Summary
Highly crosslinked gels are used in high-permeability reservoirs to achieve
appropriate fluid-loss control during well completion and workover operations.
Crosslinked gels are also used to shut off unwanted gas and/or water influx
into production wells and to improve the conformance of the near-wellbore
injection profile in naturally fractured or high-permeability reservoirs. In
all these applications, the appropriate design of the gel treatment is critical
to ensure an efficient gel placement. Important variables of gel systems are
gel rheology and gel strength during and after the gelation reaction is
completed.
The rheology of gels and gelation rates is commonly determined by rheometry or,
in a qualitative mode, through bottle testing with well-known gel-strength
codes (i.e., Sydansk’s code). Rheological measurements can be time-consuming,
while bottle testing can lead to an inconsistent gel description as a result of
the subjective nature of the gel-strength code. This paper evaluates the use of
low-field nuclear magnetic resonance (NMR) as a nonintrusive technique to
monitor gelation rates and to characterize gel strength. Because of the
nonintrusive nature of this technique, it could be considered to be a better
alternative to conventional rheological measurements and common qualitative
methods, such as gel-strength codes. In addition, NMR could offer faster and
more accurate gel-strength characterization and gelation monitoring compared to
rheological methods. Furthermore, it can be used in porous media. NMR
parameters are predicted and calibrated conducting concentration sweeps of
polymer, crosslinker, and brine, as well as gelation-time sweeps. This then
allows for a standardized method for gel characterization.
The findings of this work include a preliminary assessment of the use of
different techniques, such as low-field NMR, rheometry, and bottle testing, for
monitoring the gelation reaction and gel strength of partially hydrolyzed
polyacrylamide chromium [(HPAm)/Cr(III)] acetate gel. The experimental results
also include the initial identification of the gel point for different
formulations of the gel system using low-field NMR.
Introduction
Gels are swollen polymer networks that possess the cohesive properties of
solids and the diffusive transport properties of liquids. If some of the bonds
holding the gel network together can "make and break," the gel is
called reversible. If the bonds do not dissociate, the gel is called permanent.
A permanent gel tends to carry the history of its formation in its structure,
and it is best described as a crosslinked system of clusters. Clusters range
from small, starlike molecules to large, heavily crosslinked, and fairly
concentrated microgel cores (Silberberg 1989).
Water-based gels can be obtained by crosslinking linear flexible
water-soluble polymers by use of transition-metal ions. These gels are highly
elastic, with 98 to 99% water content trapped in the 3D polymer structure of
the gel (Vossoughi 2000). Water-based gels exhibit a wide range of static and
dynamic physical properties that make them suitable for numerous applications
in the oil and gas industry (te Nijenhuis et al. 2003), such as plugging off
lost-circulation zones during drilling operations, hydraulic fracturing to
stimulate the production of oil and gas formations, controlling excessive
water- and gas-production problems, and plugging depleted wells at the end of
their economic life (Menjivar 1986; Kabir 2001).
Currently, the most widely used polymer-gel-forming compositions use either
HPAm or an acrylamide copolymer and Cr(III) crosslinker (Bryant et al. 1997).
This network system has been studied extensively both in the laboratory and in
the field. The reliable performance of this hydrophilic-gel system in field
applications requires the appropriate understanding of its physical-chemical
properties and its viscoelastic behavior, as well as the interrelation of these
two aspects (te Nijenhuis et al. 2003). Previous studies have mainly addressed
the establishment of gelation kinetics (te Nijenhuis 2003; Menjivar 1986;
Prud’homme et al. 1983; Shu 1989; Sydansk 1988; Tackett 1989; Lockhart 1994;
Lockhart and Albonico 1994) and the evaluation of the rheological behavior and
mechanical properties of a given gel system (Chauveteau et al. 2000; Kakadjian
et al. 1999; Liu and Seright 2000; Broseta et al. 2000a; Grattoni et al. 2001;
te Nijenhuis 1997).
The polymer and crosslinker usually are mixed in surface facilities, pumped
downhole through coiled tubing, and injected into the formation over a depth of
several feet. For the operators, gelation time, or gel point, and gel
consistency after gel placement in the formation are the two most important
parameters to control. The time at which the gel is "set" is known as
"gel point." At this point, the solution just transforms into a gel (te
Nijenhuis et al. 2003), or the crosslinking reaction begins (Seymour and
Carraher 1988). Gel point, which corresponds to a sudden rise in viscosity,
must be long enough to enable placement of a sufficient gel volume before
gelation starts: Early network formation is undesirable (te Nijenhuis et al.
2003; Broseta et al. 2000a). Consequently, the rate at which this 3D gel is
formed determines how far the solution can be pushed into the rock formation
and away from the injection well before gelation occurs (Prud’homme et al.
1983). Gel consistency is related to the maximum pressure drop the gel can
sustain within the porous media (Broseta et al. 2000a).
The gelation reaction of HPAm/Cr(III) acetate gels and the determination of
gel point and the strength of this polymer network have been commonly studied
by visual observation through bottle testing by use of a strength-code table
(Sydansk 1988), through the evaluation of yield stress, or by rheological
monitoring of the gelation.
This paper evaluates the use of low-field NMR as a nonintrusive technique to
monitor gelation rates and to characterize gel strength. The main advantage of
using low-field NMR is that it allows a simple, accurate, and fast
determination of fluid physical properties, such as viscosity. Furthermore, its
nondestructive attribute makes possible the characterization of polymer gels
without disruption of the network structure, and it can be applied in rock
formations under characteristic shear rates of gel flowing through porous media
(Bryan et al. 2002b). Finally, it offers the possibility of downhole evaluation
of gelation with certain well completions.
This study aims to verify the hypothesis that there is a relationship
between NMR bulk relaxation rate and the density of the crosslinked network in
HPAm/Cr(III) acetate gels. Thus, monitoring gel formation using low-field NMR
enables the determination of gel point (the point at which crosslinking begins)
(Seymour and Carraher 1988), gel strength, and the onset of gel syneresis.
Three techniques are used in this work to evaluate the crosslinking process of
an aqueous HPAm/Cr(III) acetate gel: bottle testing, rheometry, and low-field
NMR relaxation. The characterization of the gel system is performed as a
function of polymer, crosslinker, and salinity concentration.
The first part of this paper presents a brief literature review on gelation
kinetics, basics of the rheological characterization of gels, and the
fundamental aspects of low-field NMR theory. The second part of this work
summarizes the experimental procedures and results and presents interpretation
and discussion of the experimental findings.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
5 May 2004
- Meeting paper published:
18 February 2004
- Revised manuscript received:
26 February 2007
- Manuscript approved:
19 January 2008
- Version of record:
20 June 2008
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