Modelling of Non-Hydrocarbon Gases at a Basin Scale

Abstract

Non-hydrocarbon gases such as CO₂ and H2S are an important exploration risk for which quantitative predictions are a difficult task. Multicomponent Petroleum Systems Modeling provides the basic framework with which to assess palaeo-temperatures and -pressures, the generation, expulsion and migration of petroleum and the likely fluid phases and related PVT properties. In addition, a number of additional H2S and CO₂ specific processes have to be taken into account to explain the risk for those inert gases. The main mechanism for the generation of H2S in the Gulf of Gabes, offshore Tunisia is Thermochemical Sulfate Reduction (TSR) which mainly takes place in the reservoir. A reaction scheme developed by Tang [1] has been used to model the TSR [2]. Geochemical isotope analysis indicates that the CO₂ is mainly generated by a thermally controlled breakdown and dissolution of carbonates [3]. Under subsurface conditions, both gases are highly soluble in water. Solubility models have been implemented based on Duan's paper [4]. The migration of CO₂/H2S dissolved in one of the petroleum phases (liquid/vapor) is performed with the Hybrid migration method and the Invasion Percolation approach used in PetroMod software. The diffusion of the components dissolved in water is handled by a diffusion equation based on Fick's law. By applying the approach described above to the studied area, the measured concentrations of both CO₂ and H2S could be reproduced. The authors believe that the described workflow will help to gain a deeper understanding of the relevant processes for non-hydrocarbon gases on a basin scale. [1] Tang et al. “Experimental and Theoretical Determination of the Kinetics of Thermochemical Sulfate Reduction”, Goldschmidt Conf. Davos, Switzerland (2009) [2] Kauerauf et al. “Integration of Thermochemical Sulfate Reduction into Petroleum Systems Modelling”, AAPG Hedberg (2012) [3] Grimmer et al. “Tunisia: Gulf of Gabes – 3D Modelling of Hydrocarbon and CO₂ Generation and Migration”, AAPG Milano (2011) [4] Duan et al. “An improved model for the calculation of CO₂ solubility in aqueous solutions containing Na+, K+, Ca2+, Mg2+, Cl-, and SO42-” – Marine Chemistry 98 (2006) Duan et al. “An accurate thermodynamic model for the calculation of H2S solubility in pure water and brines”, Energy & Fuels 21 (2007)

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014