Characterization of Crosslinked Gel Kinetics and Gel Strength by Use of NMR
- Laura B. Romero-Zeron (U. of New Brunswick) | Florence M. Hum (U. of Calgary) | Apostolos Kantzas (U. of Calgary)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 439 - 453
- 2008. Society of Petroleum Engineers
- 2 Well Completion, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3 Production and Well Operations, 5.8.5 Oil Sand, Oil Shale, Bitumen, 2.2.3 Fluid Loss Control, 4.6 Natural Gas, 4.1.2 Separation and Treating, 1.6 Drilling Operations
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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.
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.
|File Size||2 MB||Number of Pages||15|
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