Porosity and Permeability Constraints from Electrical Resistivity Models: Examples Using Magnetotelluric Data

Abstract

The efficiency of geothermal energy extraction from reservoirs depends not only on temperature, but also on porosity and permeability of the target rocks. Data from boreholes can constrain these parameters well at specific points but not across the entire basin. In this paper, the electrical resistivity information of the subsurface obtained from magnetotelluric data is calibrated by laws relating electrical conductivity to porosity and permeability, with the aim to extrapolate the porosity-permeability values measured in the boreholes to the whole area of study. A new approach, based on the Generalized Archie’s law, was created to determine the cementation exponent of different types of Irish rocks, from porosity and electrical conductivity data. Using sandstone as target rock, the permeability values were calculated from porosity, electrical conductivity and cementation exponent data using different approaches. The influence of the parametric variables was tested, showing which variables can create more instability in the final results. Synthetic electrical resistivity models of the subsurface were also created to analyze the sensitivity of MT data to differences on porosity - permeability of the target rock. These general results on porosity, permeability and cementation factor will be used within the geothermal project in Ireland: IRETHERM, showing which areas could have more potential as a geothermal energy reservoir.