Managed Pressure Drilling - Basics and Definitions -

Managed pressure drilling (MPD) is presented as one the techniques that can be used to overcome the drilling problems faced when using the conventional drilling techniques. Stuck Pipe, lost circulation and well control situations can be considered as the main problems faced while drilling operations leading to high level of non-productive time (NPT) and consequently an excessive operations cost. Mainly, these problems are caused by the non-control of the annular pressure losses and here comes the role of the managed pressure drilling technique to allow for better management of the annular frictional pressure losses which contributes positively in drilling operations performance.

There are many definitions of the managed pressure drilling MPD. It can be defined as an advanced form of primary well control barrier that applies a closed and pressurizable drilling fluid system that allows precise control of the annular pressure than the conventional technique (Brainard & Al, 2006).

The IADC underbalanced operations committee has defined the MPD as the following:

Managed pressure Drilling is an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the down hole pressure environment limits and to manage the annular hydraulic pressure profile accordingly.

The managed pressure drilling can be also described through some technical notes for better illustration:

- Annular pressure profiles can be proactively managed while drilling through narrow downhole environmental limits using a collection of tools and techniques which can mitigate the risks during operations.
- Managed pressure drilling performance can include back pressure control, drilling fluid density and reology, circulating friction estimation and hole geometry profile.
- Managed pressure drilling can help to avoid formation influx and safely contains the influx in case of well control situation.
- In case of pressure deviations, managed pressure drilling technique has the ability to allow for faster corrective actions. This ability to dynamically control the annular pressures facilitates drilling operations.

The MPD is becoming popular especially for offshore projects where hydrocarbon resources are not economically drillable with the conventional drilling. The risk of facing these challenged projects increases with the water depth and it is related to many obstacles such as:

- Narrow pore to fracture pressure margins
- Shallow hazards when drilling top holes
- Failure to reach the reservoir with large boreholes.

Fig 1- MPD System Layout

Managed Pressure Drilling principle

The basic behind the MPD technique is the ability to manage the bottom hole pressure profile. The bottom hole pressure in conventional drilling is calculated by combining the hydrostatic pressure of the drilling fluids and the estimated annular friction pressure. Unlike the conventional drilling, MPD applies a closed and pressurized mud system. The total bottom hole pressure in case of the MPD is a sum of the hydrostatic pressure, annular friction pressure and the back pressure applied at the surface. This principle of adjusting the back pressure helps to react quickly and accurately to maintain the required bottom hole pressure.

The basic configuration of the MPD system is consisted of rotating control device and choke manifold. The RCD is used keep the system closed and diverts flow toward the MPD choke manifold. The choke is used to generate the back pressure and allow for the pressure adjustment as required. The pressure profile is kept inside the pressure window using the automatic opening and closing of the choke.

Types of Managed Pressure Drilling

According the IADC MPD subcommittee, the MPD can be categorized into two categories: reactive and proactive.

In reactive plan, the well is designed conventionally but the MPD equipment are rigged up in order to react quickly in case for any unexpected pressure change. In this plan, the MPD equipment and technique are considered as a contingency to face any drilling problems during conventional operations. for example if any unexpected downhole pressure environments limits, at least the rig has to be equipped with rotary circulating device RCD, choke and drillstring floats to activate the MPD plan.

In proactive plan, the drilling program is designed with MPD technique to take advantage of precisely manage the wellbore pressure profile which can show many features such as:

- Optimize the casing seats and may be reducing the casing strings.
- Optimize mud density requirements and consequently the overall mud cost.
- Advanced warning signs for pressure control situations.

MPD Applications

Managed pressure drilling plan is application dependent technique. The success of the planned managed pressure drilling projects is related to the minimum of the required equipment, technology and also knowledge. Drilling industry is extending towards more challenging regions and reservoirs and facing many problems when performing operations. MPD can help to deal with such projects, for instance, depleted reservoirs, high pressure high temperature wells and extended reach wells.

Depleted reservoir drilling

Drilling through depleted zone which is above or below a high pressure formation can face loss of returns or differential sticking due to maintaining the overbalance at the high pressure formation. Drilling using the MPD technique can help to perform drilling operations effectively using the control bottom hole pressure principle. The pressure can be maintained within the required window that does not exceed the fracture pressure at the depleted zone and simultaneously can be kept overbalance at the high pressure zone.

HPHT Drilling

HPHT wells are one the most challenging wells to drill due to the high requirement of advanced planning and equipment. One of the features of the HPHT wells is the limited or low level of kick tolerance. The MPD for HPHT application is adapted with the retention of the high pressure blow preventer below the MPD control stack which can be activated or engaged when needed. Also, it can be mentioned that the use of the automated choke control system contributes effectively to perform safe HPHT drilling by compensating the bottom hole pressure variation which is related to the downhole temperature, swab / surge and drill pipe rotation. The pressure variation is compensated by adjusting the annular back pressure. The choke system is regulated using an advanced dynamic flow model which is run in real time.

Extended Reach Drilling

The extended reach well is one the types of the directional wells. It is characterized by the shallow depth of the kick off point and the great horizontal departure which can exceed at least twice the total vertical depth of the well. The features of the extended reach well have led to many operational challenges such as torque and drag, casing design and hole cleaning. Controlling the annular frictional pressure is one of the keys in the success of ERD project. The measurement of the annular pressure losses can help in the detection of the poor hole cleaning and consequently avoiding many problems such as stuck pipe. One the principle of the MPD is friction management techniques which contributes in the constant bottom pressure system.

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