ABSTRACT

Overpressure is one of the major difficulties that we have to face when exploring the deep earth. Geophysical remote sensing methods, especially seismic and electrical methods, are widely used to predict overpressure for derisking the drilling of deep boreholes in the earth’s crust. However, there are major uncertainties in relating the measured geophysical parameters to the physical properties of crustal rocks, needed for reliable overpressure estimation. We address this knowledge gap through dedicated laboratory measurements of elastic velocity, electrical conductivity, and porosity with increasing pore pressure and the analyses and interpretation of the experimental results. We find a linear increase in the porosity and an exponential increase and decrease in the electrical conductivity and compressional wave velocity, respectively, with the increasing pore pressure. We also find an exponential reduction in the cementation exponent and effective pore aspect ratio (PAR) inverted from the pore pressure induced electrical conductivity and compressional wave velocity, respectively. We further determine that electrical and elastic rock properties are affected more significantly by the changing cementation exponent and effective PAR by increasing the pore pressure rather than by dilating porosity. The results not only strengthen our understanding of the influencing mechanism of overpressure on the physical properties of crustal rocks, but also provide new insights for the more reliable detection of overpressure zones from geophysical remote sensing.

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