PETE 406 KFUPM Analysis of Material from Natural Sources that Is Used or Crosslinking Acrylamide Based Polymers Essay Background , table of content , intro

PETE 406 KFUPM Analysis of Material from Natural Sources that Is Used or Crosslinking Acrylamide Based Polymers Essay Background , table of content , intro , Body , conclusion . use the figures and refer to the paper in the references section A Natural Polymer-Based Cross-Linker
System for Conformance Gel Systems
B.R. Reddy, Larry Eoff, E. Dwyann Dalrymple, SPE, Kathy Black, David Brown, and Marcel Rietjens, Halliburton
Summary
This paper describes a material derived from natural sources that
can be used to crosslink a variety of acrylamide-based polymers
over a broad temperature range to produce gels for conformance applications.
Delayed crosslinked polymer systems have been used for many
years in conformance applications. For the past decade, the most
widely used system has been based on chromium (3+) crosslinked
polyacrylamide. Organic crosslinkers, such as phenol/formaldehyde
and polyethyleneimine (PEI) have also been used with a variety of
polymers. However, these systems are being scrutinized by governmental agencies and have been scheduled for phaseout in some
countries. Because of these issues, a single, environmentally
friendly crosslinker that could be used with a variety of polymers
over a broad temperature range was the focus of this study.
This paper details the laboratory development of an environmentally friendly, natural polyamine crosslinker system. This
crosslinker can be used with a variety of polymers, such as polyacrylamide, AMPS/acrylamide, or alkylacrylate polymers. Gels
ranging from stiff and “ringing” type to “lipping” gels have been
obtained. The data illustrate a simple, commercially available system that can be applied to field operations. Potential crosslinking
mechanism(s) of the system will be discussed.
Introduction
Water production in oil-producing wells becomes a more serious
problem as the wells mature. Remediation techniques for conformance control are selected on the basis of the water source and the
method of entry into the wellbore. Treatment options include sealant treatments and relative permeability modifiers (also referred to
as disproportionate permeability modifiers). This paper primarily
discusses water control with water-based gels for applications in
wells in which the oil- and water-producing zones are clearly
separated and can be mechanically isolated.
Chromium(III) crosslinked polyacrylamide gels can be choice
materials for matrix-fluid shut-off systems.1–4 The crosslinking reactions in these gel systems take place by the complexation of Cr(III)
oligomers with carboxylate groups on the polymer chains (Fig. 1).
Because of the nature of the chemical bond between Cr(III) and
the pendant carboxylate groups, formation of insoluble chromium
species can occur at high pH levels. Other problems with these
systems include thermal instability, unpredictable gel times, and
gel instability in the presence of chemical species that are potential
ligands. The gel times are controlled by the addition of materials
that chelate with chromium in competition with the polymerbound carboxylate groups.5,6
Another popular water-based gel system for water-control applications is based on a phenol/formaldehyde crosslinker system
for homo-, co-, and ter- polymer systems containing acrylamide.7–11
Depending on the polymer composition, these gels are thermally
stable, and the gel times are controllable over a wide temperature
range. The crosslinking mechanism involves hydroxymethylation
of the amide nitrogen, with the subsequent propagation
of crosslinking by multiple alkylation on the phenolic ring
Copyright © 2003 Society of Petroleum Engineers
This paper (SPE 84937) was revised for publication from paper SPE 75163, first presented
at the 2002 SPE/DOE Improved Oil Recovery Symposium, Tulsa, 13–17 April. Original
manuscript received for review 29 May 2002. Revised manuscript received 3 March 2003.
Manuscript peer approved 10 March 2003.
June 2003 SPE Journal
(Fig. 2).12,13 Several variations of the same technology were created to overcome the toxicity issues associated with formaldehyde
and phenol. These processes generally involve replacing formaldehyde and phenol with less toxic derivatives that generate phenol
and formaldehyde in situ, or are themselves active components of
the crosslinking system. For example, formaldehyde can be replaced with hexamethylene tetramine (HMTA), glyoxal, or 1, 3,
5-trioxane. Substitutions for phenol included phenyl acetate, phenyl salicylate, or hydroquinone, among others.12,13 Extensive patent literature for this technology exists.14–22
Recently, a less toxic crosslinker was tested extensively in field
trials worldwide and enjoyed a very high success rate.23–27 This
system is based on PEI crosslinker and a copolymer of acrylamide
and t-butyl acrylate (PA-t-BA). PEI is a low-toxicity material that
is approved in the United States for food contact.28–31 PA-t-BA is
a relatively low molecular-weight polymer. The low molecular
weight is expected to provide rigid “ringing gels.” The crosslinking is believed to take place in situ by amidation of the pendant
ester groups on the base polymer (Fig. 3). Recent test results
indicate that a variety of polymers containing acrylamide pendant
groups react with PEI nitrogens through a transamidation reaction
pathway to provide gels (Fig. 4).32
Because of recent changes in European environmental regulations, PEI is targeted for phase-out from the Norwegian section of
the North Sea within the next few years. A search for biopolymers
containing amino groups suggested that chitosan (Fig. 5) may react
with acrylamide-based polymers in a manner similar to PEI. Chitosan is a polysaccharide obtained by de-acetylating chitin, a homopolymer containing ?-(1-4)-2-acetamido-2-deoxy-D-glucose
(Fig. 6) that occurs in the shell or skin of anthropods or crustaceous water animals. Chitosan is also present in the environment,
although in lesser amounts than the chitin. The degree of deacetylation in the commercially-available chitosan materials is
usually in the 70 to 78% range. The chitosan solubility in acidified
water, for example in acetic or hydrochloric acid, is in the 1 to 2%
range. The viscosity of the solutions depends on the molecular
weight of the polymer. If the pH of the solution is increased above
6.0, polymer precipitation occurs.
This paper presents results using chitosan as an environmentally preferable crosslinker for use in combination with acrylamide-based polymers. Gel treatments using this material should
contain a biocide. The advantage here is that if inadvertently discharged, the chitosan will biodegrade.
Experimental Methods
Preparation of Chitosan Solutions. Commercial solid chitosan
samples were dissolved in fresh water solutions containing 1%
acetic acid to make 1.0 to 1.5% polymer solutions. Chitosan lactate
salt, which is also commercially available, can be dissolved directly in fresh water to prepare solutions with similar polymer
concentration. The viscosities and clarity of the solutions depended
on the polymer molecular weight and the degree of de-acetylation.
Aqueous solutions of chitosan salts are also available commercially, which can be used directly for crosslinking base polymers. The
preformed chitosan salts are insoluble in salt water or seawater.
Viscosities of Base Polymer and Crosslinker Mixtures. All the
base polymers contained acrylamide. Additionally, one polymer,
PHPA, contained acrylate groups (11 mole %) introduced by partial hydrolysis. The molecular weight of this polymer is approximately 6 million. The second polymer, PA-t-BA, contained about
5 mole % of tert-butyl acrylate as the comonomer, and hydrolysis
99
inflection point of the plot (Fig. 7). With this method, gel times are
rarely a function of the viscometer.
A typical viscosity vs. time curve obtained using this method for
chitosan lactate and PA-t-BA mixture at 150°F is shown in Fig. 7.
Method B—Sealed-Tube Method. Either a 16×150-mm borosilicate glass tube with a screw cap or a 10.2-cm pressure tube with
a screw cap was filled with the polymer composition to approximately one-half its capacity. The tube was purged with nitrogen
and the screw cap was sealed with a high-temperature silicone
sealant. In addition, a TEFLON plug was used inside the cap. The
tube was placed inside a steel bomb (container), and the bomb was
placed in a preheated oven set at the test temperature. The steel bomb
was taken out periodically, cooled to below 150°F, and the tube was
removed. The tube was laid on its side, and the liquid level was
measured. Using Eq. 1, the degree of gelation was measured.
Percentage of gelation = 100 ? ??l ? t? ? ?l ? h??, . . . . . . . . . . (1)
Fig. 1—Crosslinking reactions take place by the complexation
of carboxylate groups on the polymer chains by chromium species (Mn+=Cr3+ and/or oligomers).35
was less than 0.5% with a molecular weight of ?500,000. The third
polymer, AMPS/AA, contained about 30 mole % of 2-acrylamido2-methylpropane sulfonic acid. The molecular weight of this polymer was about 5 to 6 million, and the hydrolysis was less than 1%.
The viscosities of a solution containing 7% PA-t-BA and 0.5%
chitosan lactate in 85% fresh water and 15% synthetic seawater
were measured using a Fann 35 Rheometer and a Brookfield LVT
viscometer. The results are shown in Table 1. The viscosity values
observed are in the range typical for polymer solutions used in
conformance applications.
In the case of PHPA and AMPS/AA, because of the very high
polymer molecular weights, the concentrations of base polymers
in aqueous solutions were kept near 0.7% so that the viscosities
of the base polymer solutions were within useful range for practical applications.
Gel Time Measurements. Two methods were used to measure
gel times:
Method A—Brookfield Viscosity Method. Approximately 300
mL of the polymer composition for testing was placed in a 400-mL
beaker. The beaker was then inserted into a preheated, thermostatcontrolled Brookfield viscometer with a No. 2 spindle. The polymer solution was stirred at 10 rev/min, and the viscosity changes
were monitored as a function of time. A plot of viscosity vs. time
can be generated for estimating gel time, which is defined as the
wherein t?length of the liquid/gel level when the tube is in the
horizontal position, l?length of the tube, and h?height of the
initial fluid column when the tube is in the vertical position.
The time needed to reach a gelation of 94% or higher is the gel time.
If gel syneresis occurred after the gel was formed, the syneresis
was measured as described in Eq. 1, except that h equaled the
gel-column height before the gel was aged at a given temperature.
Thermal Stability Measurements. The gels were stored in sealed
tubes in thermostat-controlled ovens kept at 225 and 250°F. Periodically, the test tubes were visually inspected at temperature
(without cooling) for gel syneresis and/or degradation.
Single-Core Flow Testing With a Multitap Flow Cell. The following is a procedure for measuring permeability reduction to water
using the chitosan/PA-t-BA polymer gel system. A more detailed
description of the experimental setup was presented earlier.33
1. Record the test-core dimensions and place the core in a suitable holder, such as a Hassler sleeve apparatus. Place sufficient
overburden pressure on the test core to ensure that no fluid bypasses the core. (Generally, an overburden pressure of 450 to 500
psi over the treatment pressure is used for this purpose.)
2. Determine water viscosity at the temperature that will be
used in the flow tests. All flow series were performed at the desired
test temperature of 175°F.
3. Stabilize water flow through the core in the normal production direction.
4. Pump the treatment through the core in the reverse-flow direction.
5. Repeat Step 3.
6. Determine the core’s residual resistance factor (RRF) for
water by dividing the core’s pretreatment permeability-to-water to
its post-treatment permeability-to-water (RRF?pretreatment permeability-to-water ÷ post-treatment permeability-to-water).
Fig. 2—Crosslinking mechanism involving hydroxymethylation of the amide nitrogen, with subsequent propagation of crosslinking
by multiple alkylation on the phenolic ring.
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June 2003 SPE Journal
Fig. 3—Less-toxic crosslinker based on polyethyleneimine
(PEI) crosslinker and a copolymer of acrylamide and t-butylacrylate (PA-t-BA).
Fig. 4—Polymers containing acrylamide pendant groups react
with PEI nitrogens through a transamidation reaction pathway
to provide gels.
Environmental Testing. Inhouse BOD (Biochemical-OxygenDemand) tests were done according to HACH Method 10099,
and Chemical-Oxygen-Demand (COD) tests were conducted according to HACH Method 8000 described in the HACH Water
Analysis Handbook.34
very low (250°F) and
pH Effects. The results in Table 4 show that as the pH is decreased, the gel times are increased. At the lower temperatures, this
observation is in accordance with expectations for amine-type
crosslinkers. In acidic media, the lone pair of electrons on the
amine nitrogens are expected to be protonated, thus making them
unavailable to initiate a nucleophilic attack on an amide carbonyl
group of the base polymer (Fig. 4). By controlling the pH, the gel
times could be adjusted to fit the need. The gel-time decrease
appears to be independent of the organic acid used to lower the pH
of the chitosan solutions. For example, when the pH of the chitosan solution was lowered, with either acetic or lactic acid, similar
gel times were obtained. This effect is less obvious in the higher
temperature experiments because of the short gel time and inherent
experimental limits on gel-time measurement.
Mix Water. The mix water effects on the crosslinking reaction of
PA-t-BA with chitosan salts were also tested. The results are presented in Table 5.
Test results show that the reactions proceed significantly faster
in fresh water compared to seawater even though the amount of
seawater, or 2% KCl solution in the total composition, is only 15
to 18%. Similar results were observed in the crosslinking reactions
of PA-t-BA with PEI.
Temperature. The temperature effects observed are presented in
Fig. 8 and are as expected. As mentioned earlier, partially hydrolyzed polyacrylamide provides faster crosslinking rates than the
acylamide copolymers containing t-butylacrylate or AMPS as the
comonomers. Greater steric hindrance to the approach of an amino
group at the acrylamide carbonyl group flanked by bulky groups
may explain the differences in the observed differences in the
crosslinking reaction rates.
Fig. 5—Chitosan.
June 2003 SPE Journal
101
Fig. 6—Chitin.
Gel Thermal Stability. The gels turned light brown within a few
days at the oven temperatures. However, no syneresis was observed over a 6-week period in the 200 to 250°F range.
Multipressure Tap Core Flow Tests. The core flow study results
are shown in Fig. 9 and Table 6. A Berea sandstone core was used
as the test medium, the evaluation temperature was 190°F, and 7%
KCl was used as the evaluation brine. After stabilizing the flow of
the brine through the core, a 100-mL treatment (composed of 7%
polymer, 0.5% chitosan crosslinker, a pH of 5.7, using seawater as
the makeup water) was injected through the core in the reverseflow direction. The core was then shut in overnight. The effective
viscosity of the polymer/crosslinker solution within the rock during treatment was calculated using the initial permeability as a
constant and solving for viscosity using Darcy’s linear flow equation (Eq. 2):
?=
AK?P
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)
QL
The value was determined to be 13.9 cp at 190oF. As seen in Fig.
10, the permeability values were fairly constant during the treatment stage, indicating that the fluid was moving evenly throughout
the core and no “face plugging” of the core was occurring. The end
segments of the core typically show lower permeability than the
center two segments. This result is normal and is caused by “end
effects” (short segment length, contamination of the core ends via
the fluid used to drill the cores, etc.). This data is used only to help
determine the occurrence of “face plugging” in fluid treatments.
Following the overnight shut-in period, the flow of 7% KCl
was resumed to determine the effective permeability of the core
after treatment. As illustrated in Fig. 9 and Table 6, the chitosan
crosslinked polymer formed a very effective porosity seal of the
Berea sandstone core, resulting in a RRF of 3310. No wash-off or
removal of the gel treatment was observed during an overnight
flow study.
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Environmental Issues
Chitosan occurs in nature in small amounts. Chitosan-degrading
enzymes, namely chitinases, chitosanases, and lysozymes that degrade chitin-derived materials, occur in bacteria, fungi, algae
mammals, birds, fish, and so on.
Chitosan and the constituent monomer, glucosamine, are approved for human consumption as dietary supplements. It is currently used in the U.S. as an ingredient in animal feed on an
experimental basis, and as a filter medium for potable water.
Toxicity. An LD50 value of 16 grams per kilogram body weight in
mice when administered orally was reported by Arai et al.34 According to this group, the LD50 values are of the same order of
magnitude as lethal doses of sugar (sucrose) or salt.
The aquatic toxicity values (obtained by a supplier) for the
glycolate salt of chitosan are:
• Acute fish toxicity: LC 50>100 mg product/liter.
• Acute bacteria toxicity: EC 0>100 mg product/liter.
Biodegradability and Persistence. According to the supplier, chitosan is “readily and rapidly biodegradable: all individual organic
substances contained in the product achieve values in tests for
ready biodegradability (e.g., OECD 301 A-F) of at least 60%
BOD/COD or 70% DOC reduction (tolerance value for classification as ‘readily biodegradable’: >?70% DOC reduction or
>?60% BOD/COD in 28 days).”
A 5-day inhouse environmental test of chitosan showed a BOC/
COD of 54% compared to 32% for hydroxyethyl cellulose (HEC).
Conclusions
1. Chitosan is an effective crosslinker for acrylamide-based watersolu…
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