Underbalanced drilling (UBD) is an important technique for drilling mature fields with pressure-depleted reservoirs, narrow pressure windows and/or drilling issues. It can be illustrated as follows:

The bottomhole circulation pressure is a combination of hydrostatic pressure, circulation friction pressure, and surface pressure applied at the choke. When drilling underbalanced, the well is still controlled by maintaining the wellbore pressure, but this pressure is kept to always be below the reservoir pressure.

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 BOP’s. UBD increases reservoir productivity and maximizes net present value by minimizing or avoiding near-wellbore damage and enhancing reservoir characterization.

To design and manage drilling operations on an underbalanced well, as with any drilling project, significant pre-planning is required for successful implementation of the job. A better ROI is one of the main reasons to drill a well on coiled tubing, in underbalance conditions. Either the cost of drilling the well must be lower or the well productivity must increase once the well is drilled.

CTD design and manage drilling operations on an underbalanced well, as with any drilling project, significant pre-planning is required for successful implementation of the job. A better ROI is one of the main reasons to drill a well on coiled tubing, in underbalance conditions. Either the cost of drilling the well must be lower or the well productivity must increase once the well is drilled.

Using underbalanced drilling with coiled tubing can benefit a project in three ways:

1. Significantly minimises pressure related drilling problems
2. Reduces formation damage and increases production
3. Reservoir evaluation while drilling

Major benefits of underbalanced drilling over conventional overbalance techniques can be summarized as follows:

• Reduced formation damage leads to increased well productivity
• Reduction in lost circulation of drilling fluids into the formation
• Eliminates differential sticking
• Increased ROP
• Reduced completion costs
• Diminished or absent stimulation requirements and costs
• Improved formation characterisation
• Extended drill bit life

Appropriate candidate wells need to be screened for applicability of UBCTD. The key to selecting the right candidate is reaching a balance of technical, safety and economic factors.

Author: Marat Seitimov

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