Success Story Casing Design Optimization in Bentayan Structure

Abstract

One of the structures being developed and very challenging in South Sumatra is Bentayan structure with strong water drive characteristics. This structure has been developed for more than 35 years by producing heavy oil and non-erupted oil, so most of it requires an artificial lift to produce well. Typical architecture of Bentayan well were using three casing there are 13-3/8”, 9-5/8” and 7” and drilled up to 1200 meter. So far, no optimization has been carried out on the casing design. The new optimization was carried out at the end of 2021 by trying to reduce one section casing (13-3/8”) then become two casing section which size 9-5/8” and 7”.

Section casing 9-5/8” was drilled by 12-1/4” bit at shallow depth and this phase is part of the endurance test rig which is including rig up phase. The challenge in this phase is drilling without diverter up to 250 meters. It must be ensured that there is no shallow gas zone. The purpose of drilling up to 250 meter is the next phase can be drilled using BOP, not diverter. Section casing 7” then was drilled by 8-1/2” bit from spud to total depth. At this time, drilling was done using BOP with FIT calculations, kick margins and the correct MAISP for well control. Subsurface hazard analysis such as shallow gas, reservoir gas, sloughing shale must be carried out to mitigate potential hole problems. Rig operational planning must be delivered properly to all personal at the rig site.

This Casing Design Optimization in the early stages was successfully applied in two wells. Dry Hole Basis were drilled faster on average 3 - 4 days, saving costs around US $ 300,000 / per well. There had been shale sloughing at a depth of 400 - 500 meter but it's solved by increasing mud weight. In general, drilling can be implemented without significant hole problems.

With successful implementation of this optimization, this optimization will be applied for future will be applied to the next Bentayan wells and other structure to be drilled. This can reduce drilling cost so that drilling in Bentayan structure is more  economical.