Workflow for Sensitivity Analysis and Decision Making on the Lower Limit of the Mud-Weight Window in an Overpressured Formation

When drilling a well it is important to choose a reasonable mud density to ensure safe operations. This is not a trivial task
because a balanced mud-weight has a complex relation to the uncertain formation conditions and properties. The paper presents
a method for conducting sensitivity analysis over the uncertainties involved in the mud-weight window assessment, to learn
the most important input parameters for the mud density. The output from this sensitivity analysis is used to evaluate how the
assimilation of new data can improve our knowledge about the appropriate mud density. This is done in the form of value of
information analysis, coupling the uncertain variables with drilling decisions in a trade off between costs and risks. Additional
well data are valuable when they can materialize in improved decision making.
A case study from an offshore field in the North Sea is presented. The focus is on a shale layer with potential drilling
risk. The results for this case come from a workflow involving the sensitivity analysis of inputs to a wellbore stability analysis
software and the value of information analysis associated with a relevant drilling decision. The most important uncertainties in
this case turn out to be pore pressure, rock strength and horizontal stress. The value of information analysis indicates that data
about the rock’s unconfined compressive strength is the most valuable in this case. Although these results are case specific, the
authors believe that a similar workflow could be useful in other drilling situations.