fax 01-972-952-9435. AbstractThis 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) 1 . 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.
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)1. 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. Introduction In Venezuela there is a high volume of reserves in low pressure reservoirs where the use of drilling fluids with a density higher than the required could result in a partial or complete fluid loss into the formation. The Under Balanced Drilling (UBD)1 technology has been used to avoid negative effects such as fluid losses and formation damage. However, the under balanced drilling service is very expensive (about 500.000 US$/well) and the operations are more complex and riskier than those using conventional drilling fluids. Therefore, it was desirable to develop a conventional drilling fluid system, that avoids fluid losses and formation damage under the following conditions: low pore pressure, high formation permeability due to the presence of vugular porosity and microfractures, and high temperature (about 300°F). Viscoelastic drilling fluids are known for having very good fluid loss control properties and for causing minimum formation damage. However, no commercial viscoelastic fluid available withstand these extreme conditions. Consequently, PDVSA Intevep worked to develop a viscoelastic system formulation, which could overcome the fluid loss problem, with thermal stability up to a temperature of 280 °F. At the same time, could show good transport capacity, resistance to typical contaminants (crude oil, sands, etc), excellent lubricity, low environmental impact, and minimum equipment requirements for its formulation and control. The purpose of this work is to show aspects related to rheological behavior properties, formulation and procedures of the viscoelastic system developed and registered under the commercial trademark of PERMAVISC. This paper also presents the results obtained in the laboratory and in the field application of drilling an inclined well in overbalanced condition, to reach a highly depleted carbonate reservoir. Reservoirs in the Borburata oilfield has been widely developed and highly produced and this exploitation decreased their original pressure from 4,682 to 2,000 psi at 11,734 ft (about 3.0 ppg equivalent density) with botton hole temperature of about 300°F. These reservoirs consist of a fractured limestone, which has been under production for a long time. The rocks are fractured, tight, and show high axial stresses which give rise to a deferred oil production due to flushing by lost circulation. These reservoirs have a typical range for matrix permeability from 0.1 to 100 mD, however the formation permeability is enhanced due to the presence of natural fractures and vugs. Porosity in this area is about 2 percent. Figure 2 shows a picture from the limestone carbonate reservoir taken in the Borburata Oilfield, which is characterized by vugular porosity and microfractures.
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