This paper presents a case study on the role of geomechanics to identify possible failure mechanisms for non-productive time (NPT), avoid drilling risks and minimize costs in a field development drilling campaign, Offshore Sarawak Malaysia. Drilling optimization and reducing NPT for the drilling campaign was one of the key focus for maintaining the drilling time and costs. Drilling of moderately to highly deviated wells in this field has proven to be extremely challenging. Numerous lost-time incidents due to tight hole, stuck pipe, pack-off, casing held up were experienced, particularly when drilling through the shallow overburden shales and deeper reservoirs interbedded with shales and coals. Faced with continually increased NPTs, a geomechanical model was developed using regional offset wells to understand the mechanism of failures. A geomechanical model was developed to quantify the minimum recommended mud weights and optimize the wellbore trajectories. The outcome of this study was used as key input for casing and mud design. The in-situ stress state derived from field wide geomechanical model indicates the field is associated with a normal faulting stress regime, i.e., Shmin < SHmax < SV.
The presence of relatively weak rocks means the field is potentially subject to stress-induced wellbore instability problems. However, observations of numerous time-dependent failures imply secondary influences must also be considered to arrive at possible remediation strategies. It was observed that the combination of weak rocks and numerous time-dependent failures using different types of mud system have contributed to wellbore stability problems. The wellbore stability is due to reactive shale, which is time sensitive as majority of the drilling problems are observed after drilling. The major contributor to the time-dependent deterioration process is mechanical and chemical imbalances between shale and drilling fluids compounded by large open-hole exposure area and contact time resulting in rising pore pressure caused by the invasion of drilling fluid into the formations, and then exacerbated by less-than-optimal drilling practices. This finding, together with improved geomechanical understanding of the field helped to evaluate the safe mud weight windows, formulate the mud designs and optimize drilling practices. All the planned wells were drilled successfully without any loss time incidents and non-productive time. This paper presents an integrated approach and workflow that combines the drilling data and formation response to identify the most likely causative mechanisms of the time-delayed wellbore instabilities in a mature field. This knowledge was then used to develop strategies for optimizing future drilling operations in the field.