Abstract: The Development of the HPHT Elgin and Franklin Fields

Hinton, R.; K. Jones; T. Nguyen - Elf Exploration UK

The Elgin and Franklin Fields are deep (>5km), HPHT gas condensate accumulations (1100 bars, 190° C) located in Blocks 22/30c, 22/30b and 29/5b of the southern Central Graben of the UK North Sea. Franklin was discovered in 1985 and Elgin in 1991. Development drilling is currently under-way, with first production planned for 2000. The fields pose a major development challenge as the production wells must be drilled before 100bar differential pressure depletion is reached (within months of first production).

The fields comprise structural traps which formed in late Jurassic times. Elgin is a complex faulted anticline and Franklin a tilted horst block, both structures having been influenced by halokinesis of the underlying Zechstein salt. The hydrocarbons are trapped in the Late Jurassic Fulmar sandstones which comprise around 300 meters of highly bioturbated, fine to very-fine-grained, sediments deposited in a shallow marine setting. Sedimentary structures and biostratigraphic data are sparse but detailed ichnofabric analysis has enabled a depositional model to be established on the basis of lithofacies associations

Despite the extreme depths, porosities of up to 30% and permeabilities of over 2 Darcies have been encountered in the Fulmar reservoir. Reservoir quality is controlled by a complex combination of original lithofacies, porosity preservation due to high overpressure, and hydrocarbons inhibiting diagenesis. Fault seal has given rise to compartmentalization of the Elgin Field. On Franklin, additional (separate) hydrocarbons are present in the underlying Pentland Formation. Overpressure developed in response to rapid burial from Tertiary through Quaternary times, with possible additional contributions from both oil cracking and late gas generation.

Throughout the appraisal and development planning stages the constraints of data acquisition from the extreme HPHT environments have presented specific problems in terms of reservoir characterization. The depth structure mapping is recognized as the greatest uncertainty factor in the definition of the fields. A new 3D seismic coverage was acquired in 1996 to address this problem. Anti-multiple processing and post-stack depth migration (using both isotropic and anisotropic velocity models) have improved the imaging of the reservoir.

In view of the uncertainties associated with reservoir behavior under depletion, a comprehensive 3D geological model was built to integrate both field descriptions and more regional knowledge. The geological model was up-scaled to give an eleven-layer reservoir simulation model extending over both fields. The 3D model will provide a tool for sensitivity work and risk assessment related to uncertainties in reservoir parameter distributions and geomechanical constraints.

Elgin will be developed with seven producing wells, Franklin with five. These will be drilled from separate wellhead platforms feeding a centralized processing platform. Ultimate recoverable reserves are 890 bcf gas plus 245mm bbls. condensate for Elgin, and 820 bcf gas with 120mmbbls. condensate for Franklin.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil