Closed-system programmed-temperature pyrolysis on n-octadecane: Implications for the conversion of oil to gas

Abstract

The aim of this work is to determine the molecular and stable carbon isotope compositions of pyrolysates from normal octadecane and, then, to study methane generation kinetic and carbon isotope fractionation of gaseous hydrocarbons generated from n-octadecane cracking. Pyrolyses were carried out in an anhydrous closed system (gold tubes) under a constant pressure of 50 MPa at heating rates of 20°C/h and 2°C/h. Results show that secondary cracking of pyrolysates from n-octadecane largely contributes to the amount of methane generation, much more than primary cracking of noctadecane. Cracking and polymerization at relatively low temperatures and disproportionation reactions leading to light hydrocarbons and polyaromatic hydrocarbons at high temperatures are probable causes for the carbon isotope reversal of gaseous hydrocarbons that is commonly observed in pyrolysis experiments. This study of methane generation kinetics suggests that n-alkane hydrocarbon cracking begins to generate methane at 170°C but a great deal would be generated at 200°C in sedimentary basins.Keywords: n-octadecane, pyrolysis experiment, gaseous hydrocarbon, stable carbon isotope, kinetic in crude oils. It is widely accepted that the thermal evolution of oils is controlled by the kinetics of cracking reactions, which is generally modeled by the Arrhenius equation (Tissot and Welte, 1984). This allows petroleum geochemists to simulate experimentally the lowtemperature, long residence time natural processes by operating at higher temperatures than those normally occurring during geological periods.The history of experiments designed to understand oil cracking is a long one, including experiments on whole rocks and kerogens (Behar et al

A comparative experimental study on gas generation from saturated and aromatic hydrocarbons isolated from a Cambrian oil in Tarim basin

“…With the cracking of wet gases the methane gradually becomes dominant and the dryness reaches a value of 90.6% at 600°C. These observations are consistent with those obtained from the pyrolysis of single saturated compounds at similar conditions (Jackson et al, 1995;Zhang et al, 2008). However, the situation is quite different for the gases from the aromatics (Fig.…”

“…They are finally heavier than the saturate and aromatic fractions of the starting oil at maturity around 2.2%Ro for saturate and 2%Ro for aromatics. This trend also was reported by other authors (Lorant et al, 1998;Zhang et al, 2008). In some cases this also occurs in sedimentary basins.…”

“…The pyrolysis experiments were conducted in sealed gold tubes using a method described widely in the literatures (Hill et al, 2003;Tang et al, 2005;Pan et al, 2006;Zhang et al, 2008). In summary, an appropriate amount of sample (approximate 80-15 mg from low temperature to high temperature) was injected into a number of gold tubes (40 mm in length, 3 mm internal diameter and 0.2 mm in wall thickness) that were pre-welded at one end.…”

“…In recent years, many authors have conducted a series of pyrolysis experiments on various compounds, including aromatics Lorant et al, 2000;Darouich et al, 2006a, b), saturates (Jackson et al, 1995;Behar and Vandenbrouck, 1996;Burnham et al, 1997;McKinney et al, 1998;Zhang et al, 2008) and even NSO compounds (Behar et al, 2008), to investigate their individual thermal stability and gas potentials. Based on the results of model compounds, Vandenbrouck et al (1999) attempted to use a compositional kinetic model to simulate petroleum formation and cracking in the North Sea.…”

An experimental comparison of gas generation from three oil fractions: Implications for the chemical and stable carbon isotopic signatures of oil cracking gas

“…Schenk et al (1997a) also noticed that lacustrine and fluviodeltaic oils have higher frequency factors than marine oils and the onset/peak gas generation of high wax oils is approximately 10-20°C later than for marine oils in geological conditions. In recent years many authors have conducted pyrolysis experiments on various specific compounds, including aromatics Lorant et al, 2000;Al Darouich et al, 2006;Fusetti et al, 2010a,b), saturates (Jackson et al, 1995;Behar and Vandenbroucke, 1996;Burnham et al, 1997;McKinney et al, 1998;Zhang et al, 2008a) and even NSO compounds (Behar et al, 2008a), to investigate their individual thermal stability and gas potentials. Based on the results of model compounds, Vandenbroucke et al (1999) attempted to use a compositional kinetic model to simulate petroleum formation and cracking in a North Sea oil field.…”