Highly crosslinked gels are used in high-permeability reservoirs to achieve good fluid-loss control during well completion and workover operations. Crosslinked gels are also commonly used for shutting off unwanted gas and/or water influx into production wells, and for improving the conformance of the near wellbore injection profile in naturally fractured or in high-permeability reservoirs. In all such applications, the appropriate design of the gel treatment is critical to ensure an efficient gel placement. One important variable of the gel application design is the rheology of the gel system for establishing the crosslinking kinetics and the gel strength after gelation is complete.
Rheology of gels and gelation rates are commonly determined by rheological methods or in a qualitative mode through bottle testing using a well-known gel strength code (i.e., the Sydansk's Code). The rheological measurements can be both time consuming and expensive, while bottle testing can lead to an inconsistent gel description as a result of the subjective nature of the gel strength code. This paper describes the use of low field nuclear magnetic resonance (NMR0 to monitor gelation rates and to characterize gel strength. This technique provides fast and accurate gel strength characterization and gelation monitoring. Through calibration with polymer concentration, crosslinker concentration, aging of gels, and the effect of brine, it is possible to predict NMR parameters. This then allows for a standardized method for the rheological categorization of gels.
The results of this work present the correlation between the NMR spectra, rheological measurements, and the qualitative codification of gel strength.
Gels are swollen polymer networks possessing both 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 cross-linked system of clusters. Clusters range from small, starlike molecules to large heavily cross-linked, and fairly concentrated microgel cores 1.
Water-based gels are obtained by crosslinking linear flexible water soluble polymers using transition metal ions. These gels are highly elastic with a water content (98% to 99%) trapped in the three-dimensional polymer structure of the gel 2. 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 3, such as plugging off lost circulation zones during drilling operations, hydraulic fracturing for stimulating the production of oil and gas formations, controlling excessive water and gas production problems, and plugging depleted wells at the end of the economic life 4,5.
Currently, the most widely used polymer gel-forming compositions employ either a partly-hydrolyzed polyacrylamide (HPAm) or an acrylamide co-polymer and a chromium ion [Cr(III)] crosslinker 6. This network system has been studied extensively both in the laboratory and in the field. Achieving reliable performance of this hydrophilic gel system requires a good understanding of the physical chemical properties, the viscoelastic behavior of gels, as well as the interrelation of these two aspects 3. Previous research studies have addressed primarily the establishment of gelation kinetics 3,4,7–12 and the evaluation of the rheological behavior and mechanical properties of a given gel system 13–18.
These products are usually mixed in surface facilities, then pumped downhole through coiled tubing and injected into the formation over a depth of several feet. For the operators, gelation time and gel consistency after gel shut-in are the two most important parameters to control. The gelation time, which corresponds to a sudden rise in the viscosity, must be long enough to allow for placement of a sufficient gel volume16 before gelation starts: early network formation is undesired 3. Consequently, the rate at which this 3D gel is formed determines how far the solution can be pushed into the rock formation away from the injection well before gelation occurs 7. Gel consistency is related to the maximum pressure drop the gel can sustain within the porous media 16.