Managing Drilling Risk Using an Integrated Approach to Real-Time Pore Pressure Prediction

Abstract

Pore pressure and wellbore stability estimation in real-time has become a mandatory service in any situation where drilling hazards are expected and particularly in the cases of deep water, exploration, geologically complex and extended reach settings. The basic workflow assumes that under-compaction is the primary cause of abnormal pore pressure generation (Eaton 1975), where an increase in formation pressure is associated with a response in sonic, density and resistivity logs, and models are defined based on this assumption. However, structural deformities, dipping beds or alternate pressure mechanisms could result in a wrong prediction of pore pressure based on what is observed in the shales above, or what is predicted from the standard models. Besides increasing the Eaton exponent or creating different normal compaction trend lines (Malinverno et al., 2004; Sayers et al., 2006) or using an unloading parameter (Bowers 1995), there is the need to define an integrated approach using multiple data sources to construct and constrict the model. Advanced mud gas interpretation and cavings (pieces of rock not drilled by bit or reamers) analysis have become important tools in real time pore pressure and wellbore stability monitoring and are used to determine the approach to a structural deformity (faults/fractures) or the presence of elevated pressures at the crest of a permeable formation, either due to lateral transfer or gas buoyancy. Analysis of cavings shape, structure and mode of generation helps to determine the current borehole condition and whether the mud weight (MW) needs to be raised to control the problem, or just modifying the drilling parameters would suffice. The presence of connection gas peaks aids the pore pressure analyst to estimate the pore pressure across a permeable formation by associating its magnitude and its relationship to dynamic (equivalent circulating density - ECD) and static (equivalent static density - ESD) environments. The use of Managed Pressure Drilling (MPD) to maintain a constant backpressure across the annulus, negates this fluctuating static to dynamic environment and hence affects the use of mud gas behavior to determine if the prevailing MW column is sufficient to provide static overbalance, but workflows to address this issue have been defined over the years. Although, the industry is currently beginning to use these secondary indicators into their workflows, there is no standard that incorporates these sources into a single, cohesive workflow. This paper presents an integrated approach to pore pressure prediction and managing drilling risk by incorporating multiple sources of information beyond classical log-based techniques. It demonstrates the value of advanced mud gas interpretation, drilling mechanics interpretation, cavings and drilling parameter analysis to optimize the pore pressure model in real-time and enhance the traditional techniques.