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In the shale resource plays most of the activity to date has been in drilling and completing new wellbores and holding acreage by production.  In virtually all shale plays operators have increased the number of frac stages and generally obtained better production results.  This suggests that the original completions may not have adequately stimulated the length of the original wellbore and that refracture treatments in the unstimulated rock may be beneficial.  Refracs in long perforated horizontal wellbores pose several challenges in their design and execution.   Production logs typically show 80% of the production from 20% of the perforation clusters.  If the producing clusters have been drawn down below the reservoir pressure of the unstimulated rock the refrac may not initiate in the virgin rock where the majority of the reserves remain. Mechanical isolation of existing perforation clusters becomes critical, and working inside existing tubulars provides some challenges. Determining the optimum cluster and stage spacing requires an estimation of effective permeability and in-situ stress from DFIT testing which may not have been available prior to the initial completion.  A methodology is proposed to identify refracture candidates, mechanically isolate existing perforations, determine the optimum cluster and stage spacing, and successfully execute refrac treatments in a horizontal shale reservoir.