Reactive Transport Modeling to Quantify Key Reservoir Properties Affecting H2S Concentration Resulting from Thermochemical Sulfate Reduction

Abstract

Because of complicated mechanisms of hydrogen sulfide (H2S) generation, intensive reservoir souring by H2S with a high heterogeneity has been observed in many petroleum systems, for example, a H2S difference of 18 mol% observed in two similar formations lying over each other at a same well. To unravel this striking observation, a novel reactive mass transport model was developed. It is based on chemical thermodynamics and includes sulfate reduction as kinetic-controlled process. It integrates a series of geochemical hydrocarbon-water-rock interactions induced by abiotic sulfate reduction at elevated temperature (ASR) into diffusive mass transport. Calculations of various modeling scenarios demonstrate that thermochemical sulfate reduction (TSR) is more than ASR. Because TSR is a time- and space- dependent process, not all anhydrite-seal reservoirs undergo high H2S risks. To predict TSR-derived H2S concentration, any approaches have to consider TSR as a complex web of interconnected reactions instead of only ASR. Excluding several reactions selected from this web would over- or underestimate H2S concentration. The modeling shows that thin anhydrite layers interbedded in the gas cap can strongly increase the H2S concentration and produce H2S with a considerable concentration even after a short period. In contrast, primary metal (hydr-)oxides can strongly reduce H2S concentration. They can even conceal the occurrence of TSR with a high intensity, because TSR is commonly identified by a high H2S concentration. A combination of interbedded anhydrite layers and no primary iron (hydr-)oxides in the upper formation might be the main reason responsible for the significant H2S difference observed. Additionally, the modeling demonstrates that there is no simple linear relationship between H2S concentration and its controlling factors or between H2S concentration and increasing reaction time. For this, a reactive mass transport model can help in H2S risk assessment ahead of drill bit in a quantitative way.