In-situ stress orientation was measured in 17 wells throughout the Scott field in order to predict the orientation of waterflood induced fractures. The Scott field is heavily faulted and significant variations in in-situ stress orientation were found between different fault blocks. The in-situ stress orientations appear to be strongly controlled by the orientation of the local faults and tectonics. The direction of maximum horizontal stress appears rotated 30° to 50° on either side of the NNW-SSE regional compression trend generally found in the North Sea. In most cases, the maximum horizontal stress is parallel to the orientation of nearby normal and wrench faults, as would be predicted by a tectonic model of such faulting. However, the tectonics of the region suggest that the rotation of the regional stress field from fault block to fault block is due to the presence of the faults rather than active faulting. The orientation of the in-situ stress was determined from shear acoustic anisotropy measurements on cores from 5 wells and from wellbore elongation measurements in 13 wells. The two methods show very similar orientations within the same fault block. No statistical variability in stress orientation was noted between different formations or with depth. Shear acoustic anisotropy utilizes the polarization of shear acoustic waves propagating through oriented core samples and has proven to be very reliable in determining stress orientation. The observed wellbore elongations in the field do not appear due to breakouts. The character, magnitude, placement, and orientation of the wellbore elongations strongly support the premise that the measured elongations were due to drilling or coring induced wellbore fractures. Previous experience by the authors and other recently published observations support this conclusion and the reliability of these elongations for the determination of stress orientation.

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.