Porosity and tortuosity influence on geophysical properties of an artificially cemented sand

• Hydrated cement minerals are electrically charged bonds.
• Empirical relations for in situ data can be defined for such artificial materials.
• Double porosity and tortuosity are due to bonds presence filling the empty pores.
• Pore size and tortuosity define the paths for water and electrical current flow.
• Different electrical and hydraulic tortuosities must be considered in the analysis.

Electrical resistivity, saturated hydraulic permeability and P wave velocity are investigated for an artificially cemented sand. The calibration of some known equations used in geophysical characterization tests relating these properties is done considering the presence of artificial bonds. In this case bonds are the hydrated cemented minerals coating the grains. An extensive set of experimental tests was performed, which data is presented in the paper. Artificially cemented sands were prepared with different cement dosages and different water/cement ratios in weight for each dosage. Bonds distribution in the tests performed was controlled by the water used in grout preparation.Known relationships between the geophysical properties studied and porosity, and between electrical resistivity and P wave velocity or hydraulic conductivity, were adopted and calibrated to identify constants to be used for the sand-cement mixtures. Although the different scales, the values found for these constants were compared with those usually found in natural geological formations tested in similar conditions. The existence of such relationships show that it is possible to detect such materials through in situ data interpretation, however scale effects and log resolution must be considered to define the constants for field data.The results are commented considering the presence of the electrically conductive hydrated cement minerals, and how they affect porosity and tortuosity considered in the calibration of the relationships adopted. It is proposed different tortuosities for electrical current and water flow for the cases when electrical conduction through the electrically charged surfaces is more important than through the free liquid phase in the interconnected pores of the saturated material.