Yields of H 2S and gaseous hydrocarbons in gold tube experiments simulating thermochemical sulfate reduction reactions between MgSO 4 and petroleum fractions

Abstract

The aim of this work was to assess the ability of four representative fractions (saturated, aromatic, polar and asphaltene) of crude oils to participate in thermochemical sulfate reduction (TSR) and thus to evaluate their pyrolysis behaviors, which is of great relevance to the stability of reservoir hydrocarbons. Experiments simulating reactions between oil fractions and magnesium sulfates (anhydrous MgSO 4 and hydrated MgSO 4·7H 2O) at a constant temperature of 420 °C and under a pressure of 50 MPa were conducted over a period of 24 h using a gold tube confined system. The yields of non-hydrocarbon and hydrocarbon gases and their δ 13C values were measured and compared in order to reveal the main control factors on TSR under practical geological conditions. In pure blank pyrolysis experiments with oil fractions cracking without TSR, different pyrolysis rates were seen for the four petroleum fractions with a sequence of saturated > asphaltene > polar ≫ aromatic. Minimal yields of C 1–C 5 hydrocarbons in the aromatic fraction can mainly be attributed to the greater thermal stability of aromatic compounds, which was also reflected by their positive δ 13C signatures. Upon addition of anhydrous MgSO 4 to the saturated fraction, a small quantity of H 2S was generated, indicating that only a weak TSR occurred in this series. Upon addition of hydrated MgSO 4·7H 2O to the four petroleum fractions, high yields of H 2S were generated, indicating that water plays an important role in the occurrence of TSR. The presence of nitrogen and/or oxygen containing heteroatomic compounds in the polar fraction decreases the yields of H 2S when subjected to MgSO 4 TSR. Similar yields of H 2S from the saturated, aromatic and asphaltene fractions revealed that TSR occurring in the aromatic fractions is unlikely to be associated with the opening of benzene rings but closely related to availability of hydrogen atom in the alkyl moiety. The addition of MgSO 4·7H 2O, TSR can significantly enhance the yield of methane, also reflected in an increase in its δ 13C value. Besides methane, the other gaseous hydrocarbons (C 2, C 3) in the MgSO 4·7H 2O series also showed some less negative δ 13C values relative to the blank series, irrespective of the fraction type. Relative to the other fractions (saturated, polar and asphaltene), low yields of C 1–C 5 gaseous hydrocarbons derived from the aromatic fraction could be attributed to the greater stability of the benzene ring and indicate that paraffinic oils may be more susceptible to thermal or thermochemical alterations than aromatic oils under the conditions of real subsurface reservoirs.