The theory and practice of oilwell drilling continue to evolve. Recently, Jose Morales, US technical manager of controlled-pressure drilling and testing services for Weatherford International, did an outstanding job of bringing a room full of SPE and AADE members and guests up to date on the latest thinking in what Weatherford terms “controlled-pressure drilling” or CPD.

His presentation included the IADC UBO committee’s definition of underbalanced drilling: “A drilling activity employing appropriate equipment and controls where the pressure exerted in the well bore is intentionally less than the pore pressure in any part of the exposed formations with the intention of bringing formation fluids to the surface. The hydrostatic head of the fluid may naturally be less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid.

Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids which must be circulated from the well and controlled at surface.”

Morales described managed pressure drilling (MPD) as “an adaptive drilling process used to more precisely control the annular pressure profile throughout the well bore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly. It means that we control the annular pressure profile in such a way that we balance the well at all times.

MPD is intended to avoid continuous influx of formation fluids to the surface. Any flow incidental to the operation will be safely contained using an appropriate process.

“MPD process employs a collection of tools and techniques that may mitigate the risks and costs associated with drilling wells that have narrow downhole environmental limits,” Morales said. “MPD may include control of back pressure, fluid density, fluid rheology, annular fluid level, circulating friction and hole geometry, or combinations thereof. MPD may allow faster corrective action to deal with observed pressure variations. The ability to dynamically control annular pressures facilitates drilling of what might otherwise be economically unattainable prospects.”

The two main subdivisions of MPD are “reactive” and “proactive.” Reactive (or contingency) MPD uses a closed and pressurizable returns system. The technique is effectively on standby as an enhanced form of well control to manage unexpected pressures. Drilling is carried out using conventional casing and fluids programs. Backpressure is applied to control kicks. Drilling continues when circulating out (drilling through kicks) to reduce drilling flat time. HSE (health, safety, environmental) factors are also enhanced.

In proactive MPD, a fluids program is designed around ability to apply back pressure, (nearer-balanced than conventional). The casing program is designed with deeper set points, and it may be possible to eliminate a size. Morales said the technique is used to its maximum effectiveness to mitigate a wide range of drilling hazards.

“Performance drilling” is a proactive technique. “This is normally an air drilling technique to increase penetration rate. In air drilling, the bottomhole pressure is as low as possible to increase drilling performance. The objective of air drilling is to reduce the drilling costs by drilling faster. This is normally achieved by using gas or air as a circulation medium. Reducing the bottomhole circulation pressure significantly increases the penetration rate,” Morales said.

There are other proactive MPD variants. Pressurized mud-cap MPD mitigates extreme losses and reduces associate NPT when drilling highly depleted zones, avoiding well control issues resulting from the inability to maintain a full column of mud in the annulus. Constant bottomhole pressure MPD reduces NPT and enables fewer and deeper casing strings when pore-to-fracture-pressure gradient windows are narrow. Dual gradient MPD enables total well depth in the right hole size in deep-well and deepwater drilling. Returns-flow-control (or HSE) MPD reduces risk to personnel and the environment from drilling fluids and well-control incidents.

Tripping under CPD operations is a challenge, and Morales mentioned several methods: kill the well with heavy weight fluids, use a snubbing unit, flow the well to reduce surface pressures or use a downhole isolation valve (DIV). Morales said the DIV offers a number of benefits:
• No need to kill the well so formation damage is minimized;
• Eliminates time required to circulate kill fluid into and then out of the well;
• Protects against potential swabbing and kick while tripping;
• No fluid loss;
• Eliminates the need for snubbing operations, enhancing safety;
• Pipe can be tripped at conventional tripping speeds, reducing rig-time requirements and improving safety; and
• Allows long, complex assemblies such as whipstocks, slotted liners and expandable sand screens to be installed.

Whatever your flavor — UBD, MPD or CPD — one thing seems clear. This technique is getting mainstreamed. A once undesirable condition is quickly morphing into one of the drilling industry’s most effective tools.

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