Feasibility study of CO_2 huff 'n' puff process to enhance heavy oil recovery via long core experiments

摘要

In order to study a potential way to store CO2 and enhance heavy oil production performance, five experiments are implemented on the CO2 huff 'n' puff process using long cores. The production profiles of the CO2 huff 'n' puff process are analyzed, including pressure, heavy oil recovery factor, gas production, cumulative gas oil ratio, and pressure difference. The pressure drops indicate the CO2 diffusion in heavy oil. The pressure drop in the first cycle is much lower than those in the subsequent cycles. The heavy oil recovery factor is higher than 32.75% and can reach as high as 38.02% under the pressure depletion rate of 1 kPa/min. A main trend observed for each test is that the heavy oil recovery factor decreases with increases in the cycle number. With oil production, a growing space is available for CO2 injection in the core, resulting in a higher volume of injected CO2 together with increasing gas production and a cumulative gas oil ratio. With less heavy oil production, the pressure difference between the end port and the production port decreases with the cycle number increases. A novel equation is developed to study the relationship between CO2 production and heavy oil production, and the agreement between the equation and the experimental data is extremely high (R-2 0.97). This novel equation can be applied to predict the production performance in the later production period in the same cycle and/or to predict the performance in the subsequent cycles. Via the analyzation of the production performance of the CO2 huff 'n' puff process in heavy oil under different pressure depletion rates and different soaking times, the effect parameters, including pressure depletion rates, soaking time and cycle numbers, are optimized in this study. The optimized pressure depletion rates, soaking time, and cycle numbers are 1 kPa/min, 5 h and 3 cycles, respectively. The optimized parameters gained in the tests were upscaled using the scaling criteria, and the upscaled parameters can be applied in the field pilot test to enhance heavy oil recovery using the CO2 huff 'n' puff process.