Exploration gas reservoir wells demand detailed operational planning to be drilled. In the same way different issues should be overcome as a consequence of the challenges faced during this kind of operation. Yacimientos Petroliferos Fiscales S.A. (YPF S.A.) has planned to drill an exploratory gas well in Los Perales area, in Santa Cruz State southern part of Argentina. This vertical well was initially designed to a planned True Depth (TD) of 14,760 ft., in a 358掳F reservoir temperature with a 10.0 lb/gal Water Based Mud (WBM) Mud Weight. Actual TD was 15,203 ft., reservoir temperature reached 365掳F and a 17.2 lb/gal WBM was required to balance pore pressure. Provincial regulations did not permit to drill with Oil Based Mud (OBM) in this area, so it was required to apply a WBM in this well. The main challenge for this Project was to formulate the High Pressure High Temperature (HPHT) drilling fluid, to drill the potential reservoir section, as well as keeping the fluid stability at all times, specially related to fluid's rheological properties, filtration parameters and mud weight control, likewise to avoid that contaminants destabilizes the mud. This paper will describe the methodology applied to develop and to design the fluid to drill the 6 inches reservoir section for this well. It will be highlighted the lab tests results to obtain the fluid basic formulation, as well as to simulate fluid behavior under some bottom hole conditions. Finally the results will be discussed with emphasis on the main preventive drilling fluids engineering actions taken to optimize the operational performance. The aspects related to both drilling fluids formulation and engineering application will be detailed. Planning was vital to achieve the successful application of WBM in the face of such operational conditions as drilling under a high pressures and high temperatures scenario. When drilling the 6 inches reservoir hole section, no sedimentation of Barite was observed, the rheological properties and the HPHT filtration were kept stable. It should be noted that three wireline log runs were performed, maintaining adequate well stability, showing nominal and homogeneous caliper, and very good quality of electrical loggings data acquisition. The 5 inches production casing was successfully ran and cemented to planned depth.
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