Ubit Field Rejuvenation: A Case History of Reservoir Management of a Giant Oil Field, Offshore Nigeria
Authors: Clayton C. A. , Cohen M. F. , Anis M. , Cooley T. W. , Honarpour M. M. , Wallace J. P. , Chambers M. R. , Fadase A. O. , Pebdani F. N. , Odior E. G. , Nwaeri E. U. , Membere A. H. , Ekworomadu E. O. , Miner K. A. , Ogunjana B. O. , Al-Hussainy R. 
Published: December 2007

Ubit field is an example of a successful application of integrated reservoir management to an old field, which has resulted in a total change in the development strategy, an increase in recoverable reserves by a half billion barrels and a production uplift of 110 MBD. The key was an improved understanding of the reservoir hydraulics. Unlocking the genesis of elements that defined the fluid flow units identified their connectivity and distribution as related to their depositional facies, led to rejuvenating this field so completely. New data and techniques in 3-D seismic, core interpretation, computer mapping, 3-D visualization, and advanced reservoir simulation prediction capabilities were brought together to optimize recovery and production. Through the integration of the new reservoir model, horizontal drilling, and surface facilities, this old field is now producing an all-time high of 140 MBD, with ultimate recovery expected to exceed 1 billion barrels. The techniques and methodologies developed at Ubit are being leveraged in other assets.

Ubit has a STOIIP of 2.1 billion. The reservoir is cut by 3500 feet of dipping, unconsolidated sands and shales. Production is from a thin oil column, with an associated thick gas cap. Gravity-stable gas expansion is the primary recovery mechanism. For 25 years, Ubit averaged only 30 MBD with a high gas-oil ratio. Gas breakthrough in conventional directionally-drilled production wells has been problematic. Previous reservoir interpretations described the chaotic nature and poor quality reservoir properties in the eastern two-thirds of the field. Poor historical production performance seemed to confirm these observations.

A new horizontally-layered, hydraulic-focused geologic model combined with advanced reservoir simulation techniques yielded a substantially improved interpretation. The reservoir model is the primary focus of this paper. Predicted performance has guided the management of the re-development of Ubit. New technology applications and conventional techniques were brought together in the reservoir model design to capture the geologic elements controlling flow, and the dynamic processes controlling recovery.

This paper describes some of the significant reservoir engineering, geoscience, infrastructure challenges, and the technical resolutions during the development and management of this complex reservoir system. Key reservoir management strategies were applied to maximize performance and ultimate recoveries. They include: 1) implementing horizontal well drilling, 2) full-field full-life reservoir simulation results defining well placement / timing, 3) balancing a non-uniform gas cap, 4) maintaining stable gas cap movement and pressure throughout, 5) establishing a field plateau rate and 6) minimizing free-gas production.