Kinetics of thermochemical sulfate reduction based on pyrolysis gold-tube experiments on lacustrine oil: Implications for H2S prediction in Brazilian pre-salt reservoirs

Abstract

The Pre-salt section in the Santos Basin area is one of the largest oil discoveries in the world during the last 20 years. The reservoir fluid in this area contains non-hydrocarbon gases (CO₂ and H₂S) that can negatively impact the economic prospects of the field. Previous studies have indicated that the main H₂S generation process in Pre-salt reservoirs is thermochemical sulfate reduction (TSR). However, a more comprehensive understanding of these occurrences is required, particularly considering the reservoir temperatures (less than 120 °C) and the variations in δ³⁴S H₂S values. Moreover, there is currently no published research addressing the kinetic behavior of TSR in lacustrine oils. To better understand the origin of these non-hydrocarbon gases, TSR experiments with gold tubes were carried out to obtain information about the fluid changes, kinetic behavior, identification of fluid proxies, and to create a fluid composition model to predict H₂S in exploratory areas. With increasing extent of TSR reaction, the experimentally generated gases show the following patterns: i) Large generation of non-hydrocarbon gases with CO₂ being predominant; ii) The progressive increase in gas dryness (C₁/ΣC_1-5) ultimately reaching 99.9 % by the end of the experiments; iii) The carbon isotopic composition (δ¹³C) trending toward heavier values for C₁-C₃ and CO₂; and iv) The sulfur isotopic composition of H₂S approaches the δ³⁴S of sulfate. Additionally, the TSR process reduced the liquid mass by as much as 50 %, mainly focused on the gasoline range fraction (C₆-C₁₄), indicating the suitability for hydrocarbon oxidation. A large amount of residual hydrocarbons was observed at the end of each experiment, reaching 33 % of the original mass under the most extensive TSR conditions. The parameters for the TSR kinetics were: i) pre-exponential factor (Af) of 4.8x10¹³ s⁻¹, ii) unimodal activation energy (Ea) in 53 kcal/mol, iii) total potential of 767 mg H₂S/g oil. These kinetic parameters were tested in one geological scenario derived from the regional petroleum system model and compared with previous kinetic models from the literature. Two types of TSR reactions (with and without initiators/catalysts) were identified by the model predictions. The model generated with the kinetic parameters proposed by this work predicted a gas composition the closest to that observed in the modeled well. The developed kinetic model can be an important tool for better H₂S prediction in exploratory areas of the Pre-salt Santos Basin.