Journal of Petroleum Technology July 2012 : Page 30
TECHNOLOGY UPDATE the surfactant is adsorbed on the rock, leading to increased material costs; and the surfactant is prone to degradation in harsh reservoir conditions. The solution pursued in the selected projects is to continue to use foam—as it is effective in increasing CO 2 sweep effi-ciency—but to stabilize it by a different mechanism. Nanoparticles are of great interest for forming emulsions and foams because of their robust chemical stability (even in harsh reservoir conditions) and extremely strong and selective adsorp-tion at targeted fluid-to-fluid interfaces. Their surface coating can be tailored to favor CO 2 /water foam without forming oil/water emulsions. Moreover, the mechanism of gen-erating nanoparticle-stabilized foam by coinjecting CO 2 and nanoparticle disper-sion requires a threshold shear rate. In the field, high shear rates are associated with preferential flow through high per-meability zones. These properties raise the possibility of creating “self-guiding” fluids that selectively reduce CO 2 mobil-ity by generating foam only in regions where CO 2 is flowing rapidly, such as fractures and gravity override regions that contain little oil. The same foam breaks in the presence of resident oil to enable high recovery by contact with the now mobile CO 2 . UT researchers successfully gen-erated CO 2 -in-water foams in a column packed with 180 μm glass beads, using fumed silica as the stabilizer (Espinosa et al. 2010; Worthen et al. 2012). The idea of generating CO 2 -in-brine foams in situ by coinjecting or alternately injecting an aqueous dispersion of nanosilica into a porous medium followed by the injection of CO 2 was intended to provide an alter-native to surfactant-based CO 2 foams for mobility and/or conformance control. When foam was generated, it had two to 18 times more resistance to flow than the same fluids without nanoparti-cles. Nanoparticle foam was generated at temperatures up to 95°C. The researchers also observed the creation of foam with-in fractures, which is even more advan-tageous in carbonates. Foam generation occurred only above a threshold shear rate. The threshold shear rate was inde-pendent of the CO 2 /water ratio, thereby indicating that shear rate is of the first-order influence. The team at UT demonstrated that low-cost, natural nanoparticles such as nanoclays, fumed silica, or inexpensive iron oxide nanoparticles, can be coat-ed to provide foam-stabilizing perfor-mance similar to that of surfactants— with the economics comparable to or better than that of surfactants. The team also observed the remarkable capability of a component of fly ash to stabilize oil/ water emulsions and CO 2 /water foams (NETL 2011a). After constructing the apparatus to test nanoparticle foams at reservoir con-ditions, the researchers examined vari-ous coated and uncoated nanoparticles. Foam was generated by coinjecting liquid CO 2 at ambient temperature and reser-voir pressure (1800 psia) and an aqueous solution of nanoparticles. In the follow-ing example, uncoated and coated com-mercially supplied nanoparticles in the range of 6 nm to 20 nm were tested. Fig. 1 shows a photograph of the foam gener-ated with 6-nm nanoparticle dispersion, seen through a view cell window that is 1.5 cm in diameter. (Other denser and smaller-grain foams display only a homo-geneous gray picture and are not shown.) The 6-nm nanoparticle dispersion of 0.5 wt%, coated with short chain PEG molecules, was injected at 1 cm 3 / min along with 5 cm 3 /min CO 2 at 25°C and 1450 psia through a bead pack with 180 μm beads. UT has developed a capability to test for foam generation during flow in rough-walled fractures at reservoir con-ditions. A novel procedure for fracturing cores was developed by placing plastic shrink wrap around the core, warming it to induce shrinking, and placing it in a conventional load frame. Loading the core resulted in failure under tension along the length of the core, resulting in irregular fracture geom-etry similar to naturally occurring frac-tures. The shrink wrap proved effective in confining the path of the fracture. Foam was generated in the fractures by coin-jecting CO 2 and a dispersion of 5-nm PEG-coated nanoparticles at constant flow rate at ambient temperature and a pressure of 1800 psia. The apparent viscosity of the generated foam ranged from 1.1 cp to 2.7 cp, depending on the phase ratio used. The team at the Petroleum Recov-ery Research Center at New Mexico Tech investigated foam generation at dynam-ic conditions and nanosilica particle-stabilized CO 2 foam transport across porous media (NETL 2011b). Initial research showed that stable CO 2 foam could be generated when the nanopar-ticle concentration was in the range of 4,000 ppm to 6,000 ppm, using com-mercial silica nanoparticles. However, it was observed that brine salinity, reser-voir pressure, and temperature can affect foam generation by nanoparticles. CO 2 nanofoam was also dependent on phase ratio and flow rate. Recently, the New Mexico Tech team studied nanosilica particle transport in three core samples: Berea sandstone, Indiana limestone, and dolomite. It was observed that silica nanoparticles could easily pass through the sandstone core without changing the core permeability. Little adsorption was observed as nano-silica particles flooded the limestone core, but the core permeability was not changed. A high particle recovery was obtained with the dolomite core. How-ever, pressure drop across the core was observed to increase continuously, indi-cating that core plugging occurred and core permeability was changed. The coreflood performed in a sand-stone core with permeability of 57.6 md is illustrated in Fig. 2. The figure dis-plays nanoparticle concentration and pressure drop across the core during the coreflood. A slight decrease in pressure drop was observed when the coreflood was switched from nanosilica disper-sion to brine. The decrease in pressure drop occurred probably because of the viscosity change, which removed some adsorbed particles from the pore sur-face. However, there was no evidence of alteration of core permeability or core plugging by the nanosilica particles (Yu et al. 2012). 30 JPT • JULY 2012
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