Summary
Because of their thermal stability, organically cross-linked gels have been
used in conformance treatments for high-temperature applications. Most of these
gels consist of a polyacrylamide-based polymer, and an organic cross-linker.
polyethyleneimine (PEI) has been used as an organic cross-linker for
polyacrylamide-based copolymers.
Allison and Purkaple (1988) reported that PEI can form aqueous gels with
simple polyacrylamide (PAM) at room temperature. In a previous study
(Al-Muntasheri et al. 2008), we have shown that PEI can form ringing gels with
polyacrylamide homopolymers at temperatures up to 130°C (266°F). In addition,
the gelation kinetics of this system was reported up to 140°C (285°F). However,
the placement of this system in porous media was not examined.
In this study, the displacement of this gel system was visualized with
computed tomography (CT) in sandstone cores. A permeability reduction of more
than 94% was realized at 23°C, for cores shut in at 50°C and 45°C.
In addition to CT visualization, this study reports on the bulk testing of
this system over a wide range of concentrations of PAM and PEI. Visual
observation revealed the formation of stable gels for 3 weeks at 100°C (212°F).
These gels were rigid and nonflowing at PAM concentrations of 5, 7, and 9 wt%
cross-linked with PEI at concentrations from 0.5 to 3 wt%.
Introduction
Polymer gels have been used in water control treatments. These treatments
are referred to as conformance improvement treatments (CITs) (Sydansk 1990). A
polymer gel system is based on a polymer and a cross-linker mixed at the
surface. The mixture (gelant) is injected into the water-producing zone, where
it should form a barrier to unwanted or bad water.
The success rate of chemical water shut-off treatments depends strongly on
the identification of the unwanted water production mechanism. Characterization
of the reservoir and the wellbore are needed to better describe the problem
(Smith and Ott 2006).
In high-temperature applications, thermally stable gels are needed and
should be used. Polyacrylamide-based polymers cross-linked with organic
cross-linkers were reported to be applicable for reservoir temperatures up to
121°C (249.8°F) (Moradi-Araghi 2000). A widely applied organic cross-linker is
PEI (Eoff et al. 2007; Stavland et al. 2006). PEI is reported to form stable
gels (through covalent bonding) with different types of polyacrylamide-based
copolymers. For example, polyacrylamide tert-butyl acrylate (PAtBA) (Morgan et
al. 1997), mixtures of acrylamide and acrylamido-2-methylpropane sulfonic acid
(AMPSA) (Vasquez et al. 2003), mixtures of acrylamide, AMPSA and N,N-dimethyl
acrylamide (Vasquez et al. 2005). There are some environmental concerns for the
use of the PEI cross-linker in some parts of the world; hence, other
alternatives were explored (Reddy et al. 2003).
The mechanisms governing the formation of PEI cross-linked gels are reported
elsewhere (Al-Muntasheri et al. 2006). Allison and Purkaple (1988) reported
that PEI can form aqueous gels with simple PAM at room temperature. More
recently, studies indicated the possibility of using PAM polymers with PEI to
form thermally stable gels (Al-Muntasheri et al. 2008). The gelation kinetics
of this system was examined at temperatures up to 140°C (284°F). However, the
placement of this gel system in porous media was not reported.
The objectives of this work are to: (1) examine the thermal stability of the
PAM cross-linked with PEI (PAM/PEI) over a period of three weeks at 100°C
(212°F), (2) assess the performance of the PAM/PEI system in porous media at 45
and 50°C, and (3) visualize the displacement of PAM/PEI gelants (flowing
gelling mixtures) with a low-viscosity fluid using CT at different shut-in
temperatures. To the best of the authors' knowledge, this is the first study
that examined displacement of PAM/PEI gel system in sandstone cores using the
CT technique.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
15 August 2007
- Meeting paper published:
4 December 2007
- Revised manuscript received:
29 April 2008
- Manuscript approved:
21 August 2008
- Published online:
8 October 2009
- Version of record:
12 March 2010
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