One of the critical factors that control the efficiency of CO 2 geological storage process in aquifers and hydrocarbon reservoirs is the capillary-sealing potential of the caprock. This potential can be expressed in terms of the maximum reservoir overpressure that the brine-saturated caprock can sustain, i.e. of the CO 2 capillary entry pressure. It is controlled by the brine/CO 2 interfacial tension, the water-wettability of caprock minerals, and the pore size distribution within the caprock.By means of contact angle measurements, experimental evidence was obtained showing that the water-wettability of mica and quartz is altered in the presence of CO 2 under pressures typical of geological storage conditions. The alteration is more pronounced in the case of mica. Both minerals are representative of shaly caprocks and are strongly water-wet in the presence of hydrocarbons.A careful analysis of the available literature data on breakthrough pressure measurements in caprock samples confirms the existence of a wettability alteration by dense CO 2 , both in shaly and in evaporitic caprocks. The consequences of this effect on the maximum CO 2 storage pressure and on CO 2 storage capacity in the underground reservoir are discussed. For hydrocarbon reservoirs that were initially close to capillary leakage, the maximum allowable CO 2 storage pressure is only a fraction of the initial reservoir pressure.
Directed assembly of asymmetric ternary block copolymer-homopolymer blends using symmetric block copolymer into checkerboard trimming chemical patternThe phase behavior of ternary blends of an A-B random copolymer with two homopolymers (A and B) is investigated within the Flory-Huggins lattice theory. We restrict consideration to the formation of (isotropic) liquid phases. For compositionally symmetric systems in which the two homopolymers have equal molecular weights, two different topologies are found for the phase diagrams according to the length of the copolymer relative to the homopolymers. Namely, upon lowering the temperature, a three-liquid-phase region emerges either continuously via a tricritical point if the copolymer is long enough, or discontinuously otherwise. This change in phase behavior, an entropy of mixing effect, occurs when the copolymer length is a fraction 2/5 of the homopolymer molecular weight. The properties of the (symmetric) tricritical points are discussed, as well as the phase behavior of systems in which the copolymer is not symmetrical in composition and/or the two homopolymers differ in size. This Flory-Huggins approach should be valid for blends containing random copolymers, but also, at high enough temperatures, for blends that contain block copolymers, the temperature range of validity being broader for smaller block copolymers. The critical behavior of systems containing block copolymers is described by the simple Flory-Huggins theory over a range delineated by isotropic Lifshitz points. These Lifshitz points are located within the random phase approximation.
A series of viscosimetric and small-angle neutron scattering
experiments on asphaltenes diluted in
mixed toluene/heptane solvents has been conducted, with the purpose of
characterizing the size, molecular
weight, and internal structure of asphaltene aggregates as a function
of solvent conditions. With increasing
flocculant (i.e., heptane) content in the solvent, the intrinsic
viscosities of asphaltene aggregates first
decreased, went through a minimum for heptane fractions ≈ 10−20%,
and then increased at the approach
of flocculation. These variations paralleled those of the volume
of aggregate occupied per unit mass of
asphaltene, a behavior reminiscent of the Flory−Fox relationship for
polymers in a solvent. This volume,
proportional to the cubed radius of gyration of the aggregates divided
by their molecular weight, was
determined from the neutron scattering data. For increasing
heptane fractions in the solvent, the molecular
weight of the aggregates increased with their radius of gyration
according to a power law, the exponent
being in the range of 2. This exponent also characterized the
self-similar internal structure of the asphaltene
aggregates. With due care to the possible systematic effects of
the strong polydispersity of these aggregates,
these results are discussed in light of recent models of colloidal
aggregation.
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