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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Integrated Geological and Geophysical
Characterization of Gulf Coast Reservoirs
for Incremental Natural Gas Recovery
By
Incremental recovery of natural gas beyond standard development practices in structurally simple, conventional permeability reservoirs is dependent on understanding of reservoir heterogeneity. Such heterogeneity, where not structural, is a function of the original depositional system modified by diagenesis. Investigations focusing on Frio fluvial reservoirs of Seeligson and Stratton fields in South Texas, supported by the Gas Research Institute, the U.S. Department of Energy, and the Bureau of Economic Geology, illustrate how facies and diagenetic variability can result in boundaries or baffles to gas flow. Recompletions and strategic infield drilling are means of overcoming such compartmentalization and producing gas from untapped or incompletely drained reservoirs.
At Seeligson field, reservoirs tend to have better lateral
connection than at Stratton field and have been more
intensively completed. Fluvial and splay sandstone reservoirs
tend toward a multilateral geometry at Seeligson.
Nevertheless, 3-D seismic data show discrete, meandering
channel thalwegs and AVO analysis
illustrates a distribution
of gas reflecting the channel form in an
analysis
of the
19C-04 reservoir in that field. Careful use of zero-offset VSP
data have allowed the optimum tie of stratigraphy to the
geophysical data for seismic surveys completed at different
times. Recompletions made in Seeligson have been based
on cased-hole logging and have encountered pressures of
about 1,000 psi that were 2 to 3 times reservoir fieldwide
average. Five recompletions in bypassed reservoir compartments
made 1.4 Bcf of incremental gas in about 18
months and are projected to recover about 4 Bcf of gas.
At Stratton field, greater vertical separation of reservoirs
has allowed aggressive incremental development by
the operator, mostly in the late 1980's. Analysis
of publicly
available data shows33 Bcf of shallow (<7,000 ft) and 15 Bcf
of deeper gas reserve growth in an area of about one-third
the area within the field. Fieldwide production decline was
reversed beginning about 1987. Reservoirs in the F series,
for example, have seen new completions in the 1980's with
static bottom-hole pressures (BHP) of 1,800-2,300 psi in the
same reservoir where previous completions have been
abandoned with BHP's of 200-300 psi. Single-well transient
tests and interference tests are underway to characterize
the nature of the reservoir compartmentalization involved.
Two 3-D seismic grids with VSP's for
velocity
control have
been designed to further determine the nature of reservoir
variability at Stratton field. Evidence such as that from
Seeligson and Stratton fields demonstrates that strategic
targeting of heterogeneous reservoirs
using
new approaches
can lead to economic recovery of incremental gas resources
from the Frio and other major Gulf Coast fluvial-deltaic
reservoirs.
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