Asphaltenes: Fundamental Principles to Oilfield Applications

The sophisticated molecular imaging methods, atomic force microscopy (AFM) and scanning tunneling microscopy (STM), have been utilized to image individual asphaltene molecules, both their atoms and bonds, and their electronic structure. The stunning images have confirmed previous results and have all but resolved the long-standing uncertainties regarding asphaltene molecular architecture. Asphaltenes are also known to have a strong propensity to aggregate. The dominante asphaltene molecular structure and hierarchical nanocolloidal structures have been resolved and codified in the Yen-Mullins model. Use of this model in a simple polymer solution theory has given the first equation of state (EoS) for asphaltene gradients in oilfield reservoirs, the Flory-Huggins-Zuo EoS. With this EoS it is now possible to address reservoir connectivity in new ways; equilibrated asphaltenes imply reservoir connectivity. For reservoirs with disequilibrium of contained fluids, there is often a fluid process occurring in geologic time that precludes equilibrium. The collection of processes leading to equilibrium and those that preclude equilibrium constitute a new technical discipline, reservoir fluid geodynamics (RFG). Several reservoirs are reviewed employing RFG evaluation of connectivity via asphaltene thermodynamics. RFG processes in reservoris often include diffusion, RFG models incorporating simple solution to the diffusion equation coupled with quasi-equilibrium with the FHZ EoS are shown to apply for timelines up to 50 million years, the age of charge in a reservoir. When gas (or condensates) diffuse into oil, the asphaltenes are destabilized and can convect to the base of the reservoir. Increasing asphaltene onset pressure as well as viscous oil and tar mats can be consequences. Depending on specifics of the process, either gooey tar or coal-like asphaltene deposits can form. In addition, the asphaltene structures illuminated by AFM are now being used to account for interfacial properties using simple thermodynamics. At long last, asphaltenes are no longer the enigmatic component of crude oil, instead the resolution of asphaltene structures and dynamics has led to new thermodynamic applications in reservoirs, the new discipline RFG, and a new understanding of tar mats.

Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers and Activation Volumes

<p>In this theoretical study we examine several aspects of the formation, structure, and stability of the most ordered nanothreads yet made, those derived from furan and thiophene. First, we look at the enthalpic consequences and activation barriers of the first two steps of oligomerization by a Diels-Alder mechanism. The ca. 20 GPa difference in the synthetic pressures (furan lower) is explainable in terms of greater loss of aromaticity by the thiophene. Subsequent steps have understandably lower barriers. We show explicitly how pressure affects the reaction profiles, operating through the volume decrease in the transition state and onward to the product molecule. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with two opposing and cumulative dipole moments. The computed activation volumes are consistently more negative for likely initial furan (compared with thiophene) polymerization steps, in accord with the lower onset pressure of furan polymerization. In the second part of our study we examine the energetics of the likely polymers. Three ordered polymer structures compete in enthalpy -- a <i>syn</i> one, with all O/S on the same side, an <i>anti </i>one, S/O alternating, and a<i> syn-anti</i> isomer, with segments of four monomers repeating. The <i>syn</i> polymer, if not allowed to distort, is at high enthalpy relative to the other two. The origin of the destabilization is apparent, being S/O lone-pair repulsion, understandably greater for S than O at the 2.8/2.6Å separation. Set free, the <i>syn</i> isomers curve or arc, in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The <i>syn</i> polymer can also stabilize itself by the thread twisting into zig-zag or helical enthalpic minima. Release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of crystalline order in the polymer as it is returned to ambient pressure.<br></p>

Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers and Activation Volumes

<p>In this theoretical study we examine several aspects of the formation, structure, and stability of the most ordered nanothreads yet made, those derived from furan and thiophene. First, we look at the enthalpic consequences and activation barriers of the first two steps of oligomerization by a Diels-Alder mechanism. The ca. 20 GPa difference in the synthetic pressures (furan lower) is explainable in terms of greater loss of aromaticity by the thiophene. Subsequent steps have understandably lower barriers. We show explicitly how pressure affects the reaction profiles, operating through the volume decrease in the transition state and onward to the product molecule. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with two opposing and cumulative dipole moments. The computed activation volumes are consistently more negative for likely initial furan (compared with thiophene) polymerization steps, in accord with the lower onset pressure of furan polymerization. In the second part of our study we examine the energetics of the likely polymers. Three ordered polymer structures compete in enthalpy -- a <i>syn</i> one, with all O/S on the same side, an <i>anti </i>one, S/O alternating, and a<i> syn-anti</i> isomer, with segments of four monomers repeating. The <i>syn</i> polymer, if not allowed to distort, is at high enthalpy relative to the other two. The origin of the destabilization is apparent, being S/O lone-pair repulsion, understandably greater for S than O at the 2.8/2.6Å separation. Set free, the <i>syn</i> isomers curve or arc, in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The <i>syn</i> polymer can also stabilize itself by the thread twisting into zig-zag or helical enthalpic minima. Release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of crystalline order in the polymer as it is returned to ambient pressure.<br></p>

Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers and Activation Volumes

<p>In this theoretical study we examine several aspects of the formation, structure, and stability of the most ordered nanothreads yet made, those derived from furan and thiophene. First, we look at the enthalpic consequences and activation barriers of the first two steps of oligomerization by a Diels-Alder mechanism. The ca. 20 kcal/mol difference in the synthetic pressures (furan lower) is explainable in terms of greater loss of aromaticity by the thiophene. Subsequent steps have understandably lower barriers. We show explicitly how pressure affects the reaction profiles, operating through the volume decrease in the transition state and onward to the product molecule. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with two opposing and cumulative dipole moments. The computed activation volumes are consistently more negative for likely initial furan (compared with thiophene) polymerization steps, in accord with the lower onset pressure of furan polymerization. In the second part of our study we examine the energetics of the likely polymers. Three ordered polymer structures compete in enthalpy -- a <i>syn</i> one, with all O/S on the same side, an <i>anti </i>one, S/O alternating, and a<i> syn-anti</i> isomer, with segments of four monomers repeating. The <i>syn</i> polymer, if not allowed to distort, is at high enthalpy relative to the other two. The origin of the destabilization is apparent, being S/O lone-pair repulsion, understandably greater for S than O at the 2.8/2.6Å separation. Set free, the <i>syn</i> isomers curve or arc, in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The <i>syn</i> polymer can also stabilize itself by the thread twisting into zig-zag or helical enthalpic minima. Release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of crystalline order in the polymer as it is returned to ambient pressure.<br></p>

Heat Capacities of Nanostructured Wurtzite and Rock Salt ZnO: Challenges of ZnO Nano-Phase Diagram

Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2–315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K.

Pressure- and Temperature-Induced Insertion of N2, O2 and CH4 to Ag-Natrolite

This paper aimed to investigate the structural and chemical changes of Ag-natrolite (Ag16Al16Si24O80·16H2O, Ag-NAT) in the presence of different pressure transmitting mediums (PTMs), such as N2, O2 and CH4, up to ~8 GPa and 250 °C using in situ synchrotron X-ray powder diffraction and Rietveld refinement. Pressure-induced insertion occurs in two stages in the case of N2 and O2 runs, as opposed to the CH4 run. First changes of the unit cell volume in N2, O2 and CH4 runs are observed at 0.88(5) GPa, 1.05(5) GPa and 1.84(5) GPa with increase of 5.7(1)%, 5.5(1)% and 5.7(1)%, respectively. Subsequent volume changes of Ag-natrolite in the presence of N2 and O2 appear at 2.15(5) GPa and 5.24(5) GPa with a volume increase of 0.8(1)% and a decrease of 3.0(1)%, respectively. The bulk moduli of the Ag-NAT change from 42(1) to 49(7), from 38(1) to 227(1) and from 49(3) to 79(2) in the case of N2, O2 and CH4 runs, respectively, revealing that the Ag-NAT becomes more incompressible after each insertion of PTM molecules. The shape of the channel window of the Ag-NAT changes from elliptical to more circular after the uptake of N2, O2 and CH4. Overall, the experimental results of Ag-NAT from our previous data and this work establish that the onset pressure exponentially increases with the molecular size. The unit cell volumes of the expanded (or contracted) phases of the Ag-NAT have a linear relationship and limit to maximally expand and contract upon pressure-induced insertion.