Success of Geomechanical Mud Weight Window Optimisation for Drilling Through Challengeable Multiple Depleted Reservoirs

Drilling a new development well in a mature shallow water field, offshore Sarawak, Malaysia, is challenging. Reservoir depletion reduces the fracture gradient to a level such that drilling and cementing operations risk hydraulically fracturing the wellbore. To compound matters, intra-reservoir claystone sections are weak and require higher mud weights to maintain wellbore stability. As a result, the operational mud-weight window becomes very narrow or disappears, and drilling operations will be challenging. A geomechanical study was undertaken for the field to understand the evolution of the fracture gradient due to depletion of the multiple sandstone reservoirs, the drilling risks associated with the large amounts of depletion, and to quantify operational mud-weight windows. The study results showed that a tight drilling mud window exists through many of the depleted reservoirs, but these narrow mud-weight windows could be widened by casing off interim reservoirs that have undergone high depletion. The results were integrated with the managed pressure drilling and the well was successfully drilled. This paper describes the practical geomechanical workflow that assists in understanding the drilling challenges in the depleted reservoir. The integrated geomechanical study and managed pressure drilling practice provide a feasible approach to mitigate drilling risks and minimise drilling costs.