Synthesis and
Flow Modeling of Reactive Polymers
This project will: couple state-of-the-art
polymer synthesis with state-of-the-art flow modeling. The results of the
research will improve methods for tailoring polymers to specific industrial
applications.
Research Team
Primary Faculty Co-Advisors:
William H. Daly (Synthetic Polymer Chemistry)
Karsten E. Thompson (Flow and Reaction in
Porous Media)
Graduate Students:
Veronica Holmes (Chemistry)
Matt Balhoff (Chemical Engineering)
Off-Campus Participants:
Schlumberger: Modeling and Mechanics Research
Group
Paul Heyliger (
Technical
Proposal
The flow and reaction of polymers in porous materials
is of interest for a number of reasons including the manufacture of composite
materials, various improved-oil-recovery techniques, and certain environmental
processes. Effective design of these processes is challenging because the
polymer chemistry has a strong effect on reaction and fluid rheology, which in
turn have a strong effect on fluid transport.
In this research we will study how specific
modifications to polymer structure affect transport and reaction of
polymer-based fluids in porous media. The broad goal is to improve methods by
which polymer synthesis chemistry is integrated with engineering process
design. The specific application we will address involves the use of guar
polymers during hydraulic fracturing. Guar is used in delayed crosslinking
solutions to deliver proppant particles into fractured petroleum wells, and the
success of these treatments depends on how well one can control the rheology,
reaction kinetics, and polymer degradation. Hence, a thorough understanding of
how to modify the polymer chemistry and how these modifications affect flow is
important.
Fig. 1 is a flowchart that summarizes the research
project; it shows the feedback loops that exist between the chemistry and the
engineering parts of the research.
Overview
of Technical Application
To enhance hydrocarbon recovery from oil-bearing
formations, it is common, particularly in formations of low permeability, to
hydraulically fracture the hydrocarbon-bearing formation to provide flow
channels to facilitate production of the hydrocarbons to the wellbore.
Fracturing fluids typically comprise a thickened base fluid, which primarily
promotes the suspension of particulate proppant materials in the fluid. These
proppant materials, typically sand, sintered bauxite or the like, will remain
in place within the fracture when fracturing pressure is released thereby holding
the fracture open. The thickened fluids also aid in the transfer of hydraulic
fracturing pressure to the rock surfaces and help to control leak-off of the
fracturing fluid into the formation.
The most common type of fracturing fluid comprises a
polymer thickened base fluid wherein the thickening polymer comprises a
galactomannangum (guar gum), a cellulosic polymer or a synthetic polymer. To
increase the viscosity and, thus, the proppant carrying capacity as well as to
increase high temperature stability of the fracturing fluid, cross-linking of
the polymers is commonly practiced. Typical cross-linking agents comprise
soluble borates or titanates. These metal ions provide for cross-linking by
forming intra- and intermolecular polyesters with the hydroxyl groups on the
anhydroglucose repeat units. In order to
reduce the pumping friction pressure in such fluids, various methods of
delaying cross-linking of the polymers have been developed. This allows the
pumping of a relatively less viscous fracturing fluid having relatively low
friction pressures within the well tubing with cross-linking being effected at
or near the subterranean formation so that the advantageous properties of the
thickened cross-linked fluid are available at the rock face.
One difficulty with
polymer-thickened fluids is the deposit and retention of polymer residues at
the rock face and within the proppant pack, which can reduce the effectiveness
of the fracturing operation. While there have been significant advancements in
the use of oxidative or other gel breaker systems to reduce the effects of a
polymer filter cake and other polymer residue within the fracture, such methods
are never one hundred percent effective in cleaning the fracture. We would like to investigate the chemistry
associated with the crosslinking and gel breaking processes to ascertain the
factors controlling the extent of residue formation and the nature of these
polymeric residues. The goal of the
research is to produce more effective polymer thickeners as well as provide key
experimental parameters for modeling studies of the processes. The modeling
will allow us to understand how changes in rheology and reaction chemistry
affect the overall treatment.
Proposed Work
The proposed research is divided
into the following tasks to be performed by the participants in chemistry and
engineering respectively. Although the tasks are divided as such, continuous
collaboration will be necessary: rheological and kinetic parameters for new
polymers will be provided to the engineering group as they are discovered. The
effect of these factors on flow behavior will then provide direction for the
polymer synthesis work.
Proposed Research – Chemistry
Chemistry
Task I: Characterization of Guar Standard
The
solution properties of commercially available guar samples will be determined
including shear viscosity, molecular weight and molecular weight
distribution. This data will be used as
a control for further experiments with modified guar materials.
Chemistry
Task II: Preparation of Water Soluble Monomer Based Graft Copolymers
We will
prepare graft copolymers of guar with acrylic acid and/or acrylamide using
water soluble free radical initiators.
It is expected that these copolymers will exhibit substantially altered
shear viscosity and will be less subject to shear thinning at high shear
rates. The differences in properties
will provide additional parameters for the modeling studies.
Chemistry
Task III: Introduction of Initiator Functional Groups unto Guar Backbone
We will
evaluate the reaction of cyclic Barton carbonates with guar to establish the
potential for introducing Barton carbonate functional groups, which could be
used to initiate graft copolymerization of hydrophobic monomers such as styrene
and vinyl toluene. If this chemistry is
not successful, it should be possible to activate the guar for atom transfer
polymerization by introducing haloester substituents.
Chemistry
Task IV: Prepare Hydrophobically Modified Guar
By
copolymerizing styrene or vinyl toluene with the activated guar, very low
levels of grafting (<0.5 wt%) will be introduced on the guar backbone. The
water solubility of the products will be retained, but these polymers should
aggregate at low shear. These materials
will exhibit high viscosities at low shear, but will tend to shear thin
readily. The properties of these
materials should be distinctly different from the guar-g-acrylic acid
copolymers. Incorporation of the grafts
should not impact the reactivity of the guar backbone with cross linking and
gel breaking reagents, but this will need to be confirmed.
Chemistry
Task V: Impact of Crosslinking/Gel Breaking Cycle
The effect
of borate cross linking on guar and the two types of guar derivatives will be
evaluated. First the efficacy of the
cross linking agent on the rate of gel formation will be determined to
ascertain the impact of grafting on this process. Secondly, the resultant gels will be treated
with appropriate gel breakers to estimate the speed of solubilization. Finally, the polymer component of each system
that has been subjected to a gelation/gel breaking cycle will be isolated and
characterized. We need to determine if
the gel breaking process is simply a reversal of the crosslinking process or if
the gel breaking indeed results in degradation of the guar molecules. Inorganic gel breakers will compared with
enzymatic gel breakers, which are known to cleave polysaccaride chains.
Proposed Research – Engineering.
Engineering
Task I: Sphere based modeling of polymer flows.
Sphere-based
network modeling is a powerful technique for modeling flow in porous media and
is especially applicable to this problem since the proppant packs are
unconsolidated packings of sphere-like particles (Fig 2). Previously we have
used network modeling to study Newtonian flows as well as interfacial
reactions. In this task, network-modeling techniques will be adapted for use
with shear-thinning and/or viscoelastic fluids, and will be made amenable to
modeling reactive flows.
Engineering
Task II: Empirical relations for fluid rheology.
Rheological
measurements will be made of the graft copolymers of guar (see Chemistry Task
II), from which simple constitutive equations will be generated for use with
the network modeling. Of particular
interest are rheological changes that occur as the polymer structure or
molecular weight distribution are varied. Empirical equation describing these
relationships will allow us to model reactive, non-Newtonian flows in porous
materials.
Engineering
Task III: Mechanical stresses in the sphere packs.
We will
work with Dr. Paul Heyliger to model the behavior of sphere packs under the
influence of external stresses (i.e., from the fracture walls). This
information must be coupled with the flow modeling, so that changes in the pore
structure due to mechanical stresses are accounted for.
Engineering
Task IV: Sensitivity Analysis.
The network
model will be used to perform a sensitivity analysis, which will help elucidate
the variables that most strongly affect flow.
Parameters that will be studied include reaction kinetics, flowrate,
rheology, particle size, and particle size distribution. This analysis will
provide direction for the chemistry research, because it will show which
properties can most effectively improve the process at the engineering level.
Engineering
Task V: Upscaling.
The maximum
size of network-model simulations is significantly smaller than the
characteristic cell size in continuum models of the fracture. We will study
numerical approaches for upscaling, which will generate parameters used at the
continuum level (Fig. 3). The challenges lie mainly in the slow
computation times for the microscale model, which prevent it from being
integrated directly into a continuum model. Alternatives such as the generation
of a surface for multidimensional interpolation will be investigated.
Consistency with the Macromolecular Education, Research & Training
theme
The project will entail
synthesis and characterization of graft copolymers on a natural polymer
matrix. The students will be learning
synthesis techniques and standard spectral characterization techniques to
produce new polymeric materials. Starting with the guar substrate, the solution
properties of each polymer synthesized will be determined. The data obtained
will be used to develop a model for reactive flow in porous materials. Through
the modeling, the students will learn techniques for modeling non-Newtonian
behavior. They will develop new approaches for modeling the behavior of these
complex fluids in large, heterogeneous materials.
How does the project
form a vector cross-product of existing research themes by the
participants?
Daly's research group has a well-established program on the
synthesis and modification of biomacromolecules, including the production of
graft copolymers. Thompson's group
performs experimental and computational research to solve engineering problems
related to transport and reaction in porous media. A new research direction is
now possible because as new polymers are created, we can immediately assess how
they perform in a flow situation. Conversely, when as optimal rheological and
kinetic properties are discovered through computational modeling, we can focus
on defining the chemical structures that will produce these conditions. For
this project, we have focused the diverse expertise of the two groups on a very
practical problem in Louisiana: improved oil production.
How do students benefit from the team-oriented research, beyond what
would be available to them from either advisor separately?
Because of the
collaboration, the chemistry student will be able to participate in flow
simulation, which will allow the effects of new synthesis to be seen at a level
beyond the benchtop. The engineering student on the project will have access to
expertise and equipment in polymer characterization not available otherwise,
which will allow answers required for the modeling to be found. Both students
will see the practical applications of the research and will learn many key
skills required for commercial product development.
Briefly describe the support level available to each
individual faculty or off-campus participant (i.e., without IGERT).
The LSU faculty are independently supported for research in
related fields, with a net worth of 300,000. Schlumberger maintains extensive
research facilities, with Ph.D. scientists and engineers working in modeling,
polymer rheology and synthesis, and applied engineering design. A significant
part of the flow research will be performed at Schlumberger’s research facility
by the engineering student on the project. Prof. Paul Heyliger maintains an
independently funded research program on the mechanics of materials. A
collaborative proposal is currently being written that will fund the mechanics
part of this project.
Interdisciplinary
strengths of the team project:
Daly is trained as an organic polymer chemist; typical
activities in his group include synthesis of monomers, polymerization into
macromolecules, purification, and various spectral identification techniques
and simple analytical methods such as viscosity and GPC. Thompson is trained as a chemical engineer;
typical activities in his group include computational modeling of low-Reynolds
number flows, network modeling of multiphase flow, continuum modeling of reactive
polymer flows, and experimental and computational research on interfacial mass
transfer.
Commitment
of faculty & off-campus participants to work side-by-side with
apprentices:
Daly will spend at least one month during the summer
introducing an apprentice graduate student to the techniques required to
produce graft copolymers. The student
will have an opportunity to learn organic synthesis and polymer modification
techniques along with the typical approaches to structure identification and
polymer characterization. Thompson will spend two, two-week periods during the
fall of 2001 with the apprentice graduate student developing new network
modeling techniques for non-Newtonian fluids. Additionally, he will work with
the scientists at Schlumberger to define the student’s off-campus research
during the time spent at Schlumberger’s facility.
References
Taunk, K.; Behari, K.,
“Graft Copolymerization of Acrylic Acid onto Guar Gum,” J. Appl. Polym. Sci. 2000,
77, 39-44.
Wunderlich, T. et. al., “Shear and
Extensional Rheological Investigations in Solutions of Grafted and Ungrafted Polysaccharides,” J. Appl. Polym. Sci. 2000,
77, 3200-3209.
Daly, W. H.; Evenson, T. S., “Grafting of
Vinyl Polymers to Poly(aryl ether sulfone) Utilizing
Barton Ester Intermediates and Nitroxide Mediation,” Polymer, 2000, 41, 5063-5071.
Sorbie, K.S.; Clifford, P.J.; Jones, E.R.W., “The Rheology
of Pseudoplastic Fluids in Porous Media Using Network
Modeling,” J. Colloid Interface Sci., 1989, 130, 508.
Thompson,
K.E.; Fogler, H.S., “A Pore-Scale Model for Fluid
Injection and In-Situ Gelation in Porous Media,” Phys. Rev E., 1998, 57,
5825.
de Kruijf,
A.S.; Roodhart, L.P.;
Davies, D.R., “Relation between chemistry and flow mechanics of borate-crosslinked fracturing fluids,” SPE Production & Facilities, 1993, 8, 165-169.
Pope, D.S.; Leung, L.K-W.; Gulbis, Janet; Constien, V.G.,
“Effects of viscous fingering on fracture conductivity,” Proceedings - SPE
Annual Technical Conference and Exhibition, 1994, vPi npt1, 491-506.
Stromberg, J.L.; Brown, D.; Curtice, R.J., “Modeling the effects of time, temperature, and shear on the hydration of natural guar gels,” Proceedings - SPE Rocky Mountain Regional/Low Permeability Reservoirs Symposium, 1991, Denver, Apr 15-17, 533-543.