Controls on
Reservoir Performance and Recovery Factor in Fractured Gas Reservoirs: Lessons
Learned from Giant Gas Fields
Allan, Jack1, Rod Sloan2,
S. Qing Sun3 (1) C&C Reservoirs Inc,
Fullerton, CA (2) C&C Reservoirs, Maidenhead, Berks, United Kingdom (3)
C&C Reservoirs Inc, Houston, TX
Forty-eight naturally fractured carbonate
and sandstone gas reservoirs from around the world, many of them giants, were
compared to identify key controls on recovery factors, which range from 34-92%
(68% avg.). Brittle fracturing commonly aids recovery and is more common in
carbonates than sandstones, which are more likely to develop gouge-filled shear
fractures that restrict gas flow and reduce recovery factor. More than 85% of
the reservoirs were fractured by compression or uplift rather than by
extensional or wrench processes. Thus, high fracture densities and well
productivities tend to be concentrated on structural crests. Much of the
production comes from a few intensely fractured zones, which are rarely
correlative. Most reservoirs produce by gas-expansion drive. When aquifer
support is weak, highly fractured areas are the best drilling targets, but when
strong, high phi*h areas with low fracture densities are better drilling
targets.
If water incursion is minimal, hydraulic
fracture treatment is commonly used to connect widely spaced natural fractures,
open healed fractures, interconnect high phi*h areas, and penetrate near-wellbore formation damage. If water incursion is serious,
careful choice of well location and completion interval is required. Adjusting
production rates to maximize pressure drawdown in poorly fractured parts of a
reservoir, which are least susceptible to water incursion, is also effective.
Since most fractured gas reservoirs are produced to depletion without benefit
of secondary recovery programs, reservoir management strategies designed to
optimize gas recovery at the individual-well level often provide the best
approach for maximizing recovery factor for the entire reservoir.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California