Abstract
Polymer gels can be placed in fractures within subsurface reservoirs to improve sweep efficiency during subsequent floods, and its success is largely determined by the gel’s ability to completely occupy the fracture volume. Gel volumetric properties may be influenced by mechanical and chemical conditions. In this work, gel volume sensitivity to salinity contrast is investigated. Previous bulk gel studies showed that water-based gel swelled in contact with lower-salinity water and shrunk in contact with higher-salinity water. Recent core-scale experiments demonstrated that gel blocking efficiency after rupture was also impacted by the salinity of the injected water phase. Gel treatments (after gel rupture) become less efficient in controlling fracture flow with time and water throughput during water injection without salinity contrasts. However, by reducing the salinity of the injected water phase with respect to the gel, blocking efficiency may be maintained, or even improved, over time. The coupling between gel deformation during swelling/shrinking and dynamic fluid flow is complex and can initiate changes in mechanical or transport properties, included formation of fluid flow paths through the gel that are not easily distinguished during conventional core floods. In-situ imaging by positron emission tomography (PET) was utilized to gain access to local flow patterns in this work, and combined with pressure measurements to characterize complex flow phenomena in a fractured, gel-filled system. Gel rupture was quantified several consecutive times during low-salinity waterflooding. Increasing rupture pressures indicates continuous gel strengthening during low-salinity water injection. PET imaging revealed that gel swelling occurred during low-salinity waterfloods, to constrict water pathways through the fracture. Gel swelling was sufficient to restrict fracture flow completely, and injected water was diverted into the rock matrix adjacent to the fracture. Injected water continued to pass through gel at elevated pressure gradients, but continuous flow paths did not form. This observation supports the notion of gel as a compressible, porous medium.
Highlights
Polymer gel is often injected into subsurface reservoirs to reduce fracture conductivity; subsequently injected fluids may be diverted into the porous rock instead of channeling through the highly conductive fracture network
Gel rupture was achieved at a pressure gradient of 5.7 psi/ft, which is in the lower range of previously measured rupture pressures for the same core material, gel placement rate and volume (Brattekås et al 2015)
The gel rupture path was measured by positron emission tomography (PET) at t = 0.35–0.45 h, when the pressure gradient was stable after gel rupture (Fig. 5)
Summary
Polymer gel is often injected into subsurface reservoirs to reduce fracture conductivity; subsequently injected fluids may be diverted into the porous rock instead of channeling through the highly conductive fracture network (see, e.g., Sydansk and RomeroZerôn 2011). The polymer structure of fully formed gel is restricted to fractures during injection, but the water content of the gel may be reduced by leakoff, further described by Seright (2003a, b) and Brattekås et al (2019). The wormhole size increases with time and water throughput, and the gel blocking efficiency decreases as the wormhole expands due to decreased flow resistance This corroborate reports from several other authors, concluding that gel becomes decreasingly efficient in reducing fracture flow with time and water throughput when a pressure gradient is imposed on the gel (Ganguly et al 2002; Seright 2003a, b; Willhite and Pancake 2008; Brattekås et al 2015)
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