3D Analysis for Wellbore Stability: Reducing Drilling Risks in Oriente Basin, Ecuador

Abstract

This work describes a 3D approach that was used to reduce drilling risks and unscheduled events in Palo Azul field. The field is in the Oriente foreland basin (Figure 1), contiguous to the Ecuadorian Andean foothills. A portion of the field presented an unexpected igneous intrusion (laccolite) just above the reservoir formation. The 3D analysis combined 3D seismic information with the 3D structural and geological model to generate a 3D geomechanical model. The mechanical earth model (MEM), relied on onsite real-time monitoring, was used to support drilling decisions, providing predictions of well trajectory, casing points, and mud window. The 3D velocity/amplitude cube, derived from one portion of the 3D seismic program, was used as the base to construct the MEM. To capture the uncommon structural characteristics associated with the complex geological structure, an innovative methodology was developed. Steps included special reprocessing of sequences of the seismic data; structural model reinterpretation; 3D seismic velocity analysis; population of the rock properties in the 3D domain (Geostatistical approach); integration with wireline logs; core measurements analysis; and drilling history review of the offset wells. The real-time geomechanics support allowed making important decisions in time when unplanned events occurred while drilling. This new methodology created a systematic approach to well planning that reduces drilling risks in the development of a complex area. The value added by the integrated team's efforts was demonstrated by drilling the next two wells with significant reductions in costs and nonproductive time. A sensitivity analysis of different well trajectories across fractured zones was performed. Wellbore stability forecasts were compared with reference to the 3D model developed. The results obtained contribute to a better understanding of the mechanics of stress development around a magmatic intrusion and a faulted/folded zone. The methodology developed allows better insight into the parameters that must be included in the 3D MEM creation and wellbore stability forecast for similar environments. This method also serves as a good basis for further development, such as sanding studies, completion design optimization, reservoir studies, and uncertainty analysis. Introduction Palo Azul field, operated by Petrobras Energia Ecuador, is a development field located in the Oriente Basin. This field is being developed by three different well pads.During the planning stage of well PA-14, Petrobras Energia identified the risks in this area, based on an unsuccessful experience during drilling one well into this dome structure formed by igneous intrusion. The Geomechanics group from Schlumberger, integrated with the Drilling and G&G group from Petrobras Energia, developed a 3D solution that minimizes the drilling risks in this area. RockSolid[TM], wellbore stability software[1], was used to construc a 1D MEM (Mechanical Earth Model) [2], which was calibrated through drilling events analysis, image logs, and core-strength data from multistage triaxial test. The mechanical earth model is a numerical representation of the state of stress and rock mechanical properties for a specific stratigraphic section in a field or basin. Petrel[TM], 3D geological modeling software[3], used this information in conjuction with an acquired vertical seismic profile (VSP) and 3D seismic, to build the 3D-MEM. This new model uses a basic petrophysical and sonic well log curves to determine the mechanical properties. Using the available geostatistical tools, these mechanical rock properties were populated across the field. The geological and structural model previously defined in Petrel[TM] was taken into account followed by the use of the 3D seismic velocity cube. The advantage of this method is that it can handle several parameters using a 3D approach to design the optimal well trajectory.In this work, the sensitivity of well trajectory across the fracture zones, mainly limestone layers, were investigated. The sensitivity analysis is performed for several well trajectories; following velocity anomalies related to these fracture zones in combination with structural features.

Effect of Rock Properties on ROP Modeling Using Statistical and Intelligent Methods: A Case Study of an Oil Well in Southwest of Iran

“…After identifying the geological structures, geomechanical studies are necessary. One of the effective ways to improve and accelerate the drilling process is to have sufficient knowledge about the geomechanical conditions (Villalobos et al, 2005). Geomechanical parameters are important in the optimization methods of well stability and hydraulic fracturing analysis in reservoir (Zoback, 2007); However, these parameters are seldom used in estimating ROP and drillability.…”