Managed Pressure Drilling- A Food For Thought

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Managed Pressure Drilling (MPD) is a new technology that uses tools similar to those of underbalanced drilling to better control pressure variations while drilling a well. The aim of MPD is to improve the drillability of a well by alleviating drilling issues that can arise.

IADC defines MPD as “An adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly.”

MPD is further divided into two categories -“reactive” (the well is designed for conventional drilling, but equipment is rigged up to quickly react to unexpected pressure changes) and “proactive” (equipment is rigged up to actively alter the annular pressure profile, potentially extending or eliminating casing points). This category of MPD can offer the greatest benefit to the offshore drilling industry as it can deal with unforeseen problems before they occur.

PRIMARY OBJECTIVES

            The primary objectives of MPD are to mitigate drilling hazards and increase operational drilling efficiencies by diminishing the non-productive time (NPT). The operational drilling problems most associated with NPT include:

  • Lost Circulation
  • Stuck Pipe
  • Wellbore instability
  • Well control incidents

            MPD process uses a collection of tools and techniques to mitigate the risks and costs associated with drilling wells that have narrow downhole environment limits, by proactively managing the pressure profile.

MPD may include control of back pressure, fluid density, fluid rheology, annular fluid level, circulating friction, hole geometry and combinations thereof.

MPD may allow fast corrective action to deal with observed pressure variations. The ability to control annular pressures dynamically facilitates drilling of what might otherwise be economically unattainable prospects.

MPD technique may be used to avoid formation influx. Any flow incidental to the operation will be safely contained using an appropriate process.

The centerpiece of the definition is “precise control”. The technology allows drillers to control bottom hole pressure from the surface within a range of 30-50 psi. One method does not address all the problems and MPD is application specific. The vast majority of MPD while drilling in a closed vessel, using an RCD with at least one drill string, non-return valve and a DCM.

UBD vs. MPD

MPD is similar to underbalanced drilling (UBD). It uses many of the same tools that were designed for UBD

operations. The difference between the methods is that UBD is used to prevent

damage to the reservoir while the purpose of MPD is to solve drilling problems. UBD allows influx of formation fluids by drilling with the pressure of the fluid in the wellbore lower than the pore pressure. MPD manages the pressure to remain between the pore pressure and the fracture pressure of the reservoir. It is set up to handle the influx of fluids that may occur while drilling but does not encourage influx. UBD is reservoir-issue related while MPD is drilling-issue related.

PRESSURE GRADIENT WINDOWS

As a well is drilled, drilling fluid is circulated in the hole to obtain a specific bottom hole pressure. The density of the fluid is determined by the formation and pore pressure gradients and the wellbore stability.1

Fig. 1shows a pressure gradient profile of a well. This profile shows the change in pressure as the depth increases. The pressure window is the area between the pore pressure and the fracture pressure. The goal when drilling a well is to keep the pressure inside this pressure window. In a static well, the pressure is determined by the hydrostatic pressure of the mud. In conventional drilling, the only way to adjust the pressure during static conditions is to vary mud weight in the well.

Fig. 2shows the problem that can occur when dealing with tight pressure gradient windows. When the well is static, the pressure in the well is less than the pore pressure and the well takes a kick; that is, hydrocarbons flow into the well. Before drilling can begin again, the kick has to be circulated out. After a connection, the pumps restart, the BHP (Bottom Hole Pressure) increases, and the pressure goes above the fracture-pressure, resulting in lost circulation, or fluid flowing into the formation. The goal of managed pressure drilling is to walk the line of the pressure gradients. Managing the pressure and remaining inside this pressure gradient window can avoid many drilling problems.

MPD TECHNIQUES

CONTINUOUS CIRCULATION SYSTEM The continuous circulation system (CCS) is a new technology that enables a driller to make connections without stopping fluid circulation. A CCS enables a driller to maintain a constant ECD when making connections. In normal drilling operations, a driller must turn the pumps off when making a connection. Numerous problems can occur as pumps start and stop in a drilling operation. (Fig. 3)

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In a narrow drilling window, where the pore pressure and fracture pressure gradients are close, continuous circulation can prevent many problems from occurring.

Benefits of using the CCS include

• Reducing nonrotation time by eliminating the need to circulate the cuttings out of the bottom hole assembly.

• Reducing the possibility of a stuck drillstring by keeping the cuttings from dropping to the bottom.

• Constant ECD can be maintained.

ECD REDUCTION TOOL

The ECD reduction tool is designed to reduce the bottomhole pressure increase caused by friction in the annulus by providing a pressure boost up annulus.

            Equivalent circulating density (ECD) is a function of mud density, mud rheology, cuttings loading, annular geometry and flow rate. Drilling-fluid density is required for pressure control and wellbore stability. Viscosity and flow rate are needed for hole cleaning and barite-sag mitigation. Gel strengths are required to suspend drill cuttings. The goal of ECD management is to find balance between these parameters to successfully drill a well.

Reducing ECD in a well can result in many benefits. These benefits can include:

• Reducing the number of casing strings.

• Improving hole cleaning by using higher flow rates.

• Being able to remain in the pressure window for complex wells.

• Reducing lost circulation and differential sticking.

• Reducing formation damage.

FUTURE OF MPD

The challenge for the future of MPD is to convince the industry of its benefits.

The main problem in instituting MPD is that companies think that their way works well enough and do not want to take the risk of trying a newer method.

This is similar to situations that occurred when underbalanced drilling and horizontal drilling were first introduced. It is just going to take time for MPD to become an accepted method and be used in regular drilling operation.

The benefits that should be shown to companies to convince them to try MPD include the possibility of improving the drill-ability of depleted formations. Drilling through these depleted zones often result in narrow pressure windows and lost circulation issues. Drilling in these areas require a more constant bottomhole pressure to remain in the narrow pressure window. MPD would help reduce costs and improve current assets held by companies. Companies realizing these benefits and seeing them work would lead to more common use by these companies.

About author

Ahmed Bilal

I'm a Third year student of Petroleum & Gas Engineering at University of Engineering & Technology, Lahore Pakistan. I am high-spirited, devoted and an exuberant person. I am an idealist in dreaming and a realist in acting, who aspires to work in a pragmatic and cooperative environment. Equally inspired to volunteer my talent for the cause I believe in.

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