Experimental study on thermochemical composite system huff-n-puff process in ultra-heavy oil production

The thermochemical composite system huff -n-puff has been widely employed for oil reservoirs characterized by thinner layers and higher oil viscosity, which however suffers less information on residual oil distribution, the diffusion behaviors of injected fluids, and property variation of oil, etc. In this paper, a 3D physical model is employed to investigate the diffusion behaviors of the injected fluid (viscosity reducer, CO2, and steam) in porous media and their synergistic effect on ultra-heavy oil recovery. The influences of thermochemical composite system on oil properties are carried out to investigate their mechanisms on oil viscosity reduction. Based on the 3D physical model, huff -n-puff experiments are carried out to study the thermochemical composite system's advantage of enhanced heavy oil recovery. The properties of the produced oil from a production well in Shengli Oilfield are collected and analyzed to test the feasibility of thermochemical composite flooding. The results show that there is a synergistic effect on oil viscosity reduction when the viscosity reducer is combined with CO2 and steam (the oil viscosity decreases by 84.38 %). During viscosity reducer injection, most of the viscosity reducer is distributed around near-well zone of the front-well section. The uneven distribution of viscosity reducer in the 3D model is alleviated by CO2 injection, which brings the viscosity reducer to the far-well zone. For the steam, it can improve the mobility of crude oil and reduce the differences in the asphaltene content between layers. Therefore, even after five cycles of huff -n-puff, relatively high oil recovery can still be obtained (oil recovery rate above 3.35 %). In the oil field application, the oil viscosity increases gradually in one huff -n-puff cycle, while the disassociation effect of viscosity reducer becomes weaker as the huff -n-puff cycles increase. Due to the synergistic effect of thermochemical composite system, it can maintain a period of high oil production for the single well (>1700 tons/cycle) in Shengli Oilfield.