Seawater is characterized as an enhanced oil recovery (EOR) fluid for hot, fractured chalk oil reservoirs because it is able to modify the wetting conditions and improve the displacement of oil. The chemical mechanism for the wettability alteration has been described previously, and it was verified that Ca 2+ , Mg 2+ , and SO 4 2-played an important role because of their reactivity toward the chalk surface. Chalk, which is purely biogenic CaCO 3 , consists of fragmentary parts of calcite skeletons produced by plankton algae known as coccolithophorids, and it is believed to have a more reactive surface than ordinary limestone. To validate seawater as an EOR fluid also for limestone and dolomite, the affinities of these ions toward the rock surfaces must be evaluated. The present paper describes some preliminary experimental studies of the affinity of SO 4 2-, Ca 2+ , and Mg 2+ toward the surface of reservoir limestone cores at temperatures ranging from room temperature to 130°C. The results confirmed that the ions interacted with the rock surface, and that the established chemical equilibrium was sensitive to the relative concentrations of the ions. It was also observed that the adsorption of Ca 2+ and Mg 2+ from a NaCl solution onto the limestone surface was quite similar at room temperature but that Mg 2+ adsorbed more strongly at higher temperatures. At high temperatures, T ) 130°C, Mg 2+ in seawater was able to substitute Ca 2+ on the surface but the reactivity was less than for chalk. These findings indicate that seawater will act as an EOR fluid in limestone as well but its potential is probably smaller than for chalk. This was also confirmed by spontaneous imbibition tests performed at 120°C.

The work we present in this paper initiated the formal study of fractional hedonic games, coalition formation games in which the utility of a player is the average value he ascribes to the members of his coalition. Among other settings, this covers situations in which players only distinguish between friends and non-friends and desire to be in a coalition in which the fraction of friends is maximal. Fractional hedonic games thus not only constitute a natural class of succinctly representable coalition formation games, but also provide an interesting framework for network clustering. We propose a number of conditions under which the core of fractional hedonic games is non-empty and provide algorithms for computing a core stable outcome. By contrast, we show that the core may be empty in other cases, and that it is computationally hard in general to decide non-emptiness of the core. the partitions in the strict core, and give a polynomial time algorithm to compute a unique finest partition in the strict core.• We discuss how computing desirable outcomes in fractional hedonic games provides an interesting game-theoretic perspective to community detection [see, e.g., Fortunato, 2010, Newman, 2004 and network clustering. 2 RELATED WORKFractional hedonic games are related to additively separable hedonic games [see, e.g., , Olsen, 2009, Sung and Dimitrov, 2010. In both fractional hedonic games and additively separable hedonic games, each player ascribes a cardinal value to every other player. In additively separable hedonic games, utility in a coalition is derived by adding the values for the other players. By contrast, in fractional hedonic games, utility in a coalition is derived by adding the values for the other players and then dividing the sum by the total number of players in the coalition. Although conceptually, additively separable and fractional hedonic games are similar, their formal properties are quite different. As neither of the two models is obviously superior, this shows how slight modeling decisions may affect the formal analysis. For example, in unweighted and undirected graphs, the grand coalition is trivially core stable for additively separable hedonic games.On the other hand, this is not the case for fractional hedonic games. 3 A fractional hedonic game approach to social networks with only non-negative weights may help detect like-minded and densely connected communities. In comparison, when the network only has non-negative weights for the edges, any reasonable solution for the corresponding additively separable hedonic game returns the grand coalition, which is not informative. The difference between additively separable and fractional hedonic games is reminiscent of some issues in population ethics (see, e.g., Arrhenius et al., 2017), which concerns the evaluation of states of the world with different numbers of individuals alive. Two prominent principles in population ethics are total utilitarianism and average utilitarianism. The former claims that a state of the world is better than another...

[1] The Mars Exploration Rovers each carry a set of Magnetic Properties Experiments designed with the following objectives in mind: (1) to identify the magnetic mineral(s) in the dust, soil and rocks on Mars, (2) to establish if the magnetic material is present in the form of nanosized (d < 10 nm) superparamagnetic crystallites embedded in the micrometer sized airborne dust particles, and (3) to establish if the magnets are culling a subset of strongly magnetic particles or if essentially all particles of the airborne dust are sufficiently magnetic to be attracted by the magnets. To accomplish these goals, the Mars Exploration Rovers each carry a set of permanent magnets of several different strengths and sizes. Each magnet has its own specific objective. The dust collected from the atmosphere by the Capture magnet and the Filter magnet (placed on the front of each rover) will be studied by the Mössbauer spectrometer and the Alpha Particle X-ray Spectrometer, both of which are instruments located on the rover's Instrument Deployment Device. The captured dust particles will also be imaged by the Pancam and Microscopic Imager. The Sweep magnet will be imaged by Pancam and is placed near the Pancam calibration target. The four magnets in the Rock Abrasion Tool (RAT) are designed to capture magnetic particles originating from the grinding of Martian surface rocks. The magnetic particles captured by the RAT magnets will be imaged by Pancam.

Abstract. The Mars Pathfinder lander carried two magnet arrays, each containing five small permanent magnets of varying strength. The magnet arrays were passively exposed to the wind borne dust on Mars. By the end of the Mars Pathfinder mission a bull's-eye pattern was visible on the four strongest magnets of the arrays showing the presence of magnetic dust particles. From the images we conclude that the dust suspended in the atmosphere is not solely single phase particles of hematite (a-Fe203) and that single phase particles of the ferrimagnetic minerals maghemite (-y-Fe203) or magnetite (Fe304) are not present as free particles in any appreciable amount. The material on the strongest magnets seems to be indistinguishable from the bright surface material around the lander. From X-ray fluorescence it is known that the soil consists mainly of silicates. The element iron constitutes about 13% of the soil. The particles in the airborne dust seem to be composite, containing a few percent of a strongly magnetic component. We conclude that the magnetic phase present in the airborne dust particles is most likely maghemite. The particles thus appear to consist of silicate aggregates stained or cemented by ferric oxides, some of the stain and cement being maghemite. These results imply that Fe 2+ ions were leached from the bedrock, and after passing through a state as free Fe 2+ ions in liquid water, the Fe 2+ was oxidized to Fe 3+ and then precipitated. It cannot, however, be ruled out that the magnetic particles are titanomagnetite (or titanomaghemite) occurring in palagonite, having been inherited directly from the bedrock. In addition to the backhoe magnets (strong and weak), each Viking lander carried a reference test chart (RTC) magnet, which was passively exposed to the dust suspended in the Martian atmosphere. The RTC magnets were of the strong type only.The backhoe magnets and the RTC magnets on the Viking landers were constructed as disc-shaped magnets of diameter 6.5 mm surrounded by a ring magnet of an outer diameter of