Drilling horizontal wells in the Borburata field, Western Venezuela, has become a challenge due to the geological complexity of the basin. These complexities translate operationally into many unexpected drilling events while building angle in the Pagueey formation, a weak, fissile and overpressured shale. This article presents the geomechanical model (MEM) developed to re-drill BOR-30 for the Pagueey and Gobernador formations. The stress field profile (magnitude and direction), mechanical rock properties, pore pressure, optimum mud window, trajectory sensitivity analysis, risks identification and mitigation measures, as well as some recommendations for selecting appropriate drilling fluids additives and drilling operations are presented. The methodology presented was used to define the risk of borehole instability before re-drilling BOR-30. Subsequently, the results were used to optimize drilling and casing design, optimize the drilling practices and enhance safety in drilling operations in the area.
The Borburata field is located in Barinas-Apure Basin, the SSE of the Andean Mountain Range. The initial drilling campaign of the horizontal well BOR-30 was very expensive due to instability problems, where previous perforations to reach the reservoir (Gobernador Formation), presented problems such as stuck pipe, high torque and drag, excessive drillstring vibrations; resulting in the interruption of the drilling operation until the appropriate methodology to guarantee the successful reentry in the well could be encountered. PDVSA requested a study, taking information available from 3 offset wells in the whole field to develop the Mechanical Earth Model (MEM). The MEM describes the state of stress (principal stress magnitudes and direction), pore pressure, rock strength and geologic structure; it allows the estimation of the wellbore stability along an arbitrary well path. The forecast helps to optimize the drilling parameters, casing design, bit design, define the maximum and minimum mud weights required to inhibit wellbore instability, identify potential hazards to drilling and recommended remedial actions (see Figure 1). A comprehensive analysis of the drilling history was conducted for a representative number of wells.
Barinas-Apures is one of the most important Venezuelan petroliferous basins. This basin is located to the SSE of the Andean Mountain Range. The main source rock is the Navay Formation, of Late Cretaceous age and a lateral facies equivalent of La Luna Formation. The main clastic reservoir is the Escandalosa Formation (Cretaceous) that is one of the most relevant limestone reservoirs with high secondary porosity . The main reservoir belonging to the Cretaceous Formations is Escandalosa (Limestone). The source rock is still of Cretaceous age and the reservoir includes Eocene-age formations such as Gobernador and Pagueey. The formations in Borburata field are defined by Rio Yucca, Parangula, Pagueey, Masparrito, Governador and Escandalosa formations (Figure 1).
A second step in building the geomechanical model is identifying the mechanical stratigraphy. To determine it, a petrophysical model was developed to identify matrix, clay and fluids from surface to reservoir.