Tunu field is the major gas supplier of the East Kalimantan system. It started producing in 1990 and more than 200 wells have been drilled to date. The field exported an average 1.3 bcf/d in 2000.

In early 1999, extreme underbalance perforation (EUB) was introduced for monobore wells to reduce the skin effect to provide more flow capacity and maximize hydrocarbon recovery from the reservoir. The differential pressure between the wellbore and the formation creates a sufficient drawdown to instantaneously clean the perforated channels as soon as they are made. The challenge is to safely perforate under a drawdown of few 1000's psi instead of few 100's psi for classical perforations.

The anchor + stackable perforating gun system has been evaluated and improved to permit EUB perforating to be performed safely. The heart of the EUB gun system is a mechanical release anchor that secures the guns firmly during perforation avoiding the risk of lifting the tool inside the wellbore. As there is no wireline cable while perforating, the well is flowing instantaneously allowing immediate clean up. Using this system, the underbalance is now limited only by: the drawdown on the formation (limited in case of depleted sand) and the potential risk to destabilize the formation and produce sand formation.

So far, 20 jobs have been done safely on Tunu field with the differential pressure up to 4800 psi, without sand production. The paper will describe the operation procedures and the statistical approach (PBU results) demonstrating the benefit of EUB technique.

For Tunu field, this perforation technique increases in average by 15% the initial productivity compared to classical perforations for a relatively low incremental cost.

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