A seven well Big-Bore Underbalanced drilling and completion program was undertaken under challenging downhole conditions in a low pressure, high permeability carbonate reef reservoir in Indonesia. Underbalanced drilling technology was utilised as conventional drilling practices were no longer viable due to the extreme low pressure (1.1ppg e). UBD was selected as it would eliminate lost circulation and stuck pipe problems, reduce formation damage, eliminate the need for post drilling stimulation and give early analysis of reservoir behaviour and production rates. The resulting wells incorporated the largest tubingless gas completions in the world and were completed in an undamaged state, resulting in 29% higher than anticipated initial gas production rates.

This paper will describe the Front End Engineering Design, Project Management, Risk Mitigation, Detailed Engineering & Design, Operational Results and Lessons Learned for this project.


Located in a remote part of Indonesia, the field to be discussed was approximately twenty six years old at the time of the project, and the reservoir pressure had been falling rapidly from the original BHP of 7,100psis to a BHP of +/− 600psia at the time of the project. Conventional drilling techniques have proved unsuccessful for drilling new wells into the reservoir (the last attempt was in 1995) because the formation was unable to support the column of drilling fluid, leading to lost circulation and stuck pipe. In 1998, six wells were drilled underbalanced after an extensive pilot test, which utilised custom built equipment1. The ensuing studies and analysis pointed to an underbalanced Coiled Tubing Drilling (CTD) campaign. However, subsequent wellbore design, production demand and economics led to the conclusion that Big Bore (large diameter wellbore) wells drilled underbalanced with jointed pipe would be the most economical solution.

This case study will provide details of how the project was progressed from the initial concept to use CTD up to the successful implementation of a seven well UBD program:

A detailed Basis of Design (BOD) was developed to review the key drilling issues and make recommendations for how to best proceed with the project.

Tender documents were issued, basic design was undertaken and contracts were signed.

The detailed engineering & design phase was then conducted to refine the initial concept. This phase also included Quality, Health, Safety and Environment (QHSE).

Seven wells were successfully drilled; the results of these operations and the lessons learned will be discussed later.

Key Project Drivers

The key project drivers were:

  • Rapidly depleting high permeability and vugular limestone reservoir with ultra low reservoir pressure - static reservoir pressures on Cluster III ranged from 570 to 585 psia and in Cluster IV from 650 to 700psia. The reservoir pressure (1.1–1.4ppg e) was depleting rapidly at 0.3–0.4 psi/day (Figure 1). The speed of implementation was critical because with the rapid pressure depletion of the field it would soon not be possible to drill these wells underbalanced.

  • Conventional drilling was no longer viable or economic due to massive losses with a mud system, mechanical and differential sticking, low ROP and formation damage (high skin) caused by overbalanced (conventional) drilling, which in turn reduces production rates. Drilling underbalanced would minimise/eliminate all the above conventional drilling problems and reservoir concerns.

  • Reduce/eliminate the need for post drilling well stimulation.

  • Provide early analysis of reservoir behaviour and production rates.

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