Midstream on a chip: ensuring safe carbon dioxide transportation for carbon capture and storage.

Emerging technologies like enhanced oil recovery and carbon sequestration rely on carbon dioxide water content data to ensure that pipelines remain sub-saturated to avoid corrosion and hydrate flow assurance issues. To improve throughput and confidence in the hydrate phase equilibria data to avoid pipeline blockages, further research into the carbon dioxide water content must be conducted. However, the liquid carbon dioxide regime is experimentally difficult to study and the available data disagree between studies. This work aims to provide the critical and accurate data for liquid carbon dioxide for a high pressure range (13.8 to 103.4 bar) and temperature range (20 and -30 °C) utilizing a small volume microfluidic reactor (<20 microliter) coupled with Raman spectroscopy, which can reveal any phase metastability in the system. The small volume of the microfluidic system (<20 microliter) allowed experiments to be run in a few hours, compared to a whole week for prior larger scale measurements. The carbon dioxide water content results from this work agree well with both model predictions and available literature data in the gas region; however, once carbon dioxide was converted to liquid, the data showed a weak function of pressure, similar to model predictions and some previous data sets. The discrepancies between literature data are attributed to metastable phases present in the equilibrium cells, as the data is usually taken in the carbon dioxide near critical region, close to carbon dioxide's dew point, and near the hydrate phase transition. For these reasons, it is important to observe and qualify all phases in the cell, as was done in this novel study with in situ Raman spectroscopy coupled to Midstream on a chip, to ensure accurate water content of the carbon dioxide fluid phase is being measured.

Wax inhibition by comb-like polymers: support of the incorporation-perturbation mechanism from molecular dynamics simulations.

Deposition of wax on a cold surface is a serious problem in oil production. Progress in developing more effective wax inhibitors has been impeded by the lack of an established mechanism connecting the molecular structure to inhibitor efficiency. Some comb-like polymers having long alkyl side chains are known to decrease the rate of wax formation. Among several possible mechanisms, we investigate here the incorporation-perturbation mechanism. According to this mechanism, the inhibitor molecules in oil are preferentially partitioned (incorporation) toward the wax-rich (amorphous) wax deposits (soft wax), which then serves as a perturbation to slow down the ordering transition of soft amorphous wax into more stable but problematic hard wax crystals. Indeed, molecular dynamics simulations on an effective inhibitor molecule in both the oil phase and in the amorphous wax phase support the idea that the oil-to-wax partition of the inhibitor is energetically favorable. With the inhibitor molecule embedded, the structure of wax crystal is disturbed, significantly decreasing the order and significantly lowering the cohesive energy density relative to that of the pure wax crystal, supporting the slower transition from soft wax to hard wax. Thus, in the presence of an effective wax inhibitor, crystallization (formation of hard wax) is slowed dramatically, so that there is time to flush out the soft wax with a high-pressure flow inside the pipeline. This suggests design principles for developing improved wax inhibitors.

Methane hydrate formation and decomposition: structural studies via neutron diffraction and empirical potential structure refinement.

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.

Search for memory effects in methane hydrate: structure of water before hydrate formation and after hydrate decomposition.

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.