Study on the effect of CH4 dissolution on the microstructure of wax crystal in waxy crude oil

In this work, the influence of the dissolution of methane (CH4) gas on the wax crystal of waxy crude oil and the effect on the rheology of crude oil by dissolved CH4 were studied comprehensively. A self-deign high-pressure micro visualization device was developed to analyze wax crystals before and after gas dissolution. The crude oil from Shengli and Nanyang was tested by the device under various gas pressures. Results showed that the viscosity, maximum shear stress and equilibrium shear stress of Shengli crude oil decreased with the increasing pressure of the dissolved CH4. Due to the supersaturation of dissolved gas, the viscosity, maximum shear stress and equilibrium shear stress of Nanyang crude oil decreased initially and increased with the increasing pressure of dissolved CH4. The change in rheology of the dissolved gas crude oil can be a combined influence of gas pressure and dissolution mechanisms caused by CH4. Additionally, the wax precipitation point of Shengli crude oil decreased at the saturated dissolution of CH4, while Nanyang crude oil showed an increasing wax precipitation temperature. Notably, the wax precipitation area, number of wax particles, and average diameter of wax crystal in both crude oils gradually decreased with dissolution. However, a saturation of CH4 caused a small amount of precipitation of wax crystals in Nanyang crude oil, and the small wax crystals were aggregated to form the large wax crystals. The dissolution of CH4 gas can affect the wax crystallization process, crystallization ability, and morphology of wax crystals that resulted in significant variation in the rheology of crude oil.