ECD (equivalent circulating density) is the dynamic density exerted by a circulating mud at any point in the borehole. The equivalent circulating density or dynamic density at any time during fluid flow is always greater than the static mud density when flow is stopped.
To understand why this is so, consider this; the mud pumps supply the pressure that forces drilling mud down the drillstring to the bottom of the hole and up again to the surface. As the drilling mud exits the bit nozzles, they have to flow through the annular space between the drillstring and the borehole wall.
Contact is made between the drilling mud and the borehole wall as drilling mud flows upwards to the surface, this contact creates some form of “drag” as a result of friction and the drilling mud loses some of the pressure supplied by the pump in other to overcome this frictional drag due to contact.
This pressure loss is absorbed by the formation. So the equivalent circulating density (ECD) is the sum of this pressure loss (converted to density) and the original mud density of the drilling mud under static conditions. This is why the ECD is always greater than the static mud density because it also includes the pressure loss converted to its density equivalent.
ECD and Fracture Gradient
The static density is the mud weight when the fluid is not flowing such as when the pumps are shut down whereas equivalent circulating density refers to the mud density when pumps are started and fluids start flowing out of the borehole. Obviously, the greater the pressure loss from the drilling mud due to frictional drag, the higher the ECD will be.
The drilling mud always exerts pressure on the borehole. This hydrostatic pressure from the mud is a function of the mud density. The aim is to always keep the hydrostatic pressure within safe limits such that it is not too high and ends up exerting a pressure that fractures underground formations. Inducing fracture this way is undesirable and can result in loss circulation or cause a kick.
However, a mud whose density is lower than that needed to fracture the formation can still end up inducing formation fracture. The drilling mud may perform fine when the pumps are down but that same mud may fracture the formation as soon as the pumps are restarted and flow is induced. This is because under flow conditions the ECD (dynamic density) could exert a higher pressure which is greater than the pressure needed to induce formation fracture.
Factors Influencing ECD
The higher the frictional drag between the mud and the borehole, the higher the pressure drop in the mud and the higher the ECD. The equivalent circulating density (ECD) also depends on the true vertical depth (TVD) of the well. The greater the vertical distance through which the drilling mud has to travel until it gets to the surface, the higher the pressure drop and ECD.
Note that, ECD is a function of the true vertical depth and not the measured depth. Measured depth includes the horizontal section in deviated wells but ECD only depends on the true “vertical” depth.