(672b) Effects of Shear and Pressure on Wax Appearance

Wax deposition in crude oil pipelines is a challenging issue for the oil industry. Paraffin wax deposition can result in pipeline blockage, which results in safety concerns and financial losses. Factors affecting wax deposition include wax concentration, cooling rate, pressure, and flow. Despite the importance of considering shear rate on equipment and pipeline design, how shear rate may alter wax appearance temperature (WAT) and gel strength has not been widely studied. At reservoir pressures light gases, such as methane, can dissolve in oil. Thus, wax appearance might be different than the phase transition observed at ambient pressure. We investigated samples of light mineral oil and paraffin wax at different wax concentrations. Temperature ramp experiments identified an increase in viscosity related to wax appearance under steady shear. Yield stress, on the other hand, was measured using oscillatory sweep experiments. A high pressure, concentric cylinder rheology cell was used to expand study beyond ambient pressure, parallel plate measurements. WAT increased significantly by increasing wax concentration. Changing concentration from 5 to 50 wt% resulted in a~ 20 ^oCincrease in WAT at shear rate 1000 s^-1 and also at static conditions. Viscosity after wax formation increased with concentration to the power of 1.5 at shear 10 s^-1. Lowering cooling rate from 10 to 0.2 ^oC/min under shear rate of 100 s^-1 increased WAT up to 4 ^oC. Shear rate had a more modest effect on WAT of a ~ 2 ^oC decrease when shear rate increased from 1 to 1000 s^-1. However, shear rate has a significant effect on the yield stress of formed gels. An increase in shear rate from 0 to 1000 s^-1 caused the yield stress to decrease from 415 to 8 Pa. WAT decreased up to 4 ^oCwhen pressure increased from ambient pressure to 1400 psi. Overall, adding shear rate and pressure to phase behavior studies could help the oil industry better understand wax flow assurance issues.