Summary
Organically crosslinked gels have been used to control water production in
high temperature applications. These chemical systems are based on the
crosslinking of a polyacrylamide-based polymer/copolymer with an organic
crosslinker. Polyethyleneimine (PEI) has been used as an organic crosslinker
for polyacrylamide-based copolymers to provide thermally stable gels.
Literature reported that PEI can form aqueous gels with polyacrylamide (PAM) at
room temperature. In this paper, we show for the first time the possibility of
crosslinking polyacrylamide with PEI at temperatures up to 140°C (285°F) and
pressures up to 30 bars (435 psi). This paper reports data both in bulk and in
porous media.
The gelation time of the PAM crosslinked with PEI at high temperatures up to
140°C (285°F) and pressures up to 435 psi (30 bars) was measured. The effects
of polymer concentration, crosslinker concentration, temperature, salinity,
initial pH value, and the initial degree of hydrolysis of the polymer on the
gelation time were examined in detail. All measurements were conducted in the
steady shear mode. 13C Nuclear Magnetic Resonance Spectroscopy
(13C NMR) was used to relate the gelation time to changes in the
structure of the polymer and hence explain the variation in the gelation time
in terms of the gelling system chemistry.
In bulk, thermally stable gels were obtained by crosslinking PAM with PEI at
130°C (266°F) for at least 8 weeks. The performance of the PAM/PEI system in
sandstone cores at a temperature of 90°C (194°F) and pressure drops of 68.95
bars (1,000 psi) was examined. The system was found to be stable for 3 weeks,
where the permeability was reduced by a factor of 100%.
Introduction
Water production is a serious problem in petroleum-producing operations.
Additional costs are imposed by processing, treating, and disposing unwanted
water. Of the available remediation techniques, chemical methods using polymer
gels have been widely applied. The success rate of these chemical treatments
depends, among other factors, on the understanding of gelation kinetics,
gelant’s compatibility with reservoir fluids, and thermal stability of the
final gel.
Polymer gels have been used to reduce water production through the
disproportionate permeability reduction (DPR) (Zaitoun and Kohler 1988; Liang
et al. 1995). In DPR, the relative permeability to water is reduced to a
greater extent than that to oil (or gas). Polymer gels were also used to
totally block the pore space of the water producing zones in both matrix
(Vasquez et al. 2003) and fractures (Alqam et al. 2001).
Polymer gels are generally classified into two categories based on the nature
of polymer/crosslinker bonding chemistry. The first type is inorganic gel
systems based on the crosslinking of the carboxylate groups on the partially
hydrolyzed polyacrylamide chain (PHPA) with a trivalent cation like Cr(III)
(Sydansk 1990; Lockhart 1994). This crosslinking is believed to rely on
coordination covalent bonding. It should be mentioned that
Cr(III)-carboxylate/acrylamide-polymer gels (CC/AP) were reported to be stable
at temperatures up to 148.9°C (300°F) in Berea cores under pressure drops of
68.95 bars (1,000 psi) (Sydansk and Southwell 2000). The second class of
polymer gels is based on covalent bonds between the crosslinker and the
acrylamide-based polymer (Morgan et al. 1998; Moradi-Araghi 2000). High
temperature applications require the use of thermally stable covalently bonded
systems. However, these covalent bonds do not guarantee long-term stability.
Literature reports (Moradi-Araghi 2000) highlight the importance of using a
thermally stable polymer to produce thermally stable gels. Polyacrylamide-based
polymers are known to hydrolyze at high temperatures causing gel syneresis
(expulsion of water out of the gel structure due to over crosslinking)
(Moradi-Araghi 2000), especially in brines with high contents of
Mg+2 and Ca+2, where polymer precipitation may also occur
(Moradi-Araghi and Doe 1984). Therefore, more thermally stable monomers are
copolymerized with the acrylamide polymer to minimize excessive hydrolysis
(Moradi-Araghi et al. 1987; Doe et al. 1987) and enhance thermal stability of
the produced gel. .
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
15 June 2006
- Revised manuscript received:
29 October 2007
- Manuscript approved:
19 March 2008
- Version of record:
20 September 2008
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