A conceptual design of a procedure to appraise the success of high enthalpy geothermal drilling is presented in the current paper. This design problem appears in the context of establishing risk mitigation instruments for geothermal drilling whose funding intensity depend on the results of the drilling. Notice that even though the term success of geothermal drilling is used, what is meant is that the geological exploration executed with the drilling has yielded positive results. This means that not only was the well successfully drilled, i.e. drilled to target depth and cased according to the drilling program but the conditions of the geothermal reservoir have met expectations set out before starting drilling. The paper dwells into the main variables and parameters of the problem. Errors in the detection procedure are characterized. The success criteria from the viewpoint of the developer and the risk mitigation entity are analyzed and the possible influence the criteria can have on the behavior of the developer is discussed. A procedure is suggested to address the problem. The aim with the procedure is to align the success criteria as much as possible with the criteria of the developer. Practical aspects of implementing the procedure are pointed out. Finally critical aspects of the problem and the proposed procedure are discussed. These include the difficulty of providing with quantifiable criteria for successful drilling. Examples are given demonstrating the problem. Recent publications showing statistics for geothermal well capacities in the world as well as in specific fields are discussed. In continuation the inherent problem detection problem due to the skewed probability distribution of capacities of wells in a single field is also addressed.

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