After 28 years of oil and gas production from chalk of Danian and Maastrichtian age, the giant Ekofisk field is only halfway through its economic life. New surface facilities installed during 1998 offer opportunities to plan and drill as many as 50 new wells in a field with a mature waterflood under way. Locations of the new wells are based on a history-matched reservoir flow-simulation model derived from a 3-D geologic model.
Attributes (e.g., permeability, initial fluid saturation, facies type, rock composition) are distributed in the 23-million-cell geologic model from a database of well logs, cores, and 3-D seismic data inverted for porosity and thickness. An upscaled 39,000-cell flow-simulation model is used to predict the movement of injected water and to optimize the paths of wells that vary from near-vertical to long-reach horizontal, and from simple slant-holes to complex multilaterals.
Planning the new Ekofisk wells is an iterative exercise: A preliminary well path defined in the geocellular and flow-simulation models is displayed against a seismic backdrop to ensure that it avoids major faults and penetrates reservoir units with best properties. The proposed well then is compared with nearby wells to avoid collision problems and to ensure correlation of major flow units, and is plotted through the seismic volume using an automatic fault-picking routine to guard against operational or production problems resulting from minor faults. Throughout this planning process, interactions among the field geoscientists, operations geoscientists, reservoir engineers, and directional drillers ensure optimal well paths and value-based data collection programs.
Particular successes attributed to this teamwork include wells producing more than 12,000 BOPD, wells with total depths exceeding 7772 m (25,500 ft) (including horizontal reservoir sections more than 2377 m [7800 ft] long), and a total contribution to Ekofisk production from partial completions in the first 22 wells of more than 135,000 BOPD and 175 MMCFGD with minimal water production. Additional challenges to this redevelopment include (1) a gas-charged, overpressured overburden, which obscures surface seismic returns; and (2) production-induced reservoir compaction, which causes both wellbore collapse and seafloor subsidence.
Figures & Tables
Horizontal Wells: Focus on the Reservoir
This book provides an overview of the new technical approaches required for best use of horizontal and extended-reach technology in different reservoir situations. The volume is a selection from more than 50 papers presented at an AAPG/SPWLA Hedberg Research Symposium, “International Horizontal and Extended Reach Well Symposium: Focus on the Reservoir,” held in The Woodlands, Texas, on October 10 14, 1999. The 16 chapters describe horizontal and extended-reach wells and drilling programs in a variety of geologic settings all over the world.