In around 340 BC, Plato wrote in The Republic, "Necessity, who is the mother of invention." Such is the case of a relatively new offshore drilling technology - Managed Pressure Drilling (MPD), a technology that addresses a litany of issues associated with drilling into "trouble zones".
Drilling-related issues such as excessive mud cost, differentially stuck pipe and resulting well control situations associated with loss circulation issues contribute to defining the necessity for a more efficient drilling technology. A kick-Loss scenario, which may occur when drilling into narrow downhole pressure environments, is another. Drilling-related flat time further indicate a necessity for a technology that enables more precise wellbore pressure management when drilling in marine environments.
These challenges to economic drill-ability apply to a growing percentage of offshore prospects for oil and gas, becoming more troublesome and costly with water depth. These challenges have one thing in common - they all may be addressed to some degree or other with more precise wellbore pressure management. Dictated: A technology that offers more precise pressure management with fewer interruptions to drilling ahead.
Many thousands of onshore drilling programs have proven that a closed and pressurizable mud returns system enables more precise wellbore pressure management. A conventional mud returns system typically consists of an open-to-the-atmosphere drilling or bell nipple and where returns gravity flow away from the rig floor. At the essence of MPD technology is the ability to drill ahead with a closed and pressurizable mud returns, e.g., a Rotating Control Device (RCD) and drilling choke. The RCD contains and/or diverts pressurized mud returns to the choke manifold and its Bearing and Drill String Seal assembly permits drilling ahead, tripping, etc. The RCD, choke, perhaps a drill string float and other specialized tools that may have been developed primarily for the practice of Underbalanced drilling (UBD) have unique application to MPD.
However, unlike UBD, the objective of MPD is not to invite hydrocarbon influx but to contain/control/divert any that may be incidental to the operation. It may be said that the primary objective of UBD is to avoid damage to a pay zones ability to produce. UBD is reservoir-issue related. A primary objective of MPD is to address a litany of drilling-related problems or barriers to economic drill-ability. MPD is drilling-issue related.
Most but not all variations of MPD rely on an ability to apply a desired amount of surface backpressure to the mud returns.
In conventional drilling, bottom hole pressure (BHP) is determined by the sum of the mud weight (MW) hydrostatic and circulating friction pressure. The latter, also known as equivalent circulating density (ECD), is present when drilling ahead (rigs mud pumps on) and ceases to exist when shut in to make jointed pipe connections (mud pumps off). Virtually as if by definition, one can readily see why drilling within narrow formation pore pressure and fracture gradient margins, kick-loss scenarios often occur. After a shut in to make a jointed pipe connection, the same may be said when the bottomhole assembly is at a depth where the BPH is near the fracture gradient. Starting the rigs mud pumps to regain circulation increases circulating friction pressure. This additive often results in the BHP exceeding the fracture gradient. Mud losses, differential sticking, and perhaps a twist off results...again, almost as by definition. With the weight of the mud in the hole at the time, the only adjustment that can easily be made to BHP is - pumping rate, pumps on or pumps off.
When practicing MPD with a closed and pressurizable mud returns system, another important variable can be added to the equation for determining BHP. That variable is the ability to apply a desired amount of backpressure. When drilling with an essentially incompressible fluid, surface backpressure adjustments result in almost an instant change in BHP.
In the U.S., drilling with a closed and pressurizable mud returns systems has been an evolving technique on land drilling programs the last two decades. Less drilling flat time, less cost and enhanced well control have been the key drivers for land drilling programs to perfect the technique.