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Weatherford’s ECD reduction tool.
As managed pressure drilling (MPD) continues to evolve alongside its associated tools and measurement equipment, drillers are realizing that various formations and pressure regimes may require additional screening during the MPD planning process. In some scenarios, certain MPD tools might be unnecessary, creating significant cost overruns.
Conversely, some formations may require additional MPD components for added precision, or the application runs the risk of failure because of unmitigated problems.
Selecting the appropriate method of MPD ? constant bottomhole pressure (CBHP), dual gradient, pressurized mudcap (PMCD), or HSE MPD ? is crucial for project success, but variation alone might not adequately set all required parameters. Drillers must also thoroughly understand the level of complexity their drilling project constitutes and then select the proper tools and procedures ? no more, no less ? in order to effectively implement MPD.
Basic MPD
Complexity Level 1. The most elementary level of MPD complexity is all that is required when the operational pressure window is relatively wide and the goal is increased safety. Basic MPD uses only a rotating control device (RCD) and interconnecting piping for directing returns flow.
Applicable areas include regions where high rock strength and low permeability yield low rate of penetration (ROP). ROP increases, ranging from linear to exponential, result from decreased drilling fluid density across such formations.
Accepting lower kick and trip tolerances during MPD applications is necessary along with a lower margin between actual mud weight and pore pressure equivalent. Constant annulus pressure monitoring is typically unnecessary since there is no pressure maintained under the RCD, and the annulus is continually isolated from the rig floor for increased safety. In the event of a kick, the rig floor is not exposed to any flow or hazardous gases while the BOP stack is being activated. Conventional well control methods are typically used to circulate out the kick.
Enhanced kick/loss detection
Complexity Level 2. The next level on the MPD complexity scale is possible due to recent adaptations in flow measurement technology and equipment.
To offset risks involved with decreased margin between pore pressure and mud weight, MPD service companies have introduced “returns flow monitoring” into their product and service lines, either as an optional service or integral to their systems. Adding a flow meter on the return flow provides enhanced (early) kick and loss detection as well as the ability to determine whether flow anomalies are truly kicks/losses or some other phenomena.
Today’s flow meters are durable enough to handle the large volumetric flow seen during drilling and can withstand the presence of solids in the flow stream.
Manual choke MPD
Complexity Level 3. In conventional drilling, wellbore pressure is primarily adjusted by lowering or raising mud weight. Frictional pressure losses can vary significantly depending on circulation rate, mud rheology, and wellbore geometry.
The third complexity level, “Manual MPD,” uses a flow choke on the returns flow path as an additional control point and can be used with or without enhanced kick/loss detection. This provides an easily controlled variable: applied pressure from the choke, or surface backpressure. The annular pressure is described as:
BHCP = P(mw) + AFP + SBP
Where:
• BHCP is bottomhole circulating pressure;
• P(mw) is hydrostatic pressure of the drilling mud;
• AFP is annular friction pressure; and
• SBP is surface backpressure.
During conventional drilling, hydrostatic mud weight must remain above pore pressure equivalent (to avoid an influx) and collapse pressure (to avoid wellbore failure) while remaining below fracture pressure of the open wellbore.
While drilling, manual MPD replaces some of the hydrostatic pressure exerted by mud weight with friction pressure to maintain control of the well without losing returns. The objective is to maintain wellbore pressure between the highest pore pressure and the weakest fracture pressure. This is often accomplished by drilling with a hydrostatic gradient less than what is required to balance the highest pore pressure, with the difference made up using dynamic friction while circulating and SBP while static during connections and trips.
The challenge is maintaining near-constant annular pressure while transitioning between circulating and shut-in to maintain balance. Manual MPD traps pressure at the surface by gradually closing the choke on the returns flow path (until completely closed) while simultaneously reducing the circulation rate to zero (until pump speed is slowed to a stop).
Figure 1 shows an example stepwise SBP and pump rate graph for use when shutting down or bringing up the pumps on a connection. SBP should increase as the circulating rate (pump speed) decreases for a particular situation. To avoid influx, the annular pressure should always exceed the required calculated
pressure.
Automated choke MPD
Complexity Level 4. Differential sticking tendency, wellbore breathing or ballooning, pore pressure regression (depletion), and wellbore instability can demand fine control of the annulus pressure profile based on actual drilling conditions. Pre-drill models generated to predict conditions are verified and updated during MPD with actual measurements. Any differences are analyzed and acted upon as potential problems are identified.
The next MPD complexity level uses an automated control system to manage SBP. Control software uses various data to automatically operate the choke manifold to maintain a computed choke set point. The software communicates with the choke’s programmable logic controller (PLC), which controls a mechanical device that adjusts the choke.
More complex systems monitor, predict, and maintain annular pressure using hydraulics modeling software, automated chokes, and continuous surface circulating systems, sometimes working in conjunction with one another. These systems require specially trained operators.
For basic automation, the operator enters the desired SBP, and the computer and PLC maintain the desired pressure with choke position. As the allowable pressure window decreases, real-time hydraulic simulators can be employed. The simulator makes adjustments as pressure windows are recalculated from actual wellbore and surface measurements and then passes the resultant information to the choke control algorithm.
MPD with enhancements
Complexity Level 5. Recently introduced equipment systems and processes enhance MPD by providing a higher level of predicting, monitoring, and controlling annular pressure, minimizing problem issues and down time. Whereas manual MPD uses trapped SBP to maintain constant bottomhole pressure, enhanced MPD techniques typically use dynamic means to control wellbore pressure.
One way to maintain annular pressure is to use the continuous circulation system (CCS) developed by National Oilwell Varco subsidiary Shaffer. The CCS allows uninterrupted circulation through the wellbore while making connections, alleviating positive and negative pressure surges experienced when drilling conventionally.
The tool enables drilling into narrow pore pressure/fracture gradient windows and can reduce stuck pipe incidents. Ballooning effects are also minimized, as is the likelihood of formation influx. Cuttings transport and removal are also improved.
Another cutting-edge MPD technology is Weatherford’s ECD reduction tool (Figure 2), reportedly in the final stages of field trials, according to Don Hannegan, pressure control strategic manager at Weatherford. The tool, developed in collaboration with BP, is a turbine pump downhole tool that produces a “pressure boost” to the return fluid in the annulus, achieving a dual gradient in the annulus return. It is designed to counter downhole pressure increases caused by friction in the annulus by reducing the equivalent mud weight.
The ECD reduction tool is expected to have application in deepwater drilling (where drillers are historically forced to run several casing strings to reach target depth, therefore progressively reducing the hole size) and extended-reach wells (where the length of the well increases frictional pressure loss, thereby increasing ECD and causing fracturing and/or mud loss).
Planning for success
Selecting the correct MPD variation is paramount, but may not offer all the answers. “Overdoing” or “underdoing” MPD can cause cost overruns or project failure, leading operators to believe MPD failed.
Understanding and applying the proper MPD complexity level can prevent misinterpretations and provide a blueprint for successful MPD.
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