About Us Technology Applications HS&E News & Info Contact Us

Managed Pressure Drilling

...MPD Pressure Control
...MPD Open Circulation System
...MPD Closed Circulation System
...MPD Operational Considerations

Open Circulation System
To understand pressure limits encountered while drilling and the role they play in pressure control consider the simple but fundamental pressure equation:

BHPdynamic = Pstatic + Pafp

where Pstatic is the hydrostatic pressure exerted by the drilling fluid when the rig pumps are off and Pafp is the annulus frictional pressure (AFP) created by the circulating drilling fluid.

This equation applies when the rig mud pumps are on and mud is circulating. The term BHPdynamic is also referred to as the effective circulating density (ECD) and sometimes as equivalent mud weight (EMW) or mud weight equivalent (MWE). Normally when an equivalent mud weight is used the units of pressure will be in pounds per gallon, ppg, or specific gravity, SG.

When the pumps are off or when circulation stops Pafp = 0 and the fundamental pressure equation becomes

BHPstatic = Pstatic

These equations represent the dynamic and static BHP, respectively in an open circulation system. (see below).

An open circulation system -

...is one in which the drilling fluid flows out of the wellhead through surface piping open to atmospheric pressure.

In many drilling operations the formation pore pressure represents the lower boundary for the BHP and the minimum that has to be maintained to avoid influx, kicks, and subsequent losses.

The minimum pressure boundary for well control may be dictated by wellbore stability pressure not pore pressure. Wellbore stability tends to be a more complex pressure phenomenon than pore pressure because it is a function of the magnitude and direction of the maximum horizontal stress, σmax, well orientation relative to σmax, well inclination, drilling fluid rheology and in particular its density, pore pressure, rock porosity and permeability, as well as pump rate, rotary speed, and rate of penetration.

Generally, the wellbore stability pressure is greater than the pore pressure. The difference can be as small as 0.2 - 0.5 ppg EMW or as much as 2.5 - 3.0 ppg. As the drilling fluid circulates the additional frictional pressure will reduce the margin of safety with the upper limits: differential sticking, lost circulation, and fracture gradient. This is a particularly challenging situation in depleted fields with reduced fracture gradients and fields with fractured carbonates.

The risk of wellbore instability is heightened by the open system itself. Over time a circulating drilling fluid can charge an annular volume of rock near the borehole with ECD pressure. The depth of charging is limited only by the permeability and porosity of the rock itself and the degree to which the drilling fluid plugs the porosity channels at the borehole wall.

When circulation stops the charged volume relaxes and the BHP reverts to BHPstatic which will be lower than the pressure of the charged rock. This cycle of charging and relaxation is sometimes referred to as ballooning and occurs every time the mud pumps are started and stopped. It stresses the rock, induces wellbore fatigue, and ultimately leads to tensile failure.

In an open system under conventional drilling condiitions there is no way to maintain the ECD constant. The inability to maintain a constant ECD in an open system limits drilling to control pressure with static mud weight and annular friction.

©2007 At Balance Americas LLC. All rights reserved.