Abstract

In the last two years, Unocal has successfully drilled horizontal wells in the Serang Field to recover reserves from oil columns less than 25 feet, which are unrecoverable by conventional wells. Drilling horizontal wells in the geologically complicated Serang Field requires a solid teamwork and rigorous efforts by each team member in order to reduce risks, minimize costs, and optimize oil production. As a result the Serang horizontal wells have out-performed the conventional, deviated wells in the production performance and oil recovery for approximately the same or slightly higher drilling costs.

Majorities of reservoirs in the Serang Field are sandstones ranging in depositional environment from delta plain to delta front. They are fluvial channel, distributary channel, distributary-mouth bar, and tidal bar deposits. These sandstones consist of loose, fine to coarse, quartz grains and dispersed clay. Carbonaceous clay drapes are normally found in the tidally influenced sandstones. Delta progradation and agradation in the East Kalimantan area during the Late Miocene have caused stacking and accretions of sand bodies of different facies resulting in networks of reservoirs which are very heterogeneous vertically and laterally.

Oil columns in these reservoirs range in thickness from 10 to 100 feet, with large gas caps above and strong aquifer support below. Several wells in Serang cone water and gas at the same time demonstrating the strength of both gas-cap and water drive mechanisms. After less than four years of production fluid contacts have moved significantly. Oil to water contacts (OWC) have moved up as far as 50 feet reducing the oil column to 50 percent. Recent wells have recorded oil encroachment into gas cap areas as well as gas invasion into oil columns.

To learn about the reservoir quality and the present fluid contacts near a proposed horizontal section, Unocal used a real-time pilot hole, which was the same hole penetrating through the objective reservoir and hydrocarbon zones before building up and landing horizontally. Real time decisions and revisions to the plan are made onsite by geoscientists and engineers.

The horizontal section of a well was placed according to the reservoir quality, heterogeneity, and fluid contacts. The exact target location was decided as drilling in progress because of the uncertainty of the geology, reservoir quality, and present hydrocarbon accumulation limits.

The Serang horizontal wells were completed open hole. To clean out the mudcake along the horizontal section Unocal reservoir engineers surged the well at a maximum choke opening. An attempt to minimized skin damage in one of the well was to set a suing of casing just after landing horizontal, and the horizontal section was drilled using as low density drilling mud as possible.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah