An enhanced approach to real-time pore pressure prediction for optimized pressure management while drilling

Abstract

In many of today's high cost offshore areas, overpressured zones present significant challenges to optimizing the drilling process. Operators face a number of costly risks, including reduced rate of penetration (ROP); formation damage due to high mud weight; and health, safety, and environmental (HSE) risks associated with loss of well control. Operators commonly rely on logging while drilling (LWD) resistivity data and an overburden gradient to identify overpressured zones. The overburden pressure gradient is usually derived from offset density/acoustic data, seismic velocities, or regional overburden tables adjusted for water depth and air gap. One drawback of using only the resistivity-based approach is that formation resistivity is directly dependent on formation water salinity, not pore pressure. When drilling in close proximity to salt, operators have to take into account what effect this will have on water salinity. In such cases it would be better to use acoustic data due to its insensitivity to salinity changes. However, the real-time data quality of previous generation LWD sonic tools limited their use for pore pressure predictions. However, new LWD instrumentation in acoustic logging and pressure testing improves the reliability of real-time pore pressure predictions and ultimately improves the drilling process and reduces HSE risks. This article reviews a proactive approach for predicting pore pressure based on state-of-the-art LWD acoustic and formation pressure tools, in addition to discussing the application of the seismic data acquired while drilling. In order to drill safely and efficiently, the mud weight or equivalent circulating density (ECD) has to be adjusted to: stabilize wellbore/provide cuttings removal; avoid kicks/shallow flows; avoid lost circulation/fluid losses; avoid additional casing strings; increase rig safety; minimize formation damage; and optimize ROP. The task of managing the ECD becomes more demanding in areas known to have uncertain pressure versus depth trends, such as the Gulf of …