The Alpha Field located in Sarawak basin, Offshore Malaysia has been classified as a “high risk” drilling environment due to its clastic deposition and unstable coal formations. These formations are characterized by high depletion zones and narrow drilling windows, which exponentially increase the risk of experiencing severe fluid losses. Mud density represents another challenge for operators in this region. Maximum mud density predicted by formation integrity tests (FITs), sufficient to maintaining wellbore stability, typically exceeds the fracture gradient of the clastic and coal formation. Accordingly, the operator must determine the proper mud weight to avoid induced losses while maintaining well control and mitigating issues related to wellbore instability. Excessive mud weight could lead to coal destabilization, while too-low mud weight could lead to borehole collapse. Considering these challenges, it is typical to experience non-productive time (NPT) and multiple sidetracks in South China Sea wells.
While drilling Offset-1 Well, the operator experienced wellbore instability issues in the 12¼-in. section, adding $5.7 million to the total well cost. After analyzing the source of these problems, the operator determined insufficient fracture bridging material while drilling the coal formation as one of the three main contributing factors.
Following extensive lab work, offset well analysis and literature study on similar coal drilling practice, a 10.8 lb/gal synthetic-based mud (SBM) was formulated using FLC 2000 as a bridging material and wellbore stabilizer. The low-permeability barrier formed by the addition of FLC 2000 in the drilling program temporarily sealed pore throats while improving the mechanical protection to depleted and fragile formations. This formulation also prevented the initiation and propagation of fractures in the formation and significantly reduced the risk of differential sticking, lowering NPT and total well costs.
Mud weight for the 12¼-in. section was increased from 10.2 to 10.8 lb/gal prior to entering the coal formation to improve wellbore stability. FLC 2000 was maintained in the active system at 8 lb/bbl throughout the entire coal formation interval. This interval was successfully drilled with no fluid losses.
The operator successfully drilled two wells, Alpha-1 and Alpha-2, 23 days ahead of schedule with zero NPT and wellbore instability events. The operator indicated this experience as a record for the area. FLC 2000 reduced fluid invasion and stuck pipe events in the 329-ft. coal interval. FLC 2000 also provided wellbore stability and enabled the operator to drill with mud weights higher than the maximum density predicted in FITs without encountering losses while drilling the problematic formations. The addition of FLC 2000 to the system had no adverse effect on drilling fluid rheology, and facilitated logging operations while accomplishing a less costly producing well.