Kelly T. Miller

Investigation of the Hydrate Plugging and Non-Plugging Properties of Oils

Three laboratories (Norwegian Institute of Science and Technology [NTNU], Institut Français du Pétrole [IFP], and the Colorado School of Mines [CSM]) determined hydrate plug formation characteristics in three oils, each in three conditions: (1) in their natural state, (2) with asphaltenes removed, and (3) with naturally occurring acids removed from the oil. The objective was to determine the major variables that affect hydrate plugging tendencies in oil-dominated systems, to enable the flow assurance engineer to qualitatively assess the tendency of an oil to plug with hydrates. In the past, it was indicated that chemical effects, for example, water-in-oil/hydrate-in-oil (emulsion/dispersion) stability, prevented hydrate plugs. For example, deasphalted oils provided low emulsion/dispersion stability and thus hydrate particles aggregated. In contrast pH 14-extracted oils were reported to remove stabilizing naphthenic acids, causing asphaltene precipitation on water/hydrate droplets, stabilizing the emulsion/dispersion to prevent aggregation and pluggage. This work suggests that in addition to chemistry, shear can enable plug-free operation in the hydrate region. High shear can prevent hydrate particle aggregation, while low shear encourages particle aggregation and plugging. As a result, flow assurance engineers may be able to forecast hydrate plug liability of an oil by a combination of chemistry and flow variables, such as: a) measurements of live oil emulsion stability, b) predictions of flow line shear, and c) knowledge of water cut. Plug formation qualitative trends are provided for the above three variables. Implications for flow assurance are given.

Ethylene oxide hydrate non-stoichiometry: measurements and implications

Abstract Clathrate hydrates have long been known as non-stoichiometric compounds, but the relation between hydrate and overall system composition is often overlooked. Previous density measurements showed that the composition of ethylene oxide (EO) structure I (sI) hydrate changes when formed from different EO solutions. In our X-ray diffraction (XRD) experiments, it was found that at 263.15 K , the unit cell (11.987 A ) of EO hydrate formed from EO-lean ( 14%) solutions (12.020 A ) . Since all conditions except overall composition were constant, the hydrate lattice parameter could only be changed via hydrate composition. Raman spectroscopy measurements also showed that the hydrate composition changes when formed from different solutions. The hydrate composition change was predicted using the statistical mechanics model developed by van der Waals and Platteeuw. The predictions agreed well with the measured data. An EO–H 2 O phase diagram with a hydrate solid solution range is proposed based upon these measurements and calculations. It is probable that a solid solution range also exists in CH 4 and CO 2 hydrates. This may impact energy recovery from sea-floor methane hydrates and CO 2 sequestration in the deep ocean. Schematic phase diagrams are presented for CH 4 +H 2 O and CO 2 +H 2 O which account for the hydrate solid solution range.

HYDRATE PARTICLES ADHESION FORCE MEASUREMENTS: EFFECTS OF TEMPERATURE, LOW DOSAGE INHIBITORS, AND INTERFACIAL ENERGY

Micromechanical adhesion force measurements were performed on tetrahydrofuran (THF) hydrate particles in n-decane. The experiments were performed at atmospheric pressure over the temperature range 261–275 K. A scoping study characterized the effects of temperature, anti-agglomerants, and interfacial energy on the particle adhesion forces. The adhesion force between hydrate particles was found to increase with temperature and the interfacial energy of the surrounding liquid. The adhesion force of hydrates was directly proportional to the contact time and contact force. Both sorbitan monolaurate (Span20) and poly-Nvinyl caprolactam (PVCap) decreased the adhesion force between the hydrate particles. The measured forces and trends were explained by a capillary bridge between the particles.