Effects of fluid pressure on the occurrence of multi-phase oil and accumulation of light oil and condensate from crude oil cracking: Insights from modified gold tube pyrolysis experiments

An oil with an initial equivalent maturity of 0.74 %Ro, was pyrolyzed in a closed gold tube pyrolysis system with added silica sand and no added water under simulated conditions spanning 0.7 %–2.1 % EasyRo. The internal fluid pressure, ranging from 0 to >150 MPa at individual maturities states of 1.0 %, 1.5 % and 2.1 % EasyRo, was controlled by increasing sample mass and setting external confining pressure (50, 100 and 150 MPa). Results indicate that the increasing fluid pressure initially promoted and then gradually retarded crude oil cracking. The free-radical reaction mechanism (hydrogen radical supply), free space of vessels, and characteristics of pressure medium control the influence of fluid pressure on the chemical reaction process. The decreasing free space of volume-constant vessels and the difference of hydrogen radicals supplied in various thermal maturity stages together gradually reduce the reaction rate of crude oil cracking. Thus, the yields of methane, wet gas, and light and heavy hydrocarbons increase at low fluid pressure ranges and then decrease at high-pressure conditions. Moreover, the physical controls of fluid phase behaviors include the evolution of fluid phase states influencing the increased rates of fluid pressure and fluid pressure influencing the production of multi-phase hydrocarbons. The increase in fluid pressure is faster in the saturated gas–liquid phase than in the unsaturated phase; thus, the phase behaviors induce the yields of product change. The increasing fluid pressure induces the occurrence of multi-phase hydrocarbons and accumulation of light oil and condensate. This study introduces a novel approach to investigate the influence of fluid pressure on reservoir oil cracking, emphasising phase behavior analysis, and shedding light on the evolution of organic matter in deep and ultra-deep strata.