An important premise of underbalanced drilling (UBD) is the productivity improvement it delivers through mitigation of invasive damage. Characterization and quantification of such damage, therefore, becomes a pre-requisite for assessing the value delivered by UBD. Several methods are available to quantify damage. In this work, we use a novel approach that combines dynamic micro-scale reservoir simulations calibrated to special core tests to model the extent of invasive damage, and its impact on flow-back during production. The approach is based on special tests conducted on reservoir core, and a dynamic "micro-simulator" to model invasion during drilling. Special core tests designed to measure effects of overbalanced exposure to drilling fluids are first conducted. Inputs to the simulation model are based on careful interpretation of the core test results, and thus are calibrated to observation. Details of the approach were presented earlier by Suryanarayana, et. al. (SPE 95861). In this paper, we apply this approach to two field cases, and use the results to quantify the damage and its impact on production. The two field cases are discussed in detail. Both relative permeability and permanent damage effects are described. The dramatic effects of invasion on clean-up and long-term production are illustrated, thus demonstrating the incremental value of UBD in these cases. Damage can be modelled as an equivalent skin, based on the saturation and permeability profiles within the zone of invasion. Since the saturation and permeability effects are a function of location along the productive length as well as time, we obtain time- and spatially-dependent equivalent skin. The equivalent skin can then be used in field-scale reservoir models to compare different drilling and development options. The use of these results in designing an optimal drilling and completion plan to lock-in the value of UBD is demonstrated for the two field cases.

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