Pressure-sensitive characteristics of high-pour-point live oil and quantitative analysis of composition change

Abstract

Previous studies on high-pour-point (PPT) oil have focused on the effects of temperature. There has been no systematic investigation of the pressure-sensitivity of crude oil, the deposition composition change of solid phases, and the effects of asphaltene deposition on wax precipitation. In the development of a high-PPT oil field, experiments using dead oil instead of live oil will exaggerate the effect of wax precipitation. In this paper, the wax appearance temperature (WAT), PPT, and asphaltene deposition pressure were obtained by the laser transmittance method. Subsequently, a new set of experimental devices and methods were developed to analyze the solid-phase components in live oil under various conditions. Finally, the viscosity of crude oil at various shear rates was measured with a capillary viscometer, and the wax network structure was evaluated. The experimental results showed that the WAT and PPT decrease with decreased pressure, and their fast decreasing ranges were 12–18 MPa and 15–18 MPa, respectively. The oil pressure-sensitivity not only includes the influence of wax; the appearance of the platform stage and trough pattern phenomenon proves that there is also the influence of asphaltenes. The main component of the deposition is C27+, and the amount of wax precipitated can reach 2.72%. For crude oil with a pressure lower than 15.83 MPa, the amount of asphaltene deposition reaches 0.04% at temperatures above WAT. Asphaltene can inhibit the aggregation and growth of wax crystal molecules, leading to a decrease in WAT and an increase in the viscosity at temperatures above WAT; however, the inhibition may be advanced or delayed with decreased pressure. Asphaltene also enhanced the strength of the wax crystal network structure, leading to an increase in the viscosity of crude oil at the critical shear stress under low pressure. When wax precipitation is dominant, there is a positive correlation between asphaltene deposition and wax, and the growth rate of wax is higher than that of nucleation in the early stage. This study improves the understanding of the effect of pressure on the physical properties of high-PTT live oil, provides reliable parameters for numerical simulations and the adjustment of the injection scheme, and lays the foundation for future evaluation experiments on wax removal chemicals.