion between minerals or between a mineral and the rock matrix must obviously exist in polymineralic rocks. In monomineralic rocks, thermal expansion contrasts must also occur because most grains/crystals are anisotropic and randomly oriented (Fredrich & Wong, 1986; Pimienta et al., 2019). In addition to mineral thermoelasticity, the thermal expansion of the fluid can also have an important effect on microcracks (Lin, 2002; Mctigue, 1986).…”