Micro-Displacement Mechanism of Divalent Cations Activated Polymer Flooding In A Complex Conglomerate

The conglomerate reservoirs of south block Qidong, Xinjiang Oilfield, NW China are one of the largest conglomerate reservoirs worldwide and have employed polymer-based chemical flooding pilot tests since 2016. But the output is less than 10% due to the harsh divalent cations concentration and the complex fluids distribution in microscale. It is important to design a novel divalent cations resistant polymer and then to elucidate its micro-reserve displacement mechanism in a complex conglomerate. A novel core-shell like polymer (CSPAM) composes of nanosilica as the core and polyoxyethylene modified polymeric chains as the shell is synthesized via a facile method. The thickening performance of CSPAM activated by divalent cations is explained using advanced rheometer and scanning electron microscope. A visual conglomerate lamination model combined nuclear magnetic resonance online experiments are conducted to investigate the micro-unrecovered oil displacement mechanism of CSPAM flooding: the distribution of micro-remaining and micro-residual oil after the earlier waterflooding are quantified; latter, the reserve utilization and displacement behavior of CSPAM flooding on the pore space scale are characterized. Within the Ca2+/Mg2+ concentration of 12000 mg/L, the electrostatic bridging between Ca2+/Mg2+ and ethyoxyl groups of CSPAM induces an interlacing transient-network, which activates a tenfold increase of viscosity and of longest relaxation time compared to these of CSPAM solution without adding salt. The conglomerate rock shows a complex pore-and-throat structure, resulting in severe heterogeneity and rapid water cut. The oil recovery by the earlier waterflooding is approximately 35% of OOIP (original oil in place), and the cluster of remaining oil in a disconnected state and oil resident in blinds pores occupies a major proportion of unrecovered oil. Significant reduction of oil saturation is achieved by CSPAM solution at low concentration of 1000 mg/L in harsh brine. Approximately 28% of incremental oil recovery factor with cumulative oil recovery higher than 63% OOIP is achieved by 0.5 pore volume of CSPAM flooding and chase waterflooding. The reserve utilization of CSPAM is 23.1% for large pore spaces, 13.2% for intermediate pore spaces, 11.6% for confined pore spaces, and 5.67% for minimum pore spaces, respectively. This research constructs a novel water-soluble polymer CSPAM with divalent cations activating viscosification, and elucidates the micro-displacement mechanism of CSPAM through quantifying the remaining oil and residual oil distribution, and reserve utilization of CSPAM on the pore spaces scale. The results will provide the substance and technique supports for future application in harsh salinity and complex pore structure conglomerate reservoirs.