Potential Tracers of Gas Diffusion in Petroleum Reservoirs and Caprocks

Magnier, C., A. Prinzhofer, S. Kara, A. Battani, IFP, Rueil Malmaison, France

Noble gases are chemically inert but are sensitive to physical partitioning. They have been used as natural tracers for evaluating distances and direction of migration of hydrocar-bons(Prinzhofer et al. 2000). Our objectives is to test their potential as monitoring tools for gas storage and cap-rocks studies.

This methodology may be very promising to assess a risk of upward gas leakage. Our approach is based on experimental work that uses the diffusive properties of noble gases through different mineral assemblage in water saturated media. The apparatus consists of a stainless steel reactor with two separated gas chambers filled with different gases, one is carbon dioxide and noble gases, the other pure oxygen, both having similar diffusive rates and solubility in water. In the middle of the reactor, a synthetic carbonate, sandstone or a real cap-rock sample is placed.

The experiment quantifies the flux of gas through the rock and the chemical and isotopic fractionation of different components as a function of the porosity and permeability. When sampling was made early on in the diffusion experiment, important chemical (4He/20Ne etc...) and isotopic fractionations (3He/4He) were observable. The rates of diffusion in porous media were simulated by taking into account two water domains, bound water and free water. The proportion of bound water was studied and tentatively adjusted by a 1D model. The bound water has a complex behavior but clearly varies with salinity, pH and atom size. Furthermore, it induces changes in the gas solubility and rates of diffusion.