Laboratory experiments with reservoir rock samples should give input to estimation of the potentials of recovery methods. The potential for low salinity water flooding (LSWF) of sandstone oil reservoirs has been reported to depend on the amount of clay and their surface properties. The objective for the reported study was to investigate the effect of KCl water-based mud invasion on the determined LSWF-potential. The wettability of minerals and a sandstone reservoir rock was characterized by flotation tests using two brines (formation water (FW) and mud brine) and two stock tank oils (STOs). During preparation of sandstone reservoir core plugs, effluent samples were analyzed for element compositions to determine whether they were contaminated with KCl mud. Unsteady-state flooding experiments with sea water (SW) and low salinity water (LSW) were carried out in these core plugs. Core plugs were analyzed by scanning electron microscopy (SEM) both after cleaning with solvents and after water flooding experiments. Bentonite consists mainly of montmorillonite clay and is added to water-based muds for rheology and filtration control. In the flotation tests, this clay was found to be less water-wet than the dominating minerals in the original reservoir rock. Rock samples with bentonite invasion can therefore become more oil-wet than the original rock. The mud brine was also in the flotation tests found to give more water-wet reservoir rock than the FW. High permeable sandstone reservoir core plugs were during preparation found to be contaminated by the KCl mud brine. Chemical analyses confirmed that this brine was removed during cleaning of the core plugs. In some cases, production of emulsions was observed during LSWF. These emulsions may have been stabilized by bentonite contamination. The water flooded core plugs were by SEM-analysis found to contain clusters of barite and clay and also polymer. This means that all mud components were not removed by using the standard core cleaning procedure. It was not possible to conclude anything about the LSWF-potential for the studied reservoir rock, because the core plugs were contaminated by mud components that may have affected the determined LSWF-potential.

Mud contamination of the reservoir rock can affect the permeability and the established wettability conditions. Invasion of bentonite clay will increase the cation exchange capacity, and this can affect the determined LSWF-potential. It should therefore be confirmed that mud components that can affect the established flow conditions, are removed during core preparation.


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