Experimental Investigation on the Gelation Process and Gel Structure of Water-in-Waxy Crude Oil Emulsion

Abstract

The W/O emulsion formed by waxy crude oil tends to gelatinize under low-temperature gathering conditions, which may harm the flow assurance of multiphase transportation. This study mainly focused on the wax-precipitating gelation process of crude oil emulsions, and the structural differences of the emulsions with different water fractions at their gelation temperatures (GTs). First, the changes of wax appearance temperature (WAT) and precipitated wax amount at GT with variable water cut were investigated by DSC. Then, the specific structural properties of these different water cut emulsions at their own GTs were investigated by viscoelastic, yielding behavior, and structural breakdown experiments. On these bases, the gelation mechanism of a waxy crude oil emulsion was deduced. The WAT of a waxy crude oil emulsion was found to rise, evidently with the addition of dispersed water, verifying that water droplets provide necessary nucleation sites for paraffins and thus make them more prone to precipitate. When temperature was cooled down to their respective GTs, the accumulative precipitated wax amount within the emulsions was obviously reduced with increasing water cut. The combination of this phenomenon and further microscopic experiments proved the difference in gelation mechanism. These rheological tests manifested that both storage modulus and yield stress reached their minimum values at about 30% water cut when all emulsions were kept at their respective GTs, while the flow behavior index and the structural breakdown rate came to their maxima at the same water cut. These results are different from what have been known about the rheological properties at the same temperature below the gel point, demonstrating that the structural strength of the emulsion with about 30% water cut is the weakest at the GT. A conclusion could be finally deduced from the above research that, with the dispersed water increasing, the gelation mechanism of a waxy crude oil emulsion is changed, from a spanned wax crystal network structure which traps separated water droplets to an interlinking droplet flocs structure whose interface is adsorbed by a small amount of wax crystals.

New Method for Gelation Temperature Measurement without Disturbing the Crystal Network

“…Wang et al investigated the effect of water content in waxy crude oil emulsion on the gelation behavior and found that the gelation behavior of waxy crude oil is enhanced with the addition of water. A similar observation (i.e., increase in gelation temperature with an increase in water cut) was also made in previous studies. , Ekweribe et al investigated the effects of system pressure on the gelation and the yielding process using a controlled stress rheometer (CSR) and a model pipeline system. They observed that gelation temperature increases with an increase in system pressure.…”

“…A similar observation (i.e., increase in gelation temperature with an increase in water cut) was also made in previous studies. 12,35 Ekweribe et al 36 investigated the effects of system pressure on the gelation and the yielding process using a controlled stress rheometer (CSR) and a model pipeline system. They observed that gelation temperature increases with an increase in system pressure.…”

Experimental Study on the Effective Viscosity of Unstable CO2 Flooding Produced Fluid with the Energy Dissipation Method

“…As is shown in Figure , the paraffinic wax in crude oil A begins to precipitate at about 54.3 °C, and in crude oil B at about 34.2 °C. Although the WATs of the crude oils will be slightly increased after emulsification according to our previous test and the test result of Piroozian et al, it brings no difference to the conclusion. Below these temperatures, the precipitated wax crystals can attract each other and gradually form a three-dimensional network structure in the oil phase, and they can also entrap the droplets in W/O emulsions by adsorbing at the oil/water interface .…”

“…Among the physical parameters, the density was obtained according to ASTM D1298-2012b, the pour point was measured based on ASTM D5853-11, and the contents of the four fractions were measured in accordance with ASTM D4124-09. The wax appearance temperature (WAT) was tested with a differential scanning calorimeter (DSC-821 e , Mettler-Toledo, Switzerland), and the wax content was calculated according to the common method in the literature. , Water phase: To simulate the formation water, the water phase was prepared with distilled water and sodium chloride. The concentration of sodium chloride in the brine water was controlled at 0.05 mol/L. Dissolved CO 2 : The CO 2 gas used in the experiments was obtained from Airtechina Co., Ltd. Its purity was above 99.9%. …”

“…Among the physical parameters, the density was obtained according to ASTM D1298-2012b, the pour point was measured based on ASTM D5853-11, and the contents of the four fractions were measured in accordance with ASTM D4124-09. The wax appearance temperature (WAT) was tested with a differential scanning calorimeter (DSC-821 e , Mettler-Toledo, Switzerland), and the wax content was calculated according to the common method in the literature. , …”