Managed pressure and underbalanced drilling operations are predominantly conducted through the use of surface measurements and are heavily assisted by modeling subsurface conditions as downhole measurements have limited availability. Downhole information is typically absent during periods without flow and when using compressible fluids such as aerated mud.

Operational efficiency and wellsite safety would benefit by supplementing the models and surface data with direct measurements of subsurface conditions. Automation would be enabled and well control would meaningfully improve from self-sufficient downhole information by realizing the lowest pressures, while eliminating complex calculations and minimizing time spent on the choke.

Along-string annular pressure measurement and evaluation provides new downhole capabilities. Initial field deployments have demonstrated their utility and viability, but downhole ECD calculations through normalization of the absolute pressure measurements to hydrostatic depth do not properly account the distributed pressure sensing.

This paper proposes a new method for computing the along-string ECD values*, shows visualization for intuitive interpretation of the values and proposes a method for relative volume determination*. The new methodology with multiple sensors with non-hydrostatic pressure additions scaled by sensor depths show the need for two scale factors. Finally, an influx volume can be estimated across a wellbore section that is bounded by discrete drillstring pressure sensors. Examples are offered for pumping sweeps and a kick that is up the annulus and recommendations are offered for technologies that acquire pressure along the drillstring.

Managed Pressure Drilling (MPD), Under Balanced Drilling (UBD) and other operations that manage a tight pressure window benefit from the procedures and practices offered. Recommendations relevant to integration of downhole data with surface control systems for closed-loop and automated wellsite operations are offered.

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