The mechanism of superheated steam affecting the quality of in-situ pyrolysates of oil shale kerogen: Part A-saturation of pyrolytic organics

Abstract

In this work, the effect of superheated steam on pyrolysates for in-situ pyrolysis of kerogen was investigated through ReaxFF molecular dynamics simulation in terms of the saturation of pyrolytic shale gas and light shale oil. The double bond equivalents were adopted to characterize the saturation of gas, light shale oil and the hydrocarbon. The existence form of H2O molecules, introduced by superheated steam (named the environmental H2O molecule, H2Owater), were obtained using python scripts to analyze bond order file. And the reaction pathways of several representative pyrolysates (propanol, methanol, phenols and H2), generated by the participation of H2Owater molecules, were analyzed. The results indicated that, compared with pyrolysis at nitrogen atmosphere, saturation of shale gas and light shale oil were both promoted at superheated steam atmosphere, especially for the light shale oil. Under superheated steam circumstance, the inhibited dehydrogenation of kerogen is the primary reason for the saturation improvement of products. The secondary cause is the bonding of Hwater and OHwater radicals to carbon-containing radicals, which may have a hampering effect on the radical recombination reaction of short chains. During pyrolysis of kerogen with superheated steam injection, H2Owater molecules could be involved in a serious of chemical reactions: the decomposition of char and heavy shale oil, opening reaction of heterocyclic rings, transmission and exchange of OH radicals and the formation of products (hydrocarbon, alcohols, NH3, H2S, H2, etc.), principally via attacking radicals or fragments. The typical reactions in which H2Owater molecules involved were given. This study would provide theoretical support for in-situ pyrolysis technology of oil shale with superheated steam injection.