Experimental study on impedance dispersion characteristics and electrical resistivity of high temperature granite

Abstract

Electromagnetic methods are widely used in geothermal exploration. The electrical resistivity and impedance dispersion characteristics of high temperature rocks are the basis for the inversion interpretation of electromagnetic responses from geothermal reservoirs. Alternating current impedance spectroscopy experiments on granite at different temperatures and water contents were carried out. The impedance spectrum from these tests show that the resistivity of both dry and wet granite is mainly caused by grain boundaries or microfractures, rather than the mineral grains. The resistivity of granite decreases dramatically with the appearance of water, and decreases with increasing temperature. When temperature increases from 25 °C to 150 °C, the resistivity of dry granite decreases from 5690–5812 Ω·m to 5519–5638 Ω·m, while that of wet granite decreases from 3123–3278 Ω·m to 2618–2736 Ω·m. The relationship between temperature and resistivity can be described by an empirical equation, in which the temperature coefficient of dry granite is about 2.8 × 10−4, and that of wet granite is about 1.7 × 10−3. It shows the resistivity of wet granite is more significantly affected by temperature than that of dry granite. A mixing model describing resistivity of wet granite was established based on the resistivity of dry granite and water. This mixing model can also take temperature into consideration. Even though the equivalent porosity of granite is quite small, the resistivity of granite can still be described by Archie's equation, which was initially developed for high porosity sandstone. The coefficients in Archie's equation for granite were calculated from experimental results, with a tortuosity factor a = 0.005 and a cementation exponent m ranging from 1.50 to 1.75.