Diffusion Coefficient of CO2 in Light Oil Under Reservoir Conditions Using X-Ray Computed Tomography

Abstract

Molecular diffusion is the process describing the natural mixture of miscible fluids, whose main modeling parameter is the molecular diffusion coefficient. This work aims to evaluate the molecular diffusion coefficient for CO 2 -light oil systems under different experimental conditions of pressure. Experimental measurements are based on both pressure decay and computed tomography (CT) scan methods. The oil studied is light oil from Brazilian subsalt oil reservoirs. Tests were carried on a specially constructed vertical high pressure cell, from 2.76 MPa up to 28.96 MPa and at 293.15 K. The swelling effect was also evaluated during the diffusion process considering the height variation of the oil column inside the cell.Molecular diffusion is particularly important for miscible gas flooding processes, as diffusion is a key mechanism controlling the miscibility between oil and gas. The diffusion coefficient determines the rate of mass transfer during the diffusive process that will result in a miscible system. The diffusivity of solvents into light oil in porous media has become of great significance in petroleum engineering, since CO 2 injection has been proposed more and more as the enhanced oil recovery method to be applied in the reserves of conventional oils.Currently, the topic of CO 2 diffusion in light oils is scarcely described in the public literature, while diffusion in CO 2 -heavy oils systems has attracted some attention. Although both diffusion process and swelling effect have a common ground, many differences must be taken into consideration to truly model the mass transfer phenomenon.Diffusion coefficients were obtained using the CT method and the pressure decay technique throughout Etminan et al. (2013) interface resistance model. Both methods have significative discrepancies in coefficients values. The diffusivities obtained from pressure decay were more consistent with published data. Therefore pressure drop technique seems more robust even for CO 2 -light oil mixture, while CT technique needs further improvements. Furthermore, as Etminan et al. (2013) model is applicable only for VLE conditions, an improved model will be required for additional LLE situations.