Implications for improved polymer gel conformance control during low-salinity chase-floods in fractured carbonates

Polymer gel is often used to reduce flow through highly conductive fracture networks, frequently present in naturally fractured carbonate reservoirs. When in place, polymer gel efficiently reduces fracture flow and may improve sweep efficiency and oil recovery during water chase-floods. Polymer gel treatments have in some cases been less efficient than expected in reducing fracture conductivity. This may occur because the polymer gel only partially fills the fracture volume, and allows fracture channeling of injected fluids. Low-salinity waterfloods may improve polymer gel blocking of fractures and remedy less efficient polymer gel treatments. Previous experimental work has shown that low-salinity waterfloods, where the salinity is reduced with respect to the gel solvent, reduces fracture channeling and restores matrix flow. This work further investigates low-salinity waterflooding as a method to improve conformance during polymer gel treatments in fractured, low permeable, carbonate rock. The low flow capacity of carbonate may cause a less efficient gel blocking of fractures, although the gel behaves according to established models during injection. Low-permeable carbonate core plugs with open, highly conductive fractures were used for this study. Water flow paths during high-salinity and low-salinity waterflooding were evaluated by positron emission tomography (PET). We found that gel blocking efficiency during chase waterfloods depends on: 1) the salinity of the chase water. Gel blocking efficiency increases with low-salinity water throughput; opposite to high-salinity waterfloods, where the gel blocking is reduced with water throughput. 2) The core material. As expected, a higher pressure was required to maintain flow through the matrix during water chase-floods in low-permeable carbonates. Water was diverted into the matrix for all ranges of permeability: sandstone and low-permeable carbonates, during low-salinity waterfloods. 3) The presence of oil. Fracture channeling prevailed during low-salinity waterfloods in chalk cores with residual oil present.