Abstract:The heterogeneous phase combination flooding (HPCF) system which is composed of a branched-preformed particle gel (B-PPG), polymer, and surfactant has been proposed to enhance oil recovery after polymer flooding in heterogeneous reservoirs by mobility control and reducing oil-water interfacial tension. However, the high cost of chemicals can make this process economically challenging in an era of low oil prices. Thus, in an era of low oil prices, it is becoming even more essential to optimize the heterogeneous phase combination flooding design. In order to optimize the HPCF process, the injection strategy has been designed such that the incremental oil recovery can be maximized using the corresponding combination of the B-PPG, polymer, and surfactant, thereby ensuring a more economically-viable recovery process. Different HPCF injection strategies including simultaneous injection and alternation injection were investigated by conducting parallel sand pack flooding experiments and large-scale plate sand pack flooding experiments. Results show that based on the flow rate ratio, the pressure rising area and the incremental oil recovery, no matter whether the injection strategy is simultaneous injection or alternation injection of HPCF, the HPCF can significantly block high permeability zone, increase the sweep efficiency and oil displacement efficiency, and effectively improve oil recovery. Compared with the simultaneous injection mode, the alternation injection of HPCF can show better sweep efficiency and oil displacement efficiency. Moreover, when the slug of HPCF and polymer/surfactant with the equivalent economical cost is injected by alternation injection mode, as the alternating cycle increases, the incremental oil recovery increases. The remaining oil distribution at different flooding stages investigated by conducting large-scale plate sand pack flooding experiments shows that alternation injection of HPCF can recover more remaining oil in the low permeability zone than simultaneous injection. Hence, these findings could provide the guidance for developing the injection strategy of HPCF to further enhance oil recovery after polymer flooding in heterogeneous reservoirs in the era of low oil prices.

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Branched-preformed particle gel (B-PPG) is a novel agent for EOR application in mature reservoirs. To improve the enhanced oil recovery (EOR) performance of B-PPG, the heterogeneous combination flooding system composed of B-PPG, hydrolyzed polyacrylamide (HPAM), and surfactant was proposed. However, the synergistic mechanism of B-PPG and HPAM still remains unclear. Here, a series of experiments were performed to study the viscosities of HPAM, B-PPG, and mixture of HPAM and B-PPG, evaluate the suspension stability of B-PPG and mixture of HPAM and B-PPG, to investigate the propagation behavior of HPAM, B-PPG, and a mixture of HPAM and B-PPG through porous media, and to study the EOR performance of HPAM, B-PPG, and a mixture of HPAM and B-PPG in heterogeneous reservoirs by performing parallel sand pack flooding experiments. Results show that the suspension stability of B-PPG can be improved by adding polymer due to the increase in viscosity. The higher the HPAM concentration is, the higher viscosity of mixture of HPAM and B-PPG suspension and the better the suspension stability is. Due to the lubrication and increased viscosity effect of the polymer, compared with the B-PPG suspension, the HPAM enhanced B-PPG suspension can be more easily injected into the porous media and can propagate through porous media. The propagation behavior of HPAM enhanced B-PPG through porous media is achieved by blocking, deforming, and passing through the pore throat. EOR performance results in a heterogeneous reservoir demonstrate that HPAM enhanced B-PPG flooding shows better sweep efficiency improvement ability and recovers more remaining oil left unswept in the low permeability zones.

Thermo-sensitive poly(N-isoproplacrylamide)m-block-hydroxyl-terminated polybutadiene-block-poly(N-isoproplacrylamide)m (PNIPAMm-b-HTPB-b-PNIPAMm, m = 1 or 2) block copolymers, AB4 four-armed star multiblock and linear triblock copolymers, were synthesized by ATRP with HTPB as central blocks, and characterization was performed by (1)H NMR, Fourier transform infrared, and size exclusion chromatography. The multiblock copolymers could spontaneously assemble into more regular spherical core-shell nanoscale micelles than the linear triblock copolymer. The physicochemical properties were detected by a surface tension, nanoparticle analyzer, transmission electron microscope (TEM), dynamic light scattering, and UV-vis measurements. The multiblock copolymer micelles had lower critical micelle concentration than the linear counterpart, TEM size from 100 to 120 nm, and the hydrodynamic diameters below 150 nm. The micelles exhibited thermo-dependent size change, with low critical solution temperature of about 33-35 °C. The characteristic parameters were affected by the composition ratios, length of PNIPAM blocks, and molecular architectures. The camptothecin release demonstrated that the drug release was thermo-responsive, accompanied by the temperature-induced structural changes of the micelles. MTT assays were performed to evaluate the biocompatibility or cytotoxicity of the prepared copolymer micelles.