UNCERTAIN ESTIMATION OF SUBSURFACE TEMPERATURE AWAY FROM THE BOREHOLE USING MAGNETOTELLURIC INVERSIONS

Abstract

Although the magnetotelluric (MT) method has been used in geothermal exploration for some decades, there remain deficiencies in integrating resistivity data with other relevant variables and information, e.g., contemporary reservoir temperature, clay alteration, permeability and porosity. Here, we focus on the relationship between temperature and resistivity, specifically through the formation of clay minerals. We develop a methodology to extrapolate subsurface isotherms away from wellbore for which there is a temperature log. To do this, we use the resistivity distribution estimated from MT inversion assuming a correlation between shallow (clay) resistivity anomaly and clay formation temperatures. For the synthetic problem studied here, our method constrains isotherms and clay cap boundary, with uncertainties, up to 2km from a well. Our long term goal is to improve understanding of the relationship between resistivity and temperature, provide a method to image temperature away from boreholes, and develop methods to inform and validate reservoir models.