What is Equivalent Circulation Density

Equivalent Circulation Density (ECD) is the dynamic density of circulating mud in a borehole. It is used to calculate the pressure exerted on the borehole wall when the mud is circulated. Any fluid column exerts a pressure equivalent to its hydrostatic head on the walls of its container. Similarly, the drilling fluid also exerts pressure on the borehole, equivalent to the mud column's hydrostatic pressure, and is a function of the mud density. However, the pressure in dynamic conditions is more than in static conditions. This dynamic pressure expressed in terms of equivalent mud weight is called Equivalent Circulation Density (ECD)

The drilling fluid is pumped down the drill string to the bottom of the hole. After exiting the drill string, the mud flows through the annular space between the drill string and the borehole wall. The contact between the drilling mud and the borehole wall while flowing upwards causes drag because of friction. Some pressure is lost to overcome this friction, and the formation absorbs this pressure loss. Hence, the formation sees higher pressure in the circulating than in the static condition. The Equivalent Circulating Density (ECD) is the sum of this pressure loss (converted to density) and the original mud density of the drilling fluid under static conditions.

ECD has a static and a dynamic component. A simple way to calculate ECD is to convert the Annular Pressure Loss (APL) to equivalent mud density and add the original mud weight. ECD =(APL/(0.052xTVD)) + MW, where APL is the pressure loss due to annular friction in lb/sq.in (psi), TVD is ‘True Vertical Depth’ in ft, and MW is the original mud weight in lbs/gal (ppg). In reality, ECD is more than the value derived from this equation. The static component is more than the hydrostatic pressure of the mud column inside the drill string due to suspended drilled solids in the annulus. Similarly, the dynamic component includes Other Pressure Effects (OPE) and APL. OPE includes pressure pulses due to pipe velocity, inertial effects due to string acceleration or deceleration, and pressure to break mud gelation.