Recent Advances in Petroleum System Modeling of Geochemical Processes: TSR, SARA, and Biodegradation

Peters, Kenneth E.; Hantschel, Thomas; Kauerauf, Armin I.; Tang, Yongchun; Wygrala, Bjorn

Basin and petroleum system modeling (BPSM) simulates the generation, expulsion, migration, accumulation and loss of hydrocarbons in conventional and unconventional petroleum systems. This paper describes three new advances in modeling of geochemical processes: thermochemical sulfate reduction (TSR) modeling for H2S prediction, as well as saturates-aromatics-resins-asphaltene (SARA) modeling and biodegradation modeling for prediction of oil quality.

TSR is a complex redox reaction controlled by reservoir chemistry and thermal history that converts petroleum and pore water sulfate to solid bitumen, carbon dioxide, and hydrogen sulfide. Accurate TSR modeling is important because it predicts H2S, which is toxic, corrosive, and increases production costs. A new approach to model TSR enables concentrations of Mg²⁺, Ca²⁺ and SO4^2- in pore water and sulfur in oil to be estimated based on reservoir lithology and oil quality. Model output as H2S-risk distribution identifies areas where TSR can occur.

Predictions of aromatic and asphaltene content in oil cannot be made using standard published kinetics. A new SARA kinetic modeling approach includes 11 components (four bitumen, two oil, three hydrocarbon gas, CO2, and H2S) and can be used to improve predictions of oil quality. Additional features include complex secondary cracking through a multi-stage reaction network for bitumen-oil, oil-gas and bitumen-gas, as well as a special adsorption model for the bitumen components. Components are lumped according to physical and chemical properties in order to minimize processing time. The approach allows prediction of asphaltene flocculation and tar mats as well as CO2 and H2S formation.

Biodegradation modeling was previously performed using BPSM simulation results, such as reservoir charge and temperature history. Decoupled post-processing was then applied to determine biodegradation risk. However, accurate predictions of petroleum properties are not possible using this approach. The new approach features full coupling of biodegradation into the BPSM simulation. Phase kinetics (14-component model) is used with component-specific biodegradability, relative biodegradation ratios, temperature-dependent biodegradation rates, and paleopasteurization for more accurate predictions of API, GOR, viscosity, and CO2 yields.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013