E&P Synthesis on
Fractured Reservoirs provides an understanding
of the locations of fractured reservoirs
within various basin settings and the
most important controls on reservoir
performance and recovery efficiency
using 100 field analogs worldwide. The
first half of the Synthesis addresses
exploration considerations. Since fracture
location, orientation, and distribution
are for the most part tectonically determined,
fractured reservoir location and geometry
are not controlled by depositional and
diagenetic facies, as is the case in
conventional carbonate and siliciclastic
reservoirs. Instead, fractured reservoir
play types are tectonically controlled.
Accordingly, six major play types have
been defined. They include fractured
reservoirs in tectonic settings dominated
by: 1) extensional faulting and folding,
2) compressional faulting and folding,
3) wrench faulting and folding, 4) vertical
basement uplift and salt movement, 5)
regional fracturing and jointing, and
6) subunconformity weathering or karstification.
Within each play type, important exploration
lessons are summarized by examining:
1) tectonic settings and trapping configurations
in which reservoirs are found, 2) reservoir
petrophysical properties and fracture
characteristics, and 3) the relationship
of various play elements (i.e., source,
reservoir, and seal). This analysis
provides a better understanding of the
geology of fractured reservoirs that
will lead to better exploration approaches
and strategies for each play type.
The second half of the Synthesis addresses
production considerations. The production
histories of all 100 reservoirs were
examined and evaluated in order to determine
the factors most responsible for differences
in reservoir performance and recovery
efficiency. As a result of this evaluation,
the reservoirs are divided into five
groups, each with fundamentally different
production characteristics: 1) Type
I oil reservoirs, in which the matrix
is non-porous and fractures provide
all of the storage capacity and the
fluid-flow pathways in the reservoir,
2) Type II oil reservoirs, in which
a low porosity/low permeability matrix
provides the storage capacity and the
fractures provide the fluid-flow pathways,
3) Type III oil reservoirs, in which
a high porosity/low permeability matrix
provides the storage capacity and the
fractures provide the fluid-flow pathways,
4) Type IV oil reservoirs, in which
a high porosity/high permeability matrix
provides both the storage capacity and
fluid flow pathways and fractures merely
enhance permeability, and 5) Type G
gas reservoirs, which produce gas or
gas plus condensate and have much higher
recovery efficiencies than the oil reservoirs.
For each reservoir type, the effect
on recovery efficiency of porosity,
permeability, viscosity, reservoir pressure,
well spacing, and other reservoir parameters
are evaluated. Reservoir geometry and
heterogeneity of all reservoir lithologies
within each reservoir type are examined
in order to identify any effects on
recovery efficiency. Since drive mechanisms
are rather complex in fractured reservoirs,
and the performance of similar reservoirs
under different drive mechanisms and
EOR techniques is evaluated. Finally,
production histories are evaluated to
determine the most successful approaches
to reservoir management and enhanced
recovery for each reservoir type and
drive mechanism. As a result of the
better understanding of the various
controls on fractured reservoir performance
and recovery efficiency provided by
systematic evaluation of field data,
the reservoirs in the analog fields
can be used as templates for developing
and producing similar reservoirs anywhere
in the world. [view
outline]
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