2011 年 76 巻 6 号 p. 522-529
In this lecture, a cold drainage mechanism of heavy oil from sandstone cores has been presented by using immiscible CO2 gas dissolution into the oil. Dissolution curves of CO2 and CH4 gases for Japanese heavy oil were measured using with a PVT apparatus to decide the pressures of bubble point for the gases. CO2 swelling factors of the heavy oil and Oman intermediate oil were measured from surface movements of oil columns placed in a high-pressure cell which was controlled less than 10 MPa for the oil reservoir temperature 50°C. Two oil swelling factors increased with increasing gas pressure, and swelling coefficients were evaluated for CO2 and CH4 gases. The swelling-time curves for an oil column were fitted with the analytical solution of one dimensional gas diffusion derivative equation, and CO2 gas diffusion coefficients in the oils, D(m2/s), were evaluated. The values of D in the heavy oil were presented as 1.1 to 1.6 % of that of the intermediate oil, and the empirical equation for values of D has been presented with function of exponential to API gravity. Based on the observation tests on oil drainage from the sandstone cores saturated with the heavy oil, gas dissolution in the heavy oil does not make any oil drainage, however foamy heavy oil including huge number of micron CO2 gas bubbles, that were generated in depressurization process, effectively contributed to oil drainage out from the cores.