For the purpose of this paper Controlled Pressure Drilling (CPD) here is referred to as underbalanced reservoir drilling, managed pressured drilling or performance drilling. Since the operations differ between underbalanced drilling and traditional overbalanced drilling, risk management plays a key role in ensuring drilling operations are processed safely to protect people, the environment and assets.

Firstly, this paper gives a brief introduction of controlled pressure drilling. Secondly, the paper discusses the key risk management techniques applied in one organisation, such as the risk matrix, SURE, Job Safety Analysis (JSA), Hazard Identification Studies (HAZID), Hazard and Operability Studies (HAZOP), Bow-tie type risk analysis, etc. Thirdly the paper discusses the necessary HSE documentation required to integrate risk management techniques into a CPD project as a road map to a successful project. Lastly, using one project completed in Asia Pacific to elaborate the risk management process and the challenges faced over the years.


In this organisation Underbalanced Drilling is defined as drilling into any formation where the pressure exerted by the drilling fluid is less than the formation pressure.

Examples of CPD include

  • Drilling with air or gas,

  • Drilling with cable tools,

  • Drilling with any type of fluid where the pressure of the fluid column is less than formation pressure

The purposes of the different techniques are as follows:

  • Performance Drilling (PD): is used to achieve maximum penetration rates through reducing the well bore pressure to a minimum possible value.

  • Managed pressure drilling (MPD): is used to precisely manage and control the annular pressure to allow the bottom hole pressure to be within close limits.

  • Underbalanced Drilling: is used to reduce formation damage, discover potential bypassed pay, and increase reserves by allowing access to these reserves thus ultimately increasing net NPV. In underbalanced reservoir drilling the well is designed to allow the reservoir to flow to surface whilst drilling.

Although underbalanced drilling has been around since the start of the oil exploration, the modern underbalanced drilling only started when the first high pressure gas well was drilled underbalanced in Austin Chalk in 1988 and in Canada in 1993.

In 1998, the IADC UBO committee was formed in order to set standards initially for underbalanced drilling operations (UBO) and later Managed Pressure Drilling (MPD). This committee developed the underbalanced classification matrix and continues today to develop safer and more efficient methods and procedures for underbalanced drilling operations.

Compared to traditional overbalanced drilling, underbalanced drilling can identify new productive horizons when drilling, no damage or minimum damage is done to the reservoir rocks, resulting in better production. However, possible wellbore stability problems, high daily cost, more complex drilling system and generally higher risk with more inherent problems are the disadvantages of underbalanced drilling. See Figure 1.

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