A Methodology for Drilling Through Shales in Environments with Narrow Mud Weight Window (NMWW)

摘要

Shales, in general, are the major source of wellbore stability problems. A particularly critical drilling scenario is the one that combines deepwater and shallow overburden, typical of ultra deepwater fields in Brazil. This scenario is characterized by high pore fluid pressure, low effective stresses, low fracturing gradients and narrow mud weight windows (NMWW). In order to drill under these conditions, it is necessary to establish means to increase the mud weight window. In this paper, we demonstrate that it is possible to increase the mud weight window by changing conveniently both drilling fluid pressure and salt concentration. A robust computational procedure is established in order to evaluate the size of the plastic zone around the wellbore as a function of the drilling fluid pressure and salt concentration and time of rock exposition to the drilling fluid. The final width of the mud weight window will depend upon the size of the failed zone that can be tolerated by the design.