Visual Laminations Combined with Nuclear Magnetic Resonance to Study the Micro-Unrecovered Oil Distribution and Displacement Behavior of Chemical Flooding in a Complex Conglomerate

The conglomerate reservoirs of the Kexia group, Xinjiang oilfield, NW China, are one of the largest conglomerate reservoirs worldwide and have employed chemical-enhanced oil recovery (EOR) processes since the 2010s. However, compared to sandstone reservoirs, the output of chemical flooding has been less than 2.0% because of the severe heterogeneity and complex fluid distribution. This study quantitatively examined the micro-unrecovered oil distribution and displacement behavior of polymer flooding in the complex conglomerate of the Kexia group by combining visual conglomerate laminations and online nuclear magnetic resonance experiment. The results indicated that the multistream sedimentary environments and the rapid transport of the clasts prior to deposition exacerbated the complexity of nonclay minerals and variable pore-throat structures. Compared to the sandstone, the variable pore channels and disconnected seepage network in the conglomerate caused macro-permeable water channels and rapid water cuts, resulting in the recovery of less than 40% of original oil in place (OOIP) by the earlier water flooding. The cluster of disconnected oil and oil resident in blind pores occupied a major proportion of unrecovered oil, which was the significant target for the polymer flooding. The data from the T 2 responses showed that the polymer solution first navigated through dominant porous media, while simultaneously building flow resistance in the higher permeability zones. The remaining oil in the disconnected oil cluster and the four types of residual oil (ganglia- and column-type oils, oil film, and oil resident in blind pores) were effectively displaced by the viscoelastic effects of the polymer solution. In addition, star-like polyacrylamide (SHPAM) has a higher pore space utilization compared to partially hydrolyzed polyacrylamide. For SHPAM, the incremental oil recovery factor was approximately 25% with an ultimate recovery factor of 64.2% OOIP. This work implies that the lower oil recovery efficiency of water flooding gives rise to a significant potential for chemical-EOR processes in complex conglomerates.