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The general benefits of underbalanced drilling (UBD) can be explained in terms of:Drilling performanceReduced drilling problemsIncreased reservoir productivity and recovery by minimization of formation damage. However, the absence of an easily applicable method to predict which reservoirs will benefit leads to an under-utilization of UBD by the industry. For that reason, the fuzzy logic expert system (ES) introduced in this paper was developed specifically to screen underbalanced candidates with respect to maximizing reservoir performance in terms of productivity. The ES software considers basic reservoir properties and attributes such as fluid type, drive mechanism, lithology, porosity, water saturation, thickness, permeability, etc. Using algorithms and logic rules, various modules evaluate these input parameters to assess underbalanced candidacy based on the following criteria:The likelihood and severity of formation damageThe overall impact of the damage on productivity and recoveryThe ability to successfully treat this damage with conventional stimulation techniquesThe relative economics of UBD compared to conventional drilling, completion and stimulation techniques In addition to a description of the ES, this paper discusses several case studies in which the output of the fuzzy logic ES is compared with published successful and unsuccessful UBD campaigns. The results demonstrate that the ES model can accurately predict the suitability of UBD for a given reservoir. Introduction UBD is not a new idea. For decades, operators have drilled various geological formations with different drilling fluids designed to exert a bottomhole pressure lower than the formation pressure, thereby allowing oil, gas and water from the formation to flow into the wellbore as drilling proceeds. One may argue that the benefits of UBD are well documented, however, while few question its value, its perceived issues of safety and complexity - properly managing pressures and production during drilling and completion – are challenging the full acceptance of this drilling technique. In addition to these operational issues, it is more important to realize that UBD is not an economic solution for every formation. Anecdotal evidence suggests that applying UBD in unsuitable reservoirs can lead to more costly and unsuccessful wells. This paper introduces an ES that was developed to aid the successful selection of horizontal underbalanced candidate reservoirs. It is important to note that the proposed ES is specifically built to screen horizontal underbalanced candidates with respect to maximizing reservoir performance in terms of productivity and recovery.