Drilling and Completion Strategies in Naturally Fractured Reservoirs

Abstract

Traditional exploration drilling practices include targeting the structural crest; maintaining a vertical borehole; and drilling over-balanced for safety reasons. All three drilling practices may threaten success in naturally fractured reservoirs in which well and reservoir flow performance relies on flow through natural fractures. Depending on the deformation mechanism of folds, more folding related natural fractures may develop on the limb of the structure than on the crest. At typical depths, open fractures planes are likely to have a subvertical orientation; and a deviated borehole, drilled in a direction normal to the natural fractures, will intersect many more fractures than a vertical one. Over-balanced drilling can result in lost circulation of damaging drilling fluids and solids in the natural fractures. Cementing the fractured zones can further plug natural fractures, leaving little behind pipe to provide sought-after productivity from the natural fractures. This paper offers insight on drilling and completion strategies based on experiences in two South American locations with distinct structural styles. One is a Devonian gas discovery in Bolivia and the other a block southwest of Lake Maracaibo with both mature fields and exploration prospects of Cretaceous age. In both instances, the structural style and orientation provide a framework for borehole trajectory planning.