The development of some reservoirs in the South Region of México was limited by highly depleted and/or naturally fractured formations that would not allow to reach the original geological goals. Those are Cretaceous and Jurassic period formations in carbonated rocks mainly limestone and dolomite. Oil well drilling under unbalanced pressure conditions demands special techniques for hydrodynamic control and hole stabilization in order to recover formation cuttings in the surface as well as to keep navigation control during drilling and gather indirect information of geophysical logs, as well as cutting and recovering cores and minimizing productive formations damage.

Analysis formations and non conventional drilling correspond to the reservoirs named Agave, Samaria, Iride and Jujo, in which successful drilling was performed using underbalanced drilling techniques, negative pulse navigation equipment, cutting and recovering of preserved core and particularly the development and application of low density foam fluid, designed and patented by PEMEX staff workers.

The main purpose of this paper is to illustrate the non conventional oil drilling techniques used to obtain successful results in highly depleted formations; with the attempt to make the application extensive to formations and reservoirs with similar subnormal pressures and/or naturally fractured.

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