Objectives/Scope: Sidetrack drilling in depleted unconsolidated sandstone reservoirs with overlying reactive shale faces many engineering and operation challenges. Depleted reservoirs require a low mud weight to avoid stuck pipe due to differential sticking. Reactive unstable shale requires a higher mud weight to provide hole stability. Installing sand screens across the reservoir adds additional complication to drilling operations.
Proper planning is key to overcoming drilling challenges. Planning includes selection of well design, directional trajectory planning and mud selection. The selection of casing design depends on reservoir pressure and logging requirements. The requirement for a dual lateral completion also affects the hole configuration. The optimum kickoff point, maximum dog leg severity, inclination across the shale zone, shale footage and trajectory azimuth play important roles in the success of sidetracking operations. Mud weight and mud rheology are thoroughly planned and closely monitored during the operations. Drilling with a rotary steerable Bottom Hole Assembly (BHA) and deploying sand screen completion, such that it reduces chances of differential sticking, are also critical parts of the planning.
Many wells have been successfully sidetracked and completed to meet reservoir management objectives. The casing design of reentry wells has evolved from two (2) hole sections to one (1) hole section and has saved 32% of the drilling time and has reduced the well cost by 23%. Appropriate well direction (azimuth) and correct mud weight related to maximum or minimum stress direction have been identified utilizing special geomechanics software. The salinity of an oil-based mud (OBM) is critical while drilling through reactive shale. Water activity tests on shale samples is used to design the proper OBM and special monitoring protocols were established to ensure the rheology of the OBM. A modified sand screen completion has been proven to reduce the potential occurrence of stuck pipe.
Most of key factors in planning can be applied in all wells to some extent. Each well is uniquely based on its subsurface targets and reservoir pressure.