Wellbore Stability in the Deepwater Bijupira & Salema Fields, Offshore Brazil - A Probabilistic Approach.
Authors: Ritter D.G. , Grollimund B. , Zoller S.L. , Brecknock David 
Published: October 2008

The Bijupira-Salema Field was initially discovered and appraised by Petrobras. During this initial appraisal phase Petrobras drilled several vertical wells, in which wellbore stability problems were encountered within weak shales and conglomerates above the reservoir. Enterprise proposed the construction of a geomechanical model of the field for the purpose of predicting wellbore stability prior to drilling the first horizontal development well.

Due to the sparse data set from the previous wells, a probabilistic approach was used to define the initial geomechanical model and to determine the mud weight requirements and casing setting depths. With this approach, the uncertainties in the geomechanical parameters, such as pore pressure, overburden, horizontal stresses and rock strength are considered through a Monte Carlo simulation to assess the probability of success for the planned well design. The planned casing design, which omitted a 20" casing string, was confirmed as viable. Furthermore, the probability analysis showed that the mud weights for problem free drilling were higher than the previously used mud weights.

The initial well results confirmed the predicted model. Three additional horizontal wells were planned and drilled without encountering any borehole stability problems. The stable wellbores enabled improved data acquisition and completions.

Background

Petrobras discovered the Bijupira and Salema fields in 1989. Several appraisal wells were drilled at the time of discovery, three of which were completed and put on pilot production for seven years. Enterprise Oil was awarded the license to develop both fields in 1999. Drilling began at end of July 2001 with a pilot hole of well BJ-U, followed by the main hole of well BJU, which was sidetracked out of the pilot hole. The planned development by Enterprise Oil entails a total of ten producers and six injectors from two drilling clusters. (Figure 1). Production flowlines will connect the two production manifolds to one FPSO. The majority of the wells will have a horizontal tail with a length of 300-1000 meters. The heel and toe of these wells are set with regard to accommodate production and geological constraints. These result in several non-traditional well paths.

Based on the problems encountered in the previously drilled vertical wells, especially in the shales and conglomerates (the Pebbly Eocene) above the reservoir, the possibility of severe wellbore stability problems in highly deviated wells was recognized by Enterprise Oil. Specifically addressing the stability of the 12 Â¼-inch hole section, which will be drilled through the Pebbly Eocene and into the reservoir in all planned wells, was deemed crucial. The only offset well in which the 12 Â¼-inch casing was able to be set below the Pebbly conglomerate was the RSJ-471 well, a straight appraisal well, and it is this well that the current well design emulates.

Regional Tectonic Setting

Figure 2 shows the location of the Bijupira-Salema fields (red circle), located on the shelf edge off the coast of Brazil. Figure 2 also shows information on the local orientation of the maximum horizontal stress from observations of wellbore failure and from earthquake data.