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Importance of PPG
Conformance Control for Mature Oilfields
Mature reservoirs are
requiring us to control conformance
Most reservoirs from
small producers have become mature due
to long term water flooding. These reservoirs usually have a high water
cut of
more than 80% even 90%. Water production becomes a major problem as
these
oilfields mature. Higher levels of water production result in increased
levels
of corrosion and scale, increased load on fluid-handling facilities,
increased
environmental concerns, and eventually well shut-in (with associated
workover
costs). Consequently, producing zones are often abandoned in an attempt
to
avoid water contact, even when the intervals still retain large volumes
of
recoverable hydrocarbons. Controlling water production has been one
objective
of the oil industry.
Reservoir heterogeneity
severely affects the flow of gas,
oil, and water in the reservoir and thus affects the choice of
production
strategies, reservoir management, and ultimate oil recovery. Reservoir
heterogeneity is the single most important reason for low oil recovery
and
early excess water production. To maintain reservoir pressure, these
reservoirs
have usually been developed by water flooding from the early stage of
their
development. Many of them have been hydraulically-fractured,
intentionally or
unintentionally, or have channels due to mineral dissolution and
production
during waterflooding. Reservoirs with induced fractures or
high-permeability
channels are quite common in the mature oilfields. Gel treatment is one
of the
most important methods to correct the reservoir heterogeneity.
Using Preformed Gels to Control
Conformance Has Become
a New Trend for Gel Treatments
Traditionally
in-situ gels have been widely used to control conformance. The mixture
of
polymer and crosslinker called gelant is injected into target formation
and
react to form gel to fully or partially seal the formation at reservoir
temperature. So the gelation occurs in reservoir conditions. However, a
newer
trend is applying preformed gels for this purpose. Preformed gel is
formed at
surface facilities before injection, and then gel is injected into
reservoirs.
So no gelation occurs in reservoirs. Preformed gels have become a newer
trend
because they can overcome some distinct drawbacks inherent in in-situ
gelation
system such as lack of gelation time control, uncertainness of gelling
due to
shear degradation, chromatographic fractionation or change of gelant
compositions, and dilution by formation water.
Current
commercial available preformed gels include preformed bulk gels
(Seright 2004),
partially preformed gels (Sydansk 2004, 2005), and particle gels which
include
preformed particle gel – PPG (Coste 2000, Bai 2004), microgels
(Chauveteau
2000, 2001, Rousseau 2005, Zaitoun 2007, U.S. patent 6579909) and pH
sensitive
crosslinked polymer (Al-Anazi 2002, Huh 2005), “colloidal dispersion
gels” and
swelling micron-sized polymers (Pritchett 2003, Frampton 2004, U.S.
patent
6984705). Their major differences are their sizes and swelling times.
Published
documents indicate that several particle gels were economically applied
to
reduce water production in mature oilfields. For example, preformed
particle
gels-PPGs have been applied in about 2,000 wells to reduce fluid
channels in
waterfloods and polymer floods in China (Liu 2006). Recently,
Occidental
Oil Company (Pyziak 2007) and Kinder-Morgan also used similar product
to
control CO2 breakthrough for their CO2 flooding
areas and
promising results have been found. PPG treatment has been widely
accepted and
is seeing more use by operators due to their unique advantages over
traditional
in-situ gel, including:
-
Particle gels are synthesized prior to formation
contact, thus overcoming distinct drawbacks inherent in in-situ
gelation systems, such as uncontrolled gelation times and variations in
gelation due to shear degradation, and gelant changes from contact with
reservoir minerals and fluids.
-
Preformed particle gels are strength- and
size-controlled, environment-friendly, and they are stable in the
presence of almost all reservoirs minerals and formation water salinity.
-
Preformed particle gels can preferentially enter
into fractures or fracture-feature channels while minimizing gel
penetration into low permeable hydrocarbon zones/matrix. Gel particles
with the appropriate size and properties should transport through
fractures or fracture-feature channels, but they should not penetrate
into conventional rock or sand.
-
These gels usually have only one component during
injection. Thus, it is a simpler process, and does not require many of
the injection facilities and instruments that often are needed to
dissolve and mix polymer and crosslinker for conventional in-situ gels.
-
These gels can be prepared with produced water
without influencing gel stability. In contrast, traditional in-situ
gels are often very sensitive to salinity, multivalent cations, and H2S
in the produced water. This not only can save fresh water but it also
can protect our environment.
Further Research is needed for
Deploying the Technology in the USA
Unless special efforts
are made during gel placement (e.g., zone
isolation), theoretical studies and field applications demonstrate that
gel
treatments are most likely to be successful when treating fractures or
fracture-like features that cause channeling in reservoirs. Seright et
al, (2003)
have studied the propagation of preformed bulk gels through open
fractures
since 1992. They have also performed extensive core flooding
experiments and
successfully developed a series of theories and methods to characterize
the
propagation of preformed bulk gels through porous media. However,
although the preliminary
studies of particle gel propagation through porous media were performed
by the
product inventors, all core flooding tests used porous media without
channels.
The smallest gel particles, such as microgels, collodial dispersion
gels, and
micron-sized swelling polymers, were marketed only for treatment of
matrix
problems. However, we are interested in whether these particles gels
might have
applicability to fractures and fracture-like channels. The ultimate
purpose of
the project is to provide the fundamental information to select gels
for
mitigating water production and extending field life.Our proposed work
features
the following innovations.
-
The transport of particle gels through fractures and
channels will be first tested using screens and core flooding
experiments. These experiments will give a design basis for fractured
or channeled reservoir gel treatments.
-
The analysis of connecting laboratory and field data
by modern soft-computing methodologies will result in models to
optimize particle treatments in fractured reservoirs.
-
Three novel methods will be tested to improve gel
treatments, including addition of second crosslinker, inclusion of
surfactant, exploration of gravity segregation to aid gel placement.
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