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ABSRACT:The use of nanoparticles (NP) to improve reservoir characterisation or to enhance oil recovery (EOR) has recently received intensive interest; however there are still many un-resolved questions. This work reports a systematic study of the effect of rutile TiO 2 nanoparticle-assisted brine flooding. Rutile ellipsoid TiO 2 nanoparticles were synthesised and stabilised by tri-sodium citrate dehydrate for brine flooding of water-wet Berea sandstone cores. Careful characterisation of the rock samples and nanomaterials before and after the flooding was conducted, and the relative contributions to the modified flooding results from the stabiliser and the nanoparticles of different concentrations were examined. The oil recovery performance was evaluated both at break-through (BT) point and at the end of flooding (~3.2 pore volumes). Nanoparticle migration behavior was also investigated in order to understand the potential mechanisms for oil recovery. The results showed that both nanoparticle transport rate and EOR effect were strongly dependent on the particle concentration. The oil recovery efficiency at the BT point was found to increase at low nanoparticle concentrations but decrease at higher values. A maximum 33% increase of the recovery factor was observed at the BT point for a TiO 2 concentration of 20 ppm, but higher nanoparticle concentrations usually had higher ultimate recovery factors. The presence of oil phase was found to accelerate the particle migration though the core. The discussion of various mechanisms suggested that the improvement in the mobility ratio, possible wettability change and log-jamming effect were responsible for the observed phenomena.

Volumetric solar absorption using nanofluids can minimize the thermal loss by trapping the light inside the fluid volume. A strong surface boiling with the underneath fluid still subcooled could have many interesting applications, whose mechanism is however still under strong debate. This work advanced our understanding on volumetric fluid heating by performing a novel experiment under a unique uniform solar heating setup at 280 Suns, with a particular focus on the steam production phenomenon using gold nanofluids. To take the temperature distribution into account, a new integration method was used to calculate the sensible heating contribution. The results showed that the photothermal conversion efficiency was enhanced significantly by gold nanofluids. A three-stage heating scenario was identified and during the first stage most of the energy was absorbed by the surface fluid, resulting in rapid vapor generation with the underneath fluid still subcooled. The condensed vapor analysis showed no nanoparticle escaping even under vigorous boiling conditions. Such results reveal that nanoparticle enabled volumetric solar heating could have many promising applications including clean water production in arid areas where abundant solar energy is available. highlights  Novel experiment was performed for nanofluids at a focused solar flux of 280 Suns. Strong surface evaporation was enabled while the bulk fluid was still subcooled  A new integration method was used to calculate photothermal conversion efficiency  Gold nanofluid (0.04w%) increased photothermal conversion efficiency by 95%. The authors are grateful for all the constructive comments from the reviewer and the Editor. Most of the comments were concerned on the presentation of the work. We have addressed all these concerns in the revised version, and a point-by-point reply is supplied below Reviewer #1:The authors of the present work experimentally investigated the surface boiling and steam production mechanism of gold nanofluids under uniform solar heating of 280 Suns. Various concentrations of gold nanofluids were produced and the generated steam was condensed and tested to reveal the presence of any nanoparticles.The study provides good insight to the surface boiling phenomenon of nanofluids and is in consonant with recent trend of investigation. However, there are several problems that need to be addressed before considering for publication in Applied Energy.1. The Abstract, in its current state is incomplete. It is more like a conclusion and needs to be re-written.Action: the abstract was rewritten with more focus on the novelty 2. The use of "gold nanofluids" should be mentioned in the Title and Abstract.Action: The title was slightly changed to reflect the content, and the was used in the title and the abstract in the revised version.3. In the statement: "For example, researchers [43] from Rice University", the Institution name should be replaced by the Authors' name.Action: T was used to replace the institution name in the revised version....

Nanoparticle morphology is 8 expected to play a significant role in the 9 stability, aggregation behaviour and 10 ultimate fate of engineered nanomaterials 11 in natural aquatic environments. The 12 aggregation kinetics of ellipsoidal and 13 spherical titanium dioxide (TiO 2 ) 14 nanoparticles (NP) under different 15 surfactant loadings, pH values and ionic strengths are investigated in this study. The stability results 16 reveal that alteration of surface charge is the stability determining factor. Among five different 17 surfactants investigated, sodium citrate and Suwannee river fulvic acid (SRFA) were the most 18 effective stabilizers. It was observed that both types of NP were more stable in monovalent salts 19 (NaCl and NaNO 3 ) as compared with divalent salts (Ca(NO 3 ) 2 and CaCl 2 ). The aggregation of 20 spherical TiO 2 NP demonstrated a strong dependency on the ionic strength regardless of the presence 21 of mono or divalent salts; while the ellipsoids exhibited a lower dependency on the ionic strength but 22 is more stable . This work acts as a benchmark study towards understanding the fate of stabilized NP 23 in natural environments that are rich in Ca(CO 3 ) 2 , NaNO 3 , NaCl and CaCl 2 along with natural organic 24 matters. 25

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