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Evaluating Mud Weight Options in Managed Pressure Drilling for Improved Risk Management and Maximised Drilling Advantage


 
 
 The objective of this paper is to highlight the fact that while in conventional drilling there can sometimes be no mud weight solution for drilling a particular narrow margin section without either exceeding the Leak Off Test value at the shoe, or falling below the Pore Pressure at section TD. In Managed Pressure Drilling (MPD) there is often a range of mud weight solutions that can be used to drill the section, but usually one optimum mud weight that should be used, based on different risk criteria that can be evaluated. The consequences of this are that when using MPD, it could be that the section risks have not been minimised and therefore more risk than necessary has been imported into the methodology. Whereas if all MPD mud weight solutions have been evaluated, together with their associated Surface Back Pressures, and the optimum selection made, then the mitigations can be specially tailored so that the remedial actions to any system failure are clearly planned in advance, reducing the overall risk level of the operation.
 
 
 
 The methodology described in this paper demonstrates the process for choosing the optimal mud weight for drilling any well section using MPD, with worked examples. This process is especially applicable for drilling very narrow margin sections, for example with only 0.2 ppg window between Pore Pressure and Fracture Gradient. By enumerating the safety margins both at the previous casing shoe and at the proposed section TD, or any other point of interest, it is possible to rank the risks of kicks and losses in that section across a range of proposed mud weights and use this information to choose the optimal mud weight.
 
 
 
 The process of evaluating the options and outcomes of using different mud weights in MPD can not only lead to the best drilling solution for the section, but can also be used as a discussion tool between the drilling team and the subsurface team, to help elucidate the most likely risks to the operation and thereby mitigate those risks in the most appropriate way.
 
 
 
 A further benefit of this approach is that the narrowest possible drilling windows can also be evaluated and as a result, options for extending TD and potentially eliminating casing strings can be considered, leading to considerable savings, which are highly prized at all times, but especially so in a low oil price environment.
