Mechanism and Chemical Kinetics of H2S Formation from ...

Thermochemical Sulfate Reduction (TSR)Speaker: Dr. Tongwei Zhang, Research Geochemistry, Power, Environmental and Energy Research Center, (PEER), Caltech, Covina, CAAbstract:H2S generation from Thermochemical Sulfate Reduction (TSR) is a complicated organic-inorganic interaction and greatly depends on formation water chemistry, oil types, temperature and pressure. Our theoretical evidence suggests that magnesium plays a significant role in controlling the rate of TSR in petroleum reservoirs. Ab initio quantum chemical calculations have been determined the activation energy for MgSO4 contact ion-pair (CIP) reduction by hydrocarbons is about 56 kcal/mol, which is almost equivalent to that for HSO4- reduction (about 55kcal/mol). The theoretical discovery provides a base of the extrapolation from the H2S generation kinetics of HSO4- ion reduction by hydrocarbons derived from laboratory’s conditions to the geological condition with MgSO4 CIP reaction scheme. Two-stage reaction pattern of H2S generation from sulfate reduction is experimentally observed. The first stage is the sulfate reduction by hydrocarbons without H2S catalyzed, and the reaction rate is slow and depends on the concentration of HSO4 ion or MgSO4 CIP. After accumulating a sufficient amount of hydrogen sulfide, H2S catalyzed sulfate reduction kicks off and the reaction rate is much faster than the initial H2S uncatalyzed reaction. Detailed experimental work confirms that the presence of H2S is capable of catalyzing sulfate reduction and significantly increases the rate of reaction, which may help to explain why previous estimates of TSR activation energies were so divergent.
Formation water chemistry, in particular the concentration of HSO4- or MgSO4 (aq) contact ion-pair is critical to control the TSR reaction rate. The activation energy for oils reacting with HSO4- or MgSO4 (aq) contact ion-pair is experimentally determined and is in the range of 55.3kcal/mol to 58.9kcal/mol, which is close to that of the theoretical calculated activation energy. Oil types can significantly affect the rate of TSR reaction, in particular the presence of labile sulfur species (thiols, sulfides) in oils. The presence of labile sulfur can reduce the activation energy of TSR. Experimental evidence suggests that H2S catalysis of TSR actually involves the formation of labile organic sulfur compounds. The presence of H2S can significantly lower the activation energy of TSR reaction.
A new technique, PH2S prediction tool is now available that specifically assess H2S risk prediction and TSR effect to petroleum thermal stability and petroleum quality. By combining hydrocarbon types (mainly labile sulphur contents in oils), the formation water chemistry including Mg, Ca and SO4 concentration and reservoir thermal history with a PC-based model of hydrocarbon oxidation kinetics, we are able to predict: H2S risk degree, onset temperature of TSR, gas quality (e.g., sourness), and oil quality (organic sulphur incorporation).

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