Overcoming Subsurface Challenges to Develop a Thin Oil Column - A Case Study from the Five Decade Old Gekko Discovery in the Alvheim Area

Abstract

The Alvheim field has been on production since 2008 through the Alvheim FPSO. While the FPSO has produced more than twice the initial Alvheim area production estimate of 250 mmboe, the ambition is to produce a billion barrels from the area via continuous infill drilling and near-field exploration and developments. An example of the latter is the joint Kobra East and Gekko (KEG) tie-in. This paper describes the various anticipated subsurface challenges, risks and proposed solutions in developing the Gekko field, discovered in 1974 (25/4-3) and subsequently appraised in 2003 (25/4-8) and 2018 (25/4-13S&A), revealing a thin oil column (6.5m) between a gas cap and a large aquifer. The Gekko reservoir is of Late Paleocene age and consists of turbiditic sand deposits of the Heimdal Formation. Two subtle four-way closures (Gekko North and Gekko South) define the trap. Reservoir properties are excellent with high porosities and multi-Darcy permeabilities. The technical challenges associated with producing a thin oil column sandwiched between the gas cap and a strong aquifer led to the field being viewed initially as a gas discovery and not included in the Alvheim development. New appraisal wells and additional seismic data sets, advances in seismic techniques and a new area petrophysical study helped to improve the subsurface understanding. Development of the thin oil column has been evaluated using detailed reservoir simulation models to determine optimal well placement and completion solutions, resulting in a well design comprising of horizontal tri-lateral wells of approximately 4000 m reservoir lengths. New completion technologies such as autonomous inflow control devices (AICD), independent inflow control valves (ICV) installed in each lateral and downhole water cut meters are expected to play a pivotal role in unlocking the development potential. The work done has resulted in a successful PDO submission (June 2021) for the KEG development. Four tri-lateral wells will be drilled during 2023, with first oil expected in 2024. This development represents the largest ongoing tie-in project to Alvheim, adding reserves of approximately 40 mmboe. Producing a thin oil column such as Gekko presents significant subsurface challenges. Such a development will require targeted technologies and multi-disciplinary coordination. Successful development will reinforce the near-field development strategy for the Alvheim FPSO and add to its production volumes and production life. To be able to finally develop the field fifty years after it was discovered can also serve as a benchmark for future stranded developments of similar scope and complexity.