In this paper we report results from laboratory experiments designed to study the permeability sensitivity of reservoirs to seawater flooding. Core flood experiments using a new recirculating flow apparatus were conducted on the Hopeman Sandstone (Clashach Sandstone) which was used as an analog of carbonate-free North Sea sandstone reservoirs. The samples were confined to 345 bars and flooded at 0.3 cm³min^-1 under 207 bars pore pressure. During flooding, the fluid was regularly sampled for analysis of cations using high-performance liquid chromatography. The cores were examined for textural, mineralogical, and petrophysical changes after flooding using scanning electron microscopy (SEM), x-ray diffraction (XRD), probe minipermeametry, and ion and ele tron microprobe imaging.

During the tests, which lasted for 5 to 6 weeks, the permeability of the cores declined by as much as 50%. Over the same period, Mg, Ca, and K were removed from the fluid. Integration of fluid data, SEM, XRD, and ion imaging suggests that permeability reductions resulted from (1) pore-throat blockage by clays and other fine mineral particles, (2) precipitation of potassium feldspar (K-feldspar), and (3) precipitation of ferric hydroxide via reduction of ferrous iron. Although permeability damage from pore plugging by fine particles commonly recovered to near starting values, chemical reactions induced permanent permeability damage. Permeability and pore-area measurements conducted on the core after flooding have shown that permeability actually increased near the inlet end of the core but decreased in both the middle and outlet end.

These findings suggest that in reservoirs, permeability reduction due to seawater flooding would be restricted to the near-injector area, leading to loss of injectivity; however, the net effect on productivity in a particular field will depend on the relative magnitude of vertical drive due to compaction, and permeability decline within the reservoir due to creation or reactivation of faults.