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Underbalanced drilling health and safety issues

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Because UBD involves working on a live well, a hazard operational (“hazop”) analysis is required for the full process. To this end, a flow chart is created that shows all the elements in the UBD process.

UBD health and safety workflow

Using the diagram (Fig. 1), each element can be analyzed for input and output and the diagram has also been used to good effect to ensure that all items of an UBD system are reviewed during the hazop. It also allows procedures and documentation to be reviewed for all parts of the UBD system.

Fig. 1 shows an analysis path together with the interaction of the various elements. The drilling liquid system (1), the gas system (2), and the reservoir characteristics (3) specify the well system (4). The well system (4) specifies the well control system (5), which has impact on the drilling fluid system (1). This loop must be resolved before continuing to the surface separation system (6). This influences the rig fluid system (7), which must also be compatible with the drilling liquid system (1). The platform process system (8) must be consistent with the surface separation system (6) as well as the overall platform system. Multiple iterations are necessary to bring all systems into alignment.

Environmental aspects

The UBD system is a fully enclosed system. When combined with a cuttings-injection system and an enclosed mudpit system, a sour reservoir can be drilled safely using a UBD system. The pressures and flow rates are kept as low as possible. It is not the intention to drill a reservoir and produce it to its maximum capacity. A well test can be carried out while drilling underbalanced to provide some productivity information. The hydrocarbons produced during the UBD process can be routed to the platform process plant, exported, or flared. There is work currently being undertaken to reduce flaring and recover the hydrocarbons for export. In a prolific well, a significant amount of gas might be flared during the drilling process. Recovering this gas provides an environmental benefit and an economic benefit. Oil and condensate recovered are normally exported via a stock tank into the process train.

Safety aspects

Besides the full hazop, substantial crew training is required for UBD. A typical drilling crew has been instructed during its entire career that if a well kicks, it must be shut in and killed. In contrast, during UBD, the single item to be avoided is to kill the well. This may undo all the benefits of UBD. Working on a live well is not a normal operation for a drilling crew, and good training is required to ensure that accidents are avoided.

Difference between UBD and conventional drilling as it relates to health and safety

The UBD process is more complex when compared to conventional drilling operations. Gas injection, surface separation, and snubbing may be required on a well. If the hydrocarbons produced are then pumped into the process train, it is clear that drilling is no longer a standalone operation.

The reservoir is the driving force in the UBD process. To drill the well safely, the driller must understand the process and all the interaction required between the reservoir:

  • The liquid-pump rate
  • The gas-injection rate
  • The separation and process system

When tripping operations start, the well must remain under control. Snubbing pipe in and out of the hole is not a routine operation, and a specialized snubbing crew is normally brought on to snub the pipe in and out of the hole.

The extra equipment also brings a number of extra crew members to the rig. Besides a more complex operation, a number of service hands are on the rig which now must start working with the drilling crew. Once the well is completed, though, the drilling crew can move back to conventional drilling. The drilling crew must be trained in this change of operating practice.

When a number of wells will be drilled underbalanced in a field, it may be advantageous to batch drill the reservoir sections. This saves mobilization, and it also sets a routine with the drilling crew. It must be stated that few accidents occur during UBD. This is mainly because of the high emphasis on safety during live well operations.

Training and personnel

Training in UBD

The entire platform/rig crew must be trained in underbalanced techniques. Once the crew understands what is to be achieved, operations will run more smoothly with fewer problems and accidents. Documentation, policies, and procedures should not be forgotten when considering training.


Underbalanced drilling requires a larger crew than most regular drilling procedures, often needing 15 to 20 extra crewmembers for full UBD and completion.


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See also

Underbalanced drilling (UBD)