Heterogeneous Organic Gels: Rheology and Restart

Organic mixtures, such as petroleum, can form gels after flow shutdown when the fluid temperature drops below the gel point of the mixture. Organic gels are comprised of a solid matrix of paraffins of carbon number 19 and higher within a largely incompressible liquid. These gels have two general forms: homogeneous (no wax crystals present in the oil at shutdown) and heterogeneous (suspended crystals present at shutdown). Heterogeneous gels formed from solid–liquid slurries exhibit a yield stress (amount of pressure required to break the gel and restart flow) reduction of 50% and greater compared to normal homogeneous gels. The effects of solids content (volume fraction of solid wax precipitation) at shutdown was explored using a custom-built flow loop using an automated “slurry maker” called the scraped heat exchanger to control sub-wax appearance temperature (WAT) conditions. Results show that, with increased solids content at shutdown, the slope of the yield stress–temperature line decreases very slightly. Gel points (temperature at which the gel structure is maintained) of the heterogeneous gels were shown to be slightly lower than homogeneous gels, indicating that existing, sheared, and suspended wax crystals are weakening while still participating in the gel structure. In-flow microphotography shows that smaller particle size distributions led to little to no changes in the yield stress in heterogeneous gels while holding all other conditions constant. This result suggests a balance between the particle size and number and their subsequent strength of interactions. An exploration into the restart velocity profile of heterogeneous gels using laser particle imaging velocimetry (PIV) shows cohesive center-core failure at all tested pressure loading rates, suggesting that, in heterogeneous gels, the crystal–wall interaction is unchanged, while the crystal–crystal interactions are significantly weaker compared to corresponding homogeneous gels. In conclusion, it is theorized that gels formed from heterogeneous conditions are fundamentally and structurally different from homogeneous gels because of pre-existing sheared crystals disrupting and dominating the precipitation patterns.