Pyrolysis of heavy oil in supercritical multi-thermal fluid: An effective recovery agent for heavy oils

As a novel thermal agent, supercritical multi-thermal fluid (SCMTF) has great potential to be used for heavy oil reservoirs. However, its viability and enhanced oil recovery (EOR) mechanisms have not been investigated. Therefore, a series of tests were conducted in a newly designed autoclave to compare crude oil pyrolysis in steam, supercritical water (SCW), SCW + supercritical nitrogen (scN(2)), SCW + supercritical carbon dioxide (scCO(2)) and SCMTF, and the viability of SCMTF was evaluated. Then, the possible mechanisms of pyrolysis in SCMTF were proposed. Finally, the effects of temperature, pressure, and the amount of SCMTF on pyrolysis in SCMTF were investigated. The results show that the SCMTF treatment of raw crude oil at 653 K and 23 MPa resulted in molecular weight, density and C15+ content reduction rates of 44.67%, 6.45% and 12.06%, respectively, and gave a gas yield of 7.91 wt % and an increase in the light oil fraction of 10.17 wt %. SCMTF is more effective than steam, SCW and SCW + scN(2) for upgrading heavy oil. In detail, SCW dominates the pyrolysis processes; scCO(2) accelerates dealkylation, and increases oil mobility by dissolution and light hydrocarbons extraction; and scN(2) can maintain reservoir pressure and form a gas-cap, which reduces heat loss. Increases in scCO(2) fraction, temperature, pressure and the amount of SCMTF can enhance the heavy oil upgrading. However, pyrolysis at a high temperature is adverse to light oil production (the oil product yield was merely 67.10% at 683 K) due to undesirably high gas and coke yields of 13.32% and 19.58%, respectively.