Geomechanical Application for Drilling Optimization in Marginal Fields, East Venezuela

Abstract

This study is intended to show that it is possible to reduce risks, time and costs in well drilling through application of Geomechanics in marginal fields. It refers to the Oritupano-Leona field, East Venezuela, which has more than 700 wells drilled from 1938, 29 of which are horizontal wells. In spite of the production success, 6 over 29 wells, 25%, presented problems due to hole instability that lead to several stuck pipes, sidetracks and down time. Drilling parameters of 23 horizontal wells, operational problems, geomechanical information and regional geology were considered for this analysis. Problems associated to mechanical/chemical instability and operational problems were identified. Geomechanical parameters were calculated and analyzed with the specialized software considering pore pressures, mud density, minifrac and electrical logs, where the stress direction was studied with the interpretation of imaging logs and review of the regional stresses. A friction diagram adjusted to the wide of the breakout was applied, whereas vertical stresses were calculated through density log integration. Rock mechanical parameters were product of RSD tests that included rock mechanical resistance analysis (UCS), determination of the internal friction angle and the Poisson's ratio and Young modulus, calculated with the use of neural networks. Drilling events were visualized in order to define the stability frame of the area for the different operational parameters based in geomechanical parameters previously calculated. With this data a new well design was elaborated integrating trajectory optimization and operational parameters in order to diminish the stability problems. Different mechanical instability values were identified in the study in three zones of the field, and the safe operating window for drilling and operational parameters was identified for each one, guarantying hole cleaning. The application of the criteria and recommendations of the results of this study had an outstanding impact on the horizontal well drilling in the Oritupano-Leona field. In 2005, a new record was established with the drilling of the ORM-174, which is the fastest horizontal well drilled in the field history. Two other wells were drilled in 2006 with excellent results. Introduction The Oritupano-Leona unit is located in the Venezuelan Oriental Basin (Maturin sub-basin) along Anzoátegui and Monagas states. Its extension is 1600 km2 with 23 oil fields in eight (8) sets or geographical units. It was discovered in 1938 and it has been operated by different companies: TEXAS, MGO, BARCO, PDVSA, and since March 1994 to date Petrobras EnergÍa Venezuela, S.A (PEVSA). The productive intervals are represented by the Oficina Formation that belongs to the Early/Mid Miocene -composed by sandstones of outstanding petrophysical characteristics and siltstone, shale, lutites and lignite associated to a transitional fluvio-deltaic environment with tides. The multiple productive levels of the various fields are within a depth range between 4,500 and 11.000 feet. Since the discovery around 700 wells have been drilled (over 200 by PEVSA), including around 25 horizontal wells. Figure 2 shows the design of a typical horizontal well. This study emphasized regional geology, the characteristics of the field, geomechanical, well construction, optimal trajectory, well design, optimization of drilling parameters and risk analysis.