## 1 Introduction  
Nanofluids (NFs) have recently attracted attention from researchers in different disciplines. One of their fields of application is enhanced oil recovery (EOR) from petroleum reservoirs [1; 2; 3; 4]. With sizes below 100 nm and high specific surface area, nanoparticles (NPs) are suitable for subsurface porous media applications since they can pass through the pore throats of porous media without blocking them and enhance oil recovery at relatively low volume concentrations [5; 6] via wettability alteration [7; 8; 9]. The work here is performed using silica NPs. Many researchers have conducted core flooding studies and demonstrated the potential of silica NPs to increase oil recovery [2; 10; 11; 12; 13; 14; 15; 16]. Adsorption and transport of nanoparticles in porous media is of primary importance for subsurface applications as this determines the effectiveness of the nanofluid injection. However, most of the work has been focused on the adsorption and transport behaviour of NPs in sandstones [17; 18; 19]. Few investigations have addressed the applicability of NPs to carbonate reservoirs [20; 21; 22; 23; 24]. [PERSON], et al. [25] compared the performance of different types of NPs in altering the wettability of carbonate reservoirs. [PERSON], et al. [26] reported that silica NPs adhere to the calcite surface irreversibly and can alter the wettability of oil/mixed-wet to water-wet state. The efficiency of wettability change by silica NPs was shown to be enhanced at higher temperatures [20]. Apart from wettability change, silica NPs have also been shown to reduce oil-water interfacial tension thereby improving the mobility of oil phase [27; 28; 29]and stabilize oil in water emulsions [30; 31; 32; 33] for EOR application. [PERSON], et al. [34] studied the adsorption of silica NPs on calcite surfaces. They reported that salinity has a positive effect on the adsorption process. However, adsorption of silica NPs on the chalk surface is not well understood. Chalk reservoirs are generally speaking tight, with permeabilities ranging from 1-7 mD and porosity of about 50%. Low salinity water flooding has emerged as an effective technique for improving oil recovery from some reservoirs [35; 36; 37; 38; 39; 40; 41; 42]. However, increased calcite dissolution induced by low salinity interaction with chalk during flooding may lead to loss of rock integrity [43]. Previous work in our lab investigated the adsorption different silica NPs on sandstone minerals and its effect on fines migration [44]. We found the silica NPs modifies the sandstones minerals and it can be utilized to overcome the problem of formation damage induced during low salinity flooding in sandstones. To best of our knowledge, the adsorption behaviour of silica NPs on chalk and its effect on fluid/rock interactions has not been addressed previously. This work aims to address the adsorption desorption mechanisms of silica NPs at different salinity conditions and suggest a synergy between silica NPs and low salinity water flooding of chalk reservoirs. The first part of the study addresses the stability of the used nanofluids. Thereafter, static adsorption of the NPs on calcite at different salinities is addressed. The adsorption of silica NPs on chalk surface is visualized by performing scanning electron microscopy (SEM). Dynamic adsorption of NPs during injection into chalk and its effect on fluid/rock interaction was investigated. Finally, the effect of NP on the fluid/rock interactions during continuous nanofluid injection in the presence of hydrocarbons is addressed. Three types of cases are investigated, with silica NPs mixed with seawater or low salinity water (LSW) and with mix injection as the third case. The last of these represents the situation for most oil fields, especially those in the North Sea where primary flooding has been with seawater. The fluid/rock interaction has been addressed here to shed light on the possible mechanisms, which may help in deciding when to apply the technology.