Evolution of Natural Gas Composition: A Three Dimensional Modeling For Genesis, Expulsion, Migration and Trapping

K. Alex Chang, Dorothy Payne, and Peter Ortoleva

Natural gas enrichment and contamination is economically important for resource assessment and production. It involves a number of complex phenomena arising through the coupling of reaction and transport processes. Gas purification or contamination by CO₂, H₂S, N₂ or other non-petroleum constituents may result from the interaction of natural gas with iron oxide or other mineralization or through reactions among fluid species themselves. This may occur during generation, migration or while in situ in the reservoir.

In this study, we have investigated these phenomena through quantitative reaction-transport modeling to provide a method that will allow for better evaluation of economic resource potential and for the identification of key factors needed for predicting gas quality. Our test problem consists of coupling nonlinear phenomena associated with the genesis, expulsion, migration and trapping of petroleum. These phenomena include the oxidation of H₂S and methane by iron oxide mineralization. The coupling includes multiphase flow and permeability altering diagenetic reactions, destruction of seals and episodic leaking of petroleum from a reservoir. Our model is fully 3-dimensional and accounts for relative permeability, capillary and inter-extra phase, organic and inorganic reaction as well as spontaneous creation or destruction of phases (gas, aqueous, oil, CO₂-rich).

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California