Managed Pressure Drilling has gained widespread popularity and a great deal of press coverage in recent years. By applying MPD techniques, it is possible to drill holes that simultaneously expose formations with pore pressures very close to the frac pressures of other exposed formations with minimal formation influx or mud losses. Complex and expensive systems have been designed and implemented to maintain pressure on the wellbore using hydraulics modeling software, automated chokes, and continuous surface circulating systems, often working in conjunction with each other. These systems usually require several specially trained operators. This aggregation of personnel and equipment increases both the footprint and housing required for implementation, as well as substantially increasing the cost of the operation.
MPD replaces the pressure exerted by static mud weight with dynamic friction pressure to maintain control of the well without losing returns. The objective of the technique is to maintain wellbore pressure between the pore pressure of the highest pressured formation and the frac pressure of the weakest. This is usually done by drilling with a mud weight whose hydrostatic gradient is less than what is required to balance the highest pore pressure, with the difference made up using dynamic friction while circulating. That sounds quite simple but has been made extremely complicated.
The big problem is maintaining constant wellbore pressure while transitioning between circulating with little or no annular pressure and shut-in with proper annular pressure to maintain balance. A great deal of time and money has gone into trying to do this with no change in wellbore pressure throughout the process. Field experience has shown that this is not necessarily cost effective, demanding an answer to the question, "Is all this complexity really necessary?" The answer in many cases is "No."
As wells are drilled deeper, in deeper water, through additional and more severely depleted intervals, it becomes increasingly advantageous to be able to drill with smaller and smaller differences between pore and frac pressures that are simultaneously exposed to the wellbore. To accomplish this, lower kick and frac tolerances are necessary, as are lower margins between the actual mud weight and the pore pressure equivalent. This allows extending casing points and, in some cases, makes the difference between being able to drill the well to the desired depth or not.
The managed pressure concept is quite simple. So long as the combination of the hydrostatic pressure exerted by the fluid column plus some other pressure is as high as the highest formation pore pressure exposed in the wellbore, then the well will not flow. This other pressure can be either in the form of a circulating friction component of Equivalent Circulating Density (ECD) while drilling or circulating, or surface pressure imposed on the annulus while the rig pumps are shut down to make a connection.
Once an accurate pore pressure is established it is a simple matter with an accurate hydraulics model to determine what circulating friction will be and what mud weight should then be used. With this done, the well can be exactly balanced while either circulating or while shut down.
The first issue that must be addressed is how to go from static balance to dynamic (circulating) balance without either losing returns or taking a kick. This can be done by gradually reducing pump speed while simultaneously closing a surface choke to increase surface annular pressure until the rig pumps are completely stopped and surface pressure on the annulus is such that the formation "sees" the exact same pressure it saw from ECD while circulating. Note that the bottom hole pressure is constant at only one point in the annulus.