Oil cracking with different water contents: New insights into the influence of water on the phase evolution and thermal stability of deep crude oil

Water is widely distributed under geological conditions and has an significant influence on the process of oil cracking into gas. However, the mechanism of reservoir water content on the thermal stability and phase evolution of deep oil and gas reservoirs still needs further study. In this study, gold-tube pyrolysis experiments were conducted using oil, oil–water (3:2), and oil–water (1:4) mixtures, at constant heating rates of 2 °C/h and 20 °C/h. The experimental results show that water has little effect on the yield of C_1-5 hydrocarbons, but increases the yields of the non-hydrocarbon gases H₂ and CO₂. The hydrogen isotopic composition of methane, ethane, and propane generated during pyrolysis experiments with added water became increasingly enriched in deuterium with increasing water content and maturity. Our results demonstrate that disproportionation and hydrogen isotope exchange between water and crude oil significantly affect both the evolution of product yields and the hydrogen isotopic signatures of generated hydrocarbons. Extrapolation of kinetic parameters at a constant heating rate of 2 °C/Ma based on the thermal history of the Tabei area in the Tarim Basin indicates that water reduces the thermal stability of crude oil, which ultimately leads to a decrease in the depth threshold for liquid hydrocarbon preservation by about 1000 m. The PVT phase simulation results show that the effect of water on the cracking behavior of crude oil is strongest at the stage of EasyRo = 1.33 %–2.19 %. At EasyRo = 1.47 %, the presence of water leads to a decrease in the cricondentherm of the crude oil by about 33.8 °C and an increase in the cricondenbar by about 2.46 MPa.