Performance Analysis of a Gas Cap Drive Oil Reservoir: The Brent Field Case Study

Abstract

The Brent Field, located in the UK North Sea (Block 211/29), represents one of the largest hydrocarbon accumulations in the United Kingdom Continental Shelf, discovered in 1971. This comprehensive study analyzes the performance of the Brent Field as a gas cap drive oil reservoir, focusing on the Middle Jurassic Brent Group reservoir with its overlying gas cap. The field contains two major accumulations: the Brent Group reservoir (Middle Jurassic) and the Statfjord Formation reservoir (Lower Jurassic/Triassic), with the Brent Group exhibiting classic gas cap drive characteristics. Using material balance equations specifically formulated for gas cap drive reservoirs, this study evaluates gas cap expansion, gravity segregation effects, and gas-oil contact (GOC) movement during production. Analysis of production history data reveals significant pressure decline patterns and gas cap expansion dynamics that have influenced oil recovery efficiency. The original oil in place (OOIP) estimation through material balance analysis, coupled with Monte Carlo simulation for uncertainty quantification, provides a robust framework for reserve estimation. Results indicate that gas cap drive mechanisms contribute substantially to reservoir energy, with recovery factors influenced by gas cap size, expansion efficiency, and gravity segregation. Production forecasting models project future performance under various depletion scenarios, while uncertainty analysis quantifies the range of possible outcomes. This case study demonstrates the critical importance of understanding gas cap drive mechanisms for optimizing production strategies and maximizing ultimate recovery in North Sea reservoirs.