Investigating pressure effects on oil stability in deep-buried strata via chemical kinetics

Abstract

The effect of pressure on the oil stability in reservoirs, particularly in deep to ultra-deep strata, is a topic of significant interest. A series of crude oil cracking experiments were conducted in closed gold-tube pyrolysis systems under compressible and incompressible conditions without added water to quantitatively investigate this phenomenon. Crude oil samples with a thermal maturity of 0.74% Ro were heated from 354 ℃ to 600 ℃ at rates of 2 ℃/h and 20 ℃/h. Constant confining pressures of 50 MPa and 100 MPa were maintained throughout the experiments. Additionally, experiments were conducted at programmed temperatures corresponding to thermal maturities of 1.0% EasyRo (380 ℃), 1.5% EasyRo (420 ℃), and 2.1% EasyRo (454 ℃), with heating rates of 2 ℃/h, and under two variable fluid pressures of approximately 50 MPa (ranging from 0 to >50 MPa) and approximately 100 MPa (ranging from 0 to >100 MPa). The results indicate that confining pressure has a marginal impact on the hydrocarbons generation and cracking. Whereas, the fluid pressure initially promotes cracking reactions and subsequently exerts a retardation effect. This was demonstrated by the variations in alkane yields (methane, C₂–C₅, C₆–C₁₂, and C₁₃₊ hydrocarbons). A kinetic analysis shows that the temperatures required for 51% and 62.5% conversion levels of liquid hydrocarbons vary marginally (by 1–5 ℃). This indicates that from a chemical kinetics perspective, pressure has a marginal impact on oil stability compared with temperature. This study introduces a novel approach to investigate the effects of pressure on oil stability and to elucidate the complex dynamics process of hydrocarbon reactions under varying pressure conditions in deep-ultra-deep reservoirs.