When building a 1D geomechanical model for the Rang Dong field in Cuulong basin, offshore Vietnam, the rock structure was found to be very complex. In addition to inter-bedded shale and sand sequences in the overburden, there is an overpressured stratum immediately below and above the normally-pressured and highly fractured sandstones and volcanic rocks at the target depth. During the drilling of offset wells, multiple tight spots were experienced in the overburden. An inflow was detected in the volcanic rocks, and consequently the mud weight was increased in the deeper section of overpressured zone. Further, severe mud losses occurred in the underlying normally-pressured and highly fractured sandstones, and eventually stuck pipe was experienced. This study shows the development of a field-wide geomechanical model that is able to match all the drilling experiences in the offset wells.
A geomechanical model was developed for the field in which in-situ stresses and rock mechanical properties were calibrated with field data and lab measurements. The model predicted different types of borehole failures such as breakouts and drilling-induced fractures that contributed to different types of wellbore instability events in existing wells. Analysis showed that drilling with a significantly high overbalance pressure in the fractured zone increased the risk of tool string differential sticking and stuck pipe. To investigate the root causes of dynamic mud losses in the fractured intervals with major intersecting faults, a Mohr-Coulomb friction sliding criterion was applied. The analysis showed that the majority of natural fractures and faults, which are oriented in the north-east and south-west (NE-SW) direction, are critically stressed and may become conduits for drilling fluid and cause mud losses.
The geomechanical model was used to design a mud weight program for an upcoming well. This program was optimized to mitigate the issues experienced during drilling of the offset wells. The results of this study led to an optimized mud program to minimize shale breakouts in the overburden, control the risk of kicks in the overpressured zone, and reduce the risk of mud losses in the fractured formation. Additionally, an optimized casing program and the use of lower mud weight were recommended, with the addition of loss circulation material (LCM), to drill a highly deviated wellbore within the fractured formation. The drilling campaign of the planned well was successful with good drilling practices such as equivalent circulating density (ECD) control, good hole cleaning and use of the recommended mud weight programs and optimised drilling fluids suggested by this study.