2-D porous flow field reveals different EOR mechanisms between the biopolymer and chemical polymer

Biopolymers are potential flooding agents that can tolerate high temperatures and salinity. The screening of biopolymers has always followed the same rules based on viscosity and elasticity testing as chemical polymers, and the parameters are comparable. However, the current pilot field study of biopolymer flooding did not achieve the expected effect as chemical polymers. Therefore, it was necessary to examine the difference between the two polymer types in the flooding process and examine the EOR mechanisms to explain the suboptimal performance of biopolymers. Traditional microscopic flooding experiments were carried out in the present study, but the results showed no significant difference in recovery of the two polymer types (only 2% original oil-in-place (OOIP) difference) or in sweep and the microscopic displacement efficiency, indicating that a traditional experiment is not sufficient to explain the performance difference between the two polymer types. Therefore, to distinguish the two types of recovery processes, micro-PIV experiments were carried out to further quantitatively investigate the flow field characteristics of the two polymers. The results showed that the flow velocity after xanthan gum (XG) injection significantly decreased 88.6% of water injection but only 26.9% after partially hydrolysed polyacrylamide (HPAM), indicating severe pore clogging by XG. Therefore, there are differences in EOR mechanisms: HPAM improves the mobility ratio, while XG clogs pores and changes the flow direction. To understand the reasons for the distinct flow fields, further quantitative analysis was conducted. First, in the margin region with a relatively lower velocity, the effective flow diameter was reduced to 48.04% and 62.69% of the pore width after XG and HPAM injection, respectively, indicating that the adsorption of XG was stronger than that of HPAM, although both could be adsorbed. Second, both velocities after polymer injection remained in a similar low range in wide channels, while the velocity after HPAM increased up to six times higher than XG in narrow channels, indicating that the shear resistance of XG is greater than that of HPAM and is the key mechanism for its higher pore adsorption and pore clogging ability. Therefore, the strong clogging of biopolymers results in poor migration ability in reservoirs, which is the major reason why biopolymers cannot reach deep oil-rich regions and show suboptimal recovery. Therefore, understanding how the agents migrate in reservoirs is the key to improving utilization efficiency, and the combination of micro-PIV and pore scale flow experiments is essential for understanding the flow fields and characteristics.