Thermochemical sulphate reduction (TSR): experimental determination of reaction kinetics and implications of the observed reaction rates for petroleum reservoirs

Thermochemical Sulphate Reduction (TSR), responsible for substantial quantities (10–80%) of H2S in some high temperature petroleum accumulations, has been simulated in gold-titanium reaction vessels at 280–350 °C and 250–500 bars and in-situ pH values between 5.2 and 6.8 using on-line fluid sampling and analysis to monitor continuous reaction progress. The calculated activation energy of TSR is 142 kJ/mol with a half life of aqueous sulphate in the presence of aqueous acetate and elemental sulphur of 1650 years at 150 °C and 372,000 years at 100 °C. The continuing co-existence of sulphate minerals and petroleum in many reservoirs, filled millions of years ago, thus cannot be controlled by the rate of reduction. The experiments also show that neither pressure nor pH significantly affect the rate of TSR when they are limited to a range of realistic subsurface values. The experiments coincidentally revealed that the rate of acetate decarboxylation is strongly pressure dependent with elevated pressure leading to a decreased rate of reaction. This must be because decarboxylation involves a positive volume change and provides evidence that all organic breakdown processes that lead to a net volume increase, e.g. kerogen breakdown, will be relatively retarded in overpressured reservoirs.