Viscoelastic System as an Alternative to UBD for Drilling a Severely Fractured Limestone Reservoir

摘要

This document presents the drilling operations protocol designed to reach a depleted limestone carbonate reservoir, located in the Borburata Field in Southern Venezuela(Fig 1), with the use of a new viscoelastic fluid developed by PDVSA Intevep. This limestone reservoir has a very low pore pressure (about 3.0 ppg equivalent density) and it is characterized by vugular porosity and micro-fractures. Previously, when drilled with conventional fluid systems, severe fluid losses occurred; therefore, the best available alternative was Under Balanced Drilling (UBD). However, UBD service was a heavy burden on the drilling costs and represented a more complex and risky operation than using a conventional drilling fluid system. In order to have an alternative to UBD, a water-base viscoelastic fluid system was designed to avoid fluid losses in an adverse environment of very high overbalance (about 3,000 psi), very high temperature (about 300°F) and variable pore size due to the presence of vugs and microfractures. Viscoelastic drilling fluid systems have been used because they have excellent fluid loss control properties. However, for this application, the commercial viscoelastic fluids available did not comply with the extreme conditions found in this reservoir. A new formulation specifically designed for these severe conditions was proposed, tested in the laboratory, and evaluated in the field. It resulted in the elimination of fluid losses and allowed the access to this complex reservoir without requiring the UBD service. This result represents significant savings when compared with previous wells drilled in the same reservoir.