Wellbore drilling for any resource extraction-and particularly for EGS-is subject to many uncertainties, ranging from those related to geology to those related to the drilling procedures. Since well drilling and related subsurface activities for EGS represent a significant portion (above 50%) of the total EGS development cost, assessing the risk is essential. This requires that one can estimate the cost and time uncertainties for EGS well drilling. The paper describes a method and associated computer code, the Decision Aids for Tunneling (DAT) that have been adapted to EGS well drilling. Very importantly, the paper shows a detailed application to the case that was originally developed by Sandia National Laboratories. This was done in form of parametric studies, in which uncertainty in cost components, in trouble events, in geology, and in temperature related effects were investigated separately and then in combination. This not only shows that the DAT are well suited for this purpose but it also provides a guideline for interested users. Most importantly, all this shows that wellbore drilling cost and time that are subject to a variety of uncertainties can be systematically assessed. (C) 2014 Elsevier Ltd. All rights reserved.