Wax crystallization in crude oil is a main concern in flow assurance. Paraffin inhibitors (PIs) are commonly employed as additives to reduce the wax appearance temperature (WAT), thereby mitigating oil gelation, deposit formation, and pipeline blockage. The efficiency of the PI is a function of chemical structure, wax properties, and other components of the crude that may stabilize or destabilize wax in the composition. The efficacy of a family of maleic anhydride−α olefin comb copolymer-based PIs having alkyl side chains was examined by rheological and crystal morphology means in a simple wax-containing, dodecane-based synthetic crude oil model in the presence and absence of asphaltenes. PIs possessing different ranges and breadths of the alkyl chain side-chain length distributions, densities of said chains, and chemistries of their attachment to the backbone were chosen for study. PIs possessing ester attachment of the alkyl side chains to the backbone are more effective in reducing the WAT and oil viscosity than that with analogous amide/imide attachment in this model crude. Differences in the side-chain densities appeared to have the greatest effect, to the exclusion of differences in chain length distribution range or breadth, belaying the commonly held philosophy that matching the carbon distribution of the wax present in the crude with that of the alkyl side chains in the PI is an effective strategy. In small amounts, asphaltenes appear to act as a natural PI with an efficacy that is comparable with that of the aforementioned ester-functional PIs. While the addition of asphaltenes in the concentration studied appears to render the various crystal assemblies more amorphous, a molecular model describing these interactions is lacking. Further studies involving PIs that vary in their structural features in more incremental, systematic ways is needed, as are additional methods for evaluating the PI efficacy.

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The effect of wax molecular weight distribution on the efficacy of two alpha olefin-maleic anhydride paraffin inhibitors (PIs) having different densities of alkyl side-chains were examined in light West Texas crude in the absence and presence of asphaltenes. Interpretation of the data was aided by cross-polarization microscopy. Primary differences in wax crystal morphology appear to be driven by the composition of the wax, with secondary differences being associated with the choice of PI. In the absence of asphaltenes, the effect of wax composition on PI performance (i.e., reducing oil viscosity and wax appearance temperature) is greater for the PI having the higher chain density, with the one having the lower chain density being generally more effective regardless of the wax composition. These differences are diminished in the presence of asphaltenes such that the PI having the higher chain density is somewhat more effective. Trends in both morphology and viscosity suggest a steric effect associated with wax composition that is lessened on interaction of the PIs with asphaltenes.

The repeatability of steady-state foam experiments in porous media had been a major concern due to either alterations in core petrophysical properties or the impact of trapped gas. Trapped gas is an important cause of history-dependent foam behavior known as foam hysteresis. The removal of trapped gas generated in a previous foam experiment can sometimes be difficult when one tries to restore the core for a subsequent experiment. This paper presents systematic studies of steady-state foam behavior without hysteresis and foam transport with hysteresis. We developed a procedure to restore a sand-pack system after a foam flood, which ensured that every foam experiment started with the same condition of the sand pack saturated with 100% surfactant solution and ruled out the impact of surfactant adsorption or gas trapping. An experimental design composed of 24 steady-state foam core floods were carried out and the local pressure drops were measured and correlated with varying phase (gas and water) velocity and foam quality. Next, two groups of foam hysteresis experiments were conducted by varying either the foam quality or the total fluid velocity. The results showed that the repeatability of the experimental data was excellent in the hysteresis-free experiments. In addition to the steady-state data obtained in traditional high- and low-quality flow regimes, we have also identified a transition to highly viscous foam where the pressure gradient increased significantly from the entrance to the exit of the sand pack at steady state. This transition has only been observed with relatively low foam qualities and high liquid velocities. Multiple pore volumes of injection before foam reached a steady state in most of the experiments were observed. Various possible reasons, such as the requirement of minimum pressure gradient and the homogeneity of the system, are discussed. In the subsequent study of foam hysteresis, the first group of experiments showed that the final steady states of foam apparent viscosity at fixed foam quality and total fluid rate were generally repeatable regardless of initial state (phase saturations) of the sand pack in the high-quality regime; however, the initial state had a significant impact on the transient process before reaching a steady state. In the second group of experiments, the results indicated that foam hysteresis had a significant influence on the final foam steady state. To our knowledge, it is the first time that such reliable and systematic data base for steady-state foam behavior in porous media without hysteresis was provided. This comprehensive data set in combination with foam-hysteresis data could therefore be used for foam-model evaluation and improvement with high confidence.