Method for determining formation fluid entry into or drilling fluid loss from a borehole using a dynamic annular pressure control system

Abstract

A method for controlling formation pressure during drilling includes pumping a drilling fluid through a drill string in a borehole, out a drill bit at the end of the drill string into an annular space. The drilling fluid is discharged from the annular space proximate the Earth's surface. At least one of a flow rate of the drilling fluid into the borehole and a fluid flow rate out of the annular space is measured. Pressure of the fluid in the annular space proximate the Earth's surface and pressure of the fluid proximate the bottom of the borehole are measured. Pressure of the fluid proximate the bottom of the borehole is estimated using the measured flow rate, annular space pressure and density of the drilling fluid. A warning signal is generated if difference between the estimated pressure and measured pressure exceeds a selected threshold.