Study of Mechanisms Affecting the Heavy Oil Recovery in the Polymer Alternating CO2 Injection using Micro-model

In gas injection processes, due to a large viscosity contrast, sweep efficiency is incomplete and large volume of oil is left behind in the reservoir. Polymer alternating CO2 gas (CO2-PAG) injection can be used to improve sweep efficiency and increase oil recovery. This method benefits form an improved microscopic displacement with the injection of gas, and an enhanced macroscopic sweep efficiency with the injection of polymer. To the best of our knowledge, this method has not been studied in the micr scale. The purpose of this study is to investigate the mechanisms of oil production in the CO2-PAG injection process using glass micromodels. Two-dimensional water-wet micromodel with a diagonal injection-production pattern was utilized to visualize the pore-scale oil recovery mechanisms. The micromodel was well equipped with a high resolution camera to capture microscopic phenomena during the oil displacement. A paraffinic model oil with a viscosity of 0.028 Pa.s was used as the representative of viscous crude oil. Moreover, partially hydrolyzed polyacrylamide with a concentration of 1500 ppm was used to control the mobility of the aqueous phase. Initially, the micromodel was saturated with saline water i.e., fully saturation, then oil was injected to achieve an irreducible water saturation. Afterwards, to investigate the EOR potential of CO2-PAG, the alternate cycles of gas and polymer were injected at two PAG ratios of 1:1 and 2:1. Microscopic phenomena affecting oil displacement during the process of CO2-PAG injection were investigated. Therefore, it was possible to identify several effective mechanisms that improve oil recovery. Results show that both the formation of mobile gas clusters and polymer elasticity increased oil recovery. Mobile gas clusters caused double drainage and imbibition in the micromodel system. A pore to pore oil displacement by polymer solution, the formation of continuous and discontinuous polymer strings, the existence of mobile blobs and gas clusters were observed and identified during the CO2-PAG injection process. Results confirmed that an improvement in the volumetric sweep efficiency was achieved by the CO2-PAG injection. In 1: 1 PAG ratio, a higher volume sweep efficiency was obtained.