An Initial Value of Information (VOI) Framework for Geophysical Data Applied to the Exploration of Geothermal Energy

Abstract

We present a value of information (VOI) methodology for the exploration geothermal problem. VOI quantifies how relevant any particular information source is, given a decision with a highly uncertain outcome, and can be used to justify the purchase of collecting the additional, proposed data. This has been demonstrated in the literature for oil exploration, but we apply it here to the geothermal energy exploration decision. We evaluate the information reliability with forward-modeling. We begin here by quantifying how reliable magnetotellurics (MT) surveys are at detecting permeable zones containing hot fluid. We use geophysical forward models of vary geological and petro-physical properties, such as reservoir thicknesses and spatial extent, temperatures, effective permeabilities and fluid TDS. We evaluate the effectiveness of the magnetotellurics (MT) technique to detect electrically conductive targets that contain hot, saline fluids (indicating a permeable pathway) or associated with metamorphic rock (with low permeability). In our work, we define a group of prior models which have varying properties and structure. Given their property values and structure, each of the prior models is categorized as economic, marginally economic or non-economic geothermal resources (representing the “true economic viability category”). Then the forward model of MT is calculated for each prior model; noise is added to each response. Geophysical inversion is performed with the noisy data, and the inversion result is then categorized into an “interpreted economic viability category”. There is not an exclusive relationship between electrical conductivity (what MT measures) and permeability of reservoir rock. This implies that the information provided by MT data is non-unique, leading to multiple plausible interpretations. Given we know from which true economic viability category each inversion came from, we can compare the interpreted economic category: this represents our reliability measurement which accounts for possible errors from non-unique geophysical inversions and model interpretations. This reliability measure is used to calculate the VOI for MT data for geothermal exploration. This research was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.