Underbalanced drilling is not a new technology. All wells drilled up until the introduction of the rotary drilling rig were drilled underbalanced. However, knowledge of subsurface conditions was limited, well control risks were not managed and well control incidents, when wells struck over-pressured zones, were common. The introduction of the rotary rig, resulted in mud circulation systems. As wells were drilled deeper into over-pressured formations blowout preventers (BOPs) that enabled better well control were introduced. Overbalance (conventional) drilling replaced "underbalance" cable tool drilling and became the normal practice.

In conventional drilling, the hydrostatic pressure created by the mud column provides primary well control and BOPs provide secondary well control.

Underbalance drilling has emerged in the past 15 years, and, just as there have been improvements and refinements in conventional well control equipment and procedures overtime, the same holds true for underbalanced drilling technology. In underbalanced drilling, the primary well control function of the mud column is replaced by a combination of flow and pressure control, while secondary well control functionality is provided by the BOPs. The complete UBD closed-loop flow control system comprises of the drill pipe (DP) circulating system, a UBD control device (UBD-CD), which provides the sealing mechanism around the drill string at surface, a UBD choke manifold (not the rig’s well control choke manifold), a UBD separator and flare system. In addition, non-return valves (NRVs) are installed in the bottom hole assembly (BHA) and drill string to prevent flow up the DP.

This paper provides an overview of underbalanced operations primary well control. It briefly describes the equipment and techniques used and the critical issues to consider during project planning to ensure the safety of staff, the rig, the well and to ensure that the life cycle objectives of UBD wells are addressed.

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