Managed-pressure drilling (MPD) technology is a collection of techniques that has been evolving on land drilling programs over the past couple of decades. At the root of the technology is the ability to drill with a closed and pressurizable mud returns system and to drill ahead and to make jointed pipe connections with surface backpressure applied to the annulus of the well bore. The evolution of MPD technology is laced with tight-hole secrets, and drilling consultants' pet techniques to overcome conventional drilling-related challenges and enabled by the continuing development of rotating control devices (RCD).
However, it's been only recently that this litany of methods has been defined as a technology within itself. The IADC underbalanced operations (UBO) and MPD Committee has defined MPD as follows: "MPD is 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." Technical notes include the following:
• MPD processes employ a collection of tools and techniques which may mitigate the risks and costs associated with drilling wells that have narrow downhole environment limits by proactively managing the annular hydraulic pressure profile.
• MPD may include control of backpressure, 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.
• MPD techniques may be used to avoid formation influx. Any flow incidental to the operation will be safely contained using an appropriate process.
Applications in mature fields
Development drilling in brownfields is one of the more challenging opportunities the industry faces today. Much of our remaining reserves are in fields that have been produced for many years. The easier prospects have likely already been drilled and produced. Production from new wells in mature fields often is destined to be much less than first drilled. Therefore, new wells must be drilled in a most cost-effective manner to be economically viable.
These prospects are "hydraulically challenged" because it is often a requirement to drill through production zones which no longer possess their virgin pore pressure. With the mud in the hole at the time, these zones of depleted pressure often result in lost circulation of drilling fluids. This causes excessive mud costs and higher risks of differentially stuck pipe and twist-offs. It is also not uncommon to experience kicks (influx of hydrocarbons) within that same open hole from "energized stringers of near-virgin pore pressure."
If mud density is reduced to avoid exceeding the formation fracture pressure and experiencing lost circulation at one depth with depleted pressure, kicks may occur a stand of pipe or two further down if stringers of abnormal pressure are encountered. This is the classic "kick-loss" scenario that results in drilling non-productive time, excessive mud cost and, quite frequently, well control issues.
Technically, we are describing a narrow downhole pressure environment window. Such "narrow margins" are common to brownfield drilling operations. To drill efficiently within narrow pressure environments, a more constant bottomhole pressure is required, whether circulating or shut-in to make jointed pipe connections.
Constant bottomhole pressure
The requirement, then, is to adopt a technology that literally permits "walking the line" between formation pore pressure and fracture gradient, doing so with the mud in the hole at the time and with less interruptions to drilling ahead. We've just described the strengths of the constant bottomhole pressure (CBHP) variation of MPD.
Many drilling and wellbore stability-related issues stem from the significant fluctuations in bottomhole pressure that are inherent to conventional drilling practices. These fluctuations in bottomhole pressure are root causes of a litany of excessive costs to a conventional land-drilling program. Such pressure "spikes" are caused by stopping and starting of circulation for drillstring connections in jointed-pipe operations. Specifically, they result from a change in equivalent circulating density (ECD) or annulus friction pressure (AFP), which occur when the pumps are turned on and off. The AFP additive to bottomhole pressure is present when circulating and absent when not circulating.
Therefore, in conventional operations the only way to change the effective bottomhole pressure with mud in the hole is to start, stop or adjust mud pump rate.
In conventional drilling, bottomhole pressure is determined by the sum of:
Mud Weight Hydrostatic + Circulating Annulus Friction Pressure (ECD)
The CBHP variation of MPD brings a valuable and significant additional variable to the equation. That variable is the ability to apply backpressure as needed on the annulus at the surface. Now bottomhole pressure may be determined by:
Mud Weight Hydrostatic + Circulating Annulus Friction Pressure + Backpressure
How does this technique achieve a more constant bottomhole pressure whether drilling ahead (circulating) or shut-in (not circulating) to make jointed-pipe connections? First, the fluids program focuses upon a slightly less dense drilling fluid than conventional wisdom would dictate. The intent is not to invite the well to flow, as in underbalanced drilling (UBD), but to safely contain any influx of formation fluids that may be incidental to the operation. When circulating and when annulus friction pressure is an additive to the mud weight hydrostatic, no surface backpressure is required. When not circulating and when only the mud weight hydrostatic pressure determines the bottomhole pressure, the well may be hydrostatically underbalanced. However, to prevent an influx of formation fluids during connections, surface backpressure in an amount relatively equal to the most recently experienced circulating annulus friction pressure drop is applied. In most cases, the amount of surface backpressure required to effectively overbalance the well bore during connections is less than 500 psi.
Industry experience with this method has indicated that in most applications of this variation of MPD, fewer hydrocarbons are produced to the surface during the drilling process than would likely occur
if the well was drilled conventionally with an open-to-atmosphere mud returns system, e.g., drilling nipple. Although some of the tools developed for the safe practice of UBD are applicable to MPD, the intent is to discourage, not invite, influx as in UBD. UBD has a reservoir focus, avoiding damage to the well's ultimate productivity. MPD has a drilling-related focus, getting to total depth faster, cheaper and with enhanced control of the well.
Figure 1 illustrates how the CBHP variation of MPD achieves a constant bottomhole pressure compared with the bottomhole pressure fluctuations common to conventional drilling.
In its common and simplest configuration, surface backpressure may be applied to the annulus by the use of three proven and relatively low-cost tools:
• Drill string non-return valves (floats);
• RCD, also known as a rotating control head; and
• a dedicated choke manifold.
For reference, Figure 2 is from the recently published specifications for RCD and speaks to those of passive stripper rubber design. Mounted on the head of a blowout preventer stack, as for land and shallowwater rigs, this illustrates why the tool is most commonly referred to as a rotating control head.
Figure 3 compares conventional drilling to the CBHP variation of MPD. The MPD chart illustrates the required orchestration of rig mud pump rates and the amount of surface required to achieve a constant effective bottomhole pressure.
Reactive and proactive categories
Applying surface backpressure during connections is not new. This technique has been evolving in land drilling programs over the past two decades. The concept of applying annulus backpressure when making jointed-pipe connections is certainly not new to many land drilling decision-makers.
However, most of the past applications of this technique have been for more efficiently reacting to unexpected downhole pressure environments. As written in the MPD Chapter in SPE's new textbook, "Advanced Drilling Technology & Well Construction," this is the "reactive" category of MPD. Here, the drilling program "tools up" with drill string floats, an RCD and a choke so that drilling flat time may be reduced with enhanced control of the well when the mud in the hole at the time is inappropriate for the observed downhole pressure environment limits.
Most applicable to brownfields drilling, however, is the "proactive" category of MPD. The fluids and casing programs are designed from the beginning to take advantage of the ability to maintain more precise and constant bottomhole pressure. The CBHP variation of MPD is a proactive application of the techniques.
Advantages of this variation of MPD include:
• less drilling non-productive time;
• enhanced control of the well;
• more precise wellbore pressure management;
• increased rate of penetration;
• less invasive mud and cuttings damage to well productivity;
• deeper casing set points;
• fewer mud density changes to total depth objective; and
• increased recoverable assets.
Summary
Drilling in mature fields and through formations that have been heavily produced offers significant challenges. A technology is needed that reduces drilling cost and increases asset recovery. Achieving more precise wellbore pressure management, containment and diversion of mud returns with fewer interruptions to drilling ahead. The CBHP variation of MPD, when proactively applied to fluids and casing program designs, offers significant benefits for overcoming a number of hydraulic challenges associated with drilling in mature fields and produced formations.

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