Determination of diffusion coefficients of supercritical CO2 under tight oil reservoir conditions with pressure-decay method

Abstract

A generalized methodology has been developed to determine the diffusion coefficients of supercritical CO2 under tight oil reservoir conditions considering the oil swelling effect. Experimentally, a diffusion cell has been used to perform diffusion tests for CO2 in tight formation cores, saturated with crude oil, under different pressure and temperature values. The dynamic pressure-decay curves of CO2 in the annular space of the diffusion cell were monitored and recorded during the measurements. Theoretically, the Peng–Robinson equation of state (PR EOS) with a modified α function has been incorporated to develop a one-dimensional radial diffusion model for the CO2–tight formation core systems. Also, the crude oil sample has been characterized as three pseudo-components for the accurate prediction of phase behavior of the CO2 and crude oil system. The CO2 diffusion coefficients, under different pressure and temperature values, were determined once the discrepancy between the measured and the calculated times, under the same pressure decrease of the diffusion process was minimized. According to the CO2 concentration profiles in the cores, the diffusion process is divided into two distinct parts, i.e., the early stage, characterized by the fast diffusion rate, and the later stage with the slow diffusion rate. While the pressure and the temperature increase, the CO2 diffusion coefficient initially increases quickly and then reaches a plateau. Lower viscosity of crude oil, higher temperature, and higher pressure facilitate the diffusion of CO2 into the crude oil, under tight formation conditions.