Publications

Drilling in an exploration area is challenging due to limited geological information which is crucial for well planning and drilling optimization. This is especially true in a carbonate field because they tend to have a more complex geology. Constructing a geomechanical model for the planned well location and conducting a wellbore stability analysis for the planned well can help to optimize the drilling plan and achieve the well objectives by minimising non-productive time. This paper presents a case study showing the importance of geomechanical modelling in carbonate reservoirs in the Y field, Makassar Strait, Offshore Indonesia. The regional stratigraphy is composed of shallow and deeper carbonates with interbedded layers of shale. The carbonate rock is mainly wackestone with common coral fragments. These rocks have high porosity and are the main reservoir unit for the target well. 

 

Geomechanical modelling involves constraining the principal stress magnitudes and orientations, the pore pressure and the rock mechanical properties using a suite of drilling, engineering, geological and geophysical data. Drilling experiences from daily drilling reports were used to evaluate and understand relevant geomechanical and pore pressure events that were input into the geomechanical model. Daily drilling reports from the offset well show problems such as gains, pack off and tight hole events and stuck pipe occurred. The presence of shallow carbonates raised the potential issue of shallow gas hazards in the planned well which needed to be covered in the wellbore stability analysis. Since there was only one nearby offset well, some regional data (core tests to calibrate the rock mechanical properties) were used to supplement the geomechanical model. The final geomechanical model was calibrated using events recorded while drilling and also evidence of stress-induced failure observed in the image log.  

 

The planned well location is about 5 km from the offset study well. The calibrated geomechanical model was used to perform a wellbore stability analysis for the planned well. Seismic velocity data were used for modelling the pore pressure. Three cases of pore pressure were considered to account for the limited available input data and the uncertainty in the input parameters. These three cases were considered in the wellbore stability analysis and resultant mud weight design. 

 

The planned exploration well was drilled using the mid case as guidance. The mud weight recommendations and operational recommendations were all followed meticulously. The exploration well was drilled safely and successfully reached total depth by meeting all

 

 exploration objectives. There was not any non-productive time due to wellbore instability. The integrated efforts by the multi-disciplinary team from pre-planning to execution made this exploration campaign a success.