Abstract

Until 1999, three well architectures were used for the "S" wells of the Tunu field: the "standard" design (last phase in 8" and 4" multi-packer completion in 7" liner), the "standard monobore" and the "slimhole monobore" (last phase in 6" and 4" monobore completion).

The generalization of light architectures was, however, impeded by several drilling and completion limitations in high-pressure regimes as well as the complexity of Tunu field where geological prognosis and reservoir pressure profiles are very difficult to predict.

In 2000 TotalFinaElf E&P Indonesie introduced the 4" tubingless completion in the delta, firstly to take the cost cutting process in Tunu drilling operations a "step further" but also with the ultimate objective to standardize the well architecture into a "all slimhole - tubingless" development.

Three tubingless completions were run successfully in the year 2000. An additional six tubingless completions were run in early 2001 with plans to complete more by yearend. The cost of tubingless completion wells to date ranges from 2,800 to 4,000 kUS$ for an average duration of 24 days (average 4,500 kUS$/35 days in 1999/2001). Recently, the rig was equipped with High Torque 4" DP to extend the slimhole-tubingless to long reach wells. The final cost saving for the first three years of the project is expected to reach 15 MUS$.

The slimhole-tubingless design generates significant cost and duration savings but additional risks need to be evaluated and controlled such as:

• no back up phase zones

• risk of differential sticking in slim hole in depleted zones straddled by two high pressure zones

• high drilling torque in 6" phase close to the limit of the equipment

• small rat hole and shoe track (down to 2/3 meters) as the well TD may be just a few meters after the last reservoir. The reduced safety margin for cement displacement can lead to low cement bond quality around the 4" shoe

• limited workover options

In January 2000, the first tubingless completion was run on TM-41 in the Tambora field to confirm the feasibility. Then two slimhole-tubingless completions were achieved in low pressure-low departure wells: TN-I3 in Tunu and TM-42.

Meanwhile a "all slimhole-tubingless" strategy was elaborated with the drilling, completion, geology, reservoir and well servicing departments, which was made possible thanks to a clear definition of the reservoir and production objectives.

In mid 2000 the drilling swamp barge was upgraded to 7500 PSI to extend the new design to 1.40/1.60 ESG reservoir targets.

In February 2001, the first six wells of the new drilling campaign have been drilled and completed slimhole-tubingless. The well cost ranges from 2,800 to 4,000 kUS$ for an average duration of 24 days. (average 4,500 kUS$/35 days in 1999/2001). A new well duration record was established with TN-Q4 (17 days).

The next step of the project took place in April 2001 when the rig was equipped with High Torque 4" DP to extend the slimhole-tubingless to long reach wells. The final cost saving for the first three years of the project is expected to reach 15 MUS$.