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

Abstract

Summary 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. This paper presents a method for conducting sensitivity analysis (SA) over the uncertainties involved in the mud-weight window (MWW) assessment, to learn the most important input parameters for the mud density. The uncertainties in the parameters are taken into account using predrill realizations of the formation conditions (vertical stress, minimum horizontal stress, pore pressure, and temperature) and properties (unconfined compressive strength, tensile strength, friction angle, and porosity). The limit pressures of the MWW are computed to avoid shear and tensile failure. The software used to study well stability returns a mud window that could enable underbalanced drilling while still avoiding borehole collapse; this is emphasized by equating the lower stability limit with shear failure and not pore pressure. However, drilling below the pore-pressure limit entails risks with regard to kicks or blowouts. The Mohr-Coulomb failure criterion is used, but other possible shear failure criteria are discussed as well. The output from the SA is used to evaluate how the possibility of gathering more information about the most relevant parameters can improve our knowledge about the appropriate mud density. This is done in the form of value of information (VOI) analysis, coupling the uncertain variables with drilling decisions in a tradeoff between costs and risks. The interest is on deciding what information to acquire when, at a given depth, the drilling operator must decide on whether to keep drilling or set casing. Additional well data are valuable when they can materialize in improved decision making. The case study presented is from an overpressured shale layer in a field in the North Sea. The focus is on the lower allowed well pressure of the mud-weight window for a vertical well, exploring the possibility of underbalanced drilling. The SA shows that the most important uncertainties in this case are pore pressure, unconfined compressive strength, and minimum horizontal stress. The most valuable information-gathering strategy is to observe these main three parameters, which is worth USD 386,980 when acquisition costs are subtracted. The VOI analysis further indicates that data about the rock's unconfined compressive strength are the most valuable in this case, and if the decision maker can afford to acquire only one data type, this should be chosen. Although these results are case specific, we believe that a similar workflow could be useful in other drilling situations.