Performance evaluation of oil displacing agents for primary-minor layers of the Daqing Oilfield

A series of experiments were conducted to investigate the molecular coil dimension（D_h） and molecular configuration of partially hydrolyzed polyacrylamides（HPAM）,surfactant/HPAM system, and a living polymer.Compatibility between the polymer coils and the porous media was evaluated by instrumental analysis and laboratory simulation methods.Meanwhile,the performance of chemical flooding was investigated.Results indicated that the D_h decreased with an increase in water salinity and increased with an increase in polymer concentration.In aqueous solution,the polymer presented three-dimensional reticular configuration and exhibited a fractal structure characterized by self-similarity. The polymer in the surfactant/HPAM system was mainly in the form of＂surfactant-polymer＂complex compound and the living polymer had an irregular＂flaky-reticular＂configuration which resulted in relatively poor compatibility between the molecular coils and the porous media.The type of oil displacing agent and its slug size influenced the incremental oil recovery.For the same oil displacing agent,a larger slug size would lead to a better chemical flooding response.Given the final recovery efficiency and economic benefits,high-concentration polymer flooding was selected as the optimimal enhanced oil recovery（EOR） technique and the incremental recovery efficiency was forecast to be 20%.

Study of action mechanisms and properties of Cr^(3+) cross-linked polymer solution with high salinity

Performance characteristics of partially hydrolyzed polyacrylamide （HPAM） and cross- linked polymer （CLP, Cr^3＋ as the cross linker） solutions have been investigated. A Brookfield viscometer, rheometer, dynamic light scattering system, and core flow device have been used to measure the viscosity, viscoelasticity, polymer coil dimensions, molecular configuration, flow characteristics, and profile modification. The results show that, under conditions of high salinity and low HPAM and Cr^3＋ concentrations, cross-linking mainly occurred between different chains of the same HPAM molecule in the presence of Cr^3＋, and a cross-linked polymer （CLP） system with a local network structure was formed. Compared with an HPAM solution of the same concentration, the apparent viscosity of the CLP solution increased slightly or remained almost unchanged, but its viscoelasticity （namely storage modulus, loss modulus, and first normal stress difference） increased, and the resistance coefficient and residual resistance coefficient increased significantly. This indicates that the CLP solution exhibits a strong capability to divert the sequentially injected polymer flood from high-permeability zones to low- permeability zones in a reservoir. Under the same HPAM concentration conditions, the dimensions of polymer coils in the CLP solution increased slightly compared with the dimensions of polymer coils in HPAM solution, which were smaller than the rock pores, indicating that the cross-linked polymer solution was well adapted to reservoir rocks. Core flood experiments show that at the same cost of reagent, the oil recovery by CLP injection （HPAM-1, Cr^3＋ as the cross linker） is 3.1% to 5.2% higher than that by HPAM- 2 injection.