A new isomorphism invariant of matroids is introduced, in the form of a quasisymmetric function. This invariant• defines a Hopf morphism from the Hopf algebra of matroids to the quasisymmetric functions, which is surjective if one uses rational coefficients, • is a multivariate generating function for integer weight vectors that give minimum total weight to a unique base of the matroid, • is equivalent, via the Hopf antipode, to a generating function for integer weight vectors which keeps track of how many bases minimize the total weight, • behaves simply under matroid duality, • has a simple expansion in terms of P -partition enumerators, and • is a valuation on decompositions of matroid base polytopes. This last property leads to an interesting application: it can sometimes be used to prove that a matroid base polytope has no decompositions into smaller matroid base polytopes. Existence of such decompositions is a subtle issue arising in work of Lafforgue, where lack of such a decomposition implies the matroid has only a finite number of realizations up to scalings of vectors and overall change-of-basis.

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Low salinity waterflood (LSF) is a promising improved oil recovery (IOR) technology. Although, it has been demonstrated that LSF is an efficient IOR method for many sandstone reservoirs, the potential of LSF in tight oil reservoir is not well-established. This paper presents a systematic evaluation of the potential of low salinity waterfloding for the tight reservoirs in Jiyuan Oilfield, China. This investigation pushes the application envelope of low salinity waterflooding towards the reservoir with low permeability (lower than 0.5mD), formation salinity of up to 45,180ppm, reservoir temperature of 70°C and in-situ oil viscosity of 0.6 cp.Our laboratory evaluation included zeta potential tests for interface of oil/brine and brine/rock, thermodynamic analysis through disjoining pressure calculation, corefloods using representative core samples.Thermodynamic analysis showed that decreasing divalent cations and salinity makes the electrical charges at both oil/brine and brine/rock interfaces become strongly negative, which enhanced the repulsive forces between oil and rock due to the double electric layer expansion. As a result, the rock turns more water-wet. Secondary corefloods were conducted with two different brines, which include shallow aquifer water and ion tuning water with consideration of field application. Coreflooding Experimental results were history matched to obtain the relative permeability curves. Results showed that compared to shallow aquifer water, low-salinity water exhibited a higher oil relative permeability and lower water relative permeability at the same water saturation and a lower residual oil saturation to water.Laboratory results were input into a reservoir simulator to investigate the potential of low-salinity water flood in Jiyuan oilfield. It showed that suitably formulated ion tuning water (ITW) has the potential to accelerate oil production and improve displacement efficiency, thus resulting in a higher recovery factor with only a fraction of pore volume of low-salinity water injected.To conclude, this paper demonstrates that ITWF has a good potential as an IOR/EOR technology in tight reservoirs, the key points are described as follows. Firstly, the mechanism of ITWF was interpreted by thermodynamics of wettability. Secondly, laboratory experiments have shown that ITWF could improve oil recovery by accelerating the oil production rate and decrease the residual oil production.Thirdly, the potential of ITWF in a tight oil reservoir in Jiyuan oilfield is investigated using a mechanistic model based on input data of laboratory experiments.

In recent years, low salinity waterflooding (LSWF) has been a promising technique to recover oil in sandstone reservoirs. In view of research results for the last decade, it is acknowledged that substantial oil recovery beyond conventional waterflooding from sandstone is wettability alteration. However, the major contributor to wettability alteration is still uncertain. Therefore, this paper investigates this major mechanism and shows how it is involved in the process of IOR. Rock and oil surface chemistry were tested to explain the influence of zeta potential on the disjoining pressure. Coreflood experiments with permeability less than 1mD were carried out and the impact of different wettabilities ranging from strong water-wet to slight water-wet on LSWF recovery was investigated with combination of thermodynamic theory. Relative permeability curves were obtained by history matching the corefloods experiments for both slight water-wet and strong water-wet cores with consideration of salinity effect. Thermodynamics of wettability by low salinity waterflooding was analyzed to characterize the surface forces between the surfaces of oil/water and water/rock. Zeta potential results showed that decreasing divalent cations and salinity makes the electrical charges at both oil/brine and brine/rock interfaces become strongly negative, which results in elevation of the repulsive forces between oil and rock, and as a result the rock turns more water-wet, which was confirmed by thermodynamics characterization. Corefloods experiments showed that a high potential in slight water-wet reservoirs can be achieved by low salinity waterflooding due to the double layer expansion. The relative permeability curves obtained by history matching showed that LSW improves oil recovery by accelerating oil production (relative perm changes) and reducing residual oil saturation in slight water-wet rock but not in strong water-wet rock. Thermodynamics of wettability analysis indicated that the mechanism of low salinity waterflooding might be interpreted by disjoining pressure calculation. In conclusion, double layer expansion caused by highly negative zeta potential as a result of lower salinity and divalent cation plays a major role in recovering additional oil. The low salinity waterflooding EOR-Effect might be interpreted by disjoining pressure calculation. These findings can help in composition design of low salinity water to maintain higher potential to recover oil in oil field.